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

    /**     * 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

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