观察者模式Java内存管理进阶篇—

一、什么叫观察者模式

观察者模式是常用的设计模式之一；例如在下载文件时，我们可能会更新图标动画，另外在别的控件显示当前下载进度，下载完成后要对文件进行处理，可能这些处理过程都是不同的业务模块，这些模块的生命周期不一样。

简单来说观察者模式可以理解为对多个回调实体对象的管理；

二、简单案例

下面我们通过一个简单的例子来讲解如何设计观察者模式：
本案例是新建一个线程可以设定定时器，在时间到时发送当前的时间，最后把时间反馈给注册者。

首先我们定义回调接口

public interface ObserverCallback {public void onTime(String textTime);
}

然后在时间触发处管理回调实体

package com.penny.javademo;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;public class Observer implements Runnable {private static final String TAG = Observer.class.getSimpleName();private volatile boolean running = false;private long interval = 1000;private List<ObserverCallback> callbacks = new CopyOnWriteArrayList<>();public Observer(long intervalMillsec) {if (interval < 1) {throw new IllegalArgumentException("interval must be larger than 0. interval: " + interval);}this.interval = intervalMillsec;}public void addCallback(ObserverCallback cb) {if (null == cb) {return;}synchronized (this) { // 防止异步线程同时find然后插入boolean find = false;for (ObserverCallback temp : callbacks) {if (cb == temp) {find = true;break;}}if (!find) {callbacks.add(cb);}}}public void removeCallback(ObserverCallback cb) {synchronized (this) {boolean find = false;for (ObserverCallback temp : callbacks) {if (cb == temp) {find = true;break;}}if (find) {callbacks.remove(cb);}}}public void initEvent() {synchronized (this) {if (! running) {new Thread(this).start();}}}public void terminate() {synchronized (this) {this.running = false;}}@Overridepublic void run() {boolean localRun = running;synchronized (this) {this.running = true;localRun = running;}DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");while (localRun) {// 处理回掉事件for (ObserverCallback cb : callbacks) {cb.onTime(df.format(Calendar.getInstance().getTime()));}try {Thread.sleep(interval);} catch (InterruptedException e) {e.printStackTrace();}synchronized (this) {localRun = running;}}synchronized (this) {this.running = false;}}
}

回调管理者，时间触发为定时发送当前时间；

@Test
public void testObserver() {final String tag = "testObserver";Observer o = new Observer(2000);o.addCallback(new ObserverCallback() {@Overridepublic void onTime(String textTime) {LogUtils.debug(tag, "111 textTime: " + textTime);}});o.addCallback(new ObserverCallback() {@Overridepublic void onTime(String textTime) {LogUtils.debug(tag, "222 textTime: " + textTime);}});o.initEvent();try {Thread.sleep(10000);} catch (InterruptedException e) {e.printStackTrace();}LogUtils.debug(TAG, "testDemo finish.");
}

输出结果：

D/testObserver  111 textTime: 2019-03-16 15:11:06
D/testObserver  222 textTime: 2019-03-16 15:11:06
D/testObserver  111 textTime: 2019-03-16 15:11:08
D/testObserver  222 textTime: 2019-03-16 15:11:08
D/testObserver  111 textTime: 2019-03-16 15:11:10
D/testObserver  222 textTime: 2019-03-16 15:11:10
D/testObserver  111 textTime: 2019-03-16 15:11:12
D/testObserver  222 textTime: 2019-03-16 15:11:12
D/testObserver  111 textTime: 2019-03-16 15:11:14
D/testObserver  222 textTime: 2019-03-16 15:11:14
D/ObserverTest  testDemo finish.Process finished with exit code 0

上述代码虽然功能完善，但是存在一个问题，假如有观察对象注册了callback然后忘记注销，由于定时器一直持有该对象那么就会导致内存泄漏。

三、WeakReference加持

对于内存泄漏这个问题我们可以通过Java软引用来解决。

1. 首先用一个List来存储所有回调callback，该list用软引用WeakReference方式持有callback；

2. 用第二个List在存储强制类型的回调callback，该list直接持有callback；

3. 注册者在向定时器注册回调时可以设置注册引用类型，如果为强引用，那么步骤1、2的List都加入该对象；如果为软引用，那么只有步骤1的List会加入该对象。

4. 定时器触发事件时只需要遍历步骤1的Lit；

5. 注销某个注册者时需要遍历步骤1、2的callback，然后删除之。

package com.penny.javademo;import java.lang.ref.WeakReference;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;public class ObserverWeakRef3 implements Runnable {private static final String TAG = ObserverWeakRef3.class.getSimpleName();private volatile boolean running = false;private long interval = 1000;private List<WeakReference<ObserverCallback>> callbacks = new CopyOnWriteArrayList<>();private List<ObserverCallback> strongList = new CopyOnWriteArrayList<>();public ObserverWeakRef3(long intervalMillsec) {if (interval < 1) {throw new IllegalArgumentException("interval must be larger than 0. interval: " + interval);}this.interval = intervalMillsec;}public void addCallback(ObserverCallback cb, boolean isWeakRef) {if (null == cb) {return;}synchronized (this) { // 防止异步线程同时find然后插入boolean find = false;for (WeakReference<ObserverCallback> ref : callbacks) {if (cb == ref.get()) {find = true;break;}}if (!find) {callbacks.add(new WeakReference<>(cb));if (!isWeakRef) {strongList.add(cb);}}}}public void removeCallback(ObserverCallback cb) {synchronized (this) {boolean find = false;List<WeakReference<ObserverCallback>> delList = new ArrayList<>();ObserverCallback localCallback = null;for (WeakReference<ObserverCallback> ref : callbacks) {localCallback = ref.get();if (null == localCallback || cb == localCallback) {delList.add(ref);}if (cb == localCallback) {find = true;}}if (find) {strongList.remove(cb);}for (WeakReference<ObserverCallback> temp : delList) {callbacks.remove(temp);}delList.clear();}}public void initEvent() {synchronized (this) {if (! running) {new Thread(this).start();}}}public void terminate() {synchronized (this) {this.running = false;}}@Overridepublic void run() {boolean localRun = running;synchronized (this) {this.running = true;localRun = running;}ObserverCallback localCallback;DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");while (localRun) {// 处理回掉事件for (WeakReference<ObserverCallback> cb : callbacks) {localCallback = cb.get();if (null != localCallback) {localCallback.onTime(df.format(Calendar.getInstance().getTime()));} else {LogUtils.debug(TAG, "localCallback: null");}}try {Thread.sleep(interval);} catch (InterruptedException e) {e.printStackTrace();}synchronized (this) {localRun = running;}}synchronized (this) {this.running = false;}}
}

测试代码:

@Test
public void testObserverWeakRef3() {final String tag = "testObserverWeakRef3";ObserverWeakRef2 o = new ObserverWeakRef2(2000);for (int i = 0; i < 10; i++) {final String text = String.format("%d%d%d", i, i, i);o.addCallback(new ObserverCallback() {byte[] buf = new byte[1024 * 1024 * 10]; // 增大空间,诱发虚拟机回收内存@Overridepublic void onTime(String textTime) {LogUtils.debug(tag, text + " textTime: " + textTime);}}, i > 4);}o.initEvent();try {Thread.sleep(6000);System.gc(); // 手动触发gc，最终执行还是靠虚拟机的策略Thread.sleep(6000);} catch (InterruptedException e) {e.printStackTrace();}LogUtils.debug(TAG, "testDemo finish.");
}

输出结果。

D/testObserverWeakRef3  000 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  555 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  666 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  777 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  888 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:36:55
D/testObserverWeakRef3  000 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  555 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  666 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  777 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  888 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:36:57
D/testObserverWeakRef3  000 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  555 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  666 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  777 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  888 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:36:59
D/testObserverWeakRef3  000 textTime: 2019-03-16 15:37:01
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:37:01
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:37:01
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:37:01
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:37:01
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:37:01
D/testObserverWeakRef3  000 textTime: 2019-03-16 15:37:03
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:37:03
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:37:03
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:37:03
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:37:03
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:37:03
D/testObserverWeakRef3  000 textTime: 2019-03-16 15:37:05
D/testObserverWeakRef3  111 textTime: 2019-03-16 15:37:05
D/testObserverWeakRef3  222 textTime: 2019-03-16 15:37:05
D/testObserverWeakRef3  333 textTime: 2019-03-16 15:37:05
D/testObserverWeakRef3  444 textTime: 2019-03-16 15:37:05
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/ObserverWeakRef3  localCallback: null
D/testObserverWeakRef3  999 textTime: 2019-03-16 15:37:05
D/ObserverTest  testDemo finish.Process finished with exit code 0

通过打印输出可以看到，通过WeakReference软引用的callback在 2019-03-16 15:37:01时就被系统回收了，因此找不到对应的callback。证明用软引用是有效的，可以避免内存泄漏；

四、总结

当然，最重要的还是调用者本身要懂得对注册者的生命周期类型，如果把原本弱周期的注册者放到定时器是使用强引用方式注册，然后又不注销，那么也会导致内存泄漏。对于这种情况我只能说——我也很无奈啊！！！