JDK中的设计模式之观察者模式

拼搏现实的明天。 2022-06-04 06:41 293阅读 0赞

> The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

JDK中的

> java.util.Observer  
> java.util.Observable

实现了观察者模式

其中接口**Observer**定义了订阅者**update**消息接收接口

> void update(Observable o, Object arg);

**Observable**类定义了订阅者订阅、取消和通知接口

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 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);
        }

**下面是一个JDK 观察者模式的一个简单应用，代码示例如下：**

### **消费发布者** ###

重载订阅、取消、通知接口，直接调用父类Observable相关接口

import java.util.Observable;
    import java.util.Observer;
    public class Publisher extends Observable { 
        @Override
        public synchronized void addObserver(Observer o) {
            super.addObserver(o);
        }
    
        @Override
        public synchronized void deleteObserver(Observer o) {
            super.deleteObserver(o);
        }
    
        @Override
        public void notifyObservers(Object arg) {
            super.setChanged();
            super.notifyObservers(arg);
        }
    
        private String name;
    
        public Publisher(String name) {
            this.name = name;
        }
    
        @Override
        public String toString() {
            return name;
        }
    }

### **消息订阅者** ###

重载消息接收update方法

import java.util.Observable;
    import java.util.Observer;
    
    public class Subscriber implements Observer { 
        private String name;
    
        public Subscriber(String name) {
            this.name = name;
        }
    
        @Override
        public void update(Observable o, Object arg) {
            System.out.println(String.format("%s received message %s from %s", name, arg, o.toString()));
        }
    }

### **测试调用** ###

public class Main {
        public static void main(String[] args) {
            Subscriber s0 = new Subscriber("s0");
            Subscriber s1 = new Subscriber("s1");
    
            Publisher observer = new Publisher("北京气象台");
            observer.addObserver(s0);
            observer.addObserver(s1);
            observer.notifyObservers("明日大雨");
    
        }
    }

### **执行结果** ###

> s1 received message 明日大雨 from 北京气象台  
> s0 received message 明日大雨 from 北京气象台