Software Development Issues and Solutions

Detecting a Connect Disconect of USB reader in Linux

2022-05-03T00:46:00.015+05:30

Sometimes when you connect a USB device or external device through USB to a system file USB gets connected and disconnected automatically. Sometimes it get connected and disconnected automatically. That causes lot of problem while doing read and write operation. Sudden disconnect creates problem in all such operations. I too faced similar situation recently. One NFC reader was connected to my system through a USB. I have developed a desktop application which was continuously sending some command to the reader and displaying the response from the reader on the desktop GUI. The pooling command had sleep of 50ms. So what happens sometimes my operation failed as the NFC reader gets disconnected and gets connected. While sending command to reader and there is a disconnection then reader will send failed response and that helps me show the error message. Problem arises when the reader get disconnected from USB device during the sleep period and application sends commands in between. During that period the command does reach to the reader as it is disconnected.

Problem here is the application on the system should know when the USB get disconnected so that application can take necessary action so that the application does not fail.

Fix:

Frankly saying Initially I did not know what to do. So for me the first solution was reducing the sleep time so that I can notice the connection failure. Though this was to some extent helped me to partially solve the issue there is still a chance of disconnection and connection happening between the sleep time provided. So I need to find a way to notify the application that disconnection happened, and I need reset my application. Basically, we need to know when a USB device is plugged and unplugged.

So for that I did a little bit search on Google and find there are two ways one can determine when connection and disconnection happened. One is through Udev and second one is through Inotify.

Use of Inotify : As you know everything in Linux is treated as file. Inotify basically used to notify when there is any change in a file. So what you have to do is find the file name of the device connected to USB and set Inotify to get an updated on any change in the file. I am not going to give more details on that as I have not used the same. I will certainly put some reference at the end of the blog that can help others in future.

Udev : I found udev event is the most simplest way to detect USB plugged or unplugged to a device. As you know Udev is subsystem of Linux that provides device events. In Linux all devices are put under /dev category and any device added or removed usually find in this directory. There is a tool called udevadm whihc basically used to manage or monitor the udev events. To know more udevadm you can visit the link shared below. I will give few examples of udevadm here below for reference.

To run udevadm and monitor the events run the following command.

udevadm monitor –environment

If you plug or unplug any device to the system, you will see the response on the screen as shown below in the picture.

Hope this will give a fair bit of idea how the udev works. Now let us focus on how I solve the issue. I created two shell script files . One for when the USB gets connected or added while the other script was for when the USB device gets disconnected. There is nothing special in the script files. I am just writing two different values to a file based on whether the USB is plugged or unplugged.

The content of the script file is somewhat as shown below.

File Name: device_connect.sh

#!/bin/bash

echo "0 USB Device connected at $(date)">>/home/user/log/myReader.log

File Name: device_disconnect.sh

#!/bin/bash

echo "1 USB Device disconnected at $(date)">>/home/user/log/myReader.log

Note here I am writing different value to myReader.log file depending upon whether the device is plugged or unplugged.

Now the script files had been created. But we need to ensure the script files get executed when USB device plugged and removed from the system. For that we need to create some rules and remember udev allow users to create their own rules. Create a rule in /etc/udev/rules.d folder and save it.

How to create rules:

To create rule you should have prior knowledge else you need to check default rules which is there in /etc/udev/rules.d folder. If the rules are not there then it may be /lib/udev/rules.d folder. Regarding creating rules you can check the reference URL given below

For creating rules we need to know the “SUBSYSTEM”, “Vendor ID”, “Product ID”,”Action”, “RUN”. You can run the udevadmin in monitor mode to check these values. You can get all the attributes and values from there as shown below. But I got these values uing lsusb and dmesg command as shown below

pramod@pramod-laptop:~$ lsusb

Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub

Bus 001 Device 004: ID 0a5c:216d Broadcom Corp.

Bus 001 Device 003: ID 04f2:b56c Chicony Electronics Co., Ltd

Bus 001 Device 024: ID 1db2:0650

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Highlighted one is my USB device.

pramod@pramod-laptop:~$ dmesg | grep usb 1-2

[30743.688932] usb 1-2: new full-speed USB device number 24 using xhci_hcd

[30743.839216] usb 1-2: New USB device found, idVendor=1db2, idProduct=0650

[30743.839222] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3

[30743.839226] usb 1-2: Product: Card Reader

[30743.839229] usb 1-2: Manufacturer: DUALi INC.

[30743.839232] usb 1-2: SerialNumber: 082312130078

So a rule created after using the values will be look like the one shown below.

SUBSYSTEM=="usb", ACTION=="add", ATTRS{idProduct}=="0650", ATTRS{idVendor}=="1db2" , RUN+="/bin/device_connect.sh"

SUBSYSTEM=="usb", ACTION=="remove", ATTRS{idProduct}=="0650", ATTRS{idVendor}=="1db2", RUN+="/bin/device_disconnect.sh"

RUN+ = means the script you want to execute after the action happened.

Please note the script you mentioned should be placed under /bin directory and have the execute permission. Otherwise you may get “Permission denied” error.

After rules created you need to reboot you machine to reload it otherwise you need to run the following command to reload the rules.

Udevadm control --reload rules

Now you are ready and you can test by plugging and unplugging device. The script will rung and write the value to the myReader file i created.

One most important thing to note that you need to have the create, read and write permission on the folder. If you don't have the permission your script might be executed but you may not get the desired result due to permission issue

Reference :

1. https://www.linuxjournal.com/content/linux-filesystem-events-inotify

2. https://www.linuxjournal.com/content/linux-filesystem-events-inotify

3. http://www.mycpu.org/handle-usb-disconnect-properly/

4. https://unix.stackexchange.com/questions/470960/how-can-i-debug-a-udev-rule

5. https://opensource.com/article/18/11/udev

6. https://man7.org/linux/man-pages/man8/udevadm.8.html

7. https://www.linuxfordevices.com/tutorials/linux/udevadm-command

8. https://www.thegeekdiary.com/beginners-guide-to-udev-in-linux/

Permission Denied Error on Shell script

2022-05-01T23:44:00.000+05:30

Recently I was working on a shell script which is very simple in nature have only few lines of code.When I tried to run the script I got infamous “Permission denied” error . The error itself is self explanatory that the user does not have the required permission. Suppose you have logged in as root user you may think you have all the required permission. To check what all permission you have on the file run ls -l command on the directory to view the file permission. Suppose you are in home directory and if you run ls command with -l option you will view a response as shown in below image.

Fixing the Issue :

If you dont have the permission then use chmod command to provide required permission for the user. Badically the .sh script file should have the execute permission for the user to run. Run the following command to provide execute permission.

chmod + x <Name of the script file>

otherwise you can also provide execute permission by providing the following command

chmod 777 <script file name>

Please note permission denied error can be due any file permission. In my case I need execute permission to execute the script file. But you can get this error while trying to read and write file on which you don't have the permission.

But my story does not end here. It is bit longer and I thought I need to share this so that it can help some readers in future.

Please note when executing a shell script you have lots of Linux commands that is getting executed when your script runs. In my shell script I was doing a file read write operation depending upon certain condition. So even after providing execute permission I was still getting “Permission Denied” error. It took me sometime to understand why I am getting the error. On my oversight initially I could not read the error message carefully and got overawed by just viewing "Permission denied" error. I will suggest when you get this error please carefully read the error message. The message will clearly show the line or file on which you got the “Permission Denied” error. As I told you before I was doing file read/write operation on a file. As I had logged in as a root user I though I have all the permission on all the files which is a bit oversight from my end. I checked permission of the file and find root user does not have write permission on that particular file. I again use the chmod command to provide write permission to the user. After that my shell script run thoroughly without any glitch.

Reference :

1. https://fedingo.com/how-to-fix-permission-denied-error-while-running-shell-script/

Working with RXTXcomm.jar for Ubuntu 16.04 LTS for serial communication

2022-04-28T23:34:00.001+05:30

I have been working on IOT based project where I have to connect with a NFC device and communicate with the device using serial communication. I have used RXTXComm.jar on window. Windows 10 is my development environment. I started developing the application, and when almost finished I came to know that my production environment is Linux. Basically, I need to run the application where the system has Ubuntu 16.04 LTS as OS. As most developer knows RXTXComm.jar uses a dll file called “rxtxSerial.dll” for window OS to communicate with serial ports. On migrating java code to Ubuntu I need to add the shared library(.so file) for “rxtxSerial.dll” to communicate with the server. That is not a big challenge as these files are available on the web and can be downloaded from the URL below.

http://rxtx.qbang.org/pub/rxtx/rxtx-2.0-7pre2/INSTALL.

You will get all the .so files as per your system requirement. Whether your system is 32 bit or 64 bit you can use the respective .so file from the downloaded resources. Make sure your follow the instruction for installing the shared library. I got the required shared library(.so file) for RXTXComm.jar and able to run my GUI application for serial communication. The .so file should be in your java library path or you can add it to source folder. Here too there was another surprise for me. I could not get COM ports connected to my system using the API available in RXTXComm.jar. In my system (Ububtu 16.04 LTS) ttyUSB0 is the only COM port available and it was clearly visible when I run “lsusb” or “dmesg | grep ttyUSB” command from terminal. But it was not showing in my application where I was populating a combo box with available serial ports. After a lot of search on Google I came to know that we need to provide correct permission ttySxx or ttyUSBxx which is basically in /dev folder. The user you are logged in must have to be in “dialout” group. If you run a ls-ltr command as show below you can view the serial ports with their permission.

$ ls -ltr /dev/tty*

crw-rw---- 1 root dialout 4, 65 2022-04-28 01:08 /dev/ttyUSB0

crw-rw---- 1 root dialout 4, 66 2012-04-28 01:08 /dev/ttyS1

crw-rw---- 1 root dialout 4, 66 2012-04-28 01:08 /dev/ttyS2

To add the root user or any user to dialout group use the following command.

sudo usermod -a -G dialout username

After running the above command I could able to get list of COM ports. In my case there is only one USB COM port named ttyUSB0. Still there is a challenge as the production environment where my application will work is 32bit device. Here my PC on which I was testing the application is 64 bit. To make sure the application runs on 32 bit Ubuntu, I created a runnable jar file. I copied the RXTXComm.jar and librxtxSerial.so in the same location where my runnable jar. Then I used the following command to run my runnable to jar to view the GUI.

$ sudo java -jar myapp.jar

Application run successfully and I did able view the COM port, connect the COM port and communicate with the device connected to the COM port.

Hope this post helps the budding developers and they should not face the same issue as I did. Lastly I put down the refence sites that helped me to solve the above issue.

References :

1. http://rxtx.qbang.org/wiki/index.php/Installation

2. https://stackoverflow.com/questions/38753097/unable-to-find-serial-ports

3. https://stackoverflow.com/questions/15533310/rxtx-cant-list-port-on-ubuntu

Building a 32 bit application on 64 bit window

2022-04-28T12:04:00.001+05:30

Life is not that easy as it seems particularly if you are developer. I hope all of the developers reading this will agree with me. I have windows 10 system having visual studio 2017. I have to compile a C# GUI application that can run on 32 bit windows specifically that can run on a windows XP. Does not that sound very easy? You can do this by just setting the build configuration as shown in the below image then build the application. Seems like a cake walk, is not it?

I did the same thing and compile the application and copy the application on windows XP machine to run on the application. But it did not run on that particular machine I had copied to. I could not understand what could be the issue. Every time I tried to run the application it throws an error “ Not a valid win32 application”. I searched a lot on Google. People advised to compile it for x86 rather than x64. Since I had already compiled that for 32 bit it should not be a problem in my case. Though I tried to implement their suggestion it did not worked for me. Some suggest to check the “.NET target framework” for xp. Since I am not using any framework this option also ruled out. Lastly after a lot of effort someone suggested me to check platform toolset and set it to XP as shown in the below picture.

Remember platform toolset must be set to “Visual Studio 2017 -Window XP(V141_xp). If you don’t have the toolset then please download and install it from Microsoft site. After making the changes I am able to run my application on windows XP machine. Hope this helps some developers while they compile their application for targeting Windows XP or similar OS.

Below is the list of sites that helped me solve the issue and I think it is my moral obligation to mention their name.

Reference Site :

1. https://social.msdn.microsoft.com/Forums/vstudio/en-US/990fb1a1-3940-421b-ab13-7ddcde5c00d5/exe-is-not-a-valid-win32-application?forum=vcgeneral

2. https://social.msdn.microsoft.com/Forums/vstudio/en-US/47ef555b-99fb-411c-9a13-099b094884d5/not-a-valid-win32-application?forum=vbgeneral

3. https://social.msdn.microsoft.com/Forums/en-US/f699eb18-4090-479c-a2ac-a3c68bc46549/vs-2017-build-x86-for-2003-r2-exe-is-not-a-valid-win32-application?forum=vcgeneral

File Manager Crash in Raspberry Pi3

2020-09-07T00:06:00.001+05:30

Working on Raspberry Pi project, I updated the OS using normal apt-get update command. After update I suddenly found the File manager is crashing. On clicking the file manager, Raspberry Pi opens the file manager window and closes immediately after that. Searched a lot on google for the reason for the same. Came to know that the file manager application “pcmanfm” had been corrupted. Developers in the Raspberry PI forum mainly suggest to update and upgrade Raspberry Pi to get it corrected. In most of the cases if you try a normal update you may not able to resolve the issue.

I tried the following commands for fixing the issue.

sudo apt-get update

sudo apt-get full-upgrade -y

sudo autoremove -y

sudo apt-get reboot

In most of the cases the above steps will solve the file manager crash issue. Now the file manager can behave properly. But to my bad luck even trying the above four commands I could not recover the file manager, it crashed every time I tried to open file manager. To get the actual reason of crashing of the file manager try to run file manager from terminal using the following command.

sudo pcmanfm

You will get a message like the one shown below.

“** Message: 09:18:54.081: x-terminal-emulator has very limited support, consider choose another terminal

Segmentation fault

Since the above options not worked, it is better to try and reinstall “pcmanfm” again. Use the following command to reinstall “pcmanfm” on Raspberry Pi.

sudo apt-get install –reinstall pcmanfm

After successful reinstall try to run pcmanfm from the terminal. Now the file manager should open file manager and you can do what all your file operation using GUI.

Working with GSM module in Raspberry Pi 3

2020-09-06T22:46:00.005+05:30

Last few days I had tried my hands on working with GSM module with Raspberry PI 3. I had Raspberry PI 3B device on which I have used SIM 800 module having RPI GSM ADD-on V2.3. My PI device have native Raspbian buster as OS. Trying set up the SIM with the GSM module take bit of time connect to internet. I am using an Airtel 4G SIM for internet connection I need to search a bit on Google to set up the GSM module for an active internet connection and start browsing. Listing here the steps I have used.

Hardware Used:

1. Raspberry PI 3 B

2. GSM module SIM 800

3. Airtel SIM

4. 7”” inch Waveshare LCD for display

5. USB charger for (5V 1 amp for Raspberry PI)

6. Two additional USB charger for power supply to GSM Module and LCD)

7. Keyboard and mouse

Connection: Put the SIM in the SIM holder of the GSM module. Place the GSM module on top of Raspberry PI using 32 pins. Connect the LCD to raspberry pi using the cable

APN SET UP: All of you must know what APN is. It is access point name of your operator. Since I have used an Airtel SIM, I know the APN name. It is airtelgprs.com. If you have a different operator you, please note the APN that we will later use for setting PPP configuration.

PPP Configuration and installation: PPP stands for point to point protocol, which basically used for node to node communication using serial interface in PC. We will use PPP and serial connection to access internet in Raspberry pi.

To install PPP software, open the terminal. Enter the following two commands one after another.

sudo apt-get update

sudo apt-get install ppp screens elink

After successful installation of PPP software, you need to setup PPP peer configuration. For this we need login as root. By default, in raspberry pi you logged in as default user pi. At least in my case I had logged in as user pi. So, to log in as root user type the following command on the terminal and enter.

sudo -i

After logging in as root use cd command as shown below to move peers directory.

cd /etc/ppp/peers/

Now create a new file and name it anything you like and remember the name. Use any editor you want. I have used nano editor as shown below to create a file called airnet

sudo nano airnet

Enter the following text as shown below.

#airtelgprs.com is the APN for Airtel connection. Note in the above line put APN name of your operator after -T.

connect "/usr/sbin/chat -v -f /etc/chatscripts/gprs -T airtelgprs.com"

# For Raspberry Pi3 use /dev/ttyS0 as the communication port:

/dev/ttyS0

# Baudrate

115200

# Assumes that your IP address is allocated dynamically by the ISP.

noipdefault

# Try to get the name server addresses from the ISP.

usepeerdns

# Use this connection as the default route to the internet.

defaultroute

# Makes PPPD "dial again" when the connection is lost.

persist

# Do not ask the remote to authenticate.

noauth

# No hardware flow control on the serial link with GSM Modem

nocrtscts

# No modem control lines with GSM Modem

Local

There are few other parameters available for setting up the configuration and you can try those. But the above lines are sufficient for the configuration Now save the file and close it

Edit cmdline.txt file:

Now use the following command to edit the cmdline.txt in the boot directory.

sudo nano /boot/cmdline.txt

Remove all reference to console in this file as shown below

Before Edit: dwc_otg.lpm_enable=0 console=tty1 root=PARTUUID=07312df9-02 rootfstype=ext4 elevator=deadline fsck. repair=yes rootwait quiet splash plymouth.ignore-serial-consoles

After Edit: dwc_otg.lpm_enable=0 root=PARTUUID=07312df9-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait

Edit hciuart.service file: Apart from changing cmdline.txt file we need some minor change in the hciuart.service file in the /lib/system/system folder using below command.

Sudo nano /lib/systemd/system/hciuart.service

Use “#” to comment out the line having below text.

After = dev-serial.device

Execstart = /usr/bin/bt-uart

Add the new line shown below.

Execstart = /usr/bin/hciattach/dev/ttyS0 bcm43xx 460800 noflow-

Save the file and close the editor

Edit config.txt file: Open the config.txt file using any editor. I have used the following command to edit config.txt

sudo nano /boot/config.txt

Add the following lines at the bottom of the config.txt file and save the file

dtoverlay=pi3-miniuart-bt

enable_uart=1

force_tourbo=1

Check Connection

Run ifconfig command from terminal to check connection type available on your raspberry pi. You will find “eth”, “lo” and “wlan”. Now type the following command on the terminal to create a new connection

sudo pon airnet.

Now again run the ifconfig command on the terminal. Now you will see a new section called PPP0 with an IP address. Your network LED will blink quickly and continuously on your GSM module showing the connection is on.

Now you can open your web browser in your raspberry pi 3 device and browse websites.

If you want to disable or close the PPP connection, use the following command.

sudo poff airnet

Refernce:

1. https://www.instructables.com/id/Raspberry-Pi-With-a-GSM-Module/

2. https://www.rhydolabz.com/wiki/?p=16325#:~:text=Make%20the%20connections%20as%20shown,get%20registered%20to%20the%20network.

Cross compiling a C project for raspberry pi

2020-09-04T01:24:00.003+05:30

Developing an application for Rapsberry pi can be done using Python, QT or C or C++. Only thing to keep in mind while compiling is to cross compile the C and C++ files for ARM architecture. For this you can change the make file and set the ARM compiler path for compiling the files. Problem arises when you are developing a project in Python and want to use C code along with it. This is not a tough task to achieve but tricky one to do so. You can call C function from python in two ways. You can create a wrapper of the C function that you want to use in python. To know more about wrappers you can visit this page or google it. Second option is to create a shared library(.so file) and use the function with the help of ctypes. If you have more than number of C files and the files calls functions of other C files, I will prefer to use ctypes after creating a shared object file. Here I will explain how to cross compile C files to create a shared object and how to call functions of the shared object using ctypes.

In my Ubuntu 16.04 I had given the following command to compile my C files to create a shared object.

gcc - fpic -shared -o <Name of .so file> <Name of C files with path>

Example : I have to create a shared object library having some math functions and string functions, the final command for creating the shared object library will be

gcc – fpic – shared – o utility.so utility_main.c mathUtil/mathfunc.c stringUtil/stringfun.c

You can have all your C file separated by space

This will create utility.so file which can be used in my python code using ctypes. If you want to use your python code in Raspberry pi or any other ARM architecture device you need to cross compile the library using the ARM cross compiler. In my case I am using Raspberry pi 3B and my cross compiler is arm-linux-gnueabihf-gcc. So I will just change the compiler in the above command to cross compile my application for ARM architecture. You can use your cross compiler.

Here is my command for cross compiling utility.so shown above.

arm-linux-gnueabihf-gcc -fpic -shared -o utility.so utility_main.c mathUtil/mathfunc.c stringUtil/stringfun.c

Now you can copy this .so file to your python project. In your python file add “import ctypes” in the top where all import files are called. Then create object of the shared object and call the function you want to use in your Python module

Example : If I have to call addNumber function my utility shared object in my python file I have to do the following steps.

1. Import ctypes

2. obj = ctypes.CDLL(utility.so')

3. z = obj.addNumber(24,35)

How to run a program during start up on Linux using systemctl

2020-07-10T19:04:00.000+05:30

How to run a program during start up on Linux

Use the following steps to run a program on start up or booting on Linux machine. Any program that needs to be run during start or boot is termed as service. We will use systemd to run a service in the background during startup. systemd is a system and service manager for Linux, also capable of replacing the traditional SysV init system

1. To create a service run the following command

sudo nano /etc/system/system/NAME_OF_YOUR_SERVIC.service.

2. nano editor will open a blank file. Put the following command and save the file.

Description = DESRIPTION_OF_YOUR_SERVICE // A small description of your service here.

Wants=network.target // This means the service will start after the network is ready.

[Service]

Type=simple

ExecStart="Application/script name with path" ex: /home/pramod/mybgapp.sh

Restart=on-failure

RestartSec=10

KillMode=process

[install]

WantedBy=multi-user.target

3. Once the service is created we need to reload the service using systemctl. Use the following command to reload the service.

Sudo systemctl daemon-reload

4. Use the following command to enable the service.

Sudo systemctl enable YOUR_SERVICE_NAME

5. To start the service use the following command

Sudo systemctl start YOUR_SERVICE_NAME

6. To check if the service is running or not use the following command.

Systemctl status YOUR_SERVICE_NAME

7. Now reboot the system. Application will start automatically else crash if failed.

Consuming Web Service in C#

2018-04-08T23:29:00.001+05:30

Introduction

RESTful web service today becomes a norm in every mobile and web application. RESTful web services can be created by any language. Here I will explain how to consume a web service from C# application. I have developed a webservice using Java Springboot and mysql database. Note the web service created consumes and produces json data. The web application is hosted on a tomcat application server which is behind https web server.

Client Component

I have developed a C# application that will consume web service to show the data in the desktop interface. I have used windows desktop application using Visual Studio 2017 with .Net 4.5 environment. The Desktop application list of bus routes of local city transport. The sample response from the server seems like the one below.

As show in the above picture the Json response have 3 major item. One is status, which is of numeric type, other one is message which is of string type and the last one is it self a Json object. We need to create a http request to the server and the request should be done using post method as the service works only using post method. For the particular request we need to pass a Json object in the body having two parameters as shown in the picture below. Request parameter will change depending upon the service requested.

There are two parameters required for this service. These are request parameter which needs to be passed in the request body as a json object. If you have a web service that needs to send request parameter to server for processing remember to pass them in the body method as a Json object if your server application expecting a Json object in the request method.

HTTP Request :

I have created a function to make a http request to the server. The function takes four parameters as shown in the below picture,

First parameter is requestUrl is string type parameter that holds the url that needs to be called for the web service. Second parameter JSONRequest, is a Json object that holds request parameters. This is passed in the request body. Third parameter is called JSONMethod which is string type. The JSONmethod can be “GET”, “POST” or “DELETE” depending upon the service type requested. For this particular service type the method is “POST”. The last parameter is JSONContentType is a string type parameter that identifies the content type of the request that is passed in the request header. The function takes returns a JSON object as shown in the above picture. Most part of the function is self explanatory. The first line of the function is mentioned below.

ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

This is required as the server uses self signed SSL certificate and my desktop does not have the certificate. I have added this to ignore SSL certificate while doing http request from the client application. For more details you can visit this link.

Since I am using JSON object in both http request and response, I have used Newtonsoft.json to manage the json object. It has the inbuilt method to convert objects to josn and vice versa making life simpler for the developer.

string sb = JsonConvert.SerializeObject(JSONRequest);

In this line the I am using Newtonsoft.json to converts json object to String while sending request to the server.

jsonObject = JsonConvert.DeserializeObject(jsonresponse);

In the above line I am converting the http response from the server while is of string type to json object. For more information related to Newtonsoft.Json you can visit their website. For sample application using C# with Newtonsoft you can check thiss link.

nested exception is org.springframework.transaction.CannotCreateTransactionException

2017-08-26T18:30:00.000+05:30

If you are java developer and using Spring and Hibernate to develop application, you might have faced this exception sometime during the development process. The exception gives a 500 internal server and the error message will be something like the one mentioned below.

HTTP Status 500 - Request processing failed; nested exception is org.springframework.transaction.CannotCreateTransactionException

If you check the stack trace of the error you may fight the line of the code that is responsible for the exception.

Cause:

As the exception show it was not able to perform the transaction, means there is some issue either in the code, or in the database that is failing the transaction.

Solution:

So, what could be reason for the transaction failure or creation? Check if all your transaction was getting failed for different tables on different pages. If the answer is yes, then follow the following steps to find out the issue.

1. Check if your database server is up and running. If the database server is down, your application could not connect to database. Hence transaction creation will be failed. If the database is up and still facing the same issue follow the next step below.

2. Check database credential you are using to connect to server. I mean the username, password for the database. Make sure you have the correct and updated database username and password. If your application was working with the existing username and password and not working, check with your database admin if the credential had been changed.

The above steps will help you to resolve the following issue. In case if you still facing the issue comment on the post with details. I will try to get back to you

org.hibernate.PropertyAccessException: Null value was assigned to a property of primitive type.

2017-08-26T14:07:00.000+05:30

Some of you might have faced the above exception and some of you might not as the exception does not happen on normal scenario. Therefore it is a bit tricky to resolve the issue for the novice developers working on hibernate. As the error suggest assignment of Null value to primitive data type. That means you are assigning null value to any variable that has been declared as primitive data type.

Cause:

The specific reason for the above exception depends upon the application you are developing, but the main reason for the exception you are assigning null value to a variable declared as primitive data type. So, if you have a variable of boolean, char, byte, short, int, long, float, double and assigning null value in any way. Check the stack trace of the exception and find the exact line in the code.

Solution:

1. In the hibernate application check the setter method of the variable for which you are getting the exception. Make sure to check the value you are assigning is not null. In case the value is null then assign default value.

2. In case your application getting values from table of a database, check if the field in the table contains null value. Update the table with default value for existing records. Add a default value for the field in the table so that in case of new record, it will have a default value rather than null, this helping the setter function to assign default value to the variable declared in the class

3. Thirdly instead of using primitive data type, you can use wrapper class to declare the variable for which value is being assigned, for example you can use Integer for int or Float for float.

HTTP Status 404 - description The requested resource is not available in Spring MVC application

2017-08-20T22:59:00.000+05:30

Those novice java developers who had been working in spring MVC, sometimes might have faced this error while running the application. The above error usually occurs in spring mvc application. It shows name of a jsp page as not available which is associated with the application. This error is bit more confusing as the jsp page itself is there in the application. Since the page is available you are not sure why this exception is coming. It means there is something wrong in the controller class. You need to revisit your controller class again. Please check if you have defined the method in the RequestMapping of your controller class as shown below.

@RequestMapping(value="/submitAdmissionForm.html",method=RequestMethod.GET)

If you think that you have
created the method and still facing the issue, then check spelling of your
method names being called from the form or the servlet.xml. There might have
been some spelling mistake that’s why you are getting the issue. A simple
spelling mistake can take hours to debug and resolve this kind of issue if you
are new to spring MVC development.

javax.validation.ConstraintViolationException: validation failed for classes

2017-08-06T17:42:00.000+05:30

If you are working on Hibernate and face this kind of exception, then don’t be surprised. The exception is self-explanatory. It says there is a constraint violation for validation. If you check the stack trace it can show you the class and the line of the code where the exception had been occurred. The most probable cause might be validation rule applied in the model class. I am giving an example here which mostly happen during the development. Let me say I have an user_account table having Id. username, Password, email, account_status. All the fields are of String data type except the account_status which is of small Integer type and Id is auto incremented Integer number. Suppose I have created a model for the user_account table like the one below.

@Entity(name="user_account")

public class userAccount {

@Id

@GeneratedValue(strategy = GenerationType.AUTO)

private Integer id;

@NotBlank(message = "Please enter your username.")

private String username;

@Size(min = 8, max = 15, message = "Your username must between 8 and 15 characters")

@NotEmpty(message = "Please enter your password.")

@Size(min = 8, max = 15, message = "Your password must between 6 and 15 characters")

private String password;

@NotEmpty(message = "Please enter your email.")

private String email;

@Size(min=1, max=2)

private Integer account_status

// getter setter methods to be called below this line

}

So in the above model class though seems to be ok, you can get exception if you make simple mistakes. For example, you have set the password max limit to 15 and you a 15 character or less value as your password. It seems perfect till now. But if you are hashing your password for security purpose the length might exceed 15. Once the length of the password exceeds 15, your model will throw the above exception as the validation set to 15. So you have to increase the length of the password in the model

Second one is, if you have checked the model I have set the size of an account_status. Size validation does not work for Integer type field. So we can have our second exception at that above point.

If you are using hibernate validation, make sure to read the hibernate validation document and follow accordingly otherwise the above exceptions will keep bothering and can take considerable development time to debug and resolve the issue.

org.hibernate.exception.SQLGrammarException: could not execute query

2017-08-03T23:26:00.002+05:30

Developers those who are working in Hibernate must have come across the above exception sometimes. It is not a big issue but certainly is a confusing one. it certainly takes your time if you are new hibernate and not observing the exception stack trace in detail. I will explain where you might be gone wrong to get the exception and how to debug it easily.

The exception itself is self explanatory. Though the exception says “ org.hibernate.exception.SQLGrammarException”, but it has nothing to do with grammar. It means there is some error in the sql statement executed by hibernate. So where to check for the error in the sql statement?

First if you are using named query or model, please check the entity name is correctly mentioned. The entity name should be exactly same the table name in the database. For example, we declare employee in the model class as shown below

@Table(name="employee")

Hence we should have employee table in our database. Any spelling mistake in the table name can

throw the above exception.

Secondly in the model class check if you are using correct column names. Any incorrect column name

could through this exception. For example @Column(name="emp_name") should match the emp_name

column field of the associate table. If you are using hibernate-mapping  and mapping columns as shown

in the below example, make sure the column name in the property  should match the column name in the

table

Example  name="employeeName"  column="emp_name"/>

So the column name in the table should be emp_name. Any spelling mistake in the column name can

through the above section.

Hope this helps. Any more information and suggestion will be welcomed.

org.hibernate.MappingException: Unknown entity

2017-08-02T23:07:00.001+05:30

Developers working in Hibernate must have come across the above error some point of their life, particularly those working initially. I am not saying everybody faces the above issue, but sometimes most of the beginner come across the issue.

If you check the stack trace, you will see the message “org.hibernate.MappingException: Unknown entity:” followed by a model name. In the stack trace you can see list of exception somewhat like the one mentioned below.

at org.springframework.orm.hibernate3.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:679)

at org.springframework.orm.hibernate3.HibernateAccessor.convertHibernateAccessException(HibernateAccessor.java:412)

at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:411)

at org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)

at org.springframework.orm.hibernate3.HibernateTemplate.delete(HibernateTemplate.java:837)

at org.springframework.orm.hibernate3.HibernateTemplate.delete(HibernateTemplate.java:833)

I have just mentioned few lines of the exception shown in the stack trace as putting whole stack trace will take a lot of space which is not required. The error is bit confusing and takes a few minutes to few hours depending upon the experience of the developer. The reason for the exception is how ever quite simple and taking a little care will resolve the issue.

I found two issues for the above exception which I mentioned below

1. As you know both hibernate and JPA has @Entity annotation. Eclipse was automatically importing org.hibernate.annotations.Entity instead of javax.persistence.Entity annotation, or you might by mistake used the hibernate.annotation.Entity instead of Javax.persistent.entity in your model class. So make the changes to your model class, recompile the code and try to run the code now. It will solve the issue in case you have made the above mistake else follow the next instruction.

2. If you have imported javax.persistent,entity in your class and still facing the above issue then it is time to visit your application’s servlet.xml file. Check you bean for sessionFactory. Check property named annotatedClasses and see in the value section if you have mentioned the model name correctly. If you don’t have the model name here then add it or if you have misspelled it, the correct the name of the model. Save the changes and compile and run the application.

In my case I have used and hibernate 4.1 and spring 3.6 and came across the above exception when by mistake I have missed to add the model class in the annotatedClasses property file of my servlet.xml

Getting Started with R Language

2017-02-26T21:31:00.000+05:30

R is an open source powerful statistical programming language. It was modeled after S language which was developed in 1980 in AT&T laboratory. R language has a website named www.r-project.org. Any one wants to start learning R can find all the resources here. R language is available in all three major OS like Windows. Linux and Mac.

The Comprehensive R Archive Network (CRAN) website https://cran.r-project.org is a network of websites for R language across world.

MSI installer can be downloaded for both 32-bit and 64 bit version of windows.

The file with a name like R-3.3.2-win32.msi or R-3.3.2-win64.msi can be double-clicked to install on your desktop. The size of R-3.3.2 is of around 71MB and R Studio is of 85MB.

R language had many built in features that helps in data analysis and resolve many statistical data problem.

You can start learning R using R studio. The command can be written on R command prompt and result can be shown below. Using help() command on R command prompt you can get help on any topic on R. Note R command are case sensitive like Java.

R comes with lots of built in packages. Developers can also download different packages from CRAN website in case the packages are not available. To install any package on window just run the following command from R command prompt.

>Install.packages(“packagename”); Ex : install.packages("car")

Remember you need to have admin rights for installing packages.

To load library to your workspace use library command as shown below.

Library(“packagename”); Ex - library(“car”)

To know your working directory use the getwd() command from R command prompt. Similarly you can set your working directory to some other directory by using setwd() command.

With the above basic info one get ready to learn R language. I will give some more information about vectors in my next blog.

AngularJS Directives

2017-01-28T18:15:00.000+05:30

In my earlier blog I had explained the basics of AngularJS. Now onwards I will explain some of the components of of AngularJS are Directives, Scope, Controller, Data binding, Filters, Services and templates, Modules, Expressions, Dependency Injection, Modules, Providers and Factories. In today’s post I will explain what is directive and and different types of directive and it’s usage.

Directive: Directive is a class that is decorated with @Directive. Directives helps AngularJS to extend HTML.

AngularJS has a set of built-in directives which offers functionality to angular applications like ng-app, ng-model, ng-bind. There are three type of built in directives in AngularJS. They are component, structural directive and attribute directive.

Component: A component is a directive with added template feature.

Structural Directive : Structural directives change the DOM layout by adding and removing DOM elements. ngIf, ngSwitch and ngFor are some of the structural directives.

Example of structural directive

Structural Directive :

So the above statement will be displayed when the ngIf condition will be true else it will not be visible. Developer can also put a new statement to be displayed for the false statement. Since the structure of the DOM layout getting changed on run time on condition basis it is called structural directives. More about structural directives can be learn from various tutorials provided on Angular web site

Attribute Directive: The Attribute directive changes the appearance or behavior of an element. For example how an element will look or behave in a HTML. NgStyle is example of attributive directives. The NgStyle directive can be better used to set many inline styles at one go.

Example of attribute directive :

Here we have used ng-style directive to the

element. Hence all the text will be printed in blue with in the div tag whether they satisfies the condition or not. To know more about ng-style you can check documentation angular official site. Some of the major directives in AngularJS are listed below.

Ng-App – This directive tells AngularJS the root element in an AngularJS application. There is only one ng-app directive in a HTML document and it is always placed in the root element of a HTML document like in the , or

tag. For more information you can google or check angular documentation for ng-app

Ng-Model – The basic uses of this directive is to bind the value of an input field in HTML, like (input, select, textarea) to application data. ngModle responsible for binding view to model. More information regarding ng-Model can be found in ng-model documentation

Ng-Controller – This directive controls data of the AngularJS application. In other words ng-controller directive attaches a controller class to view. This helps Angular to implement MVC or MVW design pattern. For more information you can visit Angular documentation on ng-controller.

Ng-Bind – The directive helps to bind the innerHTML of the element to a specified property of data model. We can say ng-Bind replaces text or information of a HTML element to which a property of model is bind using the ng-bind directive. Expression which is recognized by double curly brace “{{}}” can be used instead of ng-Bind to display of contents of a model. ng-bind and expression used to do the same thing with little difference.

Ng-Repeat – As the name suggest the ng-Repeat directive repeats a set of HTML elements using iteration. It works the same way like a for loop. For iteration the collection must be an array or an object. For detail information on ng-repeat check Angular documentation

Ng-Hide – The directive hides an HTML element if the expression is true provided in ngHide attribute. The element is displayed if the ng-hide expression is false. For more informat. Also ngHide is a predefined CSS class and default element of display is set to none and !important. For more information visit angular document on ng-hide

Ng-Show – This directive works the same way as ng-hide. The only difference is it shows the element when the expression is true and hide when the expression is false. Check AngularJS document for further information on ng-show.

Ng-Click – This shows behavior of an element when the element is clicked or the other way ng-click directs AngularJS to do something when clicked.

Ng-Submit – When ng-click can be linked to buttons without a form, ng-submit always executes a function when a form is submitted. Means action ng-submit is always related to a form. For more information on ng-Submit and ng-Click you can visit the angular document section of their website.

Example: Though there are lots of examples available on net a lot of sites dedicated to AngularJS, I am putting a sample html page with some of the directives just discussed above to have an idea how these directives work.

AngularJS Overview

2017-01-22T15:55:00.000+05:30

AngularJS is a free open source java script framework mainly used for developing dynamic web application, mainly single page application development(SPA).

Little History:

Angular was first developed by Misko Hevery a Google employee in the year 2009. AngularJS 1.0 officially release in the year 2012. Current stable version of AngularJS 1 is AngularJs 1.6.1. Now it is officially maintained by Google by a community of developers. AngularJs 2.0 later released in late 2016. AngularJS 2.0 is very different from AngularJS 1.0. Details about different releases and versions can be found at Angular website or a little search on Google

Difference between AngularJS 1.0 and AngularJS 2.0.

So far as functionality concerned AngularJs 1.0 was primarily designed for frontend web development, AngularJS 2.0 was extended to cross platform mobile application development . Some of the major difference are.

AngularJS framework support’s client side development using MVC(Model view controller) and (MVVM) Model View View Model architecture. That’s why it is also called to support Model View Whatever(MVW) architecture. AngularJS 1.0 does not support mobile application development.

AngularJS 2.0 Supports many language for development like ES5, ES6 (ECMA Script 5 and 6), TypeScript of Microsoft and Dart, While AngularJS 1.0 supports only ES5, ES6 and Dart.

Setup for AngularJS 1.0 is very easy while the setup for AngularJS 2.0 is complex. On developer point of view there are many differences which I can put in a different post later. But one of the major difference is, if you are AngularJS 1.0 developer and used to $scope and controller then you have to forget the two in AngularJS 2.0

Prerequisite for Development:

If you want to start development or learn AngularJS it is very easy. You just need to prior knowledge of HTML, CSS, JavaScript and basic understanding of object oriented programming. You don’t need a high end IDE for development in AngularJS. If you are familiar with NotePad++, Textpad, EditPlus or Atom then you are good to go with writing your AngularJS application using any of these IDEs. For learning purpose you may need not any server to execute your AngularJS code unless you are using any service like http or any other service that needs web server intervention. You can simple put all your .js, .ts or .html files along in one folder and execute the code using any web browser. For developing a full flexed web application you may need a web server like Apache http server.

Advantage and Disadvantage

You now nothing perfect in this world, language or any other matter in that regard. So does Angular. It comes with it’s bag of pros and cons. Mentioned below the advantage and disadvantage of Angular.

Advantage of Angular

For me the biggest advantage that come with AngularJS is, it’s architecture helps it to automate unit testing using tools. I will write a different post on unit testing in AngularJS later. Since AngularJS 2.0 used for mobile application development unit testing helps developer code a bug free application.

The second biggest advantage is the support for Angular development. As Angular is supported and maintained by Google getting support for any issue is rather easy.

Thirdly as you know Angular supports MVW framework, the codes are module and reusable. Hence development and time to market is fast.

Developers can build Rich Internet application (RIA) and responsive website with Angular as it support data binding to HTML. Due to cross browser compatibility a single angular application can run on any device like, a desktop pc, a mobile phone or tablet without any code change.

Disadvantage of Angular

The biggest disadvantage of the AngularJS is it is a JavaScript based application and if JavaScript is disabled on user browser whole application will go for a toss. That mean you need the user to enable JavaScript on the browser to run your application successfully.

Second major disadvantage of Angular is Security. You need to depend upon server side authentication as client side authentication is not considered safe in Angular. In face I personally believe any java script based client side authentication as safe when compared to server side authentication.

I just mentioned few basic disadvantage of Angular. As you lean and deep dive into Angular you will find few more disadvantages of Angular

Where to Learn

Now a days learning any language is much easier with the help internet and Google. There are lot of information and study material available in the internet. Mentioned here few sites where you can learn and find relevant information regarding AngularJS.

1. https://angularjs.org/ - This is official website of AngularJS. You can get all the information you need here apart from the source code of Angular.

2. http://www.w3schools.com/angular/ - For self learning I prefer this site for Novice users.

3. https://www.tutorialspoint.com/angularjs/ : This is another site for Novice developers to learn and check sample application provided here.

The above 3 are my preferred site for learning for developers. On internet you can find different sites providing course free and paid basis like

https://www.codeschool.com/courses/shaping-up-with-angular-js , https://www.codecademy.com/learn/learn-angularjs.

You can find a lot more if you bit do a Google on Angular.

JavaMail Issue with MS Exchange Server

2016-03-11T17:50:00.001+05:30

In one of earlier mail I had explained how to send mail using Java. That is how normally we send mail using Java to any mail server. Today I am going to explain what kind of issue we can face while sending mail using Microsoft exchange server and how to resolve it. Before I proceed let me explain the background of my application.

I have a java web application which runs on Tomcat web server. The application sends mail to user on different occasions. For example if the user forgot a password, then click on reset password link then the application sends a mail to the user’s registered email address. We have a MS exchange server which is hosted on a different server. Our tomcat is hosted on another server. Our web application is developed in Java using Spring framework..

My class for sending mail was something like the one below. I am putting my sample code for JSP file to send mail for testing the issue.

############### Sample JSP file ################

<props>

</props>

</property>

</bean>

<%@ page import="java.io.*,java.util.*,javax.mail.*"%>

<%@ page import="javax.mail.internet.*,javax.activation.*"%>

<%@ page import="javax.servlet.http.*,javax.servlet.*"%>

String result;

//Recipient's email ID needs to be mentioned.

String to = "mymail@gmail.com";

// Sender's email ID needs to be mentioned

String from = "mailidfrom@mydomainname.org";

final String username = "myusername";

final String password = "mypassword";

// Assuming you are sending email through relay.jangosmtp.net

String host = "myhostname"; // I am using IP address of my MS exchange server. You can put the IP address of your exchange server here.

Properties props = new Properties();

//props.put("mail.smtp.auth", "true");

//props.put("mail.smtp.starttls.enable", "true");

props.put("mail.smtp.host", host);

props.put("mail.smtp.port", "25");

//Get the Session object.

Session mailSession = Session.getInstance(props,

new javax.mail.Authenticator() {

protected PasswordAuthentication getPasswordAuthentication() {

return new PasswordAuthentication(username,

password);

}

});

try {

// Create a default MimeMessage object.

Message message = new MimeMessage(mailSession);

// Set From: header field of the header.

message.setFrom(new InternetAddress(from));

message.setRecipients(Message.RecipientType.TO,

InternetAddress.parse(to));

message.setSubject("Testing Subject");

message.setText("Hello, this is sample for to check send "

+ "email using JavaMailAPI in JSP ");

// Send message

Transport.send(message);

System.out.println("Sent message successfully....");

result = "Sent message successfully....";

} catch (MessagingException e) {

e.printStackTrace(response.getWriter());

result = "Error: unable to send message....";

}

<html>

<head>

<title>Send Email using JSP</title>

</head>

<body>

<h1>Send Email using JSP</h1>

</center>

out.println("Result: " + result + "\n");

</p>

</body>

</html>

On trying to send mail I could not able to send the mail. When checked the error stack trace I found the following error followed by trail of errors.

“javax.mail.AuthenticationFailedException: No authentication mechansims supported by both server and client”

The error itself is self explanatory. That mean there is no authentication mechanism between the mail server and the client, which is my application. So I removed the two line below from the above class file.

// props.put("mail.smtp.auth", "true");

// props.put("mail.smtp.starttls.enable", "true");

Then I tried to send the mail again. But I got the same message again. The simple JSP file created above able to send mail using the above configuration. But to my surprise the class file I created using Spring framework does not with xml configuration does not work.

I forgot add one more thing that my exchange server does not require any authentication from my application server to send mail. So it is something to do with my java code in spring framework that was generating the error.

On checking the above I found that I am providing username and password to my mail sending method. Though I have removed authentication line but still the mail connect function is going for authentication as I am passing username and password thus producing the error.

Now I removed the two lines containing username and password from properties and try to execute the code. Now the function worked like wonder sending mail without any issue. So the bottom line here is if your exchange server does not require authentication the don’t pass username and password in the connect function to send mail. Putting the sample xml file and sample jave code for sending mail using Microsoft exchange server below for reference. Hope this helps somebody in future.

################ xml file ##################

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:context="http://www.springframework.org/schema/context"

xmlns:mvc="http://www.springframework.org/schema/mvc"

xsi:schemaLocation="http://www.springframework.org/schema/beans

http://www.springframework.org/schema/beans/spring-beans-3.1.xsd

http://www.springframework.org/schema/context

http://www.springframework.org/schema/context/spring-context-3.1.xsd

http://www.springframework.org/schema/mvc

http://www.springframework.org/schema/mvc/spring-mvc-3.1.xsd">

<props>

</props>

</property>

</bean>

</beans>

################### Mail Sender file #################

package com.nkc.ats.util;

import javax.mail.MessagingException;

import javax.mail.internet.MimeMessage;import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.mail.MailException;

import org.springframework.mail.MailParseException;

import org.springframework.mail.SimpleMailMessage;

import org.springframework.mail.javamail.JavaMailSender;

import org.springframework.mail.javamail.MimeMessageHelper;

import org.springframework.stereotype.Component;

@Component

public class MailUtil {

@Autowired

JavaMailSender mailSender;

public boolean sendEmail1(String from,String to,String messageText,String subject){

boolean result=false;

try {

SimpleMailMessage message=new SimpleMailMessage();

message.setFrom(from);

message.setTo(to);

message.setSubject(subject);

message.setText(messageText);

mailSender.send(message);

result=true;

} catch (MailException e) {

e.printStackTrace();

}

return result;

}

public void sendEmailByInfo(String from,String to,String messageText,String subject){

MimeMessage message = mailSender.createMimeMessage();

boolean result=false;

try{

MimeMessageHelper helper = new MimeMessageHelper(message, true);

helper.setFrom(from);

helper.setTo(to);

helper.setSubject(subject);

helper.setText(messageText,true);

result=true;

}

catch (MessagingException e) {

throw new MailParseException(e);

}

mailSender.send(message);

}

CREATE THUMBNAIL FROM A PDF FILE IN PHP

2015-11-09T20:49:00.000+05:30

In my last blog I had explained how to create a pdf file from an image and how to encrypt a pdf file usng password. In this blog I will explain how to create a thumbnail from a pdf file after uploading from a web page. Those develop using PHP, Java, and .Net knows how to create a thumbnail from an image file while uploading. Before I start to explain let me clarify the platform I am using for the development. I have used the 64 bit windows 7 machine with XAMPP web server with PHP 5.4. Why I had mentioned the environment because most of the development issue that developers face are more due to environment issue.

Download Ghostscript and ImageIck package

I will use Ghostsprit and Imagick to create a thumbnail from a pdf file. If any one uses Linux as OS then most of them come with Ghostsprit and Imagick. In case the package is not there they can download from the server. For Mac you can download the respective version of the Ghostsprit and Imagick from the server. For window it is bit tricky, for those using 64 bit os have to download a 64 bit compatible package and those using 32 but must download 32 bit package for both ghostscript and ImagIck package.

Uploading a PDF file

All of you know how to upload a file using PHP. So I am not going explain that here. Even if you don’t know how to upload a file a little google search will provide lots of sample code to upload a file in PHP. Same sample code can be used to upload a PDF file.

Set Target Directory

Once you had written the code to upload the file, create a directory to store the pdf files and thumbnails of pdf files. In my case I have created a folder called “uploads” where I intended to upload my pdf files. Inside the “uploads” folder I have created one more directory called “thumbnails” where I will store thumbnail images of my pdf files.

Write function to create thumbnail

Now is the time to write a function to create thumbnail from a pdf file. I have created a sample code that takes two parameters. One is source file that is the pdf file with the path that we uploaded to server. Second one is the thumbnail file name. I named the function as genPdfThumbnail().

function genPdfThumbnail($source, $target)

{

// source pdf file path

$source = realpath($source);

// target thumbnail directory

$target = realpath(dirname($source))."/thumbnails/".$target;

// Create thumbnail of the first page 0-first page, 1-second page and so on

$im = new Imagick($source."[0]");

// Prevent image colors from inverting

$im->setImageColorspace(255);

// Set the thumbnail image format

$im->setimageformat("jpeg");

// Set thumbnail image width, height

$im->thumbnailimage(160, 120);

// Create thumbnail image

$im->writeimage($target);

$im->clear();

$im->destroy();

}

Call the function

Now we have created a function above that will create a thumbnail from us. Now we have to call the function once the upload is done. So in the php file where you have written the code for uploading the pdf file we can call this function to create the thumbnail image. Here is my php code to call the genPdfThumbnail() function.

if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file))

{

//name the thumbnail image the same as the pdf file

$thumbnail = genPdfThumbnail($target_file,$thumb_file);

echo "The thumbnail file ". basename( $_FILES["fileToUpload"]["name"]). ".jpg has been created.";

}

else

{

echo "Sorry, there was an error uploading your file.";

}

Hope this helps and you enjoy creating thumbnail for your pdf file. In case any issue you can always mail me or post your comment so that I can help you.

Convert Image to PDF in PHP using FPDF

2015-10-20T20:11:00.000+05:30

In this blog I would like to explain how to convert image to pdf file and encrypt the same. Converting image to pdf can be different ways Imagick or FPDF. Here I will explain how to convert image to PDF using FPDF library and protect the same using a password. Remember FPDF is a classic PHP class which generates PDF files. To know more about FPDF you can visit their website.

Download FPDF Source Code

To use FPDF you need to download the library files from their website and copy it to your web directory.

USE FPDF TO Create PDF file

To convert image to pdf we need to include 'fpdi/FPDI.php'. If we want to secure the file then we need to use “FPDI_Protection.php”. To create a new FPDF object we sue the following command

using $pdf = new FPDF();

After crating a FPDF object, we can initialize the FPDF object using the following method.

$pdf->FPDF('p','pt','a4');

Here the first parameter stands for orientation (Landscape or Portrait) and second parameter stands for unit of measurement (mm or pt). The third or the last parameter stands for format of the PDF(A4 , Letter,A5). It is like the same format when you create a pdf using pdf writer or document using Microsoft word or any other word editor. Now you can add a page to the pdf using the following statement below

$pdf->AddPage();

Now the next step is to add the image to the pdf. The following command will help you to add the image to pdf.

$pdf->Image($srcImgFile,$x_pos,$Y_pos,$width,$height);

Protect Your Document

To protect the document yuu need to include ‘FPDI_Protection.php' as shown below.

require_once('fpdi/FPDI_Protection.php');

To protect the file to be copied by anybody use the following command

$pdf->SetProtection(array(),$password);

Here the first parameter is an array of permissions and second one is the password to protect the document. The permission array contains the following values.

Print – Print the pdf document

Copy – Copy the pdf file

Modify - Modify the pdf file

Annot-forms – Add annotation to pdf file

Save Your Document

After you set the permission to the file, now you can save the file to destination file or folder. To save the file you can use the following command

$pdf->Output($destPdfFile,'F');

Here the first parameter takes the destination file name along with file path, while the second parameter takes the destination where you want to display the file. Following options can be passed as parameter for the second parameter.

F – Save to local file path with the name

S - Return the document as string

D – Send to browser to be saved as download document

I - To be opened inline in the browser

I f the Output function called without parameter then the pdf will be opened in the web browser.

Note: To read more about the FPDF visit the manual section on the website. For tutorial you can visit the tutorial section of the website.

Apache 404 File not found error when folder already exists

2015-08-19T16:14:00.000+05:30

Some of the developers might have come across “404 File not found” error in their life. The error is self explanatory. Web server returns the "HTTP 404 - File not found" error message when it cannot retrieve the page that was requested by the client (web browser). That means the file does not exist there in the specified path. It is really annoying when we get”404 File not found” error when the file is there in the desired folder. I am going to explain why we get this error when the file is there and how to resolve the issue.

Check your DocumentRoot Value

All of you must know the default location for “DocumentRoot” is “/var/www/html”. In some server it is set to “/var/www/htdocs/”. You can find the “DocumentRoot” value in httpd.conf file. You can find this file in “/etc/httpd/conf” folder. "DocumentRoot" i.e. the directory out of which all the pages/documents are served to the client that requests a page or file. By default, all requests are taken from this directory, but symbolic links and aliases may be used to point to other locations. This can be changed or set to any other folder user set DocumentRoot to. So hence if you are getting "404 File not found" error then first point to check is look for the DocumentRoot value. Your file and folder should be in the DocumentRoot to be accessed from web. If it is not there then you will definitely get "404 File not found" error.

Check Your Virtual Host

httpd is also capable of Virtual Hosting. That means the server can have more than one domain or multiple sub domains. In that case the server can receive requests for more than one domain or sub domain hosted on the server. Different DocumentRoot can be specified for each domain, sub domain or virtual host.

A virtual host for example can be declared in the httpd.conf file as shown below.

ServerAdmin admin@samalpramod.com

DocumentRoot "/var/www/html/samalpramod"

ServerName www.samalpramod.com

ErrorLog logs/www.samalpramod.com-error_log

CustomLog logs/www.samalpramod.com-access_log common

Please check the DocumentRoot is set to "/var/www/html/samalpramod". So to access any file from DocumentRoot directory from web user have to type www.samalpramod.com followed by filename rather than www.samalpramod.com/samalpramod followed by file name. For example if any user wants to access the index.html from the “DocumentRoot” must type www.samalpramod.com/index.html rather than www.samalpramod.com/samalpramod/index.html. If any users follows the second option then the web server will show "404 File Not Found" error. So one of the main reason for getting “404 File Not Found” error is file or folder is placed in a folder while DocumentRoot is set to a different folder.

mysql_real_escape_string () Access denied for user 'root'@'localhost' (using password: NO) in cakephp

2015-08-07T20:00:00.001+05:30

Developers who have worked in PHP sometimes used mysql_real_escape_string() for escaping special character from SQL statement or Strings. Basically this mysql_real_escape_string() is a core PHP function that creates a proper SQL string or SQL query removing special character. This function was there in PHP 5.5 below. It was deprecated in PHP 5.5 and will be removed in PHP 7.0 higher version. Instead MySQLi or PDO_MySQL extension should be used in future PHP version 7.0 above. For information about mysql_real_escape_string() you can view PHP documentation.

Sometimes developer who worked in PHP frameworks like CakePHP or any other framework might have come across some error or issues. I am going to throw some light on error that comes in CakePHP.

The error basically is “mysql_real_escape_string() Access denied for user 'xxxxxx'@'localhost' (using password: NO)”. The place where I put “xxxxx” will be name of a mysql user or that can be blank depending upon the mysql user on the system.

Now I will go to explain why the error occurs in CakePHP and how it should be used. Those use this function to avoid sql injection in Cakephp they better have to use ORM methods like save(), find(), saveAll() built in method of CakePHP. If somebody wants to create their own SQL string with CakePHP, then they can use Sanitize class of CakePHP. But Sanitize class is deprecated as of 2.4, and will be removed in CakePHP 3.0. So those using CakePHP 3.0 or higher must use some native function or other libraries. Now let us come to the user that uses the native function i.e. mysql_real_escape_string() function or mysqli_real_escape_string().

Both the function needs a connection to the database to function. When calling it, it tries to establish a connection if you didn't previously establish one using mysql_connect. If your php.ini has some default credential set for mysql then mysql_real_escape_string can use the same to connect and you will have false impression that it worked perfectly for you. Otherwise if you on your local machine and you apparently have blank password for mysql and the account name for the MySQL user (mostly users keep root) is the same as the name the web server, in that case it will work for you. But when you will move to production server where there is no mysql credential set in php.ini or the account for mysql and web server is different it can't create a database connection and will through a warning. In fact your application may not work because it will not return the string which you expect mysql_connect function to return.

In that case you need to create a connection using mysql credential and provide the connection to mysql_real_escape_string function where ever called. For better flexibility, it is better you can create your own function and create a connection inside the function and call the mysql_real_escape_string() function as shown below. You can call your function where ever you need to escape a string in sql statement

public function makeSqlSafe($data)

{

$return_string = "";

// Connect

$link = mysql_connect(‘hostname’, ‘username’, 'password')

OR die(mysql_error());

$value = trim($data);

$return_string = mysql_real_escape_string($value);

return $return_string;

}

NO VIDEO WITH SUPPORTED FORMAT AND MIME TYPE FOUND WITH HTML5

2015-04-22T19:31:00.002+05:30

If your HTML5 page is able to play audio/video on local host or local server perfectly and the same page is showing the above error marked in bold then first thing to do is to check your network setting with your system admin. Please check with your system admin if they have setup any firewall or proxy that is blocking audio/video url. I have faced this many times. Most of video pages that works perfectly on my local machine does not work on the server as our firewall blocks audio or video urls.

Check audio and video MIME types.

If your HTML5 page able to play the audio and video on one browser and not able plays the same on different browser then, you need to check the MIME types. Different browsers support different MIME type for audio and video files. You need to set these MIME type in the web server. Also make sure the file type are there in the server in right place and right time. To add these MIME type you can use the AddType directive to set MIME type in the .htaccess file or httpd.conf file.

There are 3 major video types which are supported by HTML5. They are MP4, WebM, and Ogg/Ogv. MP4 is encoded with H264 codec while .ogg is encoded with Theora encoding. While Firefox and Safari widely supports .ogg/.ogv format, IE and Chrome supports mostly supports mp4 format. Webm is a new HTML5 video format supported by most of the browser(Chrome,Opera, Firefox 4+, IE 9.0 and above). So now you different browser supports different audio/video format for HTML5. Hence the behaviour of HTML5 video may differ from browser to browser. Hence it is necessary to add the audio/video type to browser and keep the supported file in the respective directory.

I will show here, how add MIME type for videos using addType directive.

Adding HTML5 Audio / Video type

Video :

Add the following lines to your httpd.conf or .htaccess file to support HTML5 video across most browser.

AddType video/mp4 .mp4 .m4v

AddType video/ogg .ogv

AddType video/webm .webm

Audio :

Add the following lines to your httpd.conf or .htacess file to support HTML5 audio across most browser.

AddType audio/aac .aac

AddType audio/mp4 .mp4 .m4a

AddType audio/ogg .ogg .oga

AddType audio/wav .wav

AddType audio/webm .webm

After adding this you can now refresh your html5 web page to play your video/audio on different browser.

Here is the sample HTML5 video code to play video.

While adding the source file make sure the MP4 files comes first followed by .WebM, .Ogg file. Sometimes few browser creates issue if the source file are not in proper sequence and you may got the above error again.

BACKUP AND RESTORE MYSQL DATABASE USING MYSQLDUMP

2015-04-06T17:54:00.000+05:30

There are different ways to backup a mysql database. One of the command or tool in mysql is mysqldump which is quite easy method to take backup of a database. It creates a .sql file having Create Database, Create table and Insert into sql statement. Drop table statement is used before Create table if the table already exists in the database. The SQL Statement will be somewhat like the one below.

Using mysql you can backup a database from one server and restore the database on a different server.

Backup Database

For backing up the database you can use the following command.

$ mysqldump –u root –p password [DB Name] > DBName.sql;

If the database size is huge then you can use the zip file to compress the .sql file and take the backup.

$mysqldump –u root –p password [DB Name] | gzip > DBName.sql.gz;

DB Name here is the database name for which backup is taken. If you want to take backup of all databases then you can use the following command.

$mysqldump –u root –p password --all-databases | gzip > alldatabase.sql.gz;

Restore Databse

To restore a single database use the following command

$mysql -u root –p password [DB Name] < DBName.sql;

Restoring a database from compressed .sql file

$gzip < DBName.sql.gz | mysql -uroot –p password [DB Name];

Restoring all database from a compressed zip file

$gzip < alldatabase.sql.gz | mysql -uroot –p password ;

Note : Make sure the database .sql file is in the current directory while restoring the database, otherwise mention the .sql file with full path while restoring the database.

To know more about mysqldump command option click here.