调用System.gc没有立即执行的解决方法

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

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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