System.gc(); //告诉垃圾收集器打算进行垃圾收集,而垃圾收集器进不进行收集是不确定的
System.runFinalization(); //强制调用已经失去引用的对象的finalize方法
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
/**
* 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
/**
* 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
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();
}
由此可见,当我们需要调用的System.gc()的时候,要这样才会执行
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
版权说明 : 本文为转载文章, 版权归原作者所有 版权申明
原文链接 : https://blog.csdn.net/qq_21383435/article/details/122907928
内容来源于网络,如有侵权,请联系作者删除!