关于Sytem.gc()主动触发Android(安卓)GC

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2016.12.31 周六晴杭州

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由于想主动的通过代码触发gc测试软引用与弱引用的不同表现，很自然我就想到了去调用System.gc();既然用到了这个函数我就想去看看这个函数是怎么样实现的

/** * Runs the garbage collector.  * Calling the gc method suggests that the Java Virtual * Machine expend effort toward recycling unused objects in order to * make the memory they currently occupy available for quick reuse. * When control returns from the method call, the Java Virtual * Machine has made a best effort to reclaim space from all discarded * objects.* The call System.gc() is effectively equivalent to the * call: Runtime.getRuntime().gc() * * @see     java.lang.Runtime#gc() */public static void gc() {    boolean shouldRunGC;   synchronized (LOCK) {        shouldRunGC = justRanFinalization;        if (shouldRunGC) {            justRanFinalization = false;        } else {            runGC = true;        }    }    if (shouldRunGC) {        Runtime.getRuntime().gc();    }}

突然感觉有点不对呢，印象中文档都说和直接调用Runtime.getRuntime().gc();一样的

Paste_Image.png

并且在jdk中也是直接调用的实现。所以由此而想到Google应该是在某个版本更改了Android中的实现。经过查看api19也就是Android 4.4还是没有变化的

   /**     * 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() {        Runtime.getRuntime().gc();    }

而在Android 5.0（API21）之后也就变成了一开始提到的样子。
具体分析一下Google对System.gc()进行了怎样的改变,调用System.gc只有当justRanFinalization 为true的时候才执行一次Runtime.getRuntime().gc();也就是说连续多次调用时不能全部都生效的，再看看justRanFinalization 是啥时候改变的

/** * Runs the finalization methods of any objects pending finalization. *  * Calling this method suggests that the Java Virtual Machine expend * effort toward running the finalize methods of objects * that have been found to be discarded but whose finalize * methods have not yet been run. When control returns from the * method call, the Java Virtual Machine has made a best effort to * complete all outstanding finalizations. *  * The call System.runFinalization() is effectively * equivalent to the call: * Runtime.getRuntime().runFinalization() * * @see     java.lang.Runtime#runFinalization() */public static void runFinalization() {    boolean shouldRunGC;    synchronized (LOCK) {        shouldRunGC = runGC;        runGC = false;    }    if (shouldRunGC) {        Runtime.getRuntime().gc();    }    Runtime.getRuntime().runFinalization();    synchronized (LOCK) {        justRanFinalization = true;    }}

忽然发现这个除了会改变justRanFinalization 的值，在 runGC 为true时调用这个方法也会触发 Runtime.getRuntime().gc();
让我们看看justRanFinalization的定义

/** * Whether or not we need to do a GC before running the finalizers. */private static boolean runGC;/** * If we just ran finalization, we might want to do a GC to free the finalized objects. * This lets us do gc/runFinlization/gc sequences but prevents back to back System.gc(). */private static boolean justRanFinalization;

也就是我们 gc/runFinlization/gc 这样调用完成一次gcAndFinalize,但是要避免多次反复的调用System.gc().

我们可以看一下ZygoteInit中的gcAndFinalize()方法中如何使用的

/** * Runs several special GCs to try to clean up a few generations of * softly- and final-reachable objects, along with any other garbage. * This is only useful just before a fork(). */ 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();}

这里的 VMRuntime.getRuntime().runFinalizationSync();的作用其实和Runtime.getRuntime().runFinalization();作用一样。Runtime.getRuntime().gc()是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;Runtime.getRuntime().runFinalization();强制调用已经失去引用的对象的finalize方法

让我们看一下谷歌为什么这样修改System.gc()这个方法呢？
从这个commit看

Avoid running Runtime.gc() until we need to run finalization.
This prevents excessive explicit GC which are called from apps to get

good GC behavior on Dalvik. Calling System.gc() does not help on ART
since GC for alloc is much rarer.
If running finalizers is requested following a System.gc we remember
that a GC was requested and perform it ahead of finalization.
Bug: 12004934
Change-Id: Id1343ce8b5378c5f53c1e5649c06ddd045f56a3b

从这里可以得到两点信息：
1.首先这是为了修复一个bug 12004934
2.其次在art模式下，直接调用gc的效果不大。

参考文章：
System.GC()并不一定执行的原因 http://www.eoeandroid.com/thread-922938-1-1.html
Android System.gc()注意点
http://blog.csdn.net/fearGod/article/details/46364599

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