JAVA高级面试进阶训练营视频教程

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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

// 强制调用gc释放内存
System.gc();
System.runFinalization();