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local-search-popup"><div class="local-search-header clearfix"><span class="search-icon"><i class="fa fa-search"></i> </span><span class="popup-btn-close"><i class="fa fa-times-circle"></i></span><div class="local-search-input-wrapper"><input autocomplete="off" placeholder="Search" spellcheck="false" type="text" id="local-search-input"></div></div><div id="local-search-result"></div></div></div></nav></div></header><main id="main" class="main"><div class="main-inner"><div class="content-wrap"><div id="content" class="content"><div id="posts" class="posts-expand"><article class="post post-type-normal" itemscope itemtype="http://schema.org/Article"><link itemprop="mainEntityOfPage" href="http://mrbird.cc/Java-Concurrent-Basis.html"><span hidden itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="name" content="MrBird"><meta itemprop="description" content=""><meta itemprop="image" content="/images/blogImage.jpg"></span><span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization"><meta itemprop="name" content="MrBird"></span><header class="post-header"><h1 class="post-title" itemprop="name headline">Java 并发基础</h1><div class="post-meta"><span class="post-time"><span class="post-meta-item-icon"><i class="fa fa-calendar-o"></i> </span><span class="post-meta-item-text">Posted on</span> <time title="创建于" itemprop="dateCreated datePublished" datetime="2016-03-14T16:54:15+08:00">2016-03-14 </time></span><span></span></div></header><div class="post-body" itemprop="articleBody"><p>Thread 类的每一个实例都表示一个线程， 进程是操作系统级别的多任务，JVM 就是运行在一个进程中的。所以在Java 中我我们只考虑线程。进程有独立的内存，一个进程间的多个线程共享进程的内存进程中至少要有一个线程。<a id="more"></a></p><h2 id="线程状态"><a href="#线程状态" class="headerlink" title="线程状态"></a>线程状态</h2><p><img src="img/Concurrent-basis.png" alt="Concurrent-basis.png"></p><ol><li><p>New：当我们创建一个线程时，该线程并没有纳入线程调度，其处于一个new状态。</p></li><li><p>Runnable：当调用线程的start方法后，该线程纳入线程调度的控制，其处于一个可运行状态，等待分配时间片段以并发运行。</p></li><li><p>Running：当该线程被分配到了时间片段后其被CPU运行，这是该线程处于running状态。</p></li><li><p>Blocked：当线程在运行过程中可能会出现阻塞现象，比如等待用户输入信息等。但阻塞状态不是百分百出现的，具体要看代码中是否有相关需求。</p></li><li><p>Dead：当线程的任务全部运行完毕，或在运行过程中抛出了一个未捕获的异常，那么线程结束，等待GC回收</p></li></ol><h2 id="创建线程"><a href="#创建线程" class="headerlink" title="创建线程"></a>创建线程</h2><p>线程有两种创建方式：</p><p>方式1：定义一个类并继承Thread，然后重写run方法，在其中书写线程任务逻辑：</p><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">Thread t1 = <span class="keyword">new</span> MyThread1();</span><br><span class="line">Thread t2 = <span class="keyword">new</span> MyThread2();</span><br><span class="line">t1.start();</span><br><span class="line">t2.start();</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyThread1</span> <span class="keyword">extends</span> <span class="title">Thread</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">5</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"are you ok?"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyThread2</span> <span class="keyword">extends</span> <span class="title">Thread</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">5</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"of course"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>输出：</p><figure class="highlight plain"><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">of course</span><br><span class="line">of course</span><br><span class="line">of course</span><br><span class="line">of course</span><br><span class="line">of course</span><br><span class="line">are you ok?</span><br><span class="line">are you ok?</span><br><span class="line">are you ok?</span><br><span class="line">are you ok?</span><br></pre></td></tr></table></figure><p></p><p>启动线程要调用start方法，不能直接调用run方法。start方法会将当前线程纳入到线程调度中，使其具有并发运行的能力。start方法很快会执行完毕。当start方法执行完毕后，当前线程的run方法会很快的被执行起来（只要获取到了cpu时间片）。但不能理解为调用start方法时run方法就执行了！</p><p>线程有几个不可控因素:</p><ol><li><p>cpu分配时间片给哪个线程我们说了不算。</p></li><li><p>时间片长短也不可控。</p></li><li><p>线程调度会尽可能均匀的将时间片分配给多个线程。</p></li></ol><p>第一种创建线程的方式存在两个不足：</p><ol><li><p>由于java是单继承的，这就导致我们若继承了Thread类就无法再继承其他类，这在写项目时会遇到很大问题；</p></li><li><p>由于我们定义线程的同时重写run方法来定义线程要执行的任务，这就导致线程与任务有一个强耦合关系，线程的重用性变得非常局限。</p></li></ol><p>方式2：定义一个类并实现Runnable接口然后在创建线程的同时将任务指定。因为是实现Runnable接口，所以不影响其继承其他类：</p><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></pre></td><td class="code"><pre><span class="line">Thread t1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">5</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"are you ok?"</span>);</span><br><span class="line"> }</span><br><span class="line">});</span><br><span class="line">t1.start();</span><br><span class="line"></span><br><span class="line">Thread t2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">5</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"of course"</span>);</span><br><span class="line"> }</span><br><span class="line">});</span><br><span class="line">t2.start();</span><br></pre></td></tr></table></figure><p></p><h2 id="线程相关API"><a href="#线程相关API" class="headerlink" title="线程相关API"></a>线程相关API</h2><p>获取当前线程：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread(); <span class="comment">// Thread[main,5,main]</span></span><br></pre></td></tr></table></figure><p></p><p>获取当前线程ID：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.getId()); <span class="comment">// 1</span></span><br></pre></td></tr></table></figure><p></p><p>获取当前线程名字：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.getName()); <span class="comment">// main</span></span><br></pre></td></tr></table></figure><p></p><p>获取当前线程优先级：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.getPriority()); <span class="comment">// 5</span></span><br></pre></td></tr></table></figure><p></p><p>判断当前线程是否还活着：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.isAlive()); <span class="comment">// true</span></span><br></pre></td></tr></table></figure><p></p><p>判断当前线程是否为守护线程：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.isDaemon()); <span class="comment">// false</span></span><br></pre></td></tr></table></figure><p></p><p>判断当前线程是否被中断：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Thread current = Thread.currentThread();</span><br><span class="line">System.out.println(current.isInterrupted()); <span class="comment">// false</span></span><br></pre></td></tr></table></figure><p></p><h2 id="线程的优先级"><a href="#线程的优先级" class="headerlink" title="线程的优先级"></a>线程的优先级</h2><p>线程优先级分为10个等级，1最低，5默认，10最高。线程提供了3个常量：</p><ul><li><p><strong>MIN_PRIORITY</strong>：1 对应最低优先级；</p></li><li><p><strong>MAX_PRIORITY</strong>： 10 对应最高优先级；</p></li><li><p><strong>NORM_PRIORITY</strong>：5 默认优先级。</p></li></ul><h2 id="线程阻塞"><a href="#线程阻塞" class="headerlink" title="线程阻塞"></a>线程阻塞</h2><p>Thread提供了一个静态方法: sleep，该方法会阻塞运行当前方法的线程指定毫秒。当超时后，线程会自动回到Runnable状态，等待再次分配时间片运行：</p><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></pre></td><td class="code"><pre><span class="line">System.out.println(<span class="string">"程序开始了"</span>);</span><br><span class="line"><span class="keyword">try</span> {</span><br><span class="line"> Thread.sleep(<span class="number">5000</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">System.out.println(<span class="string">"程序结束了"</span>);</span><br></pre></td></tr></table></figure><p></p><h2 id="守护线程"><a href="#守护线程" class="headerlink" title="守护线程"></a>守护线程</h2><p>后台线程，又叫做守护线程，当一个进程中的所有前台线程都结束了，进程就会结束，无论进程中的其他后台线程是否还在运行，都要被强制中断：</p><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></pre></td><td class="code"><pre><span class="line">Thread front = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">2</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"前端线程运行中"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</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"> System.out.println(<span class="string">"前台线程运行完毕"</span>);</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line">Thread back = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> System.out.println(<span class="string">"前台线程结束，我就gg"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</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"></span><br><span class="line">front.start();</span><br><span class="line">back.setDaemon(<span class="keyword">true</span>);</span><br><span class="line">back.start();</span><br></pre></td></tr></table></figure><p></p><p>输出如下：</p><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">前台线程结束，我就gg</span><br><span class="line">前端线程运行中</span><br><span class="line">前台线程结束，我就gg</span><br><span class="line">前端线程运行中</span><br><span class="line">前台线程结束，我就gg</span><br><span class="line">前台线程运行完毕</span><br></pre></td></tr></table></figure><p></p><h2 id="yield"><a href="#yield" class="headerlink" title="yield"></a>yield</h2><p>该方法用于使当前线程主动让出当次CPU时间片回到Runnable状态，等待分配时间片。</p><h2 id="join"><a href="#join" class="headerlink" title="join"></a>join</h2><p>允许当前线程在另一个线程上等待，直到另一个线程结束工作。通常是用来协调两个线程工作使用：</p><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">boolean</span> isFinish = <span class="keyword">false</span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Thread download = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> System.out.println(<span class="string">"开始下载图片..."</span>);</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">1</span>; i <= <span class="number">100</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"down:"</span> + i + <span class="string">"%"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.MILLISECONDS.sleep(<span class="number">10</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"> System.out.println(<span class="string">"图片下载完毕!"</span>);</span><br><span class="line"> isFinish = <span class="keyword">true</span>;</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> Thread show = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> System.out.println(<span class="string">"开始显示图片..."</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> download.join();</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 class="keyword">if</span> (!isFinish) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(<span class="string">"图片加载失败!"</span>);</span><br><span class="line"> }</span><br><span class="line"> System.out.println(<span class="string">"图片显示完毕!"</span>);</span><br><span class="line"> });</span><br><span class="line"> download.start();</span><br><span class="line"> show.start();</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/thread-join.gif" alt="thread-join.gif"></p><h2 id="synchroinzed"><a href="#synchroinzed" class="headerlink" title="synchroinzed"></a>synchroinzed</h2><p>多个线程并发读写同一个临界资源时候会发生“线程并发安全问题”</p><p>常见的临界资源:</p><ol><li><p>多线程共享实例变量；</p></li><li><p>多线程共享静态公共变量。</p></li></ol><p>若想解决线程安全问题，需要将异步的操作变为同步操作。 何为同步？那么我们来对比看一下什么是同步什么异步：</p><ol><li><p>所谓异步操作是指多线程并发的操作，相当于各干各的。</p></li><li><p>所谓同步操作是指有先后顺序的操作，相当于你干完我再干。</p></li></ol><p>而java中有一个关键字名为：synchronized，该关键字是同步锁，用于将某段代码变为同步操作，从而解决线程并发安全问题。使用锁需要注意两个方面：</p><ol><li><p>选择合适的锁对象：使用synchroinzed需要对一个锁对象上锁以保证线程同步。那么这个锁对象应当注意多个需要同步的线程在访问该同步块时，看到的应该是同一个锁对象引用。否则达不到同步效果。 通常我们会使用this来作为锁对象。</p></li><li><p>选择合适的锁范围：在使用同步块时，应当尽量在允许的情况下减少同步范围，以提高并发的执行效率。</p></li></ol><p>synchronized关键字有两个用法:</p><ol><li><p>修饰方法，这样的话，该方法就称为”同步方法”，多个线程就不能同时进入到方法内部去执行。可以避免由于线程切换不确定，导致的逻辑错误。</p></li><li><p>synchronized块，可以将某段代码片段括起来，多个线程不能同时执行里面的代码。</p></li></ol><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> <span class="keyword">final</span> Table table = <span class="keyword">new</span> Table();</span><br><span class="line"> Thread t0 = <span class="keyword">new</span> Thread() {</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> <span class="keyword">int</span> bean = table.getBean();</span><br><span class="line"> Thread.yield();</span><br><span class="line"> System.out.println(getName() + <span class="string">":"</span> + bean);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"> Thread t1 = <span class="keyword">new</span> Thread() {</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> <span class="keyword">int</span> bean = table.getBean();</span><br><span class="line"> Thread.yield();</span><br><span class="line"> System.out.println(getName() + <span class="string">":"</span> + bean);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line"> t0.start();</span><br><span class="line"> t1.start();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Table</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">int</span> beans = <span class="number">20</span>;</span><br><span class="line"> <span class="function"><span class="keyword">synchronized</span> <span class="keyword">int</span> <span class="title">getBean</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (beans == <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(<span class="string">"没有豆子了!"</span>);</span><br><span class="line"> }</span><br><span class="line"> Thread.yield();</span><br><span class="line"> <span class="keyword">return</span> beans--;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>如果不加synchronized关键字，可能beans为负数了，线程还在执行，成了死循环。</p><p>synchronized块的例子：</p><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> <span class="keyword">final</span> Shop shop = <span class="keyword">new</span> Shop();</span><br><span class="line"> Thread t1 = <span class="keyword">new</span> Thread(shop::buy);</span><br><span class="line"> Thread t2 = <span class="keyword">new</span> Thread(shop::buy);</span><br><span class="line"> t1.start();</span><br><span class="line"> t2.start();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Shop</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">void</span> <span class="title">buy</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> Thread t = Thread.currentThread();</span><br><span class="line"> System.out.println(t + <span class="string">"正在挑选衣服."</span>);</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> <span class="keyword">synchronized</span> (<span class="keyword">this</span>) {</span><br><span class="line"> System.out.println(t + <span class="string">"正在试衣服."</span>);</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> System.out.println(t + <span class="string">"结账离开."</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (Exception 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></pre></td></tr></table></figure><p></p><p>此时，“正在试衣服”这个代码块一次只能一个线程执行。</p><h2 id="wait-amp-notify"><a href="#wait-amp-notify" class="headerlink" title="wait&notify"></a>wait&notify</h2><p>Object类中定义了两个方法wait()和notify()。它们也可以实现协调线程之间同步工作的方法。当一个线程调用了某个对象的wait方法时，这个线程就进入阻塞状态，直到这个对象的notify方法被调用，这个线程才会解除wait阻塞，继续向下执行代码。</p><p>若多个线程在同一个对象上调用wait方法进入阻塞状态后，那么当该对象的notify方法被调用时，会随机解除一个线程的wait阻塞，这个不可控。若希望一次性将所有线程的wait阻塞解除，可以调用notifyAll方法。</p><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><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">boolean</span> isFinish;</span><br><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Object obj = <span class="keyword">new</span> Object();</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> <span class="keyword">final</span> Thread download = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> System.out.println(<span class="string">"down:开始下载图片..."</span>);</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">1</span>; i <= <span class="number">100</span>; i++) {</span><br><span class="line"> System.out.println(<span class="string">"down:"</span> + i + <span class="string">"%"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.MILLISECONDS.sleep(<span class="number">10</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"> System.out.println(<span class="string">"down:图片下载完毕!"</span>);</span><br><span class="line"> isFinish = <span class="keyword">true</span>;</span><br><span class="line"> <span class="comment">//通知 show线程开始工作</span></span><br><span class="line"> <span class="keyword">synchronized</span> (obj) {</span><br><span class="line"> obj.notify();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> Thread show = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> System.out.println(<span class="string">"show:开始显示图片..."</span>);</span><br><span class="line"> <span class="keyword">synchronized</span> (obj) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> obj.wait();</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"> <span class="keyword">if</span> (!isFinish) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(<span class="string">"图片加载失败!"</span>);</span><br><span class="line"> }</span><br><span class="line"> System.out.println(<span class="string">"show:图片显示完毕!"</span>);</span><br><span class="line"> });</span><br><span class="line"> download.start();</span><br><span class="line"> show.start();</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/asdfasdf.gif" alt="thread-wait-notify.gif"></p><h2 id="线程池"><a href="#线程池" class="headerlink" title="线程池"></a>线程池</h2><p>当我们的逻辑中出现了会频繁创建线程的情况时，就要考虑使用线程池来管理线程。这可以解决创建过多线程导致的系统威胁。</p><p>线程池主要解决两个问题：</p><ol><li><p>控制线程数量；</p></li><li><p>重用线程。</p></li></ol><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="comment">//创建一个固定大小的线程池</span></span><br><span class="line">ExecutorService threadPool = Executors.newFixedThreadPool(<span class="number">3</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">//指派 5个任务</span></span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">5</span>; i++) {</span><br><span class="line"> Runnable runn = () -> {</span><br><span class="line"> Thread t = Thread.currentThread();</span><br><span class="line"> System.out.println(t + <span class="string">"正在运行任务!"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">2</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"> System.out.println(t + <span class="string">"执行任务完毕"</span>);</span><br><span class="line"> };</span><br><span class="line"> threadPool.execute(runn);</span><br><span class="line">}</span><br><span class="line"><span class="comment">//停止线程池</span></span><br><span class="line">threadPool.shutdown();</span><br></pre></td></tr></table></figure><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">Thread[pool-1-thread-2,5,main]正在运行任务!</span><br><span class="line">Thread[pool-1-thread-1,5,main]正在运行任务!</span><br><span class="line">Thread[pool-1-thread-3,5,main]正在运行任务!</span><br><span class="line">Thread[pool-1-thread-2,5,main]执行任务完毕</span><br><span class="line">Thread[pool-1-thread-3,5,main]执行任务完毕</span><br><span class="line">Thread[pool-1-thread-2,5,main]正在运行任务!</span><br><span class="line">Thread[pool-1-thread-1,5,main]执行任务完毕</span><br><span class="line">Thread[pool-1-thread-3,5,main]正在运行任务!</span><br><span class="line">Thread[pool-1-thread-3,5,main]执行任务完毕</span><br><span class="line">Thread[pool-1-thread-2,5,main]执行任务完毕</span><br></pre></td></tr></table></figure><p>此时同时只能有三个线程数量。</p><script>$(".post-body a").not(".thispage").addClass("ignore-href").attr("target","_blank")</script></div><div></div><div></div><div><ul class="post-copyright"><li class="post-copyright-author"><strong>本文作者：</strong> MrBird</li><li class="post-copyright-link"><strong>本文链接：</strong> <a href="http://mrbird.cc/Java-Concurrent-Basis.html" title="Java 并发基础">http://mrbird.cc/Java-Concurrent-Basis.html</a></li><li 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