Synchronization on Threads in Java

[Deepak Lal]

Hi Ranchers,
I have a few clarifications.

Scenario 1> 
    public class A{
         public synchronized void MethodA(){ 
                   System.out.println("In synchronized  MethodA"); //Accessed by Thread (1)
         }
 
     //in the same code below here i have another synchronized method(MethodB)
          public synchronized void MethodB(){ 
                   System.out.println("In synchronized  MethodB"); //Can Thread (2) access this synchronized methodB ?
         }
    }

-----------------------------------------------------------------------------------------------------------------------------------

Scenario 2> 
    public class B{
         public synchronized void MethodA(){ 
                   System.out.println("In Synchronized MethodA"); //Access by Thread (1)
         }
 
     //in the same code below here i have another normal method
          public void MethodB(){ 
                   System.out.println("In Normal MethodB"); //Can Thread (2) access this  Normal methodB ?
         }
    }

Need your inputs/valuable suggestions on above scenarios..please help me to resolve this.

[Steve Luke]

Scenario 1:
Can Thread 2 access the synchronized methodB?
Yes, with this limitation: IF Thread 1 and Thread 2 are accessing the same instance of ClassA, then Thread2 can only access methodB when Thread1 is NOT accessing methodA. That is, if Thread1 is executing code in methodA, then Thread2 can not access code in methodB from the same instance of the class.

Scenarion2:
Can Thread 2 access the non-synchronized methodB?
Yes, with no limitation as to when.

[Deepak Lal]

Thanks Steve for your impeccable replies.

My next question is how do i complete the code for Scenario 1 and Scenario 2 i.e how should my Public static main method look like.I'm unable to figure that part for Scenario 1 and Scenario 2
Help provided will be highly appreciated.

Best Regards
Deepak lal

[Steve Luke]

Well, I don't know what the main() should look like. What do you want to do?

At the very least, I guess, you would need to:

1) Make one instance of the class A.
2) Make two Threads (or Runnables) which access the appropriate method in the class.
3) Execute both Threads (or Runnables in Threads)
4) Repeat 1-3 for class B

[Shanky Sohar]

Deepak Lal wrote:
how should my Public static main method look like.I'm unable to figure that part for Scenario 1 and Scenario 2

How could anybody know this Deepak,untill and unless we know what you want to do with that

[Shanky Sohar]

1 more things your scenerio1 will look like this for compiler..

public class A1{
public void MethodA(){
synchronized(this){
System.out.println("In synchronized MethodA"); //Accessed by Thread (1)
}
}

//in the same code below here i have another synchronized method(MethodB)
public void MethodB(){
synchronized(this){
System.out.println("In synchronized MethodB"); //Can Thread (2) access this synchronized methodB ?
}
}
}


synchronized(this)//will occupy a lock on the current object montor..

[Joe Lemmer]

Hi Deepak,

I don't know if this is what you want to do, but maybe this code might help as an example:

public class A{
    public synchronized void MethodA(){ 
        for(int i = 0; i < 100; i++) {
            System.out.println(Thread.currentThread().getName() + " in MethodA: " + i); //Accessed by Thread (1)
		}
    }
 
     //in the same code below here i have another synchronized method(MethodB)
    public synchronized void MethodB(){ 
	    for(int i = 0; i < 100; i++) {
            System.out.println(Thread.currentThread().getName() + " in MethodB: " + i);
		}
    }
}
class TestRunnableTypeOne implements Runnable {

    private A sharedA;

    public TestRunnableTypeOne(A a) {
        sharedA = a;
    }

    public void run() { // runs A's MethodA only
        sharedA.MethodA();
	}
}

class TestRunnableTypeTwo implements Runnable {

    private A sharedA;
	
	public TestRunnableTypeTwo(A a) {
	    sharedA = a;
	}
	
	public void run() { // runs A's MethodB only
	    sharedA.MethodB();
	}
}

class TestA {

    public static void main(String[] args) {
	    A a = new A();
		TestRunnableTypeOne typeOne = new TestRunnableTypeOne(a);
		TestRunnableTypeTwo typeTwo = new TestRunnableTypeTwo(a);
		Thread threadOne = new Thread(typeOne, "threadOne");
		Thread threadTwo = new Thread(typeTwo, "threadTwo");
		threadOne.start();
		threadTwo.start();
	}
}
            

So, the same instance of the A class is referenced by each instance of the classes that implement runnable. You can't tell whether threadOne or threadTwo will get the lock on that instance of A, but you can say with certainty that once one of the Threads gets the lock then the other Thread will be blocked until the first Thread's run() method has completed.

I hope that helps.

cheers

Joe

[Deepak Lal]

Thank you,
I have another scenario

Scenario 3>    
    public class c{   
        static int i =0;
         public void read(){    
                   System.out.println("In Read Method");  
         }   
    
          public void write(){    
                   System.out.println("In Write Method");          }   
    }  

How should we synchronize the read and write method for static variable integer i.?Example provided will be helpful.

[Deepak Lal]

Joe Lemmer,Can you pleasse help me with scenario 3 as stated above.

[Abimaran Kugathasan]

Deepak Lal wrote:
How should we synchronize the read and write method for static variable integer i.?Example provided will be helpful.

Synchronization is for objects! Not to primitives!

[Steve Luke]

Abimaran Kugathasan wrote:

Deepak Lal wrote:
How should we synchronize the read and write method for static variable integer i.?Example provided will be helpful.

Synchronization is for objects! Not to primitives!

That is incorrect! You must protect primitives with synchronization as well, you just can't gain a lock on the primitive itself so you need to lock on something else.

Abimaran, this situation appears special because the data you want to protect is static, and the methods are not. In reality, the situation is the same as any other synchronization problem. You have to lock on an Object which is available in the same contexts as the data you want to protect. For static content that generally means an Object stored in a static reference. A common one to use is the Class' class Object since it is the same thing used by static synchronized methods. So in you scenario3 you might have:

public class  c
{
    private static int i = 0;

    public void read()
    {
        synchronized(c.class)
        {
            System.out.println("In Protected Read Method");
            //work with i here
        }
    }

    public void write()
    {
        synchronized(c.class)
        {
            System.out.println("In Protected Write Method");
            //work with i here
        }
    }
}

Note that you should make the variable 'i' private so you can not access it directly from an outside source - you have to access it through one of the protected accessor methods.

[Deepak Lal]

Steve you just made my day...Its correct...Wow i have no words for this answer.Thanks Steve...!!!




@Steve and @Ranchers:
Where can i find such nice examples on Core Java and MultiThreading application.Please suggest some good book with examples on real time multithreaded examples.Please advise ranchers

[Deepak Lal]

@Ranchers, @Steve I'm back with another scenario.see below

 
Scenario 4>       
   public class d{      
        
         private static int i =0;    // static variables

         public static void read(){  // static method       
                   System.out.println("In Read Method");     
         }      
       
          public static void write(){   // static method      
                   System.out.println("In Write Method");          }      
    } 

How should i apply synchronization for Scenario 4 such that read and write method are executed by 2 different threads. (as demonstrated by Joe lemmer in one of the posts) ?Sample code provided will be appreciated.

[Steve Luke]

Rather than post code for you again ('cause that doesn't seem to make it stick for you), why don't you post code you think will work and we can give you pointers?

<hint>What is different between scenario 3 and scenario 4?</hint>

[Deepak Lal]

In the scenario 4 we have applied "static" to the read and write methods.i dont know whats the answer.


Please let us know the answer Steve.Your help provided will be appreciated.

@Steve.I want help/guidelines/Website links for Multi Threading concepts.Please could you share it with me.
Help provided will be appreciated.
Best Regards
Deepak lal

[Abimaran Kugathasan]

Steve Luke wrote:

Abimaran Kugathasan wrote:

Deepak Lal wrote:
How should we synchronize the read and write method for static variable integer i.?Example provided will be helpful.

Synchronization is for objects! Not to primitives!

That is incorrect! You must protect primitives with synchronization as well, you just can't gain a lock on the primitive itself so you need to lock on something else.

Abimaran, this situation appears special ....

I mean there, we can't directly synchronize the primitives like we use in objects!

[Abimaran Kugathasan]

Deepak Lal wrote:@Ranchers, @Steve I'm back with another scenario.see below

 
Scenario 4>       
   public class d{      
        
         private static int i =0;    // static variables

         public static void read(){  // static method       
                   System.out.println("In Read Method");     
         }      
       
          public static void write(){   // static method      
                   System.out.println("In Write Method");          }      
    } 

How should i apply synchronization for Scenario 4 such that read and write method are executed by 2 different threads. (as demonstrated by Joe lemmer in one of the posts) ?Sample code provided will be appreciated.

Could you please explain a bit more? I couldn't understand you question!

[Deepak Lal]

@ Abimaran Kugathasan, Scenario 4 is a different scenario...are you sure of your answer

@Steve,Can you please comment on this now.

[Steve Luke]

Deepak,

Think it through! I had said in a previous post:

You have to lock on an Object which is available in the same contexts as the data you want to protect. For static content that generally means an Object stored in a static reference.

What do you think you have to synchronize on? What is the difference between Scenario 3 and Scenario 4? What does that mean to your synchronization scheme? Think it through, show us what you come up with, and we can comment.

Note to others: I will watch this thread, and if anyone else posts code for Deepak I will hide it. Deepak has to start writing his own code rather than getting others to do it for him. Please let him work this through.

[Abimaran Kugathasan]

I mean, we can't directly synchronize on primitives.

public class TestSyn implements Runnable	{
	int i = 0;
	public static void main(String[] arg)	{
		Thread t = new Thread(new TestSyn());
		t.start();
	}
	public void run()	{
		synchronized(i)	{
			System.out.println(i);
		}
	}
}

But, we can make the int variable as Thread-safe in other ways!

PS: This just for explanation, not for the answer for the question which Deepak asked!

[Deepak Lal]

Please see the code for scenario 4,
For static methods too like static variables declared as an int,it is similar to Scenario 3 (For static content that generally means an Object stored in a static reference.)

@STEVE, please go ahead and correct my understanding if im wrong and add suitable comments to this posts.

Scenario 4>
public class  d   
{   
    private static int i = 0;   
  
    public static void read()   
    {   
        synchronized(d.class)   
        {   
            System.out.println("In Read Method");   
            //work with i here   
        }   
    }   
  
    public static void write()   
    {   
        synchronized(d.class)   
        {   
            System.out.println("In Write Method");   
            //work with i here   
        }   
    }   

:-) Deepak Lal

[Steve Luke]

Hi Deepak, that is correct. Now for a shortcut - You can use

public static synchronized void read()
{
  System.out.println(...);
}
public static synchronized void write()
{
  System.out.println(...);
}

You get the same exact behavior - it is the same thing as you wrote, just a little more compact. There are trade-offs, but for what you wrote they are the same.

[Deepak Lal]

Thank you Steve

Steve,I needed the latest multi threading tutorial in Core Java that covers the new chapters in Thread,So can you suggest me a good tutorial with ample explanation of examples.Or can you share a pdf which you use for your reference by mailing it across to my mail id.Please reply Steve

Regards
Deepak Lal

[Steve Luke]

Hi Deepak, the Concurrency chapter of the Java Tutorial is an okay starter: http://download.oracle.com/javase/tutorial/essential/concurrency/. For more details it is best to get a good book (like Java Concurrency In Practice: http://www.javaconcurrencyinpractice.com/ ...).