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post-type-normal" itemscope itemtype="http://schema.org/Article"><link itemprop="mainEntityOfPage" href="http://mrbird.cc/Java-9-Feature.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 9 新特性学习</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="2018-11-02T08:50:59+08:00">2018-11-02 </time></span><span></span></div></header><div class="post-body" itemprop="articleBody"><p>在Java 8 发布3年多后，Java 9 终于在2017年9月21日正式发布。Java 9 提供了超过150项新功能特性，包括备受期待的模块化系统、 可交互的REPL工具jShell、JDK编译工具、Java公共API和私有代码，以及安全增强、扩展提升和性能管理改善等。在学习这些新特性之前，我们得先安装好JDK 9，JDK 9 下载地址：<a href="https://www.oracle.com/technetwork/cn/java/javase/downloads/jdk9-downloads-3848520-zhs.html" target="_blank" rel="noopener">https://www.oracle.com/technetwork/cn/java/javase/downloads/jdk9-downloads-3848520-zhs.html</a>。<a id="more"></a></p><h2 id="模块化"><a href="#模块化" class="headerlink" title="模块化"></a>模块化</h2><p>Java 9 中的模块化（Module）类似于ES6中的模块化，都是为了解决项目中减少内存的开销，提供可维护性和系统性能而提出的。通俗地说，Java 9 的模块化本质上就是在包（package）上再包裹一层（Module），默认模块里的内容都是隐藏的，只能通过（exports）关键字来暴露模块里的内容，而别的模块需要用到这些内容则需要使用（requires）关键字来导入。</p><p>下面我们用代码来演示这一个新特性。</p><p>新建一个Java 工程，然后在工程下面创建一个名为<strong>ModuleOne</strong>的模块：</p><p><img src="img/QQ截图20190213094527.png" alt="QQ截图20190213094527.png"></p><p>同样的，我们再创建一个<strong>ModuleTwo</strong>模块，创建完后项目目录如下所示：</p><p><img src="img/QQ截图20190213094722.png" alt="QQ截图20190213094722.png"></p><p>接着我们在ModuleOne的src目录下创建<code>cc.mrbird.domain</code>包，并创建一个名为Person的类：</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><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> cc.mrbird.domain;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Person</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> String name;</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">int</span> age;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">Person</span><span class="params">(String name, <span class="keyword">int</span> age)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.name = name;</span><br><span class="line"> <span class="keyword">this</span>.age = age;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="string">"Person{"</span> +</span><br><span class="line"> <span class="string">"name='"</span> + name + <span class="string">'\''</span> +</span><br><span class="line"> <span class="string">", age="</span> + age +</span><br><span class="line"> <span class="string">'}'</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getName</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> name;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setName</span><span class="params">(String name)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.name = name;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getAge</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> age;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setAge</span><span class="params">(<span class="keyword">int</span> age)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.age = age;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>然后在ModuleTwo的src目录下创建一个<code>cc.mrbid.test</code>包，并且创建一个Test类，用于待会测试。</p><p>至此，工程目录结构如下所示:</p><p><img src="img/QQ截图20190213095358.png" alt="QQ截图20190213095358.png"></p><p>我们在ModuleTwo的Test类下试图引入ModuleOne的Person类，会发现是行不通的：</p><p><img src="img/QQ截图20190213095624.png" alt="QQ截图20190213095624.png"></p><p>正如上面所说，模块里的东西默认都是隐藏的，要让其可以被别的模块使用，需要通过<strong>exports</strong>关键字来暴露它们。</p><p>在ModuleOne模块下的src上右键，新建一个module-info.java：</p><p><img src="img/QQ截图20190213095840.png" alt="QQ截图20190213095840.png"></p><p>代码如下所示:</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">module</span> ModuleOne {</span><br><span class="line"> <span class="comment">// 导出包</span></span><br><span class="line"> <span class="keyword">exports</span> cc.mrbird.domain;</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>上面代码中导出了<code>cc.mrbird.domain</code>包下的所有内容，对别的模块来说，它们是可见的了。</p><p>然后同样地在ModuleTwo模块下的src上右键，也新建一个module-info.java，代码如下所示:</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">module</span> ModuleTwo {</span><br><span class="line"> <span class="comment">// 导入模块</span></span><br><span class="line"> <span class="keyword">requires</span> ModuleOne;</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>这里我们主要做的是将ModuleOne导入进来，所以其暴露的<code>cc.mrbird.domain</code>包下的内容就可以在ModuleTwo下被使用了。</p><p>这时候回到ModuleTwo模块下的Test类，IDEA会自动帮我们导入需要的类，代码就不会报错了：</p><p><img src="img/QQ截图20190213100532.png" alt="QQ截图20190213100532.png"></p><p>我们继续在Test类中加入一些内容：</p><p><img src="img/QQ截图20190213101203.png" alt="QQ截图20190213101203.png"></p><p>上面我们使用了<code>java.util.logging.Logger</code>来打印日志，但是代码编译是不通过的，使用<code>Alt+Enter</code>快捷键后选择第一项，IDEA会自动帮我们在module-info.java文件中导入需要的包，非常方便。</p><p>导入后，module-info.java下的代码如下所示：</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">module</span> ModuleTwo {</span><br><span class="line"> <span class="comment">// 导入模块</span></span><br><span class="line"> <span class="keyword">requires</span> ModuleOne;</span><br><span class="line"> <span class="keyword">requires</span> java.logging;</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>总的来说，通过模块化来构建项目可以带来以下好处:</p><ol><li><p>减少内存的开销；</p></li><li><p>简化各种类库和大型应用的开发和维护；</p></li><li><p>改进其安全性，可维护性，提高性能。</p></li></ol><h2 id="jshell命令"><a href="#jshell命令" class="headerlink" title="jshell命令"></a>jshell命令</h2><p>在Java 9 之前，我们要想使用Java来输出一句hello world或者计算两个数的和都必须创建一个Java项目，然后创建类在里面写main方法来运行。Java 9 后就不必这样了，其提供了REPL工具：jshell。利用 jshell 在没有创建类的情况下直接声明变量，计算表达式，执行语句。即开发时可以在命令行里直接运行 Java 的代码，而无需创建 Java 文件。</p><p>在JDK 9 的bin目录下有个jshell.exe文件，我们运行它：</p><p><img src="img/QQ截图20190213103825.png" alt="QQ截图20190213103825.png"></p><p>下面演示一些jshell的常用操作。</p><p><strong>基本使用</strong></p><p><img src="img/QQ截图20190213104253.png" alt="QQ截图20190213104253.png"></p><div class="note info"><p>在jshell中，代码末尾的<code>;</code>是可选的。</p></div><p><strong>导包操作</strong></p><p><img src="img/QQ截图20190213104534.png" alt="QQ截图20190213104534.png"></p><p><strong>查看所有已经导入的包</strong></p><p><img src="img/QQ截图20190213104610.png" alt="QQ截图20190213104610.png"></p><p><strong>使用Tab键补全代码</strong></p><p><img src="img/QQ截图20190213104732.png" alt="QQ截图20190213104732.png"></p><p><strong>查看当前环境下所有有效代码</strong></p><p><img src="img/QQ截图20190213104840.png" alt="QQ截图20190213104840.png"></p><p><strong>查看当前环境下所有变量</strong></p><p><img src="img/QQ截图20190213104941.png" alt="QQ截图20190213104941.png"></p><p><strong>查看当前环境下所有方法</strong></p><p><img src="img/QQ截图20190213105231.png" alt="QQ截图20190213105231.png"></p><p><strong>使用外部代码编辑器来修改add方法</strong></p><p><img src="img/QQ截图20190213105412.png" alt="QQ截图20190213105412.png"></p><p>界面弹出：</p><p><img src="img/QQ截图20190213105500.png" alt="QQ截图20190213105500.png"></p><p>修改完毕点击Accept和Exit按钮即可：</p><p><img src="img/QQ截图20190213105553.png" alt="QQ截图20190213105553.png"></p><p><strong>加载外部代码</strong></p><p>在桌面新建一个Hello.java文件：</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">hello</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"hello world"</span>);</span><br><span class="line">}</span><br><span class="line">hello();</span><br></pre></td></tr></table></figure><p></p><p>然后使用<code>/open</code>命令执行这个文件：</p><p><img src="img/QQ截图20190213110050.png" alt="QQ截图20190213110050.png"></p><p>更多关于jshell的功能介绍，可以使用<code>/help</code>命令来查看。</p><h2 id="接口中的私有方法"><a href="#接口中的私有方法" class="headerlink" title="接口中的私有方法"></a>接口中的私有方法</h2><p>我们都知道，在JDK 8之前，接口只能包含<code>public static final</code>的成员变量和<code>public abstract</code>修饰的抽象方法；而在JDK 8中，接口中可以定义静态方法和默认方法了；JDK 9的接口又加了新的特性，其允许接口中包含私有的方法，下面这个接口定义在JDK 9 中是合法的：</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TestInterface</span> </span>{</span><br><span class="line"> <span class="comment">// before jdk 7 :</span></span><br><span class="line"> <span class="function"><span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span></span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// jdk 8:</span></span><br><span class="line"> <span class="function"><span class="keyword">static</span> <span class="keyword">void</span> <span class="title">method2</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"mehtod2"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">default</span> <span class="keyword">void</span> <span class="title">method3</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"mehtod3"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// jdk 9:</span></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">method4</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"mehtod4"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><h2 id="Diamond-Operator使用升级"><a href="#Diamond-Operator使用升级" class="headerlink" title="Diamond Operator使用升级"></a>Diamond Operator使用升级</h2><p>在Java 9 之前的版本中，我们不能将钻石操作符（Diamond Operator）和匿名实现类放在一起使用，比如下面这个例子在Java 9 之前的版本中编译是不通过的：</p><p><img src="img/QQ截图20190214091025.png" alt="QQ截图20190214091025.png"></p><p>而在Java 9中取消了这个限制，我们可以通过这个特性来进行一些初始化操作:</p><p><img src="img/QQ截图20190214091154.png" alt="QQ截图20190214091154.png"></p><h2 id="try使用升级"><a href="#try使用升级" class="headerlink" title="try使用升级"></a>try使用升级</h2><p>在Java 9 之前，我们可以使用下面这种优雅的方式来进行流的关闭操作：</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span> (InputStreamReader reader = <span class="keyword">new</span> InputStreamReader(System.in)) {</span><br><span class="line"> reader.read();</span><br><span class="line">} <span class="keyword">catch</span> (IOException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>在<code>try</code>后面的括号中声明初始化的流Java会自动帮我们进行关闭操作。Java 9 对这个特性进行了升级，我们可以在<code>try</code>后面的括号中使用已经初始化过的资源，此时的资源是<code>final</code>的：</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></pre></td><td class="code"><pre><span class="line">InputStreamReader reader = <span class="keyword">new</span> InputStreamReader(System.in);</span><br><span class="line"><span class="keyword">try</span> (reader) {</span><br><span class="line"> reader.read();</span><br><span class="line">} <span class="keyword">catch</span> (IOException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>如果对<code>reader</code>再次进行赋值操作，编译将不通过：</p><p><img src="img/QQ截图20190214093344.png" alt="QQ截图20190214093344.png"></p><h2 id="下划线标识符的限制"><a href="#下划线标识符的限制" class="headerlink" title="下划线标识符的限制"></a>下划线标识符的限制</h2><p>Java 8 中，我们可以使用下划线<code>_</code>作为标识符使用，比如下面这段代码是合法的：</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">String _ = <span class="string">"mrbird"</span>;</span><br><span class="line">System.out.println(_);</span><br></pre></td></tr></table></figure><p></p><p>而在Java 9 中，下划线<code>_</code>已经不能用于标识符了：</p><p><img src="img/QQ截图20190214094709.png" alt="QQ截图20190214094709.png"></p><h2 id="String存储结构变更"><a href="#String存储结构变更" class="headerlink" title="String存储结构变更"></a>String存储结构变更</h2><p>Java 9 之前，字符串的底层是用<code>char[]</code>进行存储的，Java 9 中，字符串改用成了<code>byte[]</code>进行存储:</p><p><img src="img/QQ截图20190214101945.png" alt="QQ截图20190214101945.png"></p><p>之所以做出这个改变是因为：大多数<code>String</code>类型对象存储的都是拉丁字符，这些字符只占一个字节，而<code>char</code>能够存储两个字节，所以大部分情况下都浪费了一半的存储空间。Java 9 将<code>String</code>类的内部表示从<code>UTF-16</code>的<code>char</code>数组更改为<code>byte</code>数组加上<code>encoding-flag</code>字段。新<code>String</code>类将根据字符串的内容存储编码为<code>ISO-8859-1</code> / Latin-1（每个字符一个字节）或<code>UTF-16</code>（每个字符两个字节）的字符。<code>encoding-flag</code>编码标志将指示使用哪种编码。</p><p>基于<code>String</code>的类如<code>StringBuffer</code>和<code>StringBuilder</code>等也是做出了相同的改变。</p><p>具体可参考：<a href="http://openjdk.java.net/jeps/254" target="_blank" rel="noopener">http://openjdk.java.net/jeps/254</a>。</p><h2 id="集合的of方法"><a href="#集合的of方法" class="headerlink" title="集合的of方法"></a>集合的<code>of</code>方法</h2><p>Java 9 之前我们可以使用下面的方式来创建不可变集合：</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></pre></td><td class="code"><pre><span class="line">List<Integer> list = Collections.unmodifiableList(Arrays.asList(<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>));</span><br><span class="line"></span><br><span class="line">Set<Integer> set = Collections.unmodifiableSet(<span class="keyword">new</span> HashSet<>(Arrays.asList(<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>)));</span><br><span class="line"></span><br><span class="line">Map<String, Object> map = <span class="keyword">new</span> HashMap<>();</span><br><span class="line">map.put(<span class="string">"mrbird"</span>, <span class="number">18</span>);</span><br><span class="line">map.put(<span class="string">"kangkang"</span>, <span class="number">20</span>);</span><br><span class="line">Map<String, Object> map1 = Collections.unmodifiableMap(map);</span><br></pre></td></tr></table></figure><p></p><p>Java 9 中的集合类都添加了一个<code>of</code>方法，可以快速的构造不可变集合：</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></pre></td><td class="code"><pre><span class="line">List<Integer> list = List.of(<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>);</span><br><span class="line">Set<Integer> set = Set.of(<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>);</span><br><span class="line">Map<String, Object> map1 = Map.of(<span class="string">"mrbird"</span>, <span class="number">18</span>, <span class="string">"kangkang"</span>, <span class="number">20</span>);</span><br></pre></td></tr></table></figure><p></p><p>对不可变集合进行赋值操作将抛出异常：</p><p><img src="img/QQ截图20190214112738.png" alt="QQ截图20190214112738.png"></p><h2 id="增强的Steam-API"><a href="#增强的Steam-API" class="headerlink" title="增强的Steam API"></a>增强的Steam API</h2><p>在 Java 9 中，Stream API变得更好，Stream接口中添加了 4 个新的方法：<code>dropWhile</code>, <code>takeWhile</code>, <code>ofNullable</code>，还有个 <code>iterate</code>方法的新重载方法，可以让你提供一个<code>Predicate</code>（判断条件）来指定什么时候结束迭代。</p><p>下面举例来演示这几个的用法：</p><p><strong>takeWhile</strong></p><p>takeWhile用于从Stream中获取一部分数据，接收一个Predicate来进行选择。在有序的Stream中，takeWhile返回从开头开始的尽量多的元素。</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">List<Integer> list = Arrays.asList(<span class="number">45</span>, <span class="number">43</span>, <span class="number">76</span>, <span class="number">87</span>, <span class="number">42</span>, <span class="number">77</span>, <span class="number">90</span>, <span class="number">73</span>, <span class="number">67</span>, <span class="number">88</span>);</span><br><span class="line">list.stream().takeWhile(x -> x < <span class="number">50</span>).forEach(System.out::println);</span><br></pre></td></tr></table></figure><p></p><p>结果输出:</p><p><img src="img/QQ截图20190214135831.png" alt="QQ截图20190214135831.png"></p><p>程序从头开始判断当前值是否小于50，当判断到第三个元素76时，发现不满足，于是程序就结束了。所以<code>takeWhile</code>不同于过滤器。</p><p><strong>dropWhile</strong></p><p>dropWhile的行为与takeWhile相反，返回剩余的元素：</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">List<Integer> list = Arrays.asList(<span class="number">45</span>, <span class="number">43</span>, <span class="number">76</span>, <span class="number">87</span>, <span class="number">42</span>, <span class="number">77</span>, <span class="number">90</span>, <span class="number">73</span>, <span class="number">67</span>, <span class="number">88</span>);</span><br><span class="line">list.stream().dropWhile(x -> x < <span class="number">50</span>).forEach(System.out::println);</span><br></pre></td></tr></table></figure><p></p><p>程序输出:</p><p><img src="img/QQ截图20190214140313.png" alt="QQ截图20190214140313.png"></p><p><strong>ofNullable</strong></p><p>Java 8 中Stream不能完全为null（只有一个元素，且为null），否则会报空指针异常。而Java 9 中的ofNullable 方法允许我们创建一个单元素 Stream，可以包含一个非空元素，也可以创建一个空 Stream：</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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 报 NullPointerException</span></span><br><span class="line"><span class="comment">// Stream<Object> stream1 = Stream.of(null);</span></span><br><span class="line"><span class="comment">// System.out.println(stream1.count());</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 不报异常，允许通过</span></span><br><span class="line">Stream<String> stringStream = Stream.of(<span class="string">"AA"</span>, <span class="string">"BB"</span>, <span class="keyword">null</span>);</span><br><span class="line">System.out.println(stringStream.count()); <span class="comment">// 3</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 不报异常，允许通过</span></span><br><span class="line">List<String> list = <span class="keyword">new</span> ArrayList<>();</span><br><span class="line">list.add(<span class="string">"AA"</span>);</span><br><span class="line">list.add(<span class="keyword">null</span>);</span><br><span class="line">System.out.println(list.stream().count()); <span class="comment">// 2</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// ofNullable()：允许值完全为 null</span></span><br><span class="line">Stream<Object> stream1 = Stream.ofNullable(<span class="keyword">null</span>);</span><br><span class="line">System.out.println(stream1.count()); <span class="comment">// 0</span></span><br><span class="line"></span><br><span class="line">Stream<String> stream = Stream.ofNullable(<span class="string">"hello world"</span>);</span><br><span class="line">System.out.println(stream.count()); <span class="comment">// 1</span></span><br></pre></td></tr></table></figure><p></p><p><strong>Steam iterator的重载方法</strong></p><p><img src="img/QQ截图20190214141552.png" alt="QQ截图20190214141552.png"></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></pre></td><td class="code"><pre><span class="line"><span class="comment">// Java 8 中的</span></span><br><span class="line">Stream.iterate(<span class="number">0</span>, x -> x + <span class="number">1</span>).limit(<span class="number">10</span>).forEach(System.out::println);</span><br><span class="line"><span class="comment">// 等价于 Java 9 中的</span></span><br><span class="line">Stream.iterate(<span class="number">0</span>, x -> x < <span class="number">10</span>, x -> x + <span class="number">1</span>).forEach(System.out::println);</span><br></pre></td></tr></table></figure><p>除了上面几个新特性外，Java 9 还支持将optional转换为流的操作：</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></pre></td><td class="code"><pre><span class="line">List<String> list = <span class="keyword">new</span> ArrayList<>();</span><br><span class="line">list.add(<span class="string">"mrbird"</span>);</span><br><span class="line">list.add(<span class="string">"kangkang"</span>);</span><br><span class="line">list.add(<span class="string">"maria"</span>);</span><br><span class="line"></span><br><span class="line">Optional<List<String>> optional = Optional.of(list);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 流中的元素为 list</span></span><br><span class="line">Stream<List<String>> stream = optional.stream();</span><br><span class="line">stream.forEach(System.out::println);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 流中的元素为 list中的每个元素</span></span><br><span class="line">Stream<String> stringStream = optional.stream().flatMap(Collection::stream);</span><br><span class="line">stringStream.forEach(System.out::println);</span><br></pre></td></tr></table></figure><p></p><p>程序输出：</p><p><img src="img/QQ截图20190214143228.png" alt="QQ截图20190214143228.png"></p><h2 id="HTTP-Client"><a href="#HTTP-Client" class="headerlink" title="HTTP Client"></a>HTTP Client</h2><p>Java 9 中新增了全新的<code>HttpClient</code>来替代<code>HttpURLConnection</code>，其可以从<code>jdk.incubator.httpclient</code>模块中获取。因为在默认情况下，这个模块是不能根据<code>classpath</code>获取的，需要使用<code>add modules</code>命令选项配置这个模块，将这个模块添加到 classpath中。</p><p>我们在src下的module-info.java中引入这个模块：</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"><span class="keyword">requires</span> jdk.incubator.httpclient;</span><br></pre></td></tr></table></figure><p></p><p>写个基于<code>HttpClient</code>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></pre></td><td class="code"><pre><span class="line">HttpClient client = HttpClient.newHttpClient();</span><br><span class="line">HttpRequest req = HttpRequest.newBuilder(URI.create(<span class="string">"https://mrbird.cc"</span>))</span><br><span class="line"> .GET()</span><br><span class="line"> .build();</span><br><span class="line">HttpResponse<String> response = client.send(req, HttpResponse.BodyHandler.asString());</span><br><span class="line">System.out.println(response.statusCode());</span><br><span class="line">System.out.println(response.version().name());</span><br><span class="line">System.out.println(response.body());</span><br></pre></td></tr></table></figure><p></p><p>输出结果：</p><p><img src="img/QQ截图20190214145555.png" alt="QQ截图20190214145555.png"></p><blockquote><p>参考自：<a href="https://www.jianshu.com/u/5f70a16b98e7" target="_blank" rel="noopener">https://www.jianshu.com/u/5f70a16b98e7</a></p></blockquote><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-9-Feature.html" title="Java 9 新特性学习">http://mrbird.cc/Java-9-Feature.html</a></li><li class="post-copyright-license"><strong>版权声明： </strong>本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" rel="external nofollow" target="_blank">CC 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