Example for java.util.concurrent.locks.ReentrantReadWriteLock usage

Let's try the latest sample with another beauty of the java.util.concurrent package :ReentrantReadWriteLock

import java.util.concurrent.locks.*;

public class ReentrantReadWriteLockExample {
    int i = 0;

    public static void main (String[] args) {
        final ReentrantReadWriteLockExample example = new ReentrantReadWriteLockExample();
        final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

        final Runnable r = new Runnable () {
            public void run () {
                while (true) {
                    try {
                        lock.writeLock().lock();
                        //Here is the critical section jobs are being done securely..
                        example.printSomething ();
                        Thread.sleep (1000);
                        lock.writeLock().unlock();
                    } catch (Exception ex) {
                        System.out.println (" -- Interrupted...");
                        ex.printStackTrace ();
                    }
                }
            }
        };

        new Thread (r).start ();
        new Thread (r).start ();
        new Thread (r).start ();
    }

    public void printSomething (){
        i++;
        System.out.println (" -- current value of the i :"+ i);
    }
}

Example for java.util.concurrent.Semaphore usage

Let's try to increment a simple integer variable without any synchronization.
See what happens if we don't care about critical sections :)

Yes even we are not able to increment an integer without using any synchronization primitives.So we see the values of "i" 10-11-10 which are highlighted in the messages pane of my ide.

And...Here is the succesfull result in which we are using a Semaphore from java 1.5 (Tiger).

import java.util.concurrent.locks.*;
import java.util.concurrent.Semaphore;

public class SemaphoreExample {
    int i = 0;

    public static void main (String[] args) {
        final SemaphoreExample example = new SemaphoreExample();
        final Semaphore semaphore = new Semaphore (1);

        final Runnable r = new Runnable () {
            public void run () {
                while (true) {
                    try {
                        semaphore.acquire();
                        example.printSomething ();
                        Thread.sleep (1000);
                        semaphore.release();
                    } catch (Exception ex) {
                        System.out.println (" -- Interrupted...");
                        ex.printStackTrace ();
                    }
                }
            }
        };

        new Thread (r).start ();
        new Thread (r).start ();
        new Thread (r).start ();
    }

    public void printSomething (){
        i++;
        System.out.println (" -- current value of the i :"+ i);
    }
}
 

Take a trip into Java Monitor with your source code

                      line 1:  synchronized(obj){//Entry Set
                      line 2:       System.out.println ("I am the master of monitor");//Owner
                      line 3:       obj.wait();//Wait Set
                                 }
                      line 4:           //Out of the monitor

More than one thread can be blocked/found on line 1 , line 3 and line 4 at the same time, but there can only be one active thread at the same time at line 2 as a monitor owner.

You may want to check this sample also : http://javabender.blogspot.com/2010/09/example-for-understanding-javas-wait.html

Example for Understanding Java's Wait / Notify Mechanism and Synchronization

 public class WaitNotifyExample {

    public static void main (String[] args) {
        final Object obj = new Object();

        Runnable r = new Runnable () {
            public void run () {
                while (true) {
                    try {
                        System.out.println ("-- this lines are being printed in another thread.." + Thread.currentThread ().getName ());
                        synchronized(obj){// The current thread must own this object's monitor see javadoc guys..Object.wait ();
                            obj.wait();// just wait for somebody else to awake you here.
                        }
                    } catch (Exception ex) {
                        System.out.println (" -- Interrupted...");
                        ex.printStackTrace ();
                    }
                }
            }
        };

        new Thread (r).start ();//we crated and started a lazy guy here
        new Thread (r).start ();//another lazy guy
        new Thread (r).start ();//this can be me :)

        while (true) {
            System.out.println (" -- this lines are bing printed in main thread.." + Thread.currentThread ().getName ());
            try {
                Thread.currentThread ().sleep (5000);//Our master will sleep for 5 seconds
                synchronized(obj){//same reason with above synchronized
                    obj.notifyAll();// Then it will say others wake up !
                }
                Thread.currentThread ().sleep (500);//If we don'T stop the main thread here it will be the owner of the monitor of obj object always.
                                                    //It is in a while loop.Others will be blocked on synchronized statement.

            } catch (InterruptedException ex) {
                ex.printStackTrace();
            }
        }
    }
}

---| Lets look at to the system out , I have took a thread dump for u also  |-----

-- this lines are being printed in another thread..Thread-0
-- this lines are being printed in another thread..Thread-1
 -- this lines are bing printed in main thread..main
-- this lines are being printed in another thread..Thread-2
-- this lines are being printed in another thread..Thread-0
-- this lines are being printed in another thread..Thread-1
 -- this lines are bing printed in main thread..main
-- this lines are being printed in another thread..Thread-2
-- this lines are being printed in another thread..Thread-0
-- this lines are being printed in another thread..Thread-1
Full thread dump Java HotSpot(TM) Server VM (1.5.0_08-b03 mixed mode):

"Thread-2" prio=1 tid=0x09dcaa88 nid=0xfa6 in Object.wait() [0x6e8cb000..0x6e8cc080]
    at java.lang.Object.wait(Native Method)
    - waiting on <0xaa00e258> (a java.lang.Object)
    at java.lang.Object.wait(Object.java:474)
    at sil.WaitNotifyExample$1.run(WaitNotifyExample.java:17)//obj.wait()
    - locked <0xaa00e258> (a java.lang.Object)
    at java.lang.Thread.run(Thread.java:595)

"Thread-1" prio=1 tid=0x09dc9218 nid=0xfa5 in Object.wait() [0x6e94c000..0x6e94d100]
    at java.lang.Object.wait(Native Method)
    - waiting on <0xaa00e258> (a java.lang.Object)
    at java.lang.Object.wait(Object.java:474)
    at sil.WaitNotifyExample$1.run(WaitNotifyExample.java:17)//obj.wait()
    - locked <0xaa00e258> (a java.lang.Object)
    at java.lang.Thread.run(Thread.java:595)

"Thread-0" prio=1 tid=0x09dcb550 nid=0xfa4 in Object.wait() [0x6e9cd000..0x6e9cdd80]
    at java.lang.Object.wait(Native Method)
    - waiting on <0xaa00e258> (a java.lang.Object)
    at java.lang.Object.wait(Object.java:474)
    at sil.WaitNotifyExample$1.run(WaitNotifyExample.java:17)//obj.wait()
    - locked <0xaa00e258> (a java.lang.Object)
    at java.lang.Thread.run(Thread.java:595)

"main" prio=1 tid=0x09cecee0 nid=0xf94 waiting on condition [0xbfe77000..0xbfe776f8]
    at java.lang.Thread.sleep(Native Method)
    at sil.WaitNotifyExample.main(WaitNotifyExample.java:37)//Thread.currentThread ().sleep (500)

How to debug our code on the remote machine : remote debugging with jbuilder and intellij idea..

We write our code in our pc then we put it to a remote server. Sometimes it fails :)
Then what do you do to understand what is going on ?
Thread dump does not give what you need.
You need to connect your java process and debug.


To debug a process which is running a remote server you need to start it with the following JVM_ARGs.

-Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=3999,suspend=n

Sample usage : java -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=3999,suspend=n com.mycompany.test.WriteYourOwnCode


By the way your process will be waiting your connection from your machine whenever you want. Port number [3999] will be reserved for this debugging connections. Try to reserve a port number which is not used any other process/system.Otherwise your process will not start.You will see an error in this case something like this.


[serkans@serkansunellnx voicemail] java -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=3999,suspend=n com.mycompany.test.WriteYourOwnCode

ERROR: transport error 202: bind failed: Address already in use ["transport.c",L41]
ERROR: JDWP Transport dt_socket failed to initialize, TRANSPORT_INIT(510) ["debugInit.c",L500]
JDWP exit error JVMTI_ERROR_INTERNAL(113): No transports initializedFATAL ERROR in native method: JDWP No transports initialized, jvmtiError=JVMTI_ERROR_INTERNAL(113)

In the jvm_args you see an option : suspend=n . This means your remote code will not wait anybody to connect it before start. If you set this option to YES [suspend=y] process will wait before starting automatically. This suspend options must be set to YES [suspend=y] when you are unable to catch an error which is  happens in the beginnging of the process.


After starting our process with this options it is running and listenning for your connections...


Let me put a few picture to show how to connect to a remote process.
I'll show for both JBuilder and IntellijIdea.


1) Remote debugging with Jbuilder.


Firstly we start our code on a remote server with the remote debugging options.


[serkans@serkansunellnx voicemail] java -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=3999,suspend=n socketLogger.LogServer
Listening for transport dt_socket at address: 3999
 >> Server waiting for log messages...

It started and said that waiting on port 3999.It is not hanged.It is running, cool down :)


Then we turn to our ide.In the Jbuilder Right Click on the project and select project properties.
Click to the Run on the left pane of the project properties window.You will see RunTime Configurations. Click to New . A new window will appear called "New Runtime Configuration".Then click to the Remote in the left pane of this window.
1) Select "Enable Remote Debugging" by clicking the checkbox.
2) Select "Attach".
3) Write the IP of remote server on which your code is running as hostname.
4) Change the port number if you selected something different from 3999.


That is all.Click OK , OK  and close these windows.


It will looks like that :

After doing these right click on your Project .Select Debug..Ohh yes now you see the newly defined Runtime Configuration called "Your connection to remote process". Yes ...Click on this.




Now you are connected to your code.Go there and put your breakpoints. If your process hit your breakpoints you will notice it :-)


I got tired... I will expain it for Intellij Idea later.