Need some help, about synchronization topic

[Baaziz Hamza]

(Edit)
Hello there , I have a question about synchronization topic !
In a given method we can synchronize a block of code on a third party object
exm :

public void doStuff() {
    synchronized(OtherObject) {
      // regular code;
    }
}

I hope i understood this right : while a thread is executing the synchronized block of code, it 'll lock the OtherObject, & no other thread will be able to modify that instance or call it's methods.
So I did a simple code to test :

class Test {
    public String doStuff() { return "Iam the tested synch-on objet"; }
}
public class AppCertif {
    final private Test test = new Test();
    class Toto implements Runnable { 
        public void run() {
            synchronized(AppCertif.this.test) {
                for(int i=0; i<50; i++) {
                    System.out.println(AppCertif.this.test.doStuff()+", by Toto");
                }
            }
        }
    }
    class Titi implements Runnable {
        public void run() {
            // Titi should wait until Toto release the lock on AppCertif.this.test, before calling the following no ??
            for(int i=0; i<50; i++) {
                System.out.println(AppCertif.this.test.doStuff()+", by Titi");
            }
        }
    } 
    public static void main(String[] args) {
        AppCertif a = new AppCertif();
        new Thread(a.new Toto()).start();
        new Thread(a.new Titi()).start();
    }
}

I was expecting that while the synchronized for of Toto is running, AppCertif.test instance will be locked, & Titi won't be able to call it's method.
But I didn't get what I was expecting, instead during execution I get some

...
Iam the tested synch-on objet, by Titi
Iam the tested synch-on objet, by Toto
Iam the tested synch-on objet, by Titi
...

which shows that both threads can access the AppCertif.test methods even if we're synchronizing on it !!!

I hope someone can explains to me, what I'm missing please !

[R. Jain]

Baaziz Hamza wrote:
I hope someone can explains to me, what I'm missing please !

Suppose you are using a pen to do an assignment. And one more person sitting besides you does not have a pen..

You will happily share the pen with him occassionally, but you will certainly not want him to take away the pen out of the country unless you are done with your job..

Apply the same with Threading.

[Seetharaman Venkatasamy]

the basic key point here to understand this behavior is that if 2 threads are accessing two different methods say one is synchronized another is non-synchronized.

now thread one is entered synchronized method/block . and thread two can enter non-synchronized method simultaneously,
because to enter non-synchronized method you dont required object lock.

[Henry Wong]

Baaziz Hamza wrote:In a given method we can synchronize a block of code on a third party object
exm :

public void doStuff() {
    synchronized(OtherObject) {
      // regular code;
    }
}

I hope i understood this right : while a thread is executing the synchronized block of code, it 'll lock the OtherObject, & no other thread will be able to access that instance or call it's methods.


I was expecting that while the synchronized for of Toto is running, AppCertif.test instance will be locked, & Titi won't be able to access it or call it's method.
But I didn't get what I was expecting, instead during execution I get some [DELETED] which shows that both threads can access the AppCertif.test instance even if we're synchronizing on it !!!

I hope someone can explains to me, what I'm missing please !

Synchronization is cooperative. This....

while a thread is executing the synchronized block of code, it 'll lock the OtherObject, & no other thread will be able to access that instance or call it's methods.

is obviously incorrect, and probably better as...

while a thread is executing the synchronized block of code, it'll lock the OtherObject, & no other thread will be able to lock the same object.

If you other thread also tried to lock the object before accessing, then it would have blocked until the lock is freed.

Henry

[Baaziz Hamza]

Than you all for your answers
Henry

while a thread is executing the synchronized block of code, it'll lock the OtherObject, & no other thread will be able to lock the same object.

Can you explain to me what's your understanding of locking an instance please.
Because in my understanding, locking means preventing other threads from accessing that instance & executing it's methods, to prevent corrupted data that might happens when multiple threads are dealing with the same resource at the same time.

Regards

[R. Jain]

Baaziz Hamza wrote:
Because in my understanding, locking means preventing other threads from accessing that instance & executing it's method
Regards

Well, your understanding is quite correct, but this doesn't fit with the requirement you are having..

while a thread is executing the synchronized block of code, it'll lock the OtherObject, & no other thread will be able to lock the same object.

This statement is an improvement over the one you gave..
Because, any other thread can still access the instance that has been locked in a thread, unless it wants to achieve a lock on it..

And you were saying saomething like this: -

while a thread is executing the synchronized block of code, it'll lock the OtherObject, & no other thread will be able to access the instance of OtherObject.

Compare both the statements and my reasoning..

[R. Jain]

And that was my 100th Post.

[Baaziz Hamza]

R. Jain wrote:And that was my 100th Post.

congrats

while a thread is executing the synchronized block of code, it'll lock the OtherObject, & no other thread will be able to access the instance of OtherObject.

I made an error in saying no other thread can access the locked instance, i was trying to say that no other thread can modify it. I edited it.
But i still don't get why, another thread can still call the synchronized instance methods, even if it's locked by another thread.
Or I have a confusion & is it allowed to call the methods, but not modifying the internal state ??

[Seetharaman Venkatasamy]

R. Jain wrote:And that was my 100th Post.

keep up!

@Baaziz Hamza : Welcome to JavaRanch

[Baaziz Hamza]

Seetharaman Venkatasamy wrote:@Baaziz Hamza : Welcome to JavaRanch

Thank you

[Baaziz Hamza]

Henry Wong wrote:If you other thread also tried to lock the object before accessing, then it would have blocked until the lock is freed.

Finally i found what i was doing wrong, (I guess my ping is kind of slow or I need to go through this chapter again ), what i should have done is also getting the same lock on the other thread that's accessing my synchronizing instance :

class Titi implements Runnable {
   public void run() {
     synchronized(AppCertif.this.test) {
       for(int i=0; i<50; i++) {
         System.out.println(AppCertif.this.test.doStuff()+", by Titi");
       }
     }
   }
} 

I was thinking that once we get a lock on an instance it'll automatically block other threads (even if they don't have the same lock).

Anyway thanks every one

[Seetharaman Venkatasamy]

Baaziz Hamza wrote:Any ways thanks every one

You are Welcome