目录

1.核心思想

2.成员

3.优点

3.代码实现

3.1 主题接口和具体主题

3.2 观察者接口和具体观察者

3.3 测试类

4.源码分析

4.1 Observable类

4.2 小偷类

4.3 警察类

4.4 测试类


1.核心思想

定义了一对多的关系,当一个对象的状态发生改变,依赖于他的对象也会收到通知改变。

2.成员

Subject(主题)

Subject是被观察的对象。它包含了一个观察者列表,可以增加或删除观察者对象。当它的状态发生变化时,会通知所有注册过的观察者。

Observer(观察者)

Observer是观察主题的对象。当观察的主题状态发生变化时,Observer会接收到通知并进行相应的更新操作。

3.优点

解耦:主题和观察者之间是松耦合的,主题只知道观察者实现了特定的接口,不需要了解观察者的具体实现。

扩展性:可以灵活地增加新的观察者,或者移除现有的观察者,而不需要修改主题的代码。

支持广播通信:主题对象可以向所有注册的观察者同时发送通知,观察者收到通知后可以自行处理。

消息队列系统:发布-订阅模型中的订阅者即为观察者。

3.代码实现

3.1 主题接口和具体主题

public interface Subject {
    void  add(Observer observer);
    void delete(Observer observer);
    void update(String message);
}

public class ZookperSubject implements Subject{
    private List<Observer> observerList = new ArrayList<Observer>();

    @Override
    public void add(Observer observer) {
        observerList.add(observer);
    }

    @Override
    public void delete(Observer observer) {
        observerList.remove(observer);

    }

    @Override
    public void update(String message) {
        for (Observer observer : observerList) {
            observer.attach(message);
        }
    }
}

3.2 观察者接口和具体观察者

public interface Observer {
    void attach(String message);
}

public class ClientObserver implements Observer{
    private String ip;
    public ClientObserver(String ip) {
        this.ip = ip;
    }

    @Override
    public void attach(String message) {
        System.out.println("客户端"+ip+message);
    }
}

3.3 测试类

public class Test {
    public static void main(String[] args) {
        //创建注册中心
        ZookperSubject zookperSubject = new ZookperSubject();
        ClientObserver clientObserver = new ClientObserver("222.22.222.2");
        zookperSubject.add(new ClientObserver("111.11.111.1"));
        zookperSubject.add(clientObserver);
        zookperSubject.add(new ClientObserver("333.33.333.3"));
        zookperSubject.update("订阅");
        zookperSubject.delete(clientObserver);
    }
}

4.源码分析

4.1 Observable类

public class Observable {
    private boolean changed = false;
    private Vector<Observer> obs;//线程安全的list

    /** 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) {//新增观察者
        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() {//通知方法
        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) //需要我们调用方法给值 setChanged
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);//update调用的是Observer接口实现的方法例子中的警察
    }

    /**
     * Clears the observer list so that this object no longer has any observers.
     */
    public synchronized void deleteObservers() {//删除观察者
        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() {//update前更定状态
        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() {//取消通知
        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() {//获取通知状态
        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() {//获取观察者个数
        return obs.size();
    }
}

4.2 小偷类

public class Thief extends Observable {
    private String name;

    public Thief(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void steal(){
        System.out.println(name +"动手了");
        super.setChanged();//给观察者的前置状态
        super.notifyObservers();
    }
}

4.3 警察类

public class Police implements Observer {
    private String name;

    public Police(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("收手吧,"+((Thief)o).getName()+"外面都是警察");
    }
}

4.4 测试类

public class Test {
    public static void main(String[] args) {
        Thief thief = new Thief("阿祖");

        Police sir = new Police("sir");
        thief.addObserver(sir);

        thief.steal();
    }
}

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