Java Developers Book

Thursday, 18 September 2014

Getting JVM heap size, used memory, total memory using Java Runtime

Getting JVM heap size, used memory, total memory using Java Runtime.

Reading runtime java virtual memory usage is useful when the system/application is struggling in getting resources. System Memory is one of the main resource that an application developer has to consider while managing the application.

Java’s Runtime class provide lot of information about the resource details of Java Virtual Machine or JVM. The memory consumed by the JVM can be read by different methods in Runtime class. Following is the example of reading JVM Heap Size, Total Memory and used memory using Java Runtime api.

 public class MemoryUtilization {  
   public static void main(String [] args) {  
     int conversion = 1024*1024;       
     Runtime runtime = Runtime.getRuntime();       
     System.out.println("##### Heap utilization #####");       
     System.out.println("Used Memory:" + (runtime.totalMemory() - runtime.freeMemory()) / conversion);  
     System.out.println("Free Memory:" + runtime.freeMemory() / conversion);  
     System.out.println("Total Memory:" + runtime.totalMemory() / conversion);       
     System.out.println("Max Memory:" + runtime.maxMemory() /conversion);  
   }  
 }

Create a simple tree view effect using Jquery / Javascript

Create a tree view using javascript is one of the important needs in the UI development where document repository is implemented.The example below shows it how to do it.

Here the logic is like

The sub div for the parent div must have the same class as that of the parent div id.

An event handler is written on the parent div click which toggles all the div with class as the "one" It is shown in the script tag.

 <html>  
 <head>  
      <script src="http://code.jquery.com/jquery-1.11.0.min.js"></script>  
 </head>  
 <body>  
   <div id="one" style="" >  
     C:  
      </div>  
           <div class="one" style="margin:14px;color:#ccddaa;">  
             Program files  
           </div>  
           <div class="one" style="margin:14px;color:#ccddaa;">  
             Windows  
           </div>  
           <div class="one" style="margin:14px;color:#ccddaa;">  
             Users  
           </div>       
 <script>  
           $("#one").click(function(){  
                var id = $(this).attr('id');  
                $("."+id).toggle();  
           });  
      </script>  
  </body>  
 </html>

Wednesday, 17 September 2014

Hosting two webapps on one tomcat

Assumptions -

Assume you have a development host with two host names, localhost and localhost1(You will need to create a hosts file entry for this or you can simply put the LAN ip address if you want).
Let's also assume one instance of Tomcat running, so $CATALINA_HOME refers to wherever it's installed, may be some where in /var/lib/tomcat7 (Please use it according to the location and the operating system you are using )

What to change??

Definitely server.xml !!!
You will find server.xml in /var/lib/tomcat7/conf/server.xml or you may use find command if you have no idea where you will get it. For the list of commands in linux and their brief summary please check here

Default entry in server.xml is

 <Engine defaulthost="localhost" name="Catalina"></Engine>

You need to add this -

 <Engine name="Catalina" defaultHost="localhost">  
   <Host name="localhost"  appBase="webapps"/>  
   <Host name="localhost1" appBase="my_webapps"/>  
 </Engine>

Saturday, 13 September 2014

Remote Connection Settings for MySQL

Configuring your mysql to access it from outside i.e. from any machine across intranet / internet is one of the basic needs of the developer .

The following sequence of steps are needed to achieve this.

We have to do the following changes

Change bind address in mysql configuration file my.cnf to 0.0.0.0 This allows mysql to bind to all interfaces. Previous value may be 127.0.0.1
In mysql DB we have added new user root@% and granted all permissions to it.

Command for reference

select host,user,password from mysql.user
create user 'root'@'%' identified by 'pass';
GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' WITH GRANT OPTION;
GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION;

We ideally will not require to do anything in iptables ( Linux Firewall . ) but in some cases we would need to open 3306 port in the firewall settings.

Wednesday, 10 September 2014

Retrive System properties Java

Accessing system properties can be an important part of the code/ J2EE application . Some of the system properties and their corresponding description are given below .

Java system property	Meaning
file.encoding	The encoding used in text files on the system.
file.separator	The file separator character. This is usually \ for windows and / for UNIX like systems
java.io.tmpdir	The directory in which temporary files can be created on the local system. System environment variable %TEMP%
line.separator	The character or sequence of characters that generally mark a "new line" in a text file on the local operating system. On UNIX-like systems, this is typically a single ASCII character 10, whereas on Windows systems, this is typically a two-character sequence (ASCII 10 and ASCII 13).
os.name	The name of the local operating system, such as "Linux" or "Windows 7".
os.version	A version number of the local operating system, such as "6.0" or "2.6.18-stab02".
path.separator	The separator generally used to separate lists of paths on that operating system, for example a colon (":"). Note that this is not the directory separator used inside paths.
user.country	The ISO code of the operating system's (or local user's) configured country.
user.dir	The local directory from which the Java process has been started, and from which files will be read/written by default unless a path is specified.
user.home	The current user's "home" directory, such as `C:\Users\Fred` on Windows systems, or `/home/sandesh/` on UNIX-like systems.
user.language	The ISO code of the operating system's (or local user's) configured language, such as "en" for English.
user.name	The local user's system user name. On Windows systems, this is typically close to a "real life" name. On UNIX-like systems, it is common for user names to be all lower case letters.

 import java.util.Map;  
 public class ReadSystemProperties {  
      public static void main(String[] args) {  
           Map envVars = System.getenv(); // Gives the environment variable values.  
           System.out.println(envVars);  
           String osVersion = System.getProperty("os.version"); // Retrives the version of os you are working on.  
           System.out.println("OS Version - \t" + osVersion);  
           String osName = System.getProperty("os.name"); // Retrives the os you are working on.  
           System.out.println("OS Version - \t" + osName);  
           String fileEncoding = System.getProperty("file.encoding"); // Retrives the os you are working on.  
           System.out.println("File Encoding - \t" + fileEncoding);  
           String fileSeparator = System.getProperty("file.separator"); // Retrives the os you are working on.  
           System.out.println("File Separator - \t" + fileSeparator);  
      }  
 }

Wednesday, 3 September 2014

Servlets in Java

Servlets in breif

A servlet is a Java technology based web component, managed by a container, that generates dynamic content. A servlet can be considered as a tiny Java program which processes user request and generates dynamic content.

Following are the some advantages of using servlets.

(1) They are generally much faster than CGI scripts.

(2) They use a standard API that is supported by many web servers.

(3) They have all the advantages of the Java programming language, including

ease of development and platform independence.

(4) They can access the large set of APIs available for the Java platform.

What is a Servlet Container?

Servlet container (also known as servlet engine) is a runtime environment, which implements servlet API and manages life cycle of servlet components.Container is responsible for instantiating, invoking, and destroying servlet components.One example of container is Apache Tomcat which is an opensource container.

How to find which OS you are working on ?

For some projects you will need to find out which OS you are working on or on which Operating System the code is deployed. This may be needed in certain cases like - File System Handling ,
Data upload system, Electronic Records System etc because the file organization of different operating systems are indifferent.

For the list of other system properties/ System environment variables please click here

The following code sample in Java gives this -

 package com.util;  
 public class FindOS{  
      private static String OS = System.getProperty("os.name").toLowerCase();  
      public static void main(String[] args) {  
           System.out.println(OS);  
           if (isWindows()) {  
                System.out.println("This is Windows");  
           } else if (isMac()) {  
                System.out.println("This is Mac");  
           } else if (isUnix()) {  
                System.out.println("This is Unix or Linux");  
           } else if (isSolaris()) {  
                System.out.println("This is Solaris");  
           } else {  
                System.out.println("Your OS is not support!!");  
           }  
      }  
      public static boolean isWindows() {  
           return (OS.indexOf("win") >= 0);  
      }  
      public static boolean isMac() {  
           return (OS.indexOf("mac") >= 0);  
      }  
      public static boolean isUnix() {  
           return (OS.indexOf("nix") >= 0 || OS.indexOf("nux") >= 0 || OS.indexOf("aix") > 0 );  
      }  
      public static boolean isSolaris() {  
           return (OS.indexOf("sunos") >= 0);  
      }  
 }

Monday, 1 September 2014

How to create a readonly collection in java?

Collections is one of the most used feature in Java/J2EE applications . Read only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using get method or by using Iterator of List, This kind of List is good for certain requirement where parameters are final and can not be changed. In Java you can use Collections.unModifiableList() method to create read only List , Collections.unmodifiableSet() for creating read-only Set like read only HashSet and similarly creating a read only Map in Java.


 import java.util.ArrayList;  
 import java.util.Collections;  
 import java.util.List;  
 public class ReadOnlyCollections {  
           public static void main(String[] args) {  
                List<String> myList = new ArrayList<>();  
                myList.add("1");  
                myList.add("2");  
                myList.add("3");  
                System.out.println(myList);  
                // Convert to unmodifiable .  
                myList = Collections.unmodifiableList(myList);  
                myList.add("4");  
                System.out.println(myList);  
           }  
 }  
 Output -   
 [1, 2, 3]  
 Exception in thread "main" java.lang.UnsupportedOperationException  
      at java.util.Collections$UnmodifiableCollection.add(Unknown Source)  
      at ReadOnlyCollections.main(ReadOnlyCollections.java:15)

Friday, 29 August 2014

What is the difference between STRINGBUFFER and STRING?

String object is immutable. i.e , the value stored in the String object cannot be changed. For e.g.

String myString = “Hello”;
myString = myString + ” Guest”;

System.out.println("myString " + myString);

When you run System.out on myString the output will be “Hello Guest”. Although we made use of the same object (myString), internally a new object was created in the process because - String is an immutable class in java . That’s a performance issue.

StringBuffer/StringBuilder objects are mutable StringBuffer/StringBuilder objects are mutable i.e. we can make changes to the value stored in the object. This means is that string operations such as append would be more efficient if performed using StringBuffer/StringBuilder objects than String objects.

String str = “Be Happy With Your Salary.''
str += “Because Increments are a myth";
StringBuffer strbuf = new StringBuffer();
strbuf.append(str);
System.out.println(strbuf);

The Output of the code snippet would be: Be Happy With Your Salary. Because Increments are a myth. But here the difference is that only one object of stringbuffer is created in the example. That improves space complexity of the program.

Thursday, 28 August 2014

A sample ant build script and its explaination

A sample build script is given below . Please refer comments for explaination

<?xml version="1.0" ?>

<project name="<Your application>" default="compile">
<path id="compile.classpath">
<fileset dir="WEB-INF/lib">
<include name="*.jar"/>
</fileset>
</path>


<target name="init">
<delete dir="WEB-INF/classes"/>
<mkdir dir="WEB-INF/classes/conf"/>
<mkdir dir="dist" />
</target>

<target name="compile" depends="init" >

<javac destdir="WEB-INF/classes" debug="true" srcdir="src">
<classpath refid="compile.classpath"/>
</javac>

<copy todir="WEB-INF/classes">
<fileset dir="conf"/>
</copy>
</target>

<target name="war">
<war destfile="dist/s31.war" webxml="WEB-INF/web.xml">
<fileset dir="../dist"/>
<lib dir="WEB-INF/lib"/>
<classes dir="WEB-INF/classes"/>
</war>
</target>
</project>

Tuesday, 26 August 2014

Linux Commands

File Commands

1.	ls	Directory listing
2.	ls -al	Formatted listing with hidden files
3.	ls -lt	Sorting the Formatted listing by time modification
4.	cd dir	Change directory to dir
5.	cd	Change to home directory
6.	pwd	Show current working directory
7.	mkdir dir	Creating a directory dir
8.	cat >file	Places the standard input into the file
9.	more file	Output the contents of the file
10.	head file	Output the first 10 lines of the file
11.	tail file	Output the last 10 lines of the file
12.	tail -f file	Output the contents of file as it grows,starting with
		the last 10 lines
13.	touch file	Create or update file
14.	rm file	Deleting the file
15.	rm -r dir	Deleting the directory
16.	rm -f file	Force to remove the file
17.	rm -rf dir	Force to remove the directory dir
18.	cp file1 file2	Copy the contents of file1 to file2
19.	cp -r dir1 dir2	Copy dir1 to dir2;create dir2 if not present
20.	mv file1 file2	Rename or move file1 to file2,if file2 is an existing
		directory
21.	ln -s file link	Create symbolic link link to file
Process management
1.	ps	To display the currently working processes
2.	top	Display all running process

3.	kill pid	Kill the process with given pid
4.	killall proc	Kill all the process named proc
5.	pkill pattern	Will kill all processes matching the pattern
6.	bg	List stopped or background jobs,resume a stopped
		job in the background
7.	fg	Brings the most recent job to foreground
8.	fg n	Brings job n to the foreground

Searching

1.	grep pattern file	Search for pattern in file
2.	grep -r pattern dir	Search recursively for pattern in dir
3.	command \| grep	Search pattern in the output of a command
	pa
4.	locate file	Find all instances of file
5.	find . -name filename	Searches in the current directory (represented by
		a period) and below it, for files and directories with
		names starting with filename
6.	pgrep pattern	Searches for all the named processes , that
		matches with the pattern and, by default, returns
		their ID

System Info

1.	date	Show the current date and time
2.	cal	Show this month's calender
3.	uptime	Show current uptime
4.	w	Display who is on line
5.	whoami	Who you are logged in as

6.	finger user	Display information about user
7.	uname -a	Show kernel information
8.	cat /proc/cpuinfo	Cpu information
9.	cat proc/meminfo	Memory information
10.	man command	Show the manual for command
11.	df	Show the disk usage
12.	du	Show directory space usage
13.	free	Show memory and swap usage
14.	whereis app	Show possible locations of app
15.	which app	Show which applications will be run by default

Compression

1.	tar cf file.tar file	Create tar named file.tar containing file
2.	tar xf file.tar	Extract the files from file.tar
3.	tar czf file.tar.gz files	Create a tar with Gzip compression
4.	tar xzf file.tar.gz	Extract a tar using Gzip
5.	tar cjf file.tar.bz2	Create tar with Bzip2 compression
6.	tar xjf file.tar.bz2	Extract a tar using Bzip2
7.	gzip file	Compresses file and renames it to file.gz
8.	gzip -d file.gz	Decompresses file.gz back to file

Network

1.	ping host	Ping host and output results
2.	whois domain	Get whois information for domains
3.	dig domain	Get DNS information for domain
4.	dig -x host	Reverse lookup host
5.	wget file	Download file
6.	wget -c file	Continue a stopped download

Shortcuts
1.	ctrl+c	Halts the current command
2.	ctrl+z	Stops the current command, resume with fg in the
		foreground or bg in the background
3.	ctrl+d	Logout the current session, similar to exit
4.	ctrl+w	Erases one word in the current line
5.	ctrl+u	Erases the whole line
6.	ctrl+r	Type to bring up a recent command
7.	!!	Repeats the last command
8.	exit	Logout the current session