JVM（5）——調用System.gc沒有立即執行的解決方法

查看源碼

當我們調用System.gc()的時候，其實並不會馬上進行垃圾回收，甚至不一定會執行垃圾回收，查看系統源碼可以看到

/**
     * 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的時候才會執行

查找發現當調用runFinalization()的時候justRanFinalization變為true
下面是runFinalization()的源碼

/**
* 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;
        }
}

其實當我們直接調用System.gc()只會把這次gc請求記錄下來，等到runFinalization=true的時候才會先去執行GC，runFinalization=true之后會在允許一次system.gc()。之后在call System.gc()還會重復上面的行為。
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，這樣才有效果

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();
}

解決方案

由此可見，當我們需要調用的System.gc()的時候 要這樣才會執行

// 強制調用gc釋放內存
System.gc();
System.runFinalization();