调用System.gc没有立即执行

原文链接http://www.weyye.me/detail/System-gc-not-called/

查看源码

当我们调用System.gc()的时候，其实并不会马上进行垃圾回收，甚至不一定会执行垃圾回收，查看系统源码可以看到

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/** * Indicates to the VM 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. */ public static void gc() { boolean shouldRunGC; synchronized(lock) { shouldRunGC = justRanFinalization; if (shouldRunGC) { justRanFinalization = false; } else { runGC = true; } } if (shouldRunGC) { Runtime.getRuntime().gc(); } }

也就是

justRanFinalization=true

的时候才会执行

查找发现当调用runFinalization()的时候

justRanFinalization

变为

true

下面是runFinalization()的源码

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/*** Provides a hint to the VM that it would be useful to attempt* to perform any outstanding object finalization.*/public static void runFinalization() { boolean shouldRunGC; synchronized(lock) { shouldRunGC = runGC; runGC = false; } if (shouldRunGC) { Runtime.getRuntime().gc(); } Runtime.getRuntime().runFinalization(); synchronized(lock) { justRanFinalization = true; }}

其实当我们直接调用

System.gc()

只会把这次gc请求记录下来，等到

runFinalization=true

的时候才会先去执行GC，

runFinalization=true

之后会在允许一次system.gc()。之后在call
System.gc()还会重复上面的行为。

所以System.gc()要跟System.runFinalization()一起搭配使用才好。

查看

ZygoteInit.java

 里面 gc()和runFinalizationSync()是配合使用的，这样才有效果

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static void gcAndFinalize() { final VMRuntime runtime = VMRuntime.getRuntime(); /* runFinalizationSync() lets finalizers be called in Zygote, * which doesn't have a HeapWorker thread. */ System.gc(); runtime.runFinalizationSync(); System.gc();}

解决方案

由此可见，当我们需要调用的

System.gc()

的时候 要这样才会执行

123	System.gc();runtime.runFinalizationSync();System.gc();

不过个人建议不到万不得已不要调用,因为jvm有自己的gc策略，根本不需要我们来手动