java中常用的设计模式

1. Singleton（单例模式）

作用：　　

保证在Java应用程序中，一个类Class只有一个实例存在。

好处：

由于单例模式在内存中只有一个实例，减少了内存开销。

　单例模式可以避免对资源的多重占用，例如一个写文件时，由于只有一个实例存在内存中，避免对同一个资源文件的同时写操作。 

　　 

单例模式可以再系统设置全局的访问点，优化和共享资源访问。

使用情况：

建立目录 数据库连接的单线程操作

某个需要被频繁访问的实例对象　

第一种形式：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> Singleton {

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* 持有私有静态实例，防止被引用，此处赋值为null，目的是实现延迟加载 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Singleton instance = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* 私有构造方法，防止被实例化 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-title" style="box-sizing: border-box;">Singleton</span>() {
}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* </span></code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">    懒汉式：第一次调用时初始Singleton，以后就不用再生成了
静态方法，创建实例 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Singleton <span class="hljs-title" style="box-sizing: border-box;">getInstance</span>() {</code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (instance == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {</code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">           <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">synchronized</span><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);"> (<span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">Singleton.class</span>) {</span></code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">                   if(instance == null) {</span></code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);"><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">                  instance = </span><span class="hljs-keyword" style="font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; color: rgb(0, 0, 136); box-sizing: border-box; background-color: rgba(128, 128, 128, 0.0470588);">new</span><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);"> Singleton();</span>
</span></code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">                 }</span></code><code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span style="color: rgb(51, 51, 51); font-family: 'Source Code Pro', monospace; font-size: 14px; line-height: 20.3px; white-space: pre; background-color: rgba(128, 128, 128, 0.0470588);">              }</span>
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> instance;
}
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

　        上面这种是我自己从网上看到，并经常使用的方式。