Java同步线程无法按预期工作

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

class Worker implements Runnable {

    public void run() {
        System.out.println("Started.");
        process();

    }
    private Random random = new Random();

    private Object lock1 = new Object();
    private Object lock2 = new Object();

    private static List<Integer> list1 = new ArrayList<Integer>();
    private static List<Integer> list2 = new ArrayList<Integer>();

    public void stageOne() {

        synchronized (lock1) {

            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            list1.add(random.nextInt(100));
        }

    }

    public void stageTwo() {

        synchronized (lock2) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            list2.add(random.nextInt(100));
        }
    }

    public void process() {
        for(int i=0; i<1000; i++) {
            stageOne();
            stageTwo();
        }
    }
    public static void show() {
        System.out.println("List1: " + list1.size());
        System.out.println("List2: " + list2.size());
    }
}

public class JavaTest {

    public static void main(String[] args) {

        long start = System.currentTimeMillis();

        Thread t1 = new Thread(new Worker());
        t1.start();
        Thread t2 = new Thread(new Worker());
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("Completed.");

        long end = System.currentTimeMillis();

        System.out.println("Time taken: " + (end - start));

        Worker.show();
    }

}

当我运行这个程序时,我希望list1和list2每个包含2000个项目,并且程序需要大约2000毫秒.然而,很多时候我得到的列表少于2000个项目,尽管它确实在2000毫秒左右完成.有时我甚至会得到一个ArrayOutOfBounds异常

Started.
Started.
Exception in thread "Thread-1" java.lang.ArrayIndexOutOfBoundsException: 163
    at java.util.ArrayList.add(ArrayList.java:459)
    at Worker.stageOne(JavaTest.java:34)
    at Worker.process(JavaTest.java:53)
    at Worker.run(JavaTest.java:14)
    at java.lang.Thread.run(Thread.java:748)
Completed.
Time taken: 2217
List1: 1081
List2: 1079

我希望stageOne和stageTwo中的每个锁都应该阻止线程相互干扰.但事实似乎并非如此.这段代码有什么问题？