JVM系列之内存泄漏

素颜马尾好姑娘i 2022-05-21 00:24 220阅读 0赞

转载自http://www.wolfbe.com/detail/201609/378.html

摘要：在Java中，内存泄漏是指程序中存在某些对象已经不会再被使用，但由于仍被其它对象引用而造成内存不能被及时释放，从而造成内存被浪费。

![378.html][]

什么是内存泄漏？在Java中，内存泄漏是指程序中存在某些对象已经不会再被使用，但由于仍被其它对象引用而造成内存不能被及时释放，从而造成内存被浪费。

我们知道Java创建一个对象后，就会在堆内存中给这个对象分配一块内存，JVM会监控所有的对象创建、赋值等过程，JVM持有一个记录对象引用的一个有向图，起点为Root，把一个对象赋值给某个引用，JVM就会给这个对象在引用有向图中加多上一条路径。当一个对象不再被引用，即通过Root到这个对象不可达，在下一次GC到来时就可能回收这个对象。

举个例子，如下所示：

[?][Link 1]

<table style="border-spacing:0px;width:736px;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;"> 
 <tbody style="background:none;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
  <tr style="background:none;border-top:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
   <td style="background:none;border-width:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;color:rgb(175,175,175);"> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      1 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      2 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      3 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      4 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      5 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      6 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      7 
    </div></td> 
   <td style="width:700px;background:none;border-width:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;"> 
    <div style="background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">public</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">static</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">void</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">main(String[] args){ </code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object obj1 = </code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">new</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object();</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object obj2 = </code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">new</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object();</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">obj2 = obj1;</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
       &nbsp; 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">...</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">}</code> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

![1474986333628.png][]

调用obj2=obj1，使obj2引用指向对象OBJ1，OBJ2对象由于没有引用指向，那么如果下一次GC到来，那么就可能会回收OBJ2对象（因为垃圾回收的过程是不确定的，当一个对象没有被引用了，那么说明这个对象可以被回收，但不能保证一定会被回收掉）。

如果在程序中确定不再使用OBJ2对象，但却被其它对象引用，那么就会出现内存泄漏的现象，如下所示：

[?][Link 1]

<table style="border-spacing:0px;width:736px;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;"> 
 <tbody style="background:none;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
  <tr style="background:none;border-top:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
   <td style="background:none;border-width:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;color:rgb(175,175,175);"> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      1 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      2 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      3 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      4 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      5 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      6 
    </div> 
    <div style="background-image:none;border-width:0px 3px 0px 0px;border-right-style:solid;border-right-color:rgb(108,226,108);float:none;line-height:1.1em;text-align:right;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;">
      7 
    </div></td> 
   <td style="width:700px;background:none;border-width:0px;float:none;line-height:1.1em;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;min-height:auto;"> 
    <div style="background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;"> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object obj1 = </code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">new</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object();</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object obj2 = </code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">new</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Object();</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Vector v=</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">new</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">Vector(</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">10</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">);</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">v.add(obj1);</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">v.add(obj2);</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">...</code> 
     </div> 
     <div style="background-image:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-size:1em;min-height:auto;white-space:pre;"> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">obj2 = </code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,102,153);background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;font-weight:bold;min-height:auto;">null</code> 
      <code style="font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:#000000;background:none;border:0px;float:none;line-height:1.1em;vertical-align:baseline;min-height:auto;">;</code> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

代码中obj2 = null确定会释放obj2对OBJ2对象的引用，但是变量v仍然保持着对OBJ2对象的引用，那么OBJ2对象不会被GC回收。

我们在写程序时应该正确地释放不再引用的对象，尤其是在一些静态集合、数据库连接、内部类的代码中，应当检查是否存在不可用对象的引用仍然被持有，避免因为无法释放不可用对象而造成内存的浪费。

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[378.html]: http://www.wolfbe.com/detail/201609/378.html
[Link 1]: http://www.wolfbe.com/detail/201609/378.html#
[1474986333628.png]: http://res.img.wolfbe.com/lst/1474986333628.png