java - 多线程死锁测试

package test;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.TimeUnit;

/**
 * Created by rhwayfun on 16-4-3.
 */
public class ThreadTest {

    private static DateFormat format = new SimpleDateFormat("HH:mm:ss");

    public synchronized void tryOther(ThreadTest other) throws InterruptedException {
        System.out.println(Thread.currentThread().getName() + " enter tryOther method at " + format.format(new Date()));      
        System.out.println(Thread.currentThread().getName() + " tryOther method is about to invoke other method at " + format.format(new Date()));
        other.other();
    }

    public synchronized void other() throws InterruptedException {
        System.out.println(Thread.currentThread().getName() + " enter other method atatatatat " + format.format(new Date()));
    }

    public static void main(String[] args) throws InterruptedException {
        final ThreadTest d1 = new ThreadTest();
        final ThreadTest d2 = new ThreadTest();

        Thread t1 = new Thread(new Runnable() {
            public void run() {
                try {
                    d1.tryOther(d2);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "threadA");

        Thread t2 = new Thread(new Runnable() {
            public void run() {
                try {
                    d2.tryOther(d1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "threadB");

        t1.start();
        //让threadA先运行一秒
        TimeUnit.SECONDS.sleep(1);
        t2.start();

    }
}

如上，随便找的产生死锁的代码，问题：
TimeUnit.SECONDS.sleep(1);加上这行后，不存在死锁问题。sleep并不释放锁，为何这边死锁情况会消失。
输出结果为：
threadA enter tryOther method at 15:37:39
threadA tryOther method is about to invoke other method at 15:37:39
threadA enter other method atatatatat 15:37:39
threadB enter tryOther method at 15:37:40
threadB tryOther method is about to invoke other method at 15:37:40
threadB enter other method atatatatat 15:37:40

注掉这行，正常死锁。
输出结果为：
threadB enter tryOther method at 15:37:10
threadA enter tryOther method at 15:37:10
threadB tryOther method is about to invoke other method at 15:37:10
threadA tryOther method is about to invoke other method at 15:37:10