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<!DOCTYPE HTML>
<html lang="zh-CN">
<head><meta name="generator" content="Hexo 3.9.0">
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程序由指令和数据组成，但这些指令要运行，数据要读写，就必须将指令加载至 CPU，数据加载至内存。在 指令运行过程中还需要用到磁盘、网络等设备。进程就是用来加载指令、管理内存、管理 IO 的
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</script></head>
<div id="loading-container">
<p class="loading-text"></p>
<div class="loading-image">
<div></div>
<div></div>
<div></div>
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<h1 id="并发编程"><a href="#并发编程" class="headerlink" title="并发编程"></a>并发编程</h1><h2 id="进程与线程"><a href="#进程与线程" class="headerlink" title="进程与线程"></a>进程与线程</h2><h3 id="进程与线程-1"><a href="#进程与线程-1" class="headerlink" title="进程与线程"></a>进程与线程</h3><h4 id="进程"><a href="#进程" class="headerlink" title="进程"></a>进程</h4><ul>
<li>程序由指令和数据组成，但这些指令要运行，数据要读写，就必须将指令加载至 CPU，数据加载至内存。在 指令运行过程中还需要用到磁盘、网络等设备。进程就是用来加载指令、管理内存、管理 IO 的 </li>
<li>当一个程序被运行，从磁盘加载这个程序的代码至内存，这时就开启了一个进程。 </li>
<li>进程就可以视为程序的一个实例。大部分程序可以同时运行多个实例进程（例如记事本、画图、浏览器 等），也有的程序只能启动一个实例进程（例如网易云音乐、360 安全卫士等）</li>
</ul>
<h4 id="线程"><a href="#线程" class="headerlink" title="线程"></a>线程</h4><ul>
<li>一个进程之内可以分为一到多个线程。 </li>
<li>一个线程就是一个指令流，将指令流中的一条条指令以一定的顺序交给 CPU 执行</li>
<li>Java 中，线程作为最小调度单位，进程作为资源分配的最小单位。 在 windows 中进程是不活动的，只是作 为线程的容器</li>
</ul>
<h4 id="二者对比"><a href="#二者对比" class="headerlink" title="二者对比"></a>二者对比</h4><ul>
<li><p>进程基本上相互独立的，而线程存在于进程内，是进程的一个子集 </p>
</li>
<li><p>进程拥有共享的资源，如内存空间等，供其内部的线程共享 </p>
</li>
<li><p>进程间通信较为复杂 </p>
<p> 同一台计算机的进程通信称为 IPC（Inter-process communication） </p>
<p> 不同计算机之间的进程通信，需要通过网络，并遵守共同的协议，例如 HTTP</p>
</li>
<li><p>线程通信相对简单，因为它们共享进程内的内存，一个例子是多个线程可以访问同一个共享变量 </p>
</li>
<li><p>线程更轻量，线程上下文切换成本一般上要比进程上下文切换低</p>
</li>
</ul>
<h3 id="并行与并发"><a href="#并行与并发" class="headerlink" title="并行与并发"></a>并行与并发</h3><p>单核 cpu 下，线程实际还是<strong>串行执行</strong>的。操作系统中有一个组件叫做<strong>任务调度器</strong>，将 cpu 的时间片（windows 下时间片最小约为 15 毫秒）分给不同的程序使用，只是由于 cpu 在线程间（时间片很短）的切换非常快，人类感 觉是<strong>同时运行的</strong>。总结为一句话就是：<strong>微观串行，宏观并行</strong></p>
<p>一般会将这种 <strong>线程轮流使用</strong> CPU 的做法称为并发， concurrent</p>
<table>
<thead>
<tr>
<th>CPU</th>
<th>时间片 1</th>
<th>时间片 2</th>
<th>时间片 3</th>
<th>时间片 4</th>
</tr>
</thead>
<tbody><tr>
<td>core</td>
<td>线程 1</td>
<td>线程 2</td>
<td>线程 3</td>
<td>线程 4</td>
</tr>
</tbody></table>
<p><img src="/javaconcurrent/image-20230621115241892.png" alt="image-20230621115241892"></p>
<p>多核 cpu下，每个 <strong>核（core）</strong> 都可以调度运行线程，这时候线程可以是并行的。</p>
<p><img src="/javaconcurrent/image-20230621120130008.png" alt="image-20230621120130008"></p>
<ul>
<li>并发（concurrent）是同一时间应对（dealing with）多件事情的能力 </li>
<li>并行（parallel）是同一时间动手做（doing）多件事情的能力</li>
</ul>
<ol>
<li><p>单核 cpu 下，多线程不能实际提高程序运行效率，只是为了能够在不同的任务之间切换，不同线程轮流使用 cpu ，不至于一个线程总占用 cpu，别的线程没法干活 </p>
</li>
<li><p>多核 cpu 可以并行跑多个线程，但能否提高程序运行效率还是要分情况的 </p>
<p> 有些任务，经过精心设计，将任务拆分，并行执行，当然可以提高程序的运行效率。但不是所有计算任 务都能拆分（参考【阿姆达尔定律】）</p>
<pre><code> 也不是所有任务都需要拆分，任务的目的如果不同，谈拆分和效率没啥意义</code></pre></li>
<li><p>IO 操作不占用 cpu，只是我们一般拷贝文件使用的是【阻塞 IO】，这时相当于线程虽然不用 cpu，但需要一 直等待 IO 结束，没能充分利用线程。所以才有后面的【非阻塞 IO】和【异步 IO】优化。</p>
</li>
</ol>
<h2 id="Java-线程"><a href="#Java-线程" class="headerlink" title="Java 线程"></a>Java 线程</h2><h3 id="创建和运行线程"><a href="#创建和运行线程" class="headerlink" title="创建和运行线程"></a>创建和运行线程</h3><h4 id="方法一，直接使用-Thread"><a href="#方法一，直接使用-Thread" class="headerlink" title="方法一，直接使用 Thread"></a>方法一，直接使用 Thread</h4><pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">// 创建线程对象</span>
Thread t <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment" spellcheck="true">// 要执行的任务</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 启动线程</span>
t<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>例子</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">// 构造方法的参数是给线程指定名字，推荐</span>
Thread t1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token string">"t1"</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token comment" spellcheck="true">// run 方法内实现了要执行的任务</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
log<span class="token punctuation">.</span><span class="token function">debug</span><span class="token punctuation">(</span><span class="token string">"hello"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
t1<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出</p>
<pre><code>19:19:00 [t1] c.ThreadStarter - hello</code></pre><h4 id="方法二，使用-Runnable-配合-Thread"><a href="#方法二，使用-Runnable-配合-Thread" class="headerlink" title="方法二，使用 Runnable 配合 Thread"></a>方法二，使用 Runnable 配合 Thread</h4><p>把【线程】和【任务】（要执行的代码）分开 </p>
<ul>
<li>Thread 代表线程 </li>
<li>Runnable 可运行的任务（线程要执行的代码）</li>
</ul>
<pre class="line-numbers language-java"><code class="language-java">Runnable runnable <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token comment" spellcheck="true">// 要执行的任务</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 创建线程对象</span>
Thread t <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span> runnable <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 启动线程</span>
t<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>例如：</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">// 创建任务对象</span>
Runnable task2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
log<span class="token punctuation">.</span><span class="token function">debug</span><span class="token punctuation">(</span><span class="token string">"hello"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 参数1 是任务对象; 参数2 是线程名字，推荐</span>
Thread t2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>task2<span class="token punctuation">,</span> <span class="token string">"t2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
t2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出</p>
<pre><code>19:19:00 [t2] c.ThreadStarter - hello</code></pre><p>Java 8 以后可以使用 lambda 精简代码</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">// 创建任务对象</span>
Runnable task2 <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> log<span class="token punctuation">.</span><span class="token function">debug</span><span class="token punctuation">(</span><span class="token string">"hello"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 参数1 是任务对象; 参数2 是线程名字，推荐</span>
Thread t2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>task2<span class="token punctuation">,</span> <span class="token string">"t2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
t2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h4 id="原理"><a href="#原理" class="headerlink" title="原理"></a>原理</h4><h5 id="Thread-与-Runnable-的关系"><a href="#Thread-与-Runnable-的关系" class="headerlink" title="Thread 与 Runnable 的关系"></a>Thread 与 Runnable 的关系</h5><p>Thread实现了Runnable接口，提供了更多的可用方法和成员</p>
<p><strong>Thread和Runnable的实质是继承关系，没有可比性。无论使用Runnable还是Thread，都会new Thread，然后执行run方法。用法上，如果有复杂的线程操作需求，那就选择继承Thread，如果只是简单的执行一个任务，那就实现runnable。</strong></p>
<ul>
<li>方法1 是把线程和任务合并在了一起，方法2 是把线程和任务分开了 </li>
<li>用 Runnable 更容易与线程池等高级 API 配合 </li>
<li>用 Runnable 让任务类脱离了 Thread 继承体系，更灵活</li>
</ul>
<h4 id="方法三，FutureTask-配合-Thread"><a href="#方法三，FutureTask-配合-Thread" class="headerlink" title="方法三，FutureTask 配合 Thread"></a>方法三，FutureTask 配合 Thread</h4><p>FutureTask 能够接收 Callable 类型的参数，用来处理有返回结果的情况</p>
<pre class="line-numbers language-java"><code class="language-java"><span class="token comment" spellcheck="true">// 创建任务对象</span>
FutureTask<span class="token operator"><</span>Integer<span class="token operator">></span> task3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FutureTask</span><span class="token operator"><</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> <span class="token punctuation">{</span>
log<span class="token punctuation">.</span><span class="token function">debug</span><span class="token punctuation">(</span><span class="token string">"hello"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">100</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 参数1 是任务对象; 参数2 是线程名字，推荐</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>task3<span class="token punctuation">,</span> <span class="token string">"t3"</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">// 主线程阻塞，同步等待 task 执行完毕的结果</span>
Integer result <span class="token operator">=</span> task3<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
log<span class="token punctuation">.</span><span class="token function">debug</span><span class="token punctuation">(</span><span class="token string">"结果是:{}"</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出</p>
<pre><code> 19:22:27 [t3] c.ThreadStarter - hello
19:22:27 [main] c.ThreadStarter - 结果是:100</code></pre><h4 id="多个线程同时运行"><a href="#多个线程同时运行" class="headerlink" title="多个线程同时运行"></a>多个线程同时运行</h4><ul>
<li>交替执行 </li>
<li>谁先谁后，不由我们控制</li>
</ul>
<h4 id="原理-1"><a href="#原理-1" class="headerlink" title="原理"></a>原理</h4><p>线程运行</p>
<h5 id="栈与栈帧"><a href="#栈与栈帧" class="headerlink" title="栈与栈帧"></a>栈与栈帧</h5><p>Java Virtual Machine Stacks （Java 虚拟机栈） </p>
<p> JVM 中由堆、栈、方法区所组成，其中栈内存其实就是给线程用，每个线程启动后，虚拟机就会为其分配一块栈内存。 每个栈由多个栈帧（Frame）组成，对应着每次方法调用时所占用的内存 每个线程只能有一个活动栈帧，对应着当前正在执行的那个方法。</p>
<h5 id="线程上下文切换（Thread-Context-Switch）"><a href="#线程上下文切换（Thread-Context-Switch）" class="headerlink" title="线程上下文切换（Thread Context Switch）"></a>线程上下文切换（Thread Context Switch）</h5><p>因为以下一些原因导致 cpu 不再执行当前的线程，转而执行另一个线程的代码 </p>
<ul>
<li>线程的 cpu 时间片用完 </li>
<li>垃圾回收 </li>
<li>有更高优先级的线程需要运行 </li>
<li>线程自己调用了 sleep、yield、wait、join、park、synchronized、lock 等方法 </li>
</ul>
<p>当 Context Switch 发生时，需要由操作系统保存当前线程的状态，并恢复另一个线程的状态，Java 中对应的概念 就是程序计数器（Program Counter Register），它的作用是记住下一条 jvm 指令的执行地址，是线程私有的 </p>
<ul>
<li>状态包括程序计数器、虚拟机栈中每个栈帧的信息，如局部变量、操作数栈、返回地址等 </li>
<li>Context Switch 频繁发生会影响性能</li>
</ul>
<h3 id="常见方法"><a href="#常见方法" class="headerlink" title="常见方法"></a>常见方法</h3><p>sleep 不释放锁、释放cpu<br>join 释放锁、抢占cpu<br>yiled 不释放锁、释放cpu<br>wait 释放锁、释放cpu</p>
<p>LockSupport.park()不会释放锁，只负责阻塞当前线程，释放锁资源实际上是在Condition的await()方法中实现的。</p>
<table>
<thead>
<tr>
<th>方法名</th>
<th>static</th>
<th>功能说明</th>
<th>注意</th>
</tr>
</thead>
<tbody><tr>
<td>start()</td>
<td></td>
<td>启动一个新线 程，在新的线程 运行 run 方法 中的代码</td>
<td>start 方法只是让线程进入就绪，里面代码不一定立刻 运行（CPU 的时间片还没分给它）。每个线程对象的 start方法只能调用一次，如果调用了多次会出现 IllegalThreadStateException</td>
</tr>
<tr>
<td>run()</td>
<td></td>
<td>新线程启动后会 调用的方法</td>
<td>如果在构造 Thread 对象时传递了 Runnable 参数，则 线程启动后会调用 Runnable 中的 run 方法，否则默 认不执行任何操作。但可以创建 Thread 的子类对象， 来覆盖默认行为。</td>
</tr>
<tr>
<td>join()</td>
<td></td>
<td>等待线程运行结 束</td>
<td><strong>「<code>Thread.join()</code>方法表示调用此方法的线程被阻塞，仅当该方法完成以后，才能继续运行」</strong></td>
</tr>
<tr>
<td>join(long n)</td>
<td></td>
<td>等待线程运行结 束,最多等待 n 毫秒</td>
<td></td>
</tr>
<tr>
<td>getId()</td>
<td></td>
<td>获取线程长整型 的 id</td>
<td>id 唯一</td>
</tr>
<tr>
<td>getName()</td>
<td></td>
<td>获取线程名</td>
<td></td>
</tr>
<tr>
<td>setName(String)</td>
<td></td>
<td>修改线程名</td>
<td></td>
</tr>
<tr>
<td>getPriority()</td>
<td></td>
<td>获取线程优先级</td>
<td></td>
</tr>
<tr>