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<h1 id="线程信号量及wait，notify方法"><a href="#线程信号量及wait，notify方法" class="headerlink" title="线程信号量及wait，notify方法"></a>线程信号量及wait，notify方法</h1><p>本篇主要介绍线程之间如何进行信号的通知。同时介绍wait，notify底层的一些实现。</p>
<h2 id="通过共享对象进行信号通知"><a href="#通过共享对象进行信号通知" class="headerlink" title="通过共享对象进行信号通知"></a>通过共享对象进行信号通知</h2><p>最简单的进行线程之间通知的方式就是采用共享变量的方式。比如下面的代码<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MySignal</span>{</span><br><span class="line"> <span class="keyword">protected</span> <span class="type">boolean</span> <span class="variable">hasDataToProcess</span> <span class="operator">=</span> <span class="literal">false</span>;</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">synchronized</span> <span class="type">boolean</span> <span class="title function_">hasDataToProcess</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">this</span>.hasDataToProcess;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">synchronized</span> <span class="keyword">void</span> <span class="title function_">setHasDataToProcess</span><span class="params">(<span class="type">boolean</span> hasData)</span>{</span><br><span class="line"> <span class="built_in">this</span>.hasDataToProcess = hasData; </span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>线程A和线程B共享同一个MySingal实例，当线程A处理好数据后，可以设置hasDataToProcess属性为true，然后线程B获取到此属性。从而完成线程之间的信号通知。当然，如果线程A和线程B不是在同一个MySingal实例上进行的，则不能进行信号的传递。</p>
<h2 id="忙等待"><a href="#忙等待" class="headerlink" title="忙等待"></a>忙等待</h2><p>在采用MySingal的例子中，一般会采用下面的代码来判断一个线程是否可以进行处理了。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">protected</span> <span class="type">MySignal</span> <span class="variable">sharedSignal</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MySingal</span>()</span><br><span class="line"><span class="keyword">while</span>(!sharedSignal.hasDataToProcess()){</span><br><span class="line"> <span class="comment">//do nothing... busy waiting</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>从代码中可以看到，检测hasDataToProcess属性是在一个while循环中，如果hasDataToProcess为false，这就会造成线程一直在执行while语句，造成忙等待。这会造成CPU资源的浪费。</p>
<h2 id="wait，notify，notifyAll使用"><a href="#wait，notify，notifyAll使用" class="headerlink" title="wait，notify，notifyAll使用"></a>wait，notify，notifyAll使用</h2><p>一般在Java中，我们一般会采用wait，notify或notifyAll进行线程之间信号的传递。线程调用某一个对象上的wait方法前，必须首先获取该对象上的锁，才能执行此对象上的wait方法。在调用notify或者notifyAll之前，也是要获取对应对象上的锁。调用wait方法时，会释放此对象上的锁，线程进入阻塞状态，等待信号。而调用notify后，会随机唤醒一个同对象上的线程，但是必须是退出了notify对应的synchronized块后，被唤醒的线程才能继续执行，因为被唤醒的线程还要获取对象上的锁。如下面的代码所示：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MonitorObject</span>{</span><br><span class="line">}</span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MyWaitNotify</span>{</span><br><span class="line"> <span class="type">MonitorObject</span> <span class="variable">myMonitorObject</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MonitorObject</span>();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doWait</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> myMonitorObject.wait();</span><br><span class="line"> } <span class="keyword">catch</span>(InterruptedException e){...}</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doNotify</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> myMonitorObject.notify();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>从代码中，可以看到在myMonitorObject上调用wait方法之前，会先获取myMonitorObject上的锁。在调用notify方法之前，也是要先获取myMonitorObject上的锁。在下面的内容中我会简单介绍wait和notify的底层原理。</p>
<h3 id="wait-notify-notifyAll在Object源码中的介绍"><a href="#wait-notify-notifyAll在Object源码中的介绍" class="headerlink" title="wait notify notifyAll在Object源码中的介绍"></a>wait notify notifyAll在Object源码中的介绍</h3><p>在JDK1.8中，Object对象中有三个wait(),wait(long timeout),wait(long timeout, int nanos)这三个方法，它们的主要区别是后两个方法增加了等待的时间。<br>下面是JDK1.8中关于wait(long timeout)方法的描述：</p>
<blockquote>
<p>/**</p>
<pre><code> * 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.
* <p>
* The current thread must own this object's monitor.
* <p>
* This method causes the current thread (call it <var>T</var>) to
* place itself in the wait set for this object and then to relinquish
* any and all synchronization claims on this object. Thread <var>T</var>
* becomes disabled for thread scheduling purposes and lies dormant
* until one of four things happens:
* <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>
* The thread <var>T</var> is then removed from the wait set for this
* object and re-enabled for thread scheduling. It then competes in the
* usual manner with other threads for the right to synchronize on the
* object; once it has gained control of the object, all its
* synchronization claims on the object are restored to the status quo
* ante - that is, to the situation as of the time that the {@code wait}
* method was invoked. Thread <var>T</var> then returns from the
* invocation of the {@code wait} method. Thus, on return from the
* {@code wait} method, the synchronization state of the object and of
* thread {@code T} is exactly as it was when the {@code wait} method
* was invoked.
* <p>
* A thread can also wake up without being notified, interrupted, or
* timing out, a so-called <i>spurious wakeup</i>. While this will rarely
* occur in practice, applications must guard against it by testing for
* 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 (&lt;condition does not hold&gt;)
* obj.wait(timeout);
* ... // Perform action appropriate to condition
* }
* </pre>
* (For more information on this topic, see Section 3.2.3 in Doug Lea's
* "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
* 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
* Language Guide" (Addison-Wesley, 2001).
*
* <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
* interrupted} by any thread before or while it is waiting, then an
* {@code InterruptedException} is thrown. This exception is not
* thrown until the lock status of this object has been restored as
* described above.
*
* <p>
* Note that the {@code wait} method, as it places the current thread
* into the wait set for this object, unlocks only this object; any
* other objects on which the current thread may be synchronized remain
* locked while the thread waits.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @param timeout the maximum time to wait in milliseconds.
* @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
* current thread before or while the current thread
* was waiting for a notification. The <i>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
* @see java.lang.Object#notify()
* @see java.lang.Object#notifyAll()
*/
public final native void wait(long timeout) throws InterruptedException;
</code></pre></blockquote>
<p>从代码的注释中，我们可以获得以下一些内容：</p>
<ol>
<li>调用wait的线程一定要获取对象上的monitor，在调用wait后，会释放该对象上的锁。</li>
<li>发生以下四种情况线程会被唤醒：<ul>
<li>其他的线程在相同的对象上调用了notify</li>
<li>其他的线程在相同的对象上调用了notifyAll</li>
<li>其他的线程终止了当前线程(interrupt方法)，会扔出InterruptedException异常</li>
<li>指定的最大等待时间到了</li>
</ul>
</li>
<li>当线程被唤醒时，当前的线程会从对象的等待集合中移除，重新进入线程调度阶段。之后会跟其他线程竞争获取对象上的锁。</li>
<li>线程可能在没有上述四种情况发生的时候，被唤醒。这被称为伪唤醒，下面会有详细介绍。</li>
</ol>
<h3 id="浅析Object-monitor的底层原理"><a href="#浅析Object-monitor的底层原理" class="headerlink" title="浅析Object monitor的底层原理"></a>浅析Object monitor的底层原理</h3><p>在JVM实现获取对象上的锁，是通过monitor进行实现的。图示如下：<br><img src="/Thread-Signaling/monitor.png" class="" title="java-memory-model"><br>当一个线程需要获取 Object 的锁时，会被放入 EntrySet 中进行等待，如果该线程获取到了锁，成为当前锁的 owner。如果根据程序逻辑，一个已经获得了锁的线程缺少某些外部条件，而无法继续进行下去（例如生产者发现队列已满或者消费者发现队列为空），那么该线程可以通过调用 wait 方法将锁释放，进入 wait set 中阻塞进行等待，其它线程在这个时候有机会获得锁，去干其它的事情，从而使得之前不成立的外部条件成立，这样先前被阻塞的线程就可以重新进入 EntrySet 去竞争锁。</p>
<h3 id="notify和notifyAll区别"><a href="#notify和notifyAll区别" class="headerlink" title="notify和notifyAll区别"></a>notify和notifyAll区别</h3><p>乍一看，notify和notifyAll的区别很简单，就是notify只能随机选择一个处于等待状态的线程进行唤醒；而notifyAll可以唤醒所有处于等待状态下的线程，但是也是只有一个线程能够继续执行。<br>如果结合上面的图片，我们就能更好地进行理解。当线程在对象上调用notify方法时，随机选择一个处于等待状态的线程，并且把该线程放置在该对象的EntrySet列表中。而如果调用的是notifyAll方法时，所有的处于等待状态下的线程都会进入到EntrySet中，从而多个线程进行对象上的锁。notify有点类似于网络中的单播，而notifyAll类似于多播。</p>
<h2 id="丢失信号"><a href="#丢失信号" class="headerlink" title="丢失信号"></a>丢失信号</h2><p>设想一下这种情况，有一个线程首先调用了notify方法，然后其他的线程调用了wait方法。那么处于wait下的线程将不会接受到之前线程发送的notify信号。如下面代码所示，<a href="https://github.com/lightnine/daydayup/blob/master/src/main/java/com/leon/concurrent/waitNotify/WaitNotifyMissSingal.java">代码地址</a>：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">WaitNotifyMissSingal</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> {</span><br><span class="line"> <span class="keyword">final</span> <span class="type">Object</span> <span class="variable">lock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Object</span>();</span><br><span class="line"> <span class="keyword">new</span> <span class="title class_">Thread</span>(<span class="keyword">new</span> <span class="title class_">Runnable</span>() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> System.out.println(<span class="string">"thread B is waiting to get lock"</span>);</span><br><span class="line"> <span class="keyword">synchronized</span> (lock) {</span><br><span class="line"> System.out.println(<span class="string">"thread B get lock"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">5</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> lock.notify();</span><br><span class="line"> System.out.println(<span class="string">"thread B do notify method"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }).start();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">new</span> <span class="title class_">Thread</span>(<span class="keyword">new</span> <span class="title class_">Runnable</span>() {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> System.out.println(<span class="string">"thread A is waiting to get lock"</span>);</span><br><span class="line"> <span class="keyword">synchronized</span> (lock) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(<span class="string">"thread A get lock"</span>);</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> System.out.println(<span class="string">"thread A do wait method"</span>);</span><br><span class="line"> lock.wait();</span><br><span class="line"> System.out.println(<span class="string">"wait end"</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }).start();</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>运行上面的代码，程序会一直运行下去。那么如何解决这个问题呢，其实在Object的wait的方法注释中也有对应的说明。我们可以把通知信号保存在信号类的成员变量中。<a href="https://github.com/lightnine/daydayup/blob/master/src/main/java/com/leon/concurrent/waitNotify/MyWaitNotify2.java">代码地址</a><br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MyWaitNotify2</span>{</span><br><span class="line"> <span class="type">MonitorObject</span> <span class="variable">myMonitorObject</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MonitorObject</span>();</span><br><span class="line"> <span class="type">boolean</span> <span class="variable">wasSignalled</span> <span class="operator">=</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doWait</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> <span class="keyword">if</span>(!wasSignalled){</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> myMonitorObject.wait();</span><br><span class="line"> } <span class="keyword">catch</span>(InterruptedException e){...}</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//clear signal and continue running.</span></span><br><span class="line"> wasSignalled = <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doNotify</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> wasSignalled = <span class="literal">true</span>;</span><br><span class="line"> myMonitorObject.notify();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>在上面的代码中，将信号保存在了wasSingalled变量中，只要调用了notify方法，就是将wasSignalled设置为true，表示有线程执行了notify。在doWait方法中，如果wasSignalled为false，则当前的线程会执行wait方法，进入等待状态；而当wasSignalled为true时，不会执行wait方法，只会将wasSignalled设置为false。</p>
<h2 id="伪唤醒"><a href="#伪唤醒" class="headerlink" title="伪唤醒"></a>伪唤醒</h2><p>因为一些底层操作系统的原因，具体的可以查看unix操作系统相关内容。简单理解，就是当线程没有接受到唤醒信号时，而线程被错误的唤醒。为了防止伪唤醒，一定要在while循环中检查信号变量的值。这样的循环也叫自旋锁。代码如下：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MyWaitNotify3</span>{</span><br><span class="line"></span><br><span class="line"> <span class="type">MonitorObject</span> <span class="variable">myMonitorObject</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MonitorObject</span>();</span><br><span class="line"> <span class="type">boolean</span> <span class="variable">wasSignalled</span> <span class="operator">=</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doWait</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> <span class="keyword">while</span>(!wasSignalled){</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> myMonitorObject.wait();</span><br><span class="line"> } <span class="keyword">catch</span>(InterruptedException e){...}</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//clear signal and continue running.</span></span><br><span class="line"> wasSignalled = <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doNotify</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> wasSignalled = <span class="literal">true</span>;</span><br><span class="line"> myMonitorObject.notify();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="不要在常量字符串或全局对象上调用wait方法"><a href="#不要在常量字符串或全局对象上调用wait方法" class="headerlink" title="不要在常量字符串或全局对象上调用wait方法"></a>不要在常量字符串或全局对象上调用wait方法</h2><p>请看下面的例子<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MyWaitNotify</span>{</span><br><span class="line"></span><br><span class="line"> <span class="type">String</span> <span class="variable">myMonitorObject</span> <span class="operator">=</span> <span class="string">""</span>;</span><br><span class="line"> <span class="type">boolean</span> <span class="variable">wasSignalled</span> <span class="operator">=</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doWait</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> <span class="keyword">while</span>(!wasSignalled){</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> myMonitorObject.wait();</span><br><span class="line"> } <span class="keyword">catch</span>(InterruptedException e){...}</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//clear signal and continue running.</span></span><br><span class="line"> wasSignalled = <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">doNotify</span><span class="params">()</span>{</span><br><span class="line"> <span class="keyword">synchronized</span>(myMonitorObject){</span><br><span class="line"> wasSignalled = <span class="literal">true</span>;</span><br><span class="line"> myMonitorObject.notify();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><br>从代码中我们可以看到，在上锁时是对myMonitorObject对象上锁，也是在myMonitorObject上调用wait方法，而myMonitorObject是一个空的字符串。我们知道，JVM会将相同的字符串看成同一个对象。也就说说，如果有两个MyWaitNotify实例，则这两个实例中的myMonitorObject是指向同一个对象的。那么一个在实例MyWaitNotify上调用wait的线程会被在另一个实例MyWaitNotify上调用doNotify方法的线程唤醒。下面的图表示了实例的成员指向了相同的对象。<br><img src="/Thread-Signaling/strings-wait-notify.png" class="" title="strings-wait-notify"><br>在上图中，4个线程是在相同的String常量上调用wait和notify方法，但是信号wasSignalled仍然是单独存放在对应的实例对象上。即一个在MyWaitNotify1实例上调用doNotify方法的线程可能会唤醒在MyWaitNotify2实例上等待的线程，但是唤醒信号仍然是单独保存在MyWaitNotify1实例中。<br>如果仔细看上面的程序，发现当在第二个MyWaitNotify2实例上调用doNotify方法时，会唤醒线程A或者B，但是由于在while循环中的wasSignalled变量，对于MyWaitNotify1实例仍然是false。所以被唤醒的线程A或者B从wait启动，但是会再次进入while循环调用wait，再次进入阻塞状态。这跟伪唤醒很像。<br>由于在doNotify方法中调用的notify方法，此方法不像notifyAll方法，notify方法只会唤醒一个线程，如果是线程C调用的doNotify，本来想唤醒的是线程D。但是有可能会错误的唤醒线程A或B，并且线程A或B会修改对应实例上的wasSignalled变量。而发给D的信号就丢失了，就有点像信号丢失的情况。如果将doNotify方法中的notify替换成notifyAll就不会有这个问题。但是这是一个坏主意，当应用仅仅只需要唤醒一个线程时，没有任何理由要把所有的线程都唤醒。<br>注意，对于wait/notify的情况，永远不要使用全局的对象例如string常量。在wait和notify上使用的对象针对对应的实例必须是独一的。</p>
<h2 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h2><ol>
<li><a href="http://tutorials.jenkov.com/java-concurrency/thread-signaling.html">http://tutorials.jenkov.com/java-concurrency/thread-signaling.html</a></li>
<li><a href="http://www.php.cn/java-article-410323.html">http://www.php.cn/java-article-410323.html</a></li>
</ol>
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<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#%E7%BA%BF%E7%A8%8B%E4%BF%A1%E5%8F%B7%E9%87%8F%E5%8F%8Await%EF%BC%8Cnotify%E6%96%B9%E6%B3%95"><span class="nav-number">1.</span> <span class="nav-text">线程信号量及wait，notify方法</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#%E9%80%9A%E8%BF%87%E5%85%B1%E4%BA%AB%E5%AF%B9%E8%B1%A1%E8%BF%9B%E8%A1%8C%E4%BF%A1%E5%8F%B7%E9%80%9A%E7%9F%A5"><span class="nav-number">1.1.</span> <span class="nav-text">通过共享对象进行信号通知</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E5%BF%99%E7%AD%89%E5%BE%85"><span class="nav-number">1.2.</span> <span class="nav-text">忙等待</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#wait%EF%BC%8Cnotify%EF%BC%8CnotifyAll%E4%BD%BF%E7%94%A8"><span class="nav-number">1.3.</span> <span class="nav-text">wait，notify，notifyAll使用</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#wait-notify-notifyAll%E5%9C%A8Object%E6%BA%90%E7%A0%81%E4%B8%AD%E7%9A%84%E4%BB%8B%E7%BB%8D"><span class="nav-number">1.3.1.</span> <span class="nav-text">wait notify notifyAll在Object源码中的介绍</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E6%B5%85%E6%9E%90Object-monitor%E7%9A%84%E5%BA%95%E5%B1%82%E5%8E%9F%E7%90%86"><span class="nav-number">1.3.2.</span> <span class="nav-text">浅析Object monitor的底层原理</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#notify%E5%92%8CnotifyAll%E5%8C%BA%E5%88%AB"><span class="nav-number">1.3.3.</span> <span class="nav-text">notify和notifyAll区别</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E4%B8%A2%E5%A4%B1%E4%BF%A1%E5%8F%B7"><span class="nav-number">1.4.</span> <span class="nav-text">丢失信号</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E4%BC%AA%E5%94%A4%E9%86%92"><span class="nav-number">1.5.</span> <span class="nav-text">伪唤醒</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E4%B8%8D%E8%A6%81%E5%9C%A8%E5%B8%B8%E9%87%8F%E5%AD%97%E7%AC%A6%E4%B8%B2%E6%88%96%E5%85%A8%E5%B1%80%E5%AF%B9%E8%B1%A1%E4%B8%8A%E8%B0%83%E7%94%A8wait%E6%96%B9%E6%B3%95"><span class="nav-number">1.6.</span> <span class="nav-text">不要在常量字符串或全局对象上调用wait方法</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E5%8F%82%E8%80%83"><span class="nav-number">1.7.</span> <span class="nav-text">参考</span></a></li></ol></li></ol></div>
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