Android Handler之内存泄露

前言

在Android开发中，使用Handler的地方很多，大致通常写法如下：

    private Handler handler = new Handler() {
        public void handleMessage(android.os.Message msg) {
            if (msg.what == 1) {
                mAdapter.notifyDataSetChanged();
            }
        }
    };

这段代码看似没什么问题，但是里面却有一个警告，警告信息如下：

This Handler class should be static or leaks might occur

大致意思：建议使用静态声明否者可能出现内存泄露
Handler会内存泄露？下面来说明：

当使用内部类（包括匿名类）来创建Handler的时候，Handler对象会隐式地持有一个外部类对象（通常是一个Activity）的引用（不然你怎么可能通过Handler来操作Activity中的View？）。而Handler通常会伴随着一个耗时的后台线程（例如从网络拉取图片）一起出现，这个后台线程在任务执行完毕（例如图片下载完毕）之后，通过消息机制通知Handler，然后Handler把图片更新到界面。然而，如果用户在网络请求过程中关闭了Activity，正常情况下，Activity不再被使用，它就有可能在GC检查时被回收掉，但由于这时线程尚未执行完，而该线程持有Handler的引用（不然它怎么发消息给Handler？），这个Handler又持有Activity的引用，就导致该Activity无法被回收（即内存泄露），直到网络请求结束（例如图片下载完毕）。另外，如果你执行了Handler的postDelayed()方法，该方法会将你的Handler装入一个Message，并把这条Message推到MessageQueue中，那么在你设定的delay到达之前，会有一条MessageQueue
-> Message -> Handler -> Activity的链，导致你的Activity被持有引用而无法被回收。

内存泄漏简介

Java使用有向图机制，通过GC自动检查内存中的对象（什么时候检查由虚拟机决定），如果GC发现一个或一组对象为不可到达状态，则将该对象从内存中回收。也就是说，一个对象不被任何引用所指向，则该对象会在被GC发现的时候被回收；另外，如果一组对象中只包含互相的引用，而没有来自它们外部的引用（例如有两个对象A和B互相持有引用，但没有任何外部对象持有指向A或B的引用），这仍然属于不可到达，同样会被GC回收。

内存泄露的危害

内存泄露的危害就是会使虚拟机占用内存过高，导致OOM（内存溢出），程序出错。

对于Android应用来说，就是你的用户打开一个Activity，使用完之后关闭它，内存泄露；又打开，又关闭，又泄露；几次之后，程序占用内存超过系统限制，FC。

解决方案

• 通过程序逻辑来进行保护

1，在关闭Activity的时候停掉你的后台线程。线程停掉了，就相当于切断了Handler和外部连接的线，Activity自然会在合适的时候被回收。
2，如果你的Handler是被delay的Message持有了引用，那么使用相应的Handler的removeCallbacks()方法，把消息对象从消息队列移除就行了。

• 将Handler声明为静态类

静态类不持有外部类的对象，所以你的Activity可以随意被回收。由于Handler不再持有外部类对象的引用，导致程序不允许你在Handler中操作Activity中的对象了。所以你需要在Handler中增加一个对Activity的弱引用（WeakReference 文章尾部会有说明）。
PS：在Java 中，非静态的内部类和匿名内部类都会隐式地持有其外部类的引用，静态的内部类不会持有外部类的引用。

    static class MyHandler extends Handler {
        WeakReference<Activity> mWeakReference;

        public MyHandler(Activity activity) {
            mWeakReference = new WeakReference<Activity>(activity);
        }

        @Override
        public void handleMessage(Message msg) {
            final Activity activity = mWeakReference.get();
            if (activity != null) {
                if (msg.what == 1) {
                    mAdapter.notifyDataSetChanged();
                }
            }
        }
    }

什么是WeakReference？

WeakReference弱引用，与强引用（即我们常说的引用）相对，它的特点是，GC在回收时会忽略掉弱引用，即就算有弱引用指向某对象，但只要该对象没有被强引用指向（实际上多数时候还要求没有软引用，但此处软引用的概念可以忽略），该对象就会在被GC检查到时回收掉。对于上面的代码，用户在关闭Activity之后，就算后台线程还没结束，但由于仅有一条来自Handler的弱引用指向Activity，所以GC仍然会在检查的时候把Activity回收掉。这样，内存泄露的问题就不会出现了。

在此推荐一个写好的Handler WeakReference，可直接使用。
WeakHandler.java

import java.lang.ref.WeakReference;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.VisibleForTesting;

public class WeakHandler {
    private final Handler.Callback mCallback; // hard reference to Callback. We need to keep callback in memory
    private final ExecHandler mExec;
    private Lock mLock = new ReentrantLock();
    @VisibleForTesting
    final ChainedRef mRunnables = new ChainedRef(mLock, null);

    public WeakHandler() {
        mCallback = null;
        mExec = new ExecHandler();
    }

    public WeakHandler(@Nullable Handler.Callback callback) {
        mCallback = callback; // Hard referencing body
        mExec = new ExecHandler(new WeakReference<>(callback)); // Weak referencing inside ExecHandler
    }

    public WeakHandler(@NonNull Looper looper) {
        mCallback = null;
        mExec = new ExecHandler(looper);
    }

    public WeakHandler(@NonNull Looper looper, @NonNull Handler.Callback callback) {
        mCallback = callback;
        mExec = new ExecHandler(looper, new WeakReference<>(callback));
    }

    public final boolean post(@NonNull Runnable r) {
        return mExec.post(wrapRunnable(r));
    }

    public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {
        return mExec.postAtTime(wrapRunnable(r), uptimeMillis);
    }

    public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) {
        return mExec.postAtTime(wrapRunnable(r), token, uptimeMillis);
    }

    public final boolean postDelayed(Runnable r, long delayMillis) {
        return mExec.postDelayed(wrapRunnable(r), delayMillis);
    }

    public final boolean postAtFrontOfQueue(Runnable r) {
        return mExec.postAtFrontOfQueue(wrapRunnable(r));
    }

    public final void removeCallbacks(Runnable r) {
        final WeakRunnable runnable = mRunnables.remove(r);
        if (runnable != null) {
            mExec.removeCallbacks(runnable);
        }
    }

    public final void removeCallbacks(Runnable r, Object token) {
        final WeakRunnable runnable = mRunnables.remove(r);
        if (runnable != null) {
            mExec.removeCallbacks(runnable, token);
        }
    }

    public final boolean sendMessage(Message msg) {
        return mExec.sendMessage(msg);
    }

    public final boolean sendEmptyMessage(int what) {
        return mExec.sendEmptyMessage(what);
    }

    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        return mExec.sendEmptyMessageDelayed(what, delayMillis);
    }

    public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        return mExec.sendEmptyMessageAtTime(what, uptimeMillis);
    }

    public final boolean sendMessageDelayed(Message msg, long delayMillis) {
        return mExec.sendMessageDelayed(msg, delayMillis);
    }

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        return mExec.sendMessageAtTime(msg, uptimeMillis);
    }

    public final boolean sendMessageAtFrontOfQueue(Message msg) {
        return mExec.sendMessageAtFrontOfQueue(msg);
    }

    public final void removeMessages(int what) {
        mExec.removeMessages(what);
    }

    public final void removeMessages(int what, Object object) {
        mExec.removeMessages(what, object);
    }

    public final void removeCallbacksAndMessages(Object token) {
        mExec.removeCallbacksAndMessages(token);
    }

    public final boolean hasMessages(int what) {
        return mExec.hasMessages(what);
    }

    public final boolean hasMessages(int what, Object object) {
        return mExec.hasMessages(what, object);
    }

    public final Looper getLooper() {
        return mExec.getLooper();
    }

    private WeakRunnable wrapRunnable(@NonNull Runnable r) {
        //noinspection ConstantConditions
        if (r == null) {
            throw new NullPointerException("Runnable can't be null");
        }
        final ChainedRef hardRef = new ChainedRef(mLock, r);
        mRunnables.insertAfter(hardRef);
        return hardRef.wrapper;
    }

    private static class ExecHandler extends Handler {
        private final WeakReference<Callback> mCallback;

        ExecHandler() {
            mCallback = null;
        }

        ExecHandler(WeakReference<Callback> callback) {
            mCallback = callback;
        }

        ExecHandler(Looper looper) {
            super(looper);
            mCallback = null;
        }

        ExecHandler(Looper looper, WeakReference<Callback> callback) {
            super(looper);
            mCallback = callback;
        }

        @Override
        public void handleMessage(@NonNull Message msg) {
            if (mCallback == null) {
                return;
            }
            final Callback callback = mCallback.get();
            if (callback == null) { // Already disposed
                return;
            }
            callback.handleMessage(msg);
        }
    }

    static class WeakRunnable implements Runnable {
        private final WeakReference<Runnable> mDelegate;
        private final WeakReference<ChainedRef> mReference;

        WeakRunnable(WeakReference<Runnable> delegate, WeakReference<ChainedRef> reference) {
            mDelegate = delegate;
            mReference = reference;
        }

        @Override
        public void run() {
            final Runnable delegate = mDelegate.get();
            final ChainedRef reference = mReference.get();
            if (reference != null) {
                reference.remove();
            }
            if (delegate != null) {
                delegate.run();
            }
        }
    }

    static class ChainedRef {
        @Nullable
        ChainedRef next;
        @Nullable
        ChainedRef prev;
        @NonNull
        final Runnable runnable;
        @NonNull
        final WeakRunnable wrapper;

        @NonNull
        Lock lock;

        public ChainedRef(@NonNull Lock lock, @NonNull Runnable r) {
            this.runnable = r;
            this.lock = lock;
            this.wrapper = new WeakRunnable(new WeakReference<>(r), new WeakReference<>(this));
        }

        public WeakRunnable remove() {
            lock.lock();
            try {
                if (prev != null) {
                    prev.next = next;
                }
                if (next != null) {
                    next.prev = prev;
                }
                prev = null;
                next = null;
            } finally {
                lock.unlock();
            }
            return wrapper;
        }

        public void insertAfter(@NonNull ChainedRef candidate) {
            lock.lock();
            try {
                if (this.next != null) {
                    this.next.prev = candidate;
                }

                candidate.next = this.next;
                this.next = candidate;
                candidate.prev = this;
            } finally {
                lock.unlock();
            }
        }

        @Nullable
        public WeakRunnable remove(Runnable obj) {
            lock.lock();
            try {
                ChainedRef curr = this.next; // Skipping head
                while (curr != null) {
                    if (curr.runnable == obj) { // We do comparison exactly how Handler does inside
                        return curr.remove();
                    }
                    curr = curr.next;
                }
            } finally {
                lock.unlock();
            }
            return null;
        }
    }
}

by anonymous