Use CountDownLatch as a CyclicBarrier

Following is a program to use CountDownLatch as a CyclicBarrier: (However, unlike CyclicBarrier, CountDownLatch cannot be restarted!) Just to understand how/if CountDownLatch can be used instead of CyclicBarrier.

public class CountDownLatchTry {

      private static CountDownLatch latch = new CountDownLatch(2);

      /**

      * @param args

      */

      public static void main(String[] args) {

            // TODO Auto-generated method stub

            Runnable r1 = new Runnable() {

                  public void run() {

                        System.out.println("T1: Before countDown()");

                        latch.countDown();

                        System.out.println("T1: After countDown()... sleeping");

                        try {

                              Thread.sleep(2000);

                        } catch (InterruptedException e1) {

                              // TODO Auto-generated catch block

                              e1.printStackTrace();

                        }

                        try {

                              latch.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T1: After await()");

                  }

            };

           

            Runnable r2 = new Runnable() {

                  public void run() {

                        System.out.println("T2: Before countDown()");

                        latch.countDown();

                        System.out.println("T2: After countDown()... sleeping");

                        try {

                              Thread.sleep(2000);

                        } catch (InterruptedException e1) {

                              // TODO Auto-generated catch block

                              e1.printStackTrace();

                        }

                        try {

                              latch.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T2: After await()");

                  }

            };

            Thread t1 = new Thread(r1, "T1");

            Thread t2 = new Thread(r2, "T2");

           

            t1.start();

            t2.start();

            System.out.println("Done!");

      }

}

Corresponding CyclicBarrier example:

public class CyclicBarrierTry {

      private static CyclicBarrier cb = new CyclicBarrier(2);

      /**

      * @param args

      */

      public static void main(String[] args) {

            // TODO Auto-generated method stub

           

            Runnable r1 = new Runnable() {

                  public void run() {

                        System.out.println("T1: Before await()");

                        try {

                              cb.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        } catch (BrokenBarrierException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T1: After await()");

                  }

            };

           

            Runnable r2 = new Runnable() {

                  public void run() {

                        System.out.println("T2: Before await()");

                        try {

                              cb.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        } catch (BrokenBarrierException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T2: After await()");

                  }

            };

            Thread t1 = new Thread(r1, "T1");

            Thread t2 = new Thread(r2, "T2");

           

            t1.start();

            t2.start();

            System.out.println("Done!");

      }

}

Potential perf issues with String.substring()

This post is relevant for Oracle’s java implementation of 1.6!

Following is the implementation of substring method in String class:

    public String substring(int beginIndex, int endIndex) {

      if (beginIndex < 0) {

          throw new StringIndexOutOfBoundsException(beginIndex);

      }

      if (endIndex > count) {

          throw new StringIndexOutOfBoundsException(endIndex);

      }

      if (beginIndex > endIndex) {

          throw new StringIndexOutOfBoundsException(endIndex - beginIndex);

      }

      return ((beginIndex == 0) && (endIndex == count)) ? this :

          new String(offset + beginIndex, endIndex - beginIndex, value);

    }

Now, if you take a look at the highlighted part of the code, it says a new String is created with same char sequence (‘value’), but with different offset!

Let’s take a scenario where we have a huge string, say of 100MB and we take a substring containing last 100 chars of that string.

String s100MB = <100MB String>; //memory occupied is 100MB

String substring = s100MB.subString(s100MB.length() – 100); //does not occupy any additional memory for the chars of substring as it uses the same char array as the parent string

s100MB = null; //‘substring’ still occupies 100MB of memory where as what it requires is only 200k (for 100 chars)

Now, in real world this may not be a very serious issue as GC is not as instantaneous but cases where ‘substring’ hangs around in memory for very long time – this can be unnecessary wastage of memory! (Worse…. think of the original string being 1GB instead of 100MB!)

<![if !supportLists]>-          <![endif]>Sarang

final Collection in java

An interesting aspect about final Collection that many java developers may not be familiar with is that making a collection final means only the reference cannot be changed but you can add, remove or change an object inside collection.

 

Example:

            private final List<String> names = new ArrayList<String>();

            names.add("Name1"); //Allowed

            names.add("Name2"); //Allowed

                       

            names = new ArrayList<String>(); //Error

 

Setup yum repository from a linux iso shared on remote windows machine

1. Mount remote windows share on linux system

In general, a useful stuff – specially when important software-installers are stored on a windows share. It can be useful to mount the windows share on your linux system to use it efficiently.

Following are the steps:

• Login to Linux as root user
• > mkdir /mtn/winshare 
• > mount -t cifs //<windows-machine-IP-addr>/<folder-path/ -o username=<user>,password=<pw> /mnt/winshare (For RHEL > 4)
• > mount -t smbfs //<windows-machine-IP-addr>/<folder-path/ -o username=<user>,password=<pw> /mnt/winshare (For RHEL < 4)

2. Extract Files from iso image and create yum repository

• > mkdir -p /mnt/iso/{1,2,3}
• > mount -o loop /mnt/winshare/disk.iso /mnt/iso/1
• > rpm -ivh  /mnt/iso/1/Packages/<createrepo-package-name>.rpm
• > cd /mnt/iso
• > createrepo .
• > yum clean all

3. Create yum repository configuration file

• > vi /etc/yum.repos.d/iso.repo
• Place following text in iso.repo

           [iso] 

           name=My ISO Repo

           baseurl=file:///mnt/iso
           enabled=1

           gpgcheck=0 

Now you should be able to install any package from iso using yum.

- Sarang Anajwala

Debug class-not-found exception

Print ClassNotFoundException

A simple problem that I have found many people struggling with – how to print class-path. Following is the line that can be used to print classpath.

Arrays.toString((((URLClassLoader) Test.class.getClassLoader().getURLs()));

This code returns an array list of all jars and directories on the classpath of the classloader.

-          Sarang Anajwala

 

End of Public Updates for Java SE 6

A quick update on future of Java 6.

From oracle blog -

“The last publicly available release of Oracle JDK 6 is to be released in February, 2013. This means that after 19 February 2013, all new security updates, patches and fixes for Java SE 6 and Java SE 5 will only be available through My Oracle Support and will thus require a commercial license with Oracle.“

More details available @ https://blogs.oracle.com/java/entry/end_of_public_updates_for

Presents a case for upgrade to Java 7!

- Sarang Anajwala

Log4j 2.0 - Some important features

Log4j’s latest 2.0 version has been recently released. Following are few of the important and very useful features of Log4j-2.0:

1.       Supports parameter substitution (same as slf4j)

For example:

   LOGGER.info (“User Id is { } and User Email is { }”, userId, userEmail);

2.       Flow Tracing:

To print the entry and exit of a method.

Example:

  public String getUserEmail(int userId) {

    logger.entry(userId);

    User user = UserDao.getUser(userId)

    String email = user.getEmail();

    return logger.exit(email);

  }

  This method will log following messages:

    19:08:07.061 TRACE com.class.MyClass 10 getUserEmail -  entry parms(1)

    19:08:07.061 TRACE com.class.MyClass 13 getUserEmail -  exit with (User.Email@gmail.com)

3.       Markers:

To log some special messages.

For example:

 public class MyApp {

   private Logger logger = LogManager.getLogger(MyApp.class.getName());

   private static final Marker QUERY_MARKER = MarkerManager.getMarker("SQL");

   public String doQuery(String table) {

     logger.entry(param);

     logger.debug(QUERY_MARKER, "SELECT * FROM {}", table);

     return logger.exit();

   }

} 

4.       Messages:

Very useful to standardize the format of an application’s log messages.

Example:

            logger.info("User {} has logged in using id {}", username, userId);        

      The above log message can be standardized by implementing a class ‘LoggedInMessage’ implementing ‘Message’ interface.

            logger.info(new LoggedInMessage(userName, userId)); 

                    (‘getFormattedMessage’ method of ‘Message’ interface is used to log the message string.)

Some more important types of messages are ThreadDumpMessage, TimestampMessage, MapMessage and few more.

5.       Plug-ins:

Log4j 2 uses a Plugin system that makes it extremely easy to extend the framework by adding new Appenders, Filters, Layouts, Lookups, and Pattern Converters without requiring any changes to Log4j.           

-          Sarang Anajwala      

git - tips

• git remote –v                    

o   Shows remote branches

• git remote add <remote branch name> <remote branch url>

o   Add a new remote branch

• git push <remote branch name> <local branch name>

o   push changes from local branch to remote branch

Ref:

-          http://git-scm.com/book/en/Git-Basics-Working-with-Remotes

-          https://help.github.com/articles/which-remote-url-should-i-use

Maven - include a lib manually in local repo

Use following command to include a lib manually in local maven repository:

 

mvn install:install-file -Dfile=./EWSJavaAPIWithJars_1.2.0.jar -DgroupId=local.disk -DartifactId=EWSJavaAPI -Dversion=1.2 -Dpackaging=jar