multi-thread synchronization in Java

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public class Test {

	public static void main(String[] args) {

		for(int i=0; i<5; i++) {
			ThreadEX ex = new ThreadEX();
			new Thread(ex).start();
		}
		
	}

}

class ThreadEX implements Runnable {

	private static Hashtable<Integer, String> table;
	private boolean thCheck = false;
	
	public ThreadEX() {
		table = new Hashtable<Integer,String>();
	}
	
	public void run() {
		synchronized(this) {
			table.put(this.hashCode(), "tmp");		
			System.out.println(this.hashCode() + " added, size : " + table.size());
		}
	}
}

Hello, everyone.
I like to ask about multi-thread synchronization in Java.
There is a code above, which I used as an example.
I excepted it would make a result like this:

6413875 added, size : 1
5442986 added, size : 2
28737396 added, size : 3
827574 added, size : 4
24355087 added, size : 5

(hash code doesn't matter).

However, it doesn't at all. I think I have a problem with synchronizing hashtable, but I don't know what the wrong is for now.

Could you anyone give me any advice or correction of my synchronization code, to make a result like what I excepted?

Thank you!

Posted 10-Jan-10 8:01am

sunku89

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Dimitri Witkowski · Accepted Answer · 2010-01-10T08:13:00

You have written:
synchronized(this) {

But this in all thread points to a different instance of your ThreadEX class

You can do one of the following:
1. Write in such way: synchronized(ThreadEx.class) {
2. Use ConcurrentHashTable class.

Also, you re-initialize table in each thread. You should definitely fix this.
I have slightly changed your version:

Java

public class ThreadEX implements Runnable {
	private static Hashtable<Integer, String> table = new Hashtable<Integer,String>();
	private boolean thCheck = false;
	
	public ThreadEX() {
	}
	
	@Override
	public void run() {
		synchronized(ThreadEX.class) {
			table.put(this.hashCode(), "tmp");		
			System.out.println(this.hashCode() + " added, size : " + table.size());
		}
	}
}

Good luck!