synchronized Java Keyword with Examples

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The synchronized keyword may be applied to a method or statement block and provides protection for critical sections that should only be executed by one thread at a time.

We can synchronize our code in either of two ways. Both involve the use of the synchronized keyword.

1. Using Synchronized Methods
2. Using Synchronized Statement Or Block

1. Using Synchronized Methods

To make a method synchronized, simply add the synchronized keyword to its declaration:

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}

If a count is an instance of SynchronizedCounter, then making these methods synchronized has two effects:

• First, 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.
• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

2. Using Synchronized Statement or Block

You do not have to synchronize a whole method. Sometimes it is preferable to synchronize only part of a method. Java synchronized blocks inside methods make this possible.

Here is a synchronized block of Java code inside an unsynchronized Java method:

  public void add(int value){

       synchronized(this){
           this.count += value;   
       }
  }

This example uses the Java synchronized block construct to mark a block of code as synchronized. This code will now execute as if it was a synchronized method.

Summary

1. The synchronized keyword prevents a critical section of code from being executed by more than one thread at a time.
2. When applied to a static method, the entire class is locked while the method is being executed by one thread at a time.
3. When applied to an instance method, the instance is locked while being accessed by one thread at a time.
4. When applied to an object or array, the object or array is locked while the associated code block is executed by one thread at a time.

Read more at Synchronization in Multithreading Java

All Java Keywords 

1. abstract Java Keyword
2. assert Java Keyword
3. boolean Java Keyword
4. break Java Keyword
5. byte Java Keyword
6. case Java Keyword
7. catch Java Keyword
8. char Java Keyword
9. class Java Keyword
10. continue Java Keyword
11. default Java Keyword
12. do Java Keyword
13. double Java Keyword
14. else Java Keyword
15. enum Java Keyword
16. extends Java Keyword
17. final Java Keyword
18. finally Java Keyword
19. float Java Keyword
20. for Java Keyword
21. if Java Keyword
22. implements Java Keyword
23. import Java Keyword
24. instanceof Java Keyword
25. int Java Keyword
26. interface Java Keyword
27. long Java Keyword
28. native Java Keyword
29. new Java Keyword
30. package Java Keyword
31. private Java Keyword
32. protected Java Keyword
33. public Java Keyword
34. return Java Keyword
35. short Java Keyword
36. static Java Keyword
37. strictfp Java Keyword
38. super Java Keyword
39. switch Java Keyword
40. synchronized Java Keyword
41. this Java Keyword
42. throw Java Keyword
43. throws Java Keyword
44. transient Java Keyword
45. try Java Keyword
46. void Java Keyword
47. volatile Java Keyword
48. while Java Keyword