调用System.gc()会发生什么？

• 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

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

/**
 * 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();
}

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

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