javascript的垃圾回收机制与内存管理

曾经终败给现在 2022-07-13 07:53 211阅读 0赞

**一、垃圾回收机制—GC**

Javascript具有自动垃圾回收机制(GC:Garbage Collecation)，也就是说，执行环境会负责管理代码执行过程中使用的内存。

原理：**垃圾收集器会定期（周期性）**找出那些不在继续使用的变量，然后释放其内存。

JavaScript垃圾回收的机制很简单：找出不再使用的变量，然后释放掉其占用的内存，但是这个过程不是实时的，因为其开销比较大，所以垃圾回收器会按照固**定的时间间隔周期性的执行**。

不再使用的变量也就是生命周期结束的变量，当然只可能是局部变量，全局变量的生命周期直至浏览器卸载页面才会结束。局部变量只在函数的执行过程中存在，而在这个过程中会为局部变量在栈或堆上分配相应的空间，以存储它们的值，然后在函数中使用这些变量，直至函数结束，而闭包中由于内部函数的原因，外部函数并不能算是结束。

还是上代码说明吧：

[?][Link 1]

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       &nbsp; 
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      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> obj = {name:</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important; color:blue!important">'hanzichi'</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">, age: 10};</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">&nbsp;</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">return</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> obj;</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">}</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
       &nbsp; 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> a = fn1();</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> b = fn2();</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

我们来看代码是如何执行的。首先定义了两个function，分别叫做fn1和fn2，当fn1被调用时，进入fn1的环境，会开辟一块内存存放对象\{name: 'hanzichi', age: 10\}，而当调用结束后，出了fn1的环境，那么该块内存会被js引擎中的垃圾回收器自动释放；在fn2被调用的过程中，返回的对象被全局变量b所指向，所以该块内存并不会被释放。

这里问题就出现了：到底哪个变量是没有用的？所以垃圾收集器必须跟踪到底哪个变量没用，对于不再有用的变量打上标记，以备将来收回其内存。用于标记的无用变量的策略可能因实现而有所区别，通常情况下有两种实现方式：标记清除和引用计数。引用计数不太常用，标记清除较为常用。

**二、标记清除**

js中最常用的垃圾回收方式就是标记清除。当变量进入环境时，例如，在函数中声明一个变量，就将这个变量标记为“进入环境”。从逻辑上讲，永远不能释放进入环境的变量所占用的内存，因为只要执行流进入相应的环境，就可能会用到它们。而当变量离开环境时，则将其标记为“离开环境”。

[?][Link 1]

垃圾回收器在运行的时候会给存储在内存中的所有变量都加上标记（当然，可以使用任何标记方式）。然后，它会去掉环境中的变量以及被环境中的变量引用的变量的标记（闭包）。而在此之后再被加上标记的变量将被视为准备删除的变量，原因是环境中的变量已经无法访问到这些变量了。最后，垃圾回收器完成内存清除工作，销毁那些带标记的值并回收它们所占用的内存空间。

到目前为止，IE、Firefox、Opera、Chrome、Safari的js实现使用的都是标记清除的垃圾回收策略或类似的策略，只不过垃圾收集的时间间隔互不相同。

**三、引用计数**

引用计数的含义是跟踪记录每个值被引用的次数。当声明了一个变量并将一个引用类型值赋给该变量时，则这个值的引用次数就是1。如果同一个值又被赋给另一个变量，则该值的引用次数加1。相反，如果包含对这个值引用的变量又取得了另外一个值，则这个值的引用次数减1。当这个值的引用次数变成0时，则说明没有办法再访问这个值了，因而就可以将其占用的内存空间回收回来。这样，当垃圾回收器下次再运行时，它就会释放那些引用次数为0的值所占用的内存。

[?][Link 1]

<table style="border-collapse:collapse; border-spacing:0px; border:1px solid rgb(204,204,204); margin-top:0.54em; text-align:center; width:643px; clear:both; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin-right:0px!important; margin-bottom:0px!important; margin-left:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important"> 
 <tbody style="background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; text-align:left!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
  <tr style="background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
   <td style="padding:0.2em 0.46em; border:1px solid rgb(204,204,204); margin:0px!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important; color:rgb(175,175,175)!important"> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      1 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      2 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      3 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      4 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      5 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      6 
    </div> </td> 
   <td style="padding:0.2em 0.46em; border:1px solid rgb(204,204,204); width:611px; margin:0px!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important"> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">function</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> test(){ </code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">&nbsp;</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> a = {} ; </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important">//a的引用次数为0 </code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">&nbsp;</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> b = a ; </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important">//a的引用次数加1，为1 </code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">&nbsp;</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> c =a; </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important">//a的引用次数再加1，为2</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">&nbsp;</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">var</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> b ={}; </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important">//a的引用次数减1，为1</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

Netscape Navigator3是最早使用引用计数策略的浏览器，但很快它就遇到一个严重的问题：循环引用。循环引用指的是对象A中包含一个指向对象B的指针，而对象B中也包含一个指向对象A的引用。

[?][Link 1]

以上代码a和b的引用次数都是2，fn()执行完毕后，两个对象都已经离开环境，在标记清除方式下是没有问题的，但是在引用计数策略下，因为a和b的引用次数不为0，所以不会被垃圾回收器回收内存，如果fn函数被大量调用，就会造成内存泄露。在IE7与IE8上，内存直线上升。

我们知道，IE中有一部分对象并不是原生js对象。例如，其内存泄露DOM和BOM中的对象就是使用C++以COM对象的形式实现的，而COM对象的垃圾回收机制采用的就是引用计数策略。因此，即使IE的js引擎采用标记清除策略来实现，但js访问的COM对象依然是基于引用计数策略的。换句话说，只要在IE中涉及COM对象，就会存在循环引用的问题。

[?][Link 1]

这个例子在一个DOM元素（element)与一个原生js对象（myObject)之间创建了循环引用。其中，变量myObject有一个名为element的属性指向element对象；而变量element也有一个属性名为o回指myObject。由于存在这个循环引用，即使例子中的DOM从页面中移除，它也永远不会被回收。

看上面的例子，有同学回觉得太弱了，谁会做这样无聊的事情，其实我们是不是就在做

[?][Link 1]

这段代码看起来没什么问题，但是obj引用了document.getElementById(“element”)，而document.getElementById(“element”)的onclick方法会引用外部环境中德变量，自然也包括obj，是不是很隐蔽啊。

**解决办法**

最简单的方式就是自己手工解除循环引用，比如刚才的函数可以这样

[?][Link 1]

将变量设置为null意味着切断变量与它此前引用的值之间的连接。当垃圾回收器下次运行时，就会删除这些值并回收它们占用的内存。

要注意的是，IE9+并不存在循环引用导致Dom内存泄露问题，可能是微软做了优化，或者Dom的回收方式已经改变

**四、内存管理**

**1、什么时候触发垃圾回收？**

垃圾回收器周期性运行，如果分配的内存非常多，那么回收工作也会很艰巨，确定垃圾回收时间间隔就变成了一个值得思考的问题。IE6的垃圾回收是根据内存分配量运行的，当环境中存在256个变量、4096个对象、64k的字符串任意一种情况的时候就会触发垃圾回收器工作，看起来很科学，不用按一段时间就调用一次，有时候会没必要，这样按需调用不是很好吗？但是如果环境中就是有这么多变量等一直存在，现在脚本如此复杂，很正常，那么结果就是垃圾回收器一直在工作，这样浏览器就没法儿玩儿了。

微软在IE7中做了调整，触发条件不再是固定的，而是动态修改的，初始值和IE6相同，如果垃圾回收器回收的内存分配量低于程序占用内存的15%，说明大部分内存不可被回收，设的垃圾回收触发条件过于敏感，这时候把临街条件翻倍，如果回收的内存高于85%，说明大部分内存早就该清理了，这时候把触发条件置回。这样就使垃圾回收工作职能了很多

**2、合理的GC方案**

1）、Javascript引擎基础GC方案是（simple GC）：mark and sweep（标记清除），即：

*  （1）遍历所有可访问的对象。
 *  （2）回收已不可访问的对象。

2）、GC的缺陷

和其他语言一样，javascript的GC策略也无法避免一个问题：GC时，停止响应其他操作，这是为了安全考虑。而Javascript的GC在100ms甚至以上，对一般的应用还好，但对于JS游戏，动画对连贯性要求比较高的应用，就麻烦了。这就是新引擎需要优化的点：避免GC造成的长时间停止响应。

3）、GC优化策略

David大叔主要介绍了2个优化方案，而这也是最主要的2个优化方案了：

**（1）分代回收（Generation GC）**  
这个和Java回收策略思想是一致的。目的是通过区分“临时”与“持久”对象；多回收“临时对象”区（young generation），少回收“持久对象”区（tenured generation），减少每次需遍历的对象，从而减少每次GC的耗时。如图：

![20151123144005806.png_20151023144017][]

这里需要补充的是：对于tenured generation对象，有额外的开销：把它从young generation迁移到tenured generation，另外，如果被引用了，那引用的指向也需要修改。

**（2）增量GC**  
这个方案的思想很简单，就是“每次处理一点，下次再处理一点，如此类推”。如图：

[![20151123144026796.png_20151023144035][]][20151123144026796.png_20151023144035]

这种方案，虽然耗时短，但中断较多，带来了上下文切换频繁的问题。

因为每种方案都其适用场景和缺点，因此在实际应用中，会根据实际情况选择方案。

比如：低 (对象/s) 比率时，中断执行GC的频率，simple GC更低些；如果大量对象都是长期“存活”，则分代处理优势也不大。

参考：

以上就是关于javascript的垃圾回收机制与内存管理的全部内容，希望对大家的学习有所帮助。

[Link 1]: http://www.jb51.net/article/75292.htm#
[20151123144005806.png_20151023144017]: /images/20220713/363fe1dbae394cf393f27ea234e7c6bb.png
[20151123144026796.png_20151023144035]: /images/20220713/6a19c08adfd2444b96117dfea64536a2.png