java多线程中关于使用notify()方法的几个疑问

Question

代码：

public class NotifyAndWaitTest2 implements Runnable {
    public int i = 0;
    public Object lock;
    public SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss.SSS");

    public NotifyAndWaitTest2(Object o) {
        this.lock = o;
    }

    @Override
    public void run() {
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " enter the SYNCHRONIZED block --- "+ sdf.format(new Date()));
            try {
                while (i < 9) {
                    Thread.sleep(500);
                    lock.notify();
                    lock.wait();
                    System.out.println(Thread.currentThread().getName() + " say：" + i++ + " --- " + sdf.format(new Date()));
                }
                lock.notify();
                return;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        Object lock = new Object();
        NotifyAndWaitTest2 test = new NotifyAndWaitTest2(lock);
        Thread t1 = new Thread(test,"Thread A");
        Thread t2 = new Thread(test,"Thread B");
        Thread t3 = new Thread(test,"Thread C");
        Thread t4 = new Thread(test,"Thread D");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}

执行结果：

Thread A enter the SYNCHRONIZED block --- 10:47:07.242
Thread B enter the SYNCHRONIZED block --- 10:47:07.743
Thread C enter the SYNCHRONIZED block --- 10:47:08.243
Thread D enter the SYNCHRONIZED block --- 10:47:08.743
Thread C say：0 --- 10:47:09.243
Thread D say：1 --- 10:47:09.744
Thread C say：2 --- 10:47:10.244
Thread D say：3 --- 10:47:10.744
Thread C say：4 --- 10:47:11.245
Thread D say：5 --- 10:47:11.745
Thread C say：6 --- 10:47:12.246
Thread D say：7 --- 10:47:12.746
Thread C say：8 --- 10:47:13.247
Thread D say：9 --- 10:47:13.247
Thread B say：10 --- 10:47:13.247
Thread A say：11 --- 10:47:13.247

这段代码我在 jdk1.7 和 jdk1.8 中都执行过，结果是"相同的"

我主要有两个问题：（以我贴出的这个执行结果为例）

1、在线程A 和线程B 进入了同步代码块之后，并且线程B 调用了 wait() 方法，为什么接下来一定是另外两个线程进入同步代码块？而不是线程A 开始打印 say。是"让新的线程进入同步代码块"的优先级比"调用wait()方法的线程被唤醒"的优先级高吗？

2、为什么调用 notify() 方法只能唤醒最后一个调用了 wait() 方法的线程？就像上面这个执行结果，只有线程C 和线程D 互相唤醒对方，为什么不能唤醒其他的线程，比如线程A 和线程B？

我认为抢对象锁应该是不规律的、随机的，比如这样：

Thread A enter the SYNCHRONIZED block
Thread B enter the SYNCHRONIZED block
Thread A say：0
Thread C enter the SYNCHRONIZED block
Thread B say：1
Thread A say：2
Thread D enter the SYNCHRONIZED block
Thread B say：3
Thread C say：4

Answer 1

理论上是随机的，哪个抢到cpu资源就执行哪个，线程初始化的时候会设置默认优先级为5，可以通过设置线程优先级让线程更有抢的cpu资源的优势，但是也只是相对随机的，代码实现的时候就当成随机，不要依赖这一点就行了，Thread类有三个静态变量可以设置线程执行优先级分别为高中低
Thread.MAX_PRIORITY;
Thread.MIN_PRIORITY;
Thread.NORM_PRIORITY;