Multithreading Dilemna

[Surinder Mehra]

Below code throw outofBound Exception on arraylist if executed continuously(immediately)2-3 times.
I am aware that this exception can be avoided by synchronizing alterlist() methods or synchronized bocks using different objectslock1 and lock2 I have created. I want to understand why this exception is thrown. Arraylist auto increase its size when needed.

Can anyone explain what is happening behind the scene

package com.suri.threads;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class MultipleLocks {
	List<Integer> list1 = new ArrayList<Integer>();
	List<Integer> list2 = new ArrayList<Integer>();
	Object lock1 = new Object();
	Object lock2 = new Object();
	Random random = new Random();

	public static void main(String[] args) {
		
		new MultipleLocks().doWork();
		
	}

	private void doWork() {
		System.out.println("Starting...");
		long start = System.currentTimeMillis();
		Thread t1 = new Thread(new Runnable() {

			@Override
			public void run() {
				process();

			}
		});

		Thread t2 = new Thread(new Runnable() {

			@Override
			public void run() {
				process();

			}
		});
		t1.start();
		t2.start();
		try {
			t1.join();
			t2.join();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		long end = System.currentTimeMillis();
		System.out.println("Time taken : "+(end-start));
		System.out.println("List 1 size : "+list1.size()+" List 2 Size : "+list2.size());
	}

	protected void process() {
		for(int i=0;i<1000;i++){
			alterList1();
			alterList2();
		}

	}

	private  void alterList1() {
		try {
			Thread.sleep(1);
		} catch (Exception e) {
		}
		list1.add(random.nextInt(100));

	}

	private  void alterList2() {
		try {
			Thread.sleep(1);
		} catch (Exception e) {
		}
		list2.add(random.nextInt(100));		
	}
}

Exception:

Starting...
Exception in thread "Thread-1" java.lang.ArrayIndexOutOfBoundsException: 25
at java.util.ArrayList.add(ArrayList.java:352)
at com.suri.threads.MultipleLocks.alterList2(MultipleLocks.java:75)
at com.suri.threads.MultipleLocks.process(MultipleLocks.java:56)
at com.suri.threads.MultipleLocks$2.run(MultipleLocks.java:36)
at java.lang.Thread.run(Thread.java:662)
Time taken : 2067
List 1 size : 1013 List 2 Size : 1013

[Henry Wong]

The ranch office forum is used for discussions regarding the ranch itself ... let's move this topic to a more appropriate forum.

Henry

[Henry Wong]

Surinder Mehra wrote:
Can anyone explain what is happening behind the scene

The behavior of using classes in a non-thread-safe manner is completely undefined, so, I would argue that it is probably better to just *not* do that, than to figure out how broken code is behaving. Regardless...

Surinder Mehra wrote:
I am aware that this exception can be avoided by synchronizing alterlist() methods or synchronized bocks using different objectslock1 and lock2 I have created. I want to understand why this exception is thrown. Arraylist auto increase its size when needed.

A quick look at the code shows that the add() method checks if there is enough room, increases it if there isn't, and then add the element. So, if there is room for one more element, we can speculate that the two threads checks and both confirm that there is enough room for its one element. The first thread then adds and complete successfully, and the second fails.

In my opinion, I am surprised you didn't get something more complicated. It could be possible to corrupt data structures, and you could wind up with all sorts of other errors.

Henry