观察者模式--众多通知，一步到位（行为模式06）

什么是观察者模式
定义对象间一种一对多的依赖关系，使得每当一个对象改变时，所有依赖于他的对象都会得到通知并被自动更新。
观察者模式在实际应用中非常的广泛，他的重要作用就是把观察者和被观察者解耦，使得彼此依赖更小。好比我们android开发中给listview设定的Adapter，如果我们改变数据并通知Adapter，则我们listview的所有目标item数据内容就会相应改变。

观察者模式的适用场景

• 可拆分的关联行为
• 事件多级触发
• 跨系统的消息交换，比如消息队列、事件总线

观察者模式用例
客户端可订阅服务器消息通知，服务器有新消息需要通知客户端，则客户端可收到服务器发布的消息。

UML类图（Obserable和Observer是jdk内置类型，我们不用关心实现）

Client类：

public class Client implements Observer{    private String name;    public Client(String name){        this.name = name;    }    @Override    public void update(Observable o, Object arg) {        System.out.println("你好，"+ name +",最新消息："+arg);    }}

Server类：

public class Server extends Observable{    public void postNews(String content){        setChanged();        notifyObservers(content);    }}

测试类：

public class Test {    public static void main(String[] args) {        //服务器        Server server = new Server();        //多个观察者（客户端）        Client client1 = new Client("张三");        Client client2 = new Client("李四");        Client client3 = new Client("王五");        Client client4 = new Client("马六");        //将观察者注册到可观察列表        server.addObserver(client1);        server.addObserver(client2);        server.addObserver(client3);        server.addObserver(client4);        //网站更新，通知观察者        server.postNews("国家新规定，下个月基本工资涨1000！！！");    }}

运行结果：

你好，马六,最新消息：国家新规定，下个月基本工资涨1000！！！你好，王五,最新消息：国家新规定，下个月基本工资涨1000！！！你好，李四,最新消息：国家新规定，下个月基本工资涨1000！！！你好，张三,最新消息：国家新规定，下个月基本工资涨1000！！！

---

---

这里就结束了代码，非常简洁。我们使用的是jdk内部提供的类，由此可见观察者模式的地位是举足轻重的，我们可以看一下源代码：
Observer：

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);}

Observable：

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();    }}

代码注释说的很明白了，总结一下流程，Observer是抽象观察者角色，而Client是Observer的实现类，Server类继承了Observable类有了可观察的功能，Client订阅了Server的可观察对象，每当Server有更新时通过遍历观察者对象，然后给所有观察者发布更新的消息，也就是调用了update方法，这样实现了一对多的通知功能，在这个系统中，Client和Server的通信都是依赖于Observer和Observable这些抽象，没有直接耦合，保证系统高灵活性。

优点：耦合度低，应对业务变化灵活。
缺点：我们可以看到 synchronized字样，说明他是线程安全的，也就是同步执行，我们在效率方面，是按顺序通知观察者的，如果某个卡顿，则其后也就势必出现问题。应考虑采用异步方式优化。