notify() -从对象的等待集中随机选择一个线程，并将其置于BLOCKED状态。对象的等待集中的其余线程仍然处于WAITING状态。

notifyAll() -将所有线程从对象的等待集移动到BLOCKED状态。使用notifyAll()后，共享对象的等待集中没有剩余线程，因为所有线程现在都处于BLOCKED状态，而不是WAITING状态。

BLOCKED—锁定获取阻塞。 WAITING -等待通知(或阻塞连接完成)。

这里有一个例子。运行它。然后将notifyAll()中的一个更改为notify()，看看会发生什么。

ProducerConsumerExample类

public class ProducerConsumerExample {

    private static boolean Even = true;
    private static boolean Odd = false;

    public static void main(String[] args) {
        Dropbox dropbox = new Dropbox();
        (new Thread(new Consumer(Even, dropbox))).start();
        (new Thread(new Consumer(Odd, dropbox))).start();
        (new Thread(new Producer(dropbox))).start();
    }
}

Dropbox类

public class Dropbox {

    private int number;
    private boolean empty = true;
    private boolean evenNumber = false;

    public synchronized int take(final boolean even) {
        while (empty || evenNumber != even) {
            try {
                System.out.format("%s is waiting ... %n", even ? "Even" : "Odd");
                wait();
            } catch (InterruptedException e) { }
        }
        System.out.format("%s took %d.%n", even ? "Even" : "Odd", number);
        empty = true;
        notifyAll();

        return number;
    }

    public synchronized void put(int number) {
        while (!empty) {
            try {
                System.out.println("Producer is waiting ...");
                wait();
            } catch (InterruptedException e) { }
        }
        this.number = number;
        evenNumber = number % 2 == 0;
        System.out.format("Producer put %d.%n", number);
        empty = false;
        notifyAll();
    }
}

消费阶层

import java.util.Random;

public class Consumer implements Runnable {

    private final Dropbox dropbox;
    private final boolean even;

    public Consumer(boolean even, Dropbox dropbox) {
        this.even = even;
        this.dropbox = dropbox;
    }

    public void run() {
        Random random = new Random();
        while (true) {
            dropbox.take(even);
            try {
                Thread.sleep(random.nextInt(100));
            } catch (InterruptedException e) { }
        }
    }
}

生产类

import java.util.Random;

public class Producer implements Runnable {

    private Dropbox dropbox;

    public Producer(Dropbox dropbox) {
        this.dropbox = dropbox;
    }

    public void run() {
        Random random = new Random();
        while (true) {
            int number = random.nextInt(10);
            try {
                Thread.sleep(random.nextInt(100));
                dropbox.put(number);
            } catch (InterruptedException e) { }
        }
    }
}

当你调用wait()的“对象”(期望对象锁)、实习生这将释放锁,物体和帮助的其他线程锁在这个“对象”,在这种情况下,将会有超过1线程等待“资源/对象”(考虑到其他线程也发布了等待上面相同的对象,将会有一个线程的方式填补资源/对象并调用通知/ notifyAll)。

在这里，当您(从进程/代码的同一/另一端)发出同一对象的通知时，这将释放一个阻塞和等待的单个线程(不是所有等待的线程——这个释放的线程将由JVM thread Scheduler挑选，对象上的所有锁获取进程与常规进程相同)。

如果只有一个线程共享/处理这个对象，那么可以在wait-notify实现中单独使用notify()方法。

如果您处于基于业务逻辑的多个线程对资源/对象进行读写的情况，那么您应该使用notifyAll()

现在我正在寻找JVM是如何识别和打破等待线程时，我们发出通知()在一个对象…

有用的差异:

Use notify() if all your waiting threads are interchangeable (the order they wake up doesn't matter), or if you only ever have one waiting thread. A common example is a thread pool used to execute jobs from a queue--when a job is added, one of threads is notified to wake up, execute the next job and go back to sleep. Use notifyAll() for other cases where the waiting threads may have different purposes and should be able to run concurrently. An example is a maintenance operation on a shared resource, where multiple threads are waiting for the operation to complete before accessing the resource.

notify()将唤醒一个线程，而notifyAll()将唤醒所有线程。据我所知，没有中间立场。但是如果你不确定notify()会对你的线程做什么，使用notifyAll()。每次都很灵验。

显然，notify唤醒等待集中的一个线程(any)， notifyAll唤醒等待集中的所有线程。下面的讨论应能消除任何疑问。大多数时候应该使用notifyAll。如果您不确定使用哪个，那么使用notifyAll。请看下面的解释。

仔细阅读并理解。如果您有任何问题，请发邮件给我。

查看生产者/消费者(假设是一个具有两个方法的ProducerConsumer类)。IT IS BROKEN(因为它使用notify) -是的，它可能工作-甚至大多数时候，但它也可能导致死锁-我们将看到为什么:

public synchronized void put(Object o) {
    while (buf.size()==MAX_SIZE) {
        wait(); // called if the buffer is full (try/catch removed for brevity)
    }
    buf.add(o);
    notify(); // called in case there are any getters or putters waiting
}

public synchronized Object get() {
    // Y: this is where C2 tries to acquire the lock (i.e. at the beginning of the method)
    while (buf.size()==0) {
        wait(); // called if the buffer is empty (try/catch removed for brevity)
        // X: this is where C1 tries to re-acquire the lock (see below)
    }
    Object o = buf.remove(0);
    notify(); // called if there are any getters or putters waiting
    return o;
}

首先,

为什么我们需要一个while循环围绕等待?

我们需要一个while循环，以防出现这种情况:

消费者1 (C1)进入同步块，缓冲区是空的，因此C1被放入等待集(通过等待调用)。消费者2 (C2)即将进入同步方法(在上面的Y点)，但生产者P1在缓冲区中放入一个对象，然后调用notify。唯一等待的线程是C1，因此它被唤醒，现在试图重新获得点X(上面)的对象锁。

Now C1 and C2 are attempting to acquire the synchronization lock. One of them (nondeterministically) is chosen and enters the method, the other is blocked (not waiting - but blocked, trying to acquire the lock on the method). Let's say C2 gets the lock first. C1 is still blocking (trying to acquire the lock at X). C2 completes the method and releases the lock. Now, C1 acquires the lock. Guess what, lucky we have a while loop, because, C1 performs the loop check (guard) and is prevented from removing a non-existent element from the buffer (C2 already got it!). If we didn't have a while, we would get an IndexArrayOutOfBoundsException as C1 tries to remove the first element from the buffer!

NOW,

为什么我们需要notifyAll?

在上面的生产者/消费者示例中，我们似乎可以使用notify。看起来是这样的，因为我们可以证明生产者和消费者的等待循环上的守卫是互斥的。也就是说，看起来我们不能让一个线程同时在put方法和get方法中等待，因为，为了使它为真，那么下面的条件必须为真:

buf.size() == 0 AND buf.size() == MAX_SIZE(假设MAX_SIZE不为0)

然而，这还不够好，我们需要使用notifyAll。让我们看看为什么……

Assume we have a buffer of size 1 (to make the example easy to follow). The following steps lead us to deadlock. Note that ANYTIME a thread is woken with notify, it can be non-deterministically selected by the JVM - that is any waiting thread can be woken. Also note that when multiple threads are blocking on entry to a method (i.e. trying to acquire a lock), the order of acquisition can be non-deterministic. Remember also that a thread can only be in one of the methods at any one time - the synchronized methods allow only one thread to be executing (i.e. holding the lock of) any (synchronized) methods in the class. If the following sequence of events occurs - deadlock results:

步骤1: - P1将1个字符放入缓冲区

步骤2: P2尝试put -检查等待循环-已经是一个字符-等待

步骤3: P3尝试put -检查等待循环-已经是一个字符-等待

步骤4: - C1尝试获取1个char C2尝试在进入get方法时获取1个字符块 C3尝试在进入get方法时获取1个字符块

步骤5: C1正在执行get方法——获取char，调用notify，退出方法 —notify唤醒P2 但是，C2在P2之前进入方法(P2必须重新获得锁)，所以P2在进入put方法时阻塞 C2检查等待循环，缓冲区中没有更多字符，所以等待 C3在C2之后进入方法，但在P2之前，检查等待循环，缓冲区中没有更多字符，所以等待

步骤6: -现在:有P3, C2，和C3等待! -最后P2获取锁，在缓冲区中放入一个字符，调用notify，退出方法

第七步: P2的通知会唤醒P3(记住任何线程都可以被唤醒) P3检查等待循环条件，缓冲区中已经有一个字符，所以等待。 没有更多线程调用通知和三个线程永久挂起!

解决方案:在生产者/消费者代码(上面)中用notifyAll替换notify。