Java基础笔记 - JDK中提供的观察者模式支持

JDK中也提供了对观察者模式的内置支持。

1、JDK中观察者模式的实现原理：

Observable类用于创建可以观测到你的程序中其他部分的子类。当这种子类的对象发生变化时，观测类被通知。观测类必须实现定义了update()方法的Observer接口。当一个观测程序被通知到一个被观察对象的改变时，update()方法被调用。

2、Observable类：

public class Observable extends Object

此类表示模型视图范例中的 observable 对象，或者说“数据”。可将其子类化，表示应用程序想要观察的对象。

一个 observable 对象可以有一个或多个观察者。观察者可以是实现了 Observer 接口的任意对象。一个 observable 实例改变后，调用 Observable 的 notifyObservers 方法的应用程序会通过调用观察者的 update 方法来通知观察者该实例发生了改变。

2.1、相关方法：

public class Observable {
    private boolean changed = false;
    private Vector<Observer> obs;

    /** Construct an Observable with zero Observers. */

    public Observable() {
        obs = new Vector<>();
    }

    /**
     * Adds an observer to the set of observers for this object, provided
     * that it is not the same as some observer already in the set.
     * The order in which notifications will be delivered to multiple
     * observers is not specified. See the class comment.
     *
     * @param   o   an observer to be added.
     * @throws NullPointerException   if the parameter o is null.
     */
    public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
    }

    /**
     * Deletes an observer from the set of observers of this object.
     * Passing <CODE>null</CODE> to this method will have no effect.
     * @param   o   the observer to be deleted.
     */
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    }

    /**
     * If this object has changed, as indicated by the
     * <code>hasChanged</code> method, then notify all of its observers
     * and then call the <code>clearChanged</code> method to
     * indicate that this object has no longer changed.
     * <p>
     * Each observer has its <code>update</code> method called with two
     * arguments: this observable object and <code>null</code>. In other
     * words, this method is equivalent to:
     * <blockquote><tt>
     * notifyObservers(null)</tt></blockquote>
     *
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * If this object has changed, as indicated by the
     * <code>hasChanged</code> method, then notify all of its observers
     * and then call the <code>clearChanged</code> method to indicate
     * that this object has no longer changed.
     * <p>
     * Each observer has its <code>update</code> method called with two
     * arguments: this observable object and the <code>arg</code> argument.
     *
     * @param   arg   any object.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
     */
    public void notifyObservers(Object arg) {
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
        Object[] arrLocal;

        synchronized (this) {
            /* We don't want the Observer doing callbacks into
             * arbitrary code while holding its own Monitor.
             * The code where we extract each Observable from
             * the Vector and store the state of the Observer
             * needs synchronization, but notifying observers
             * does not (should not).  The worst result of any
             * potential race-condition here is that:
             * 1) a newly-added Observer will miss a
             *   notification in progress
             * 2) a recently unregistered Observer will be
             *   wrongly notified when it doesn't care
             */
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }

    /**
     * Clears the observer list so that this object no longer has any observers.
     */
    public synchronized void deleteObservers() {
        obs.removeAllElements();
    }

    /**
     * Marks this <tt>Observable</tt> object as having been changed; the
     * <tt>hasChanged</tt> method will now return <tt>true</tt>.
     */
    protected synchronized void setChanged() {
        changed = true;
    }

    /**
     * Indicates that this object has no longer changed, or that it has
     * already notified all of its observers of its most recent change,
     * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
     * This method is called automatically by the
     * <code>notifyObservers</code> methods.
     *
     * @see     java.util.Observable#notifyObservers()
     * @see     java.util.Observable#notifyObservers(java.lang.Object)
     */
    protected synchronized void clearChanged() {
        changed = false;
    }

    /**
     * Tests if this object has changed.
     *
     * @return  <code>true</code> if and only if the <code>setChanged</code>
     *          method has been called more recently than the
     *          <code>clearChanged</code> method on this object;
     *          <code>false</code> otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

    /**
     * Returns the number of observers of this <tt>Observable</tt> object.
     *
     * @return  the number of observers of this object.
     */
    public synchronized int countObservers() {
        return obs.size();
    }
}

3、Observer接口：

public interface Observer

一个可在观察者要得到 observable 对象更改通知时可实现 Observer 接口的类。

3.1、相关方法：

public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    void update(Observable o, Object arg);
}

4、被观察对象的工作原理按照如下规则：

如果它被改变了，它必须调用setChanged()方法。

当它准备通知观测程序它的改变时，它必须调用notifyObservers()方法。该方法导致了在观测对象中对update()方法的调用。

注意：当对象在调用notifyObservers()方法之前没有调用setChanged()方法，就不会有动作发生。也就是说在update()方法被调用之前，被观测对象必须调用setChanged()和notifyObservers()两个方法。

5、下面写一个例子实现从被观察对象向观察者发送执行动作：

其中，update方法，第一个参数为被观察者，也叫事件源；第二个参数，也就是sendMessage方法参数，为事件对象。

被观察者：

/**
 * 被观察者
 */
public class Monitored extends Observable {
  
    public void sendMessage(String str){
        // 需要先调用setChanged方法
        this.setChanged();
        this.notifyObservers(str);
    }
}

观察者1：

/**
 * 观察者1
 */
public class Watcher1 implements Observer {
  
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Watcher1 receive:" + arg);
    }
}

观察者2：

/**
 * 观察者2
 */
public class Watcher2 implements Observer {
  
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Watcher2 receive:" + arg);
    }
}

在客户端使用观察者角色：

public static void main(String[] args) {
    //创建被观察对象
    Monitored monitored = new Monitored();
    //创建两个观察者
    Watcher1 watcher1 = new Watcher1();
    Watcher2 watcher2 = new Watcher2();
    //添加观察者
    monitored.addObserver(watcher1);
    monitored.addObserver(watcher2);
    //被观察者发送消息给观察者
    monitored.sendMessage("这是一条消息");
}