JDK源码笔记--Object

public final native Class<?> getClass();
 
public native int hashCode();
 
public boolean equals(Object obj) {
        return (this == obj);
    }
 
protected native Object clone() throws CloneNotSupportedException;
 
public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }
 
public final native void notify();
 
public final native void notifyAll();
 
public final native void wait(long timeout) throws InterruptedException;
 
public final void wait() throws InterruptedException {
        wait(0);
    }
 
protected void finalize() throws Throwable { }

重点看看wait()，notify()，notifyAll()方法

首先是都是final native 修饰的。

monitor暂且译成监听器，相当于对象锁。

notify的注释：

    /**
     * Wakes up a single thread that is waiting on this object's
     * monitor. If any threads are waiting on this object, one of them
     * is chosen to be awakened. The choice is arbitrary and occurs at
     * the discretion of the implementation. A thread waits on an object's
     * monitor by calling one of the {@code wait} methods.

唤醒正在等待该对象的单个线程，如果有多个线程在等待获取某个对象的监听器，只唤醒其中一个。

@throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.

如果当前线程没拥有监听器就抛异常 IllegalMonitorStateException，这就是为什么notify，notifyAll必须在synchronized代码中执行。

notifyAll的注释：

/**
     * Wakes up all threads that are waiting on this object's monitor. A
     * thread waits on an object's monitor by calling one of the
     * {@code wait} methods.

唤醒在该对象监视器上等待的所有线程，但是只有一个最终获得监听器。

 @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.

同上。

wait的注释：

/**
     * Causes the current thread to wait until either another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or a
     * specified amount of time has elapsed.

     * <ul>
     * <li>Some other thread invokes the {@code notify} method for this
     * object and thread <var>T</var> happens to be arbitrarily chosen as
     * the thread to be awakened.
     * <li>Some other thread invokes the {@code notifyAll} method for this
     * object.
     * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
     * thread <var>T</var>.
     * <li>The specified amount of real time has elapsed, more or less.  If
     * {@code timeout} is zero, however, then real time is not taken into
     * consideration and the thread simply waits until notified.
     * </ul>

导致当前线程等待，直到另一个线程调用notify()，notifyAll()，线程中断，或者超过指定的时间。

当调用wait(),实际上是调用的wait(0)表示无限等下去，也可以指定如wait(1000)表示等待一秒就继续执行。

wait()方法会放弃对象锁，sleep()不会。

     * the condition that should have caused the thread to be awakened, and
     * continuing to wait if the condition is not satisfied.  In other words,
     * waits should always occur in loops, like this one:
     * <pre>
     *     synchronized (obj) {
     *         while (<condition does not hold>)
     *             obj.wait(timeout);
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>

建议wait(time)的方法放在while循环中。

     * @throws  IllegalArgumentException      if the value of timeout is
     *               negative.
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @throws  InterruptedException if any thread interrupted the

当时间参数not in the range 0-999999，抛异常IllegalArgumentException

当线程未获得对象锁，抛异常IllegalMonitorStateException。这就是wait()要在synchronized中执行。

当线程被中断，抛异常InterruptedException

一个栗子：

public class Test2 implements Runnable {
    static Object a = new Object();
    public static void main(String[] args) throws InterruptedException {
        Test2 t2 = new Test2();
        Thread thread = new Thread(t2);
        thread.start();
        //TimeUnit.MILLISECONDS.sleep(1000);
        synchronized (a) {
            System.out.println("notify start"+System.currentTimeMillis());
            a.notify();
            System.out.println("notify end"+System.currentTimeMillis());
        }
    }
 
    @Override
    public void run() {
        synchronized (a) {
            try {
                System.out.println("wait start" + System.currentTimeMillis());
                a.wait();
                System.out.println("wait end" + System.currentTimeMillis());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
 
        }
    }
}

当TimeUnit.MILLISECONDS.sleep(1000);注释掉时，控制台：

notify start1517303479751
notify end1517303479752
wait start1517303479752

notify执行完了才执行wait，导致wait不能结束。意味着执行顺序不能依赖代码先后顺序。