Tuesday, 31 March 2015

Multithreading in Java

Multithreading in Java

  1. Multithreading
  2. Multitasking
  3. Process-based multitasking
  4. Thread-based multitasking
  5. What is Thread
Multithreading in java is a process of executing multiple threads simultaneously.
Thread is basically a lightweight sub-process, a smallest unit of processing. Multiprocessing and multithreading, both are used to achieve multitasking.
But we use multithreading than multiprocessing because threads share a common memory area. They don't allocate separate memory area so saves memory, and context-switching between the threads takes less time than process.
Java Multithreading is mostly used in games, animation etc.

Advantage of Java Multithreading

1) It doesn't block the user because threads are independent and you can perform multiple operations at same time.
2) You can perform many operations together so it saves time.
3) Threads are independent so it doesn't affect other threads if exception occur in a single thread.

Multitasking

Multitasking is a process of executing multiple tasks simultaneously. We use multitasking to utilize the CPU. Multitasking can be achieved by two ways:
  • Process-based Multitasking(Multiprocessing)
  • Thread-based Multitasking(Multithreading)

1) Process-based Multitasking (Multiprocessing)

  • Each process have its own address in memory i.e. each process allocates separate memory area.
  • Process is heavyweight.
  • Cost of communication between the process is high.
  • Switching from one process to another require some time for saving and loading registers, memory maps, updating lists etc.

2) Thread-based Multitasking (Multithreading)

  • Threads share the same address space.
  • Thread is lightweight.
  • Cost of communication between the thread is low.

Note: At least one process is required for each thread.

What is Thread in java

A thread is a lightweight sub process, a smallest unit of processing. It is a separate path of execution.
Threads are independent, if there occurs exception in one thread, it doesn't affect other threads. It shares a common memory area.
As shown in the above figure, thread is executed inside the process. There is context-switching between the threads. There can be multiple processes inside the OS and one process can have multiple threads.




Note: At a time one thread is executed only.





Do You Know ?
·         Why your class, which extends the Thread class, object is treated as thread ? Who is responsible for it ?
·         How to perform two tasks by two threads ?
·         How to perform multithreading by annonymous class ?
·         What is the Thread Schedular and what is the difference between preemptive scheduling and time slicing ?
·         What happens if we start a thread twice ?
·         What happens if we call the run() method instead of start() method ?
·         What is the purpose of join method ?
·         Why JVM terminates the daemon thread if there is no user threads remaining ?
·         What is the shutdown hook?
·         What is garbage collection ?
·         What is the purpose of finalize() method ?
·         What does gc() method ?
·         What is synchronization and why use synchronization ?
·         What is the difference between synchronized method and synchronized block ?
·         What are the two ways to perform static synchronization ?
·         What is deadlock and when it can occur ?
·         What is interthread-communication or cooperation ?
What we will learn in Multithreading ?
·         Multithreading
·         Life Cycle of a Thread
·         Two ways to create a Thread
·         How to perform multiple tasks by multiple threads
·         Thread Schedular
·         Sleeping a thread
·         Can we start a thread twice ?
·         What happens if we call the run() method instead of start() method ?
·         Joining a thread
·         Naming a thread
·         Priority of a thread
·         Daemon Thread
·         ShutdownHook
·         Garbage collection
·         Synchronization with synchronized method
·         Synchronized block
·         Static synchronization
·         Deadlock
·         Inter-thread communication



Life cycle of a Thread (Thread States)

1.    Life cycle of a thread
1.    New
2.    Runnable
3.    Running
4.    Non-Runnable (Blocked)
5.    Terminated
A thread can be in one of the five states. According to sun, there is only 4 states in thread life cycle in javanew, runnable, non-runnable and terminated. There is no running state.
But for better understanding the threads, we are explaining it in the 5 states.
The life cycle of the thread in java is controlled by JVM. The java thread states are as follows:
1.    New
2.    Runnable
3.    Running
4.    Non-Runnable (Blocked)
5.    Terminated

1) New

The thread is in new state if you create an instance of Thread class but before the invocation of start() method.

2) Runnable

The thread is in runnable state after invocation of start() method, but the thread scheduler has not selected it to be the running thread.

3) Running

The thread is in running state if the thread scheduler has selected it.

4) Non-Runnable (Blocked)

This is the state when the thread is still alive, but is currently not eligible to run.

5) Terminated

A thread is in terminated or dead state when its run() method exits.

How to create thread

There are two ways to create a thread:
  1. By extending Thread class
  2. By implementing Runnable interface.

Thread class:

Thread class provide constructors and methods to create and perform operations on a thread.Thread class extends Object class and implements Runnable interface.

Commonly used Constructors of Thread class:

·         Thread()
·         Thread(String name)
·         Thread(Runnable r)
·         Thread(Runnable r,String name)

Commonly used methods of Thread class:

1.    public void run(): is used to perform action for a thread.
2.    public void start(): starts the execution of the thread.JVM calls the run() method on the thread.
3.    public void sleep(long miliseconds): Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds.
4.    public void join(): waits for a thread to die.
5.    public void join(long miliseconds): waits for a thread to die for the specified miliseconds.
6.    public int getPriority(): returns the priority of the thread.
7.    public int setPriority(int priority): changes the priority of the thread.
8.    public String getName(): returns the name of the thread.
9.    public void setName(String name): changes the name of the thread.
10. public Thread currentThread(): returns the reference of currently executing thread.
11. public int getId(): returns the id of the thread.
12. public Thread.State getState(): returns the state of the thread.
13. public boolean isAlive(): tests if the thread is alive.
14. public void yield(): causes the currently executing thread object to temporarily pause and allow other threads to execute.
15. public void suspend(): is used to suspend the thread(depricated).
16. public void resume(): is used to resume the suspended thread(depricated).
17. public void stop(): is used to stop the thread(depricated).
18. public boolean isDaemon(): tests if the thread is a daemon thread.
19. public void setDaemon(boolean b): marks the thread as daemon or user thread.
20. public void interrupt(): interrupts the thread.
21. public boolean isInterrupted(): tests if the thread has been interrupted.
22. public static boolean interrupted(): tests if the current thread has been interrupted.

Runnable interface:

The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. Runnable interface have only one method named run().

1.    public void run(): is used to perform action for a thread.

Starting a thread:

start() method of Thread class is used to start a newly created thread. It performs following tasks:
·         A new thread starts(with new callstack).
·         The thread moves from New state to the Runnable state.
·         When the thread gets a chance to execute, its target run() method will run.

1)By extending Thread class:

1.    class Multi extends Thread{  
2.    public void run(){  
3.    System.out.println("thread is running...");  
4.    }  
5.    public static void main(String args[]){  
6.    Multi t1=new Multi();  
7.    t1.start();  
8.     }  
9.    }  
Output:thread is running...

Who makes your class object as thread object?

Thread class constructor allocates a new thread object.When you create object of Multi class,your class constructor is invoked(provided by Compiler) fromwhere Thread class constructor is invoked(by super() as first statement).So your Multi class object is thread object now.

2)By implementing the Runnable interface:

1.    class Multi3 implements Runnable{  
2.    public void run(){  
3.    System.out.println("thread is running...");  
4.    }  
5.      
6.    public static void main(String args[]){  
7.    Multi3 m1=new Multi3();  
8.    Thread t1 =new Thread(m1);  
9.    t1.start();  
10.  }  
11. }  
Output:thread is running...
If you are not extending the Thread class,your class object would not be treated as a thread object.So you need to explicitely create Thread class object.We are passing the object of your class that implements Runnable so that your class run() method may execute.

Thread Scheduler in Java

Thread scheduler in java is the part of the JVM that decides which thread should run.
There is no guarantee that which runnable thread will be chosen to run by the thread scheduler.
Only one thread at a time can run in a single process.
The thread scheduler mainly uses preemptive or time slicing scheduling to schedule the threads.

Difference between preemptive scheduling and time slicing

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Sleep method in java

The java sleep() method of Thread class is used to sleep a thread for the specified milliseconds of time.


Syntax of sleep() method in java

The Thread class provides two methods for sleeping a thread:
·         public static void sleep(long miliseconds)throws InterruptedException
·         public static void sleep(long miliseconds, int nanos)throws InterruptedException

Example sleep method in java

1.    class TestSleepMethod1 extends Thread{  
2.     public void run(){  
3.      for(int i=1;i<5;i++){  
4.        try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);}  
5.        System.out.println(i);  
6.      }  
7.     }  
8.     public static void main(String args[]){  
9.      TestSleepMethod1 t1=new TestSleepMethod1();  
10.   TestSleepMethod1 t2=new TestSleepMethod1();  
11.    
12.   t1.start();  
13.   t2.start();  
14.  }  
15. }  
Output:1
       1
       2
       2
       3
       3
       4
       4
       5
       5
As you know well that at a time only one thread is executed. If you sleep a thread for the specified time,the thread shedular picks up another thread and so on.
Can we start a thread twice?
No. After staring a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown. For Example:
1.    public class TestThreadTwice1 extends Thread{  
2.     public void run(){  
3.       System.out.println("running...");  
4.     }  
5.     public static void main(String args[]){  
6.      TestThreadTwice1 t1=new TestThreadTwice1();  
7.      t1.start();  
8.      t1.start();  
9.     }  
10. }  

Output:running
       Exception in thread "main" java.lang.IllegalThreadStateException

What if we call run() method directly instead start() method?
·         Each thread starts in a separate call stack.
·         Invoking the run() method from main thread, the run() method goes onto the current call stack rather than at the beginning of a new call stack.
1.    class TestCallRun1 extends Thread{  
2.     public void run(){  
3.       System.out.println("running...");  
4.     }  
5.     public static void main(String args[]){  
6.      TestCallRun1 t1=new TestCallRun1();  
7.      t1.run();//fine, but does not start a separate call stack  
8.     }  
9.  }  
10.Output:running...
 Problem if you direct call run() method
1.    class TestCallRun2 extends Thread{  
2.     public void run(){  
3.      for(int i=1;i<5;i++){  
4.        try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);}  
5.        System.out.println(i);  
6.      }  
7.     }  
8.     public static void main(String args[]){  
9.      TestCallRun2 t1=new TestCallRun2();  
10.   TestCallRun2 t2=new TestCallRun2();  
11.    
12.   t1.run();  
13.   t2.run();  
14.  }  
15. }  

Output:1
       2
       3
       4
       5
       1
       2
       3
       4
       5

As you can see in the above program that there is no context-switching because here t1 and t2 will be treated as normal object not thread object.

The join() method:

The join() method waits for a thread to die. In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task.

Syntax:

public void join()throws InterruptedException
public void join(long milliseconds)throws InterruptedException
Example of join() method
1.    class TestJoinMethod1 extends Thread{  
2.     public void run(){  
3.      for(int i=1;i<=5;i++){  
4.       try{  
5.        Thread.sleep(500);  
6.       }catch(Exception e){System.out.println(e);}  
7.      System.out.println(i);  
8.      }  
9.     }  
10. public static void main(String args[]){  
11.  TestJoinMethod1 t1=new TestJoinMethod1();  
12.  TestJoinMethod1 t2=new TestJoinMethod1();  
13.  TestJoinMethod1 t3=new TestJoinMethod1();  
14.  t1.start();  
15.  try{  
16.   t1.join();  
17.  }catch(Exception e){System.out.println(e);}  
18.   
19.  t2.start();  
20.  t3.start();  
21.  }  
22. }  
Test it Now
Output:1
       2
       3
       4
       5
       1
       1
       2
       2
       3
       3
       4
       4
       5
       5
 
As you can see in the above example,when t1 completes its task then t2 and t3 starts executing.
Example of join(long miliseconds) method
1.    class TestJoinMethod2 extends Thread{  
2.     public void run(){  
3.      for(int i=1;i<=5;i++){  
4.       try{  
5.        Thread.sleep(500);  
6.       }catch(Exception e){System.out.println(e);}  
7.      System.out.println(i);  
8.      }  
9.     }  
10. public static void main(String args[]){  
11.  TestJoinMethod2 t1=new TestJoinMethod2();  
12.  TestJoinMethod2 t2=new TestJoinMethod2();  
13.  TestJoinMethod2 t3=new TestJoinMethod2();  
14.  t1.start();  
15.  try{  
16.   t1.join(1500);  
17.  }catch(Exception e){System.out.println(e);}  
18.   
19.  t2.start();  
20.  t3.start();  
21.  }  
22. }  
Test it Now
Output:1
       2
       3
       1
       4
       1
       2
       5
       2
       3
       3
       4
       4
       5
       5
 
In the above example,when t1 is completes its task for 1500 miliseconds(3 times) then t2 and t3 starts executing.

getName(),setName(String) and getId() method:

public String getName()
public void setName(String name)
public long getId()
1.    class TestJoinMethod3 extends Thread{  
2.      public void run(){  
3.       System.out.println("running...");  
4.      }  
5.     public static void main(String args[]){  
6.      TestJoinMethod3 t1=new TestJoinMethod3();  
7.      TestJoinMethod3 t2=new TestJoinMethod3();  
8.      System.out.println("Name of t1:"+t1.getName());  
9.      System.out.println("Name of t2:"+t2.getName());  
10.   System.out.println("id of t1:"+t1.getId());  
11.   
12.   t1.start();  
13.   t2.start();  
14.   
15.   t1.setName("Sonoo Jaiswal");  
16.   System.out.println("After changing name of t1:"+t1.getName());  
17.  }  
18. }  

Output:Name of t1:Thread-0
       Name of t2:Thread-1
       id of t1:8
       running...
       After changling name of t1:Sonoo Jaiswal
       running...
     
 

The currentThread() method:

The currentThread() method returns a reference to the currently executing thread object.

Syntax:

public static Thread currentThread()
Example of currentThread() method
1.    class TestJoinMethod4 extends Thread{  
2.     public void run(){  
3.      System.out.println(Thread.currentThread().getName());  
4.     }  
5.     }  
6.     public static void main(String args[]){  
7.      TestJoinMethod4 t1=new TestJoinMethod4();  
8.      TestJoinMethod4 t2=new TestJoinMethod4();  
9.      
10.   t1.start();  
11.   t2.start();  
12.  }  
13. }  

Output:Thread-0
       Thread-1

Naming a thread:

The Thread class provides methods to change and get the name of a thread.
1.    public String getName(): is used to return the name of a thread.
2.    public void setName(String name): is used to change the name of a thread.

Example of naming a thread:

1.    class TestMultiNaming1 extends Thread{  
2.      public void run(){  
3.       System.out.println("running...");  
4.      }  
5.     public static void main(String args[]){  
6.      TestMultiNaming1 t1=new TestMultiNaming1();  
7.      TestMultiNaming1 t2=new TestMultiNaming1();  
8.      System.out.println("Name of t1:"+t1.getName());  
9.      System.out.println("Name of t2:"+t2.getName());  
10.    
11.   t1.start();  
12.   t2.start();  
13.   
14.   t1.setName("Ram");  
15.   System.out.println("After changing name of t1:"+t1.getName());  
16.  }  
17. }  

Output:Name of t1:Thread-0
       Name of t2:Thread-1
       id of t1:8
       running...
       After changeling name of t1:Ram
       running...
     
 

The currentThread() method:

The currentThread() method returns a reference to the currently executing thread object.

Syntax of currentThread() method:

  • public static Thread currentThread(): returns the reference of currently running thread.
  •  

Example of currentThread() method:

1.    class TestMultiNaming2 extends Thread{  
2.     public void run(){  
3.      System.out.println(Thread.currentThread().getName());  
4.     }  
5.     }  
6.     public static void main(String args[]){  
7.      TestMultiNaming2 t1=new TestMultiNaming2();  
8.      TestMultiNaming2 t2=new TestMultiNaming2();  
9.      
10.   t1.start();  
11.   t2.start();  
12.  }  
13. }  

Output:Thread-0
       Thread-1

Priority of a Thread (Thread Priority):

Each thread have a priority. Priorities are represented by a number between 1 and 10. In most cases, thread schedular schedules the threads according to their priority (known as preemptive scheduling). But it is not guaranteed because it depends on JVM specification that which scheduling it chooses.

3 constants defiend in Thread class:

1.    public static int MIN_PRIORITY
2.    public static int NORM_PRIORITY
3.    public static int MAX_PRIORITY

Default priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY is 1 and the value of MAX_PRIORITY is 10.

Example of priority of a Thread:

1.    class TestMultiPriority1 extends Thread{  
2.     public void run(){  
3.       System.out.println("running thread name is:"+Thread.currentThread().getName());  
4.       System.out.println("running thread priority is:"+Thread.currentThread().getPriority());  
5.      
6.      }  
7.     public static void main(String args[]){  
8.      TestMultiPriority1 m1=new TestMultiPriority1();  
9.      TestMultiPriority1 m2=new TestMultiPriority1();  
10.   m1.setPriority(Thread.MIN_PRIORITY);  
11.   m2.setPriority(Thread.MAX_PRIORITY);  
12.   m1.start();  
13.   m2.start();  
14.    
15.  }  
16. }     

Output:running thread name is:Thread-0
       running thread priority is:10
       running thread name is:Thread-1
       running thread priority is:1


Daemon Thread in Java

Daemon thread in java is a service provider thread that provides services to the user thread. Its life depend on the mercy of user threads i.e. when all the user threads dies, JVM terminates this thread automatically.
There are many java daemon threads running automatically e.g. gc, finalizer etc.
You can see all the detail by typing the jconsole in the command prompt. The jconsole tool provides information about the loaded classes, memory usage, running threads etc.

Points to remember for Daemon Thread in Java

  • It provides services to user threads for background supporting tasks. It has no role in life than to serve user threads.
  • Its life depends on user threads.
  • It is a low priority thread.

Why JVM terminates the daemon thread if there is no user thread?

The sole purpose of the daemon thread is that it provides services to user thread for background supporting task. If there is no user thread, why should JVM keep running this thread. That is why JVM terminates the daemon thread if there is no user thread.

Methods for Java Daemon thread by Thread class

The java.lang.Thread class provides two methods for java daemon thread.
No.
Method
Description
1)
public void setDaemon(boolean status)
is used to mark the current thread as daemon thread or user thread.
2)
public boolean isDaemon()
is used to check that current is daemon.

Simple example of Daemon thread in java

File: MyThread.java
1.    public class TestDaemonThread1 extends Thread{  
2.     public void run(){  
3.      if(Thread.currentThread().isDaemon()){//checking for daemon thread  
4.       System.out.println("daemon thread work");  
5.      }  
6.      else{  
7.      System.out.println("user thread work");  
8.     }  
9.     }  
10.  public static void main(String[] args){  
11.   TestDaemonThread1 t1=new TestDaemonThread1();//creating thread  
12.   TestDaemonThread1 t2=new TestDaemonThread1();  
13.   TestDaemonThread1 t3=new TestDaemonThread1();  
14.   
15.   t1.setDaemon(true);//now t1 is daemon thread  
16.     
17.   t1.start();//starting threads  
18.   t2.start();  
19.   t3.start();  
20.  }  
21. }  

Output

daemon thread work
user thread work
user thread work

Note: If you want to make a user thread as Daemon, it must not be started otherwise it will throw IllegalThreadStateException.

File: MyThread.java
1.    class TestDaemonThread2 extends Thread{  
2.     public void run(){  
3.      System.out.println("Name: "+Thread.currentThread().getName());  
4.      System.out.println("Daemon: "+Thread.currentThread().isDaemon());  
5.     }  
6.      
7.     public static void main(String[] args){  
8.      TestDaemonThread2 t1=new TestDaemonThread2();  
9.      TestDaemonThread2 t2=new TestDaemonThread2();  
10.   t1.start();  
11.   t1.setDaemon(true);//will throw exception here  
12.   t2.start();  
13.  }  
14. }  

Output:exception in thread main: java.lang.IllegalThreadStateException


Java Thread Pool

Java Thread pool represents a group of worker threads that are waiting for the job and reuse many times.
In case of thread pool, a group of fixed size threads are created. A thread from the thread pool is pulled out and assigned a job by the service provider. After completion of the job, thread is contained in the thread pool again.

Advantage of Java Thread Pool

Better performance It saves time because there is no need to create new thread.

Real time usage

It is used in Servlet and JSP where container creates a thread pool to process the request.

Example of Java Thread Pool

Let's see a simple example of java thread pool using ExecutorService and Executors.
File: WorkerThrad.java
1.    import java.util.concurrent.ExecutorService;  
2.    import java.util.concurrent.Executors;  
3.    class WorkerThread implements Runnable {  
4.        private String message;  
5.        public WorkerThread(String s){  
6.            this.message=s;  
7.        }  
8.         public void run() {  
9.            System.out.println(Thread.currentThread().getName()+" (Start) message = "+message);  
10.         processmessage();//call processmessage method that sleeps the thread for 2 seconds  
11.         System.out.println(Thread.currentThread().getName()+" (End)");//prints thread name  
12.     }  
13.     private void processmessage() {  
14.         try {  Thread.sleep(2000);  } catch (InterruptedException e) { e.printStackTrace(); }  
15.     }  
16. }  
File: JavaThreadPoolExample.java
1.    public class TestThreadPool {  
2.         public static void main(String[] args) {  
3.            ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads  
4.            for (int i = 0; i < 10; i++) {  
5.                Runnable worker = new WorkerThread("" + i);  
6.                executor.execute(worker);//calling execute method of ExecutorService  
7.              }  
8.            executor.shutdown();  
9.            while (!executor.isTerminated()) {   }  
10.   
11.         System.out.println("Finished all threads");  
12.     }  
13.  }  

Output:

pool-1-thread-1 (Start) message = 0
pool-1-thread-2 (Start) message = 1
pool-1-thread-3 (Start) message = 2
pool-1-thread-5 (Start) message = 4
pool-1-thread-4 (Start) message = 3
pool-1-thread-2 (End)
pool-1-thread-2 (Start) message = 5
pool-1-thread-1 (End)
pool-1-thread-1 (Start) message = 6
pool-1-thread-3 (End)
pool-1-thread-3 (Start) message = 7
pool-1-thread-4 (End)
pool-1-thread-4 (Start) message = 8
pool-1-thread-5 (End)
pool-1-thread-5 (Start) message = 9
pool-1-thread-2 (End)
pool-1-thread-1 (End)
pool-1-thread-4 (End)
pool-1-thread-3 (End)
pool-1-thread-5 (End)
Finished all threads


Shutdown Hook

The shutdown hook can be used to perform cleanup resource or save the state when JVM shuts down normally or abruptly. Performing clean resource means closing log file, sending some alerts or something else. So if you want to execute some code before JVM shuts down, use shutdown hook.

When does the JVM shut down?

The JVM shuts down when:
  • user presses ctrl+c on the command prompt
  • System.exit(int) method is invoked
  • user logoff
  • user shutdown etc.

The addShutdownHook(Runnable r) method

The addShutdownHook() method of Runtime class is used to register the thread with the Virtual Machine. Syntax:
1.    public void addShutdownHook(Runnable r){}  


The object of Runtime class can be obtained by calling the static factory method getRuntime(). For example:
Runtime r = Runtime.getRuntime();

Factory method

The method that returns the instance of a class is known as factory method.

Simple example of Shutdown Hook

1.    class MyThread extends Thread{  
2.        public void run(){  
3.            System.out.println("shut down hook task completed..");  
4.        }  
5.    }  
6.      
7.    public class TestShutdown1{  
8.    public static void main(String[] args)throws Exception {  
9.      
10. Runtime r=Runtime.getRuntime();  
11. r.addShutdownHook(new MyThread());  
12.       
13. System.out.println("Now main sleeping... press ctrl+c to exit");  
14. try{Thread.sleep(3000);}catch (Exception e) {}  
15. }  
16. }  

Output:Now main sleeping... press ctrl+c to exit
       shut down hook task completed..
       
 

Note: The shutdown sequence can be stopped by invoking the halt(int) method of Runtime class.

Same example of Shutdown Hook by annonymous class:

1.    public class TestShutdown2{  
2.    public static void main(String[] args)throws Exception {  
3.      
4.    Runtime r=Runtime.getRuntime();  
5.      
6.    r.addShutdownHook(new Runnable(){  
7.    public void run(){  
8.        System.out.println("shut down hook task completed..");  
9.        }  
10. }  
11. );  
12.       
13. System.out.println("Now main sleeping... press ctrl+c to exit");  
14. try{Thread.sleep(3000);}catch (Exception e) {}  
15. }  
16. }  

Output:Now main sleeping... press ctrl+c to exit
       shut down hook task completed..
      


How to perform single task by multiple threads?

If you have to perform single task by many threads, have only one run() method.For example:
Program of performing single task by multiple threads
1.    class TestMultitasking1 extends Thread{  
2.     public void run(){  
3.       System.out.println("task one");  
4.     }  
5.     public static void main(String args[]){  
6.      TestMultitasking1 t1=new TestMultitasking1();  
7.      TestMultitasking1 t2=new TestMultitasking1();  
8.      TestMultitasking1 t3=new TestMultitasking1();  
9.      
10.   t1.start();  
11.   t2.start();  
12.   t3.start();  
13.  }  
14. }  

Output:task one
       task one
       task one
Program of performing single task by multiple threads
1.    class TestMultitasking2 implements Runnable{  
2.    public void run(){  
3.    System.out.println("task one");  
4.    }  
5.      
6.    public static void main(String args[]){  
7.    Thread t1 =new Thread(new TestMultitasking2());//passing annonymous object of TestMultitasking2 class  
8.    Thread t2 =new Thread(new TestMultitasking2());  
9.      
10. t1.start();  
11. t2.start();  
12.   
13.  }  
14. }  

Output:task one
       task one

Note: Each thread run in a separate callstack.

How to perform multiple tasks by multiple threads (multitasking in multithreading)?

If you have to perform multiple tasks by multiple threads,have multiple run() methods.For example:
Program of performing two tasks by two threads
1.    class Simple1 extends Thread{  
2.     public void run(){  
3.       System.out.println("task one");  
4.     }  
5.    }  
6.      
7.    class Simple2 extends Thread{  
8.     public void run(){  
9.       System.out.println("task two");  
10.  }  
11. }  
12.   
13.  class TestMultitasking3{  
14.  public static void main(String args[]){  
15.   Simple1 t1=new Simple1();  
16.   Simple2 t2=new Simple2();  
17.   
18.   t1.start();  
19.   t2.start();  
20.  }  
21. }  

Output:task one
       task two

Same example as above by annonymous class that extends Thread class:

Program of performing two tasks by two threads
1.    class TestMultitasking4{  
2.     public static void main(String args[]){  
3.      Thread t1=new Thread(){  
4.        public void run(){  
5.          System.out.println("task one");  
6.        }  
7.      };  
8.      Thread t2=new Thread(){  
9.        public void run(){  
10.       System.out.println("task two");  
11.     }  
12.   };  
13.   
14.   
15.   t1.start();  
16.   t2.start();  
17.  }  
18. }  

Output:task one
       task two

Same example as above by annonymous class that implements Runnable interface:

Program of performing two tasks by two threads
1.    class TestMultitasking5{  
2.     public static void main(String args[]){  
3.      Runnable r1=new Runnable(){  
4.        public void run(){  
5.          System.out.println("task one");  
6.        }  
7.      };  
8.      
9.      Runnable r2=new Runnable(){  
10.     public void run(){  
11.       System.out.println("task two");  
12.     }  
13.   };  
14.       
15.   Thread t1=new Thread(r1);  
16.   Thread t2=new Thread(r2);  
17.   
18.   t1.start();  
19.   t2.start();  
20.  }  
21. }  

Output:task one
       task two

Garbage Collection:

In java, garbage means unreferenced objects.
Garbage Collection is process of reclaiming the runtime unused memory automatically.

Advantage of Garbage Collection:

·         It makes java memory efficient because garbage collector removes the unreferenced objects from heap memory.
·         It is automatically done by the garbage collector so we don't need to make extra efforts.

How can an object be unreferenced?

There are many ways:
·         By nulling the reference
·         By assigning a reference to another
·         By annonymous object etc.

1) By nulling a reference:

1.    Employee e=new Employee();  
2.    e=null;  

2) By assigning a reference to another:

1.    Employee e1=new Employee();  
2.    Employee e2=new Employee();  
3.      
4.    e1=e2;//now the first object referred by e1 is available for garbage collection  

3) By annonymous object:

1.    new Employee();  

finalize() method:

The finalize() method is invoked each time before the object is garbage collected. This method can be used to perform cleanup processing. This method is defined in System class as:
1.    protected void finalize(){}  

Note: The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created any object without new, you can use finalize method to perform cleanup processing (destroying remaining objects).

gc() method:

The gc() method is used to invoke the garbage collector to perform cleanup processing. The gc() is found in System and Runtime classes.
1.    public static void gc(){}  

Note: Garbage collection is performed by a daemon thread called Garbage Collector(GC). This thread calls the finalize() method before object is garbage collected.

Simple Example of garbage collection:

1.    public class TestGarbage1{  
2.      
3.     public void finalize(){System.out.println("object is garbage collected");}  
4.      
5.     public static void main(String args[]){  
6.      TestGarbage1 s1=new TestGarbage1();  
7.      TestGarbage1 s2=new TestGarbage1();  
8.      s1=null;  
9.      s2=null;  
10.   System.gc();  
11.  }  
12. }  

Output:object is garbage collected
       object is garbage collected
Note: Neither finalization nor garbage collection is guaranteed.
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NOTE: To access an attribute in a  component , use expressions as  {! v.<Attribute Name>} . ----------------------------------------...