Java Developers Book: September 2014

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)