Difference between object Lock and class Lock in java




It is very important multithreading topic. We must understand this difference to answer interview, ocjp answers correctly.




Difference between object Lock and class Lock >



Thread can acquire object lock by-
  1. Entering synchronized block or
  2. by entering synchronized methods.
Thread can acquire lock on class’s class object by-
  1. Entering synchronized block or
  2. by entering static synchronized methods.

Multiple threads may exist on same or different objects of class but only one thread can enter static synchronized method at a time.
Multiple objects of class may exist and every object has it’s own lock.
Multiple objects of class may exist but there is always one class’s class object lock available.
First let’s acquire object lock by entering synchronized block.

Example- Let’s say there is one class MyClass and we have created it’s object and reference to that object is myClass. Now we can create synchronization block, and parameter passed with synchronization tells which object has to be synchronized. In below code, we have synchronized object reference by myClass.
MyClass myClass=new Myclass();
      synchronized (myClass) {
     }
As soon thread entered Synchronization block, thread acquired object lock on object referenced by myClass (by acquiring object’s monitor.)
Thread will leave lock when it exits synchronized block.
First let’s acquire lock on class’s class object by entering synchronized block.

Example- Let’s say there is one class MyClass. Now we can create synchronization block, and parameter passed with synchronization tells which class has to be synchronized. In below code, we have synchronized MyClass
   synchronized (MyClass.class) {
   }

As soon as thread entered Synchronization block, thread acquired MyClass’s class object. Thread will leave lock when it exits synchronized block.
public synchronized void method1() {
}

As soon as thread entered Synchronization method, thread acquired object lock.
Thread will leave lock when it exits synchronized method.
public static synchronized void method1() {}
As soon as thread entered static Synchronization method, thread acquired lock on class’s class object.
Thread will leave lock when it exits synchronized method.




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Object and class lock >

Acquiring object lock - synchronization blocks and methods- multiple threads may exist on same object but only one thread of that object can enter synchronized block/method at a time.
Acquiring lock on class, 2 Ways to acquire lock on class

Difference between object Lock and class Lock



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Interviews >

THREADS - Top 75 interview questions and answers (detailed explanation with programs) Set-1 >  Q1- Q54

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