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javathread-safetysynchronize
Eagle
Eagle

Reputation: 217

How multiple synchronized key words work in a single class?

package anonymous;

public class A {
public static int counter=0;

public static void main(String[] args) {

    synchronized (args) {
        //some logic
    }

    synchronized (args) {
        //some logic

    }
 }        

}

Let say one thread is executing in one synchronized block. Can another thread acquire lock on other synchronized block?
What will happen if a method call happened within a synchronized block to a nonsynchronized method? will that method be thread safe?

What if we try to access a static variable from a synchronized instance method?? At a time each thread accessing a synchronized block in each instance will try to modify the static variable. Is n't it? In this case how we can have thread safety??

Upvotes: 1

Views: 1158

Answers (4)

Dhiraj
Dhiraj

Reputation: 1

First and foremost, there can be multiple synchronized methods in a class. That is done by:

  • declaring the methods as synchronized

  • synchronized methods .

As per your question, if the threads are of the same class(multiple threads requesting the same resource, then no two objects of the same class can access a particular resource at the same time(concurrently). The thread that is holding the mutually exclusive lock(mutex) is in the monitor. All other such threads has to undergo waiting for the monitor.

Java Thread Scheduler ensures that any of the synchronized method might be required by the thread which is currently in the monitor, i.e. the one which is holding the lock. So, does not allow other threads to execute them concurrently. That is not the case with non-synchronized methods, they can execute concurrently.

Upvotes: 0

bitkot
bitkot

Reputation: 4504

  1. No, any other thread will not be able execute another block which acquires lock on the same object in your case args.

  2. No. the other method which is being called from synchronized code block will not be thread safe. It can be called by any other thread from any other code block if it does not have synchronized keyword, because the thread need not to acquire any lock to execute that method.

This is common misunderstanding that synchronized keyword locks piece of code.

What synchronized keyword does?

It locks the object and not the method. So if you put synchronized keyword in front of a method then it will lock this object. so any other method with synchronized keyword can not be executed by any other thread.

This is not the same with static and not static method because when you have synchronized static method then the it will not lock this but it will lock default class object i.e A.class object. If you have another static sync method then that will not be executed by any other thread.

In case of sync blocks it will acquire lock on the object which is passed as argument in your case it is args. so if there is another sync block anywhere else which acquires lock on the same object then it will have to wait until the first thread completes and releases the lock.

Upvotes: 1

Mustafa sabir
Mustafa sabir

Reputation: 4360

There is always a lock present on synchronised code. To access the code threads have to acquire the key , unlock , run and then handback the key.

There is a object level lock present , the lock is of the entire object and not of some method or block. So even if there are multiple synchronized block or methods on a single instance only one of the threads can acquire the key and access the synchronized code. It will then hand back the key to the object and then other threads can resume to acquire key and access synchronized code.

Incase you are wondering what happens to the static methods declared syncrhonized (since they have no objects/instances associated with them), in that case there is always also a lock present with the class itself. To access any synchronized static part of a class, a thread has to acquire the key from the class to unlock the lock.

Note:- the lock and key thing I mentioned is just to help understand, there is no methods or way to access keys or anything it is all internally maintained by JVM.

Upvotes: 0

Keppil
Keppil

Reputation: 46239

Can another thread acquire lock on other synchronized block?

No, only one synchronized method at a time can run. A call to the other synchronized method will have to wait until the first method is done. This is described in the Java tutorials:

...it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

What will happen if a method call happened within a synchronized block to a nonsynchronized method? will that method be thread safe?

If the non-synchronized method is only called from synchronized methods belonging to this Object, then it can be considered thread-safe, since only one thread can execute one of the calling (synchronized) methods at a time.

Note that, as @Ordous points out below, static synchronized methods lock on the class, and non-static synchronized methods lock on the instance. Therefore a non-static method can interleave with a static method belonging to the class in question.

Upvotes: 2

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