System.gc() in Java

Java的的一个显著特点就是垃圾回收机制，关于垃圾回收机制大家都不生疏了，网上一大片关于他的论述，所以就不多说了。

 

http://stackoverflow.com/questions/66540/system-gc-in-java

 

在这个问题里面，可能仔细看的人会注意到一个问题：

因为有一种说法就是，System.gc() ; 会提醒虚拟机清理内存，但不一定会立即执行，还有一种说法是这个方法会停止所有线程而扫描一遍，找出并清理垃圾对象释放内存，

写道

while system.gc works,it will stop the world:all respones are stopped so garbage collector can scan every object to check if it is needed deleted. if the application is a web project, all request are stopped until gc finishes,and this will cause your web project can not work in a monent.

 

写道

You have no control over GC in java -- the VM decides. I've never run across a case where System.gc() is needed. Since a System.gc() call simply SUGGESTS that the VM do a garbage collection and it also does a FULL garbage collection (old and new generations in a multi-generational heap), then it can actually cause MORE cpu cycles to be consumed than necessary.

In some cases, it may make sense to suggest to the VM that it do a full collection NOW as you may know the application will be sitting idle for the next few minutes before heavy lifting occurs. For example, right after the initialization of a lot of temporary object during application startup (i.e., I just cached a TON of info, and I know I won't be getting much activity for a minute or so). Think of an IDE such as eclipse starting up -- it does a lot to initialize, so perhaps immediately after initialization it makes sense to do a full gc at that point.

 

写道

I can't think of a specific example when it is good to run explicit GC.

In general, running explicit GC can actually cause more harm than good, because an explicit gc will trigger a full collection, which takes significantly longer as it goes through every object. If this explicit gc ends up being called repeatedly it could easily lead to a slow application as a lot of time is spent running full GCs.

Alternatively if going over the heap with a heap analyzer and you suspect a library component to be calling explicit GC's you can turn it off adding: gc=-XX:+DisableExplicitGC to the JVM parameters.

 

我不敢妄作评论，最终得出结论就是：最好不要调用。  

 

从http://bbs.csdn.net/topics/390011420 这里你或许也可以得出点结论来。

System.gc() 源码：

 

写道

/**
* Runs the garbage collector.
* <p>
* Calling the <code>gc</code> method suggests that the Java Virtual
* Machine expend effort toward recycling unused objects in order to
* make the memory they currently occupy available for quick reuse.
* When control returns from the method call, the Java Virtual
* Machine has made a best effort to reclaim space from all discarded
* objects.
* <p>
* The call <code>System.gc() </code> is effectively equivalent to the
* call:
* <blockquote><pre>
* Runtime.getRuntime(). gc()
* </pre></blockquote>
*
* @see java.lang.Runtime#gc()
*/
public static void gc() {
Runtime.getRuntime().gc();
}

 

    

写道

/**
* Indicates to the virtual machine that it would be a good time to run the
* garbage collector. Note that this is a hint only. There is no guarantee
* that the garbage collector will actually be run.
*
* @since Android 1.0
*/
public native void gc();

 

贴两张图在这里：对比了一下显示执行该方法与不显示执行的区别。 很显然  这里一个两次一个一次，GC_EXTERNAL_ALLOC  , GC_EXTERNAL