Lets see how synchronization works in Java. Below is a sample program which has three parts
- TestSynchronization is a the main class
- TestThread is a private class extends Thread.
- IamSynchronized is one more private class which has synchronized code logic. one part is just a synchronous block and the other is a synchronous method.
public class TestSynchronization {
public static void main(String args[]) {
IamSynchronized IS = new IamSynchronized();
TestThread T1 = new TestThread( "Thread - 1 ", IS );
TestThread T2 = new TestThread( "Thread - 2 ", IS );
try{Thread.sleep(60000);}catch(Exception e){}
T1.start();
T2.start();
}
}
class TestThread extends Thread {
private Thread t;
private String threadName;
IamSynchronized IS ;
TestThread(String name, IamSynchronized is){
threadName = name;
IS = is;
}
public void run() {
IS.justIterate();
System.out.println("calling sync method - Thread: "+threadName);
IS.syncMethod();
System.out.println("Thread " +threadName + " done");
}
public void start ()
{
System.out.println("Starting " + threadName );
if (t == null)
{ t = new Thread (this, threadName);
t.start ();
}
}
}
class IamSynchronized {
public void justIterate(){
synchronized(this){
try {
for(int i = 5; i > 0; i--) {
System.out.println("Iteration -- " + i );
Thread.sleep(1000);
}} catch (Exception e) { System.out.println("Interrupted"+e); }
}
}
public synchronized void syncMethod(){
{
try {
for(int i = 5; i > 0; i--) {
System.out.println("Iteration -- " + i );
Thread.sleep(1000);
}
} catch (Exception e) {
System.out.println("Interrupted"+e);
}
}
}
}
Lets see how the locking works and how the control flow happens when 2 parallel threads trying to execute the same logic.
Starting Thread - 1
Starting Thread - 2
I am in the non synchronous part of justIterate method - Thread Thread - 1
I am Thread Thread - 1 in sync block Iteration -- 5
I am in the non synchronous part of justIterate method - Thread Thread - 2 -- THREAD 2 IS BLOCKED BY THREAD1
I am Thread Thread - 1 in sync block Iteration -- 4
I am Thread Thread - 1 in sync block Iteration -- 3
I am Thread Thread - 1 in sync block Iteration -- 2
I am Thread Thread - 1 in sync block Iteration -- 1
I am Thread Thread - 2 in sync block Iteration -- 5
calling sync method - Thread: Thread - 1 -- THREAD 1 IS BLOCKED BY THREAD2
I am Thread Thread - 2 in sync block Iteration -- 4
I am Thread Thread - 2 in sync block Iteration -- 3
I am Thread Thread - 2 in sync block Iteration -- 2
I am Thread Thread - 2 in sync block Iteration -- 1
calling sync method - Thread: Thread - 2 -- THREAD 2 IS AGAIN BLOCKED BY THREAD1 BUT IN SYNC MTHOD
I am Thread Thread - 1 in sync method Iteration -- 10
I am Thread Thread - 1 in sync method Iteration -- 9
I am Thread Thread - 1 in sync method Iteration -- 8
I am Thread Thread - 1 in sync method Iteration -- 7
I am Thread Thread - 1 in sync method Iteration -- 6
Thread Thread - 1 done
I am Thread Thread - 2 in sync method Iteration -- 10 -- SINCE THREAD1 RELEASES LOCK, THREAD2 CONTINUES
I am Thread Thread - 2 in sync method Iteration -- 9
I am Thread Thread - 2 in sync method Iteration -- 8
I am Thread Thread - 2 in sync method Iteration -- 7
I am Thread Thread - 2 in sync method Iteration -- 6
Thread Thread - 2 done
Below stack trace gives an idea on how the locks being handled by the JVM
"Thread - 2 " prio=6 tid=0x000000000b550800 nid=0x2478 waiting for monitor entry [0x000000000d48f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at IamSynchronized.justIterate(TestSynchronization.java:47)
- waiting to lock <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:24)
at java.lang.Thread.run(Thread.java:722)
"Thread - 1 " prio=6 tid=0x000000000b550000 nid=0x18c4 waiting on condition [0x000000000d17f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at IamSynchronized.justIterate(TestSynchronization.java:49)
- locked <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:24)
at java.lang.Thread.run(Thread.java:722)
--------------------------------------------------
"Thread - 2 " prio=6 tid=0x000000000b550800 nid=0x2478 waiting on condition [0x000000000d48f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at IamSynchronized.justIterate(TestSynchronization.java:49)
- locked <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:24)
at java.lang.Thread.run(Thread.java:722)
"Thread - 1 " prio=6 tid=0x000000000b550000 nid=0x18c4 waiting for monitor entry [0x000000000d17f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at IamSynchronized.syncMethod(TestSynchronization.java:56)
- waiting to lock <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:26)
at java.lang.Thread.run(Thread.java:722)
--------------------------------------------------
"Thread - 2 " prio=6 tid=0x000000000b550800 nid=0x2478 waiting for monitor entry [0x000000000d48f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at IamSynchronized.syncMethod(TestSynchronization.java:56)
- waiting to lock <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:26)
at java.lang.Thread.run(Thread.java:722)
"Thread - 1 " prio=6 tid=0x000000000b550000 nid=0x18c4 waiting on condition [0x000000000d17f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at IamSynchronized.syncMethod(TestSynchronization.java:58)
- locked <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:26)
at java.lang.Thread.run(Thread.java:722)
--------------------------------------------------
"Thread - 2 " prio=6 tid=0x000000000b550800 nid=0x2478 waiting on condition [0x000000000d48f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at IamSynchronized.syncMethod(TestSynchronization.java:58)
- locked <0x00000007d5fbab18> (a IamSynchronized)
at TestThread.run(TestSynchronization.java:26)
at java.lang.Thread.run(Thread.java:722)
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