浅谈java中内置的观察者模式与动态代理的实现

Posted 2020-10-02 专注Java后端技术

一.关于观察者模式

1.将观察者与被观察者分离开来，当被观察者发生变化时，将通知所有观察者，观察者会根据这些变化做出对应的处理。

2.jdk里已经提供对应的Observer接口(观察者接口)与Observable(被观察者类)用于实现观察者模式

3.关于Observer接口，该接口只有一个update方法，当被观察者发生相关变化时，会通知所有的观察者，观察者接受到通知时，调用update方法进行处理。贴出源代码：

 1 /*
 2  * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved.
 3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 4  *
 5  *
 6  *
 7  *
 8  *
 9  *
10  *
11  *
12  *
13  *
14  *
15  *
16  *
17  *
18  *
19  *
20  *
21  *
22  *
23  *
24  */
25 package java.util;
26 
27 /**
28  * A class can implement the <code>Observer</code> interface when it
29  * wants to be informed of changes in observable objects.
30  *
31  * @author  Chris Warth
32  * @see     java.util.Observable
33  * @since   JDK1.0
34  */
35 public interface Observer {
36     /**
37      * This method is called whenever the observed object is changed. An
38      * application calls an <tt>Observable</tt> object‘s
39      * <code>notifyObservers</code> method to have all the object‘s
40      * observers notified of the change.
41      *
42      * @param   o     the observable object.
43      * @param   arg   an argument passed to the <code>notifyObservers</code>
44      *                 method.
45      */
46     void update(Observable o, Object arg);
47 }

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4：关于被观察者Observable的常用方法：

    1.  public synchronized void addObserver(Observer o)；//添加观察者对象

    2. public void notifyObservers()；//通知所有观察者

    3. protected synchronized void setChanged()；//设置观察项已经做出改变，此方法很重要

贴出源代码，注意内部实现：

  1 /*
  2  * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
  3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  *
 24  */
 25 
 26 package java.util;
 27 
 28 /**
 29  * This class represents an observable object, or "data"
 30  * in the model-view paradigm. It can be subclassed to represent an
 31  * object that the application wants to have observed.
 32  * <p>
 33  * An observable object can have one or more observers. An observer
 34  * may be any object that implements interface <tt>Observer</tt>. After an
 35  * observable instance changes, an application calling the
 36  * <code>Observable</code>‘s <code>notifyObservers</code> method
 37  * causes all of its observers to be notified of the change by a call
 38  * to their <code>update</code> method.
 39  * <p>
 40  * The order in which notifications will be delivered is unspecified.
 41  * The default implementation provided in the Observable class will
 42  * notify Observers in the order in which they registered interest, but
 43  * subclasses may change this order, use no guaranteed order, deliver
 44  * notifications on separate threads, or may guarantee that their
 45  * subclass follows this order, as they choose.
 46  * <p>
 47  * Note that this notification mechanism has nothing to do with threads
 48  * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
 49  * mechanism of class <tt>Object</tt>.
 50  * <p>
 51  * When an observable object is newly created, its set of observers is
 52  * empty. Two observers are considered the same if and only if the
 53  * <tt>equals</tt> method returns true for them.
 54  *
 55  * @author  Chris Warth
 56  * @see     java.util.Observable#notifyObservers()
 57  * @see     java.util.Observable#notifyObservers(java.lang.Object)
 58  * @see     java.util.Observer
 59  * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
 60  * @since   JDK1.0
 61  */
 62 public class Observable {
 63     private boolean changed = false;
 64     private Vector<Observer> obs;
 65 
 66     /** Construct an Observable with zero Observers. */
 67 
 68     public Observable() {
 69         obs = new Vector<>();
 70     }
 71 
 72     /**
 73      * Adds an observer to the set of observers for this object, provided
 74      * that it is not the same as some observer already in the set.
 75      * The order in which notifications will be delivered to multiple
 76      * observers is not specified. See the class comment.
 77      *
 78      * @param   o   an observer to be added.
 79      * @throws NullPointerException   if the parameter o is null.
 80      */
 81     public synchronized void addObserver(Observer o) {
 82         if (o == null)
 83             throw new NullPointerException();
 84         if (!obs.contains(o)) {
 85             obs.addElement(o);
 86         }
 87     }
 88 
 89     /**
 90      * Deletes an observer from the set of observers of this object.
 91      * Passing <CODE>null</CODE> to this method will have no effect.
 92      * @param   o   the observer to be deleted.
 93      */
 94     public synchronized void deleteObserver(Observer o) {
 95         obs.removeElement(o);
 96     }
 97 
 98     /**
 99      * If this object has changed, as indicated by the
100      * <code>hasChanged</code> method, then notify all of its observers
101      * and then call the <code>clearChanged</code> method to
102      * indicate that this object has no longer changed.
103      * <p>
104      * Each observer has its <code>update</code> method called with two
105      * arguments: this observable object and <code>null</code>. In other
106      * words, this method is equivalent to:
107      * <blockquote><tt>
108      * notifyObservers(null)</tt></blockquote>
109      *
110      * @see     java.util.Observable#clearChanged()
111      * @see     java.util.Observable#hasChanged()
112      * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
113      */
114     public void notifyObservers() {
115         notifyObservers(null);
116     }
117 
118     /**
119      * If this object has changed, as indicated by the
120      * <code>hasChanged</code> method, then notify all of its observers
121      * and then call the <code>clearChanged</code> method to indicate
122      * that this object has no longer changed.
123      * <p>
124      * Each observer has its <code>update</code> method called with two
125      * arguments: this observable object and the <code>arg</code> argument.
126      *
127      * @param   arg   any object.
128      * @see     java.util.Observable#clearChanged()
129      * @see     java.util.Observable#hasChanged()
130      * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
131      */
132     public void notifyObservers(Object arg) {
133         /*
134          * a temporary array buffer, used as a snapshot of the state of
135          * current Observers.
136          */
137         Object[] arrLocal;
138 
139         synchronized (this) {
140             /* We don‘t want the Observer doing callbacks into
141              * arbitrary code while holding its own Monitor.
142              * The code where we extract each Observable from
143              * the Vector and store the state of the Observer
144              * needs synchronization, but notifying observers
145              * does not (should not).  The worst result of any
146              * potential race-condition here is that:
147              * 1) a newly-added Observer will miss a
148              *   notification in progress
149              * 2) a recently unregistered Observer will be
150              *   wrongly notified when it doesn‘t care
151              */
152             if (!changed)
153                 return;
154             arrLocal = obs.toArray();
155             clearChanged();
156         }
157 
158         for (int i = arrLocal.length-1; i>=0; i--)
159             ((Observer)arrLocal[i]).update(this, arg);
160     }
161 
162     /**
163      * Clears the observer list so that this object no longer has any observers.
164      */
165     public synchronized void deleteObservers() {
166         obs.removeAllElements();
167     }
168 
169     /**
170      * Marks this <tt>Observable</tt> object as having been changed; the
171      * <tt>hasChanged</tt> method will now return <tt>true</tt>.
172      */
173     protected synchronized void setChanged() {
174         changed = true;
175     }
176 
177     /**
178      * Indicates that this object has no longer changed, or that it has
179      * already notified all of its observers of its most recent change,
180      * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
181      * This method is called automatically by the
182      * <code>notifyObservers</code> methods.
183      *
184      * @see     java.util.Observable#notifyObservers()
185      * @see     java.util.Observable#notifyObservers(java.lang.Object)
186      */
187     protected synchronized void clearChanged() {
188         changed = false;
189     }
190 
191     /**
192      * Tests if this object has changed.
193      *
194      * @return  <code>true</code> if and only if the <code>setChanged</code>
195      *          method has been called more recently than the
196      *          <code>clearChanged</code> method on this object;
197      *          <code>false</code> otherwise.
198      * @see     java.util.Observable#clearChanged()
199      * @see     java.util.Observable#setChanged()
200      */
201     public synchronized boolean hasChanged() {
202         return changed;
203     }
204 
205     /**
206      * Returns the number of observers of this <tt>Observable</tt> object.
207      *
208      * @return  the number of observers of this object.
209      */
210     public synchronized int countObservers() {
211         return obs.size();
212     }
213 }

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5.举一个例子吧：当婴儿哭泣时，则通知家人来哄宝宝，那么这里很明显婴儿是一个被观察者，当婴儿哭泣时，立刻通知家人(观察者)

package com.bdqn.s2.javaoop.study.proxy;

import java.lang.reflect.Proxy;
import java.util.Observable;
import java.util.Observer;

/**
 * 婴儿类，被观察者
 */
public class Baby extends Observable {

    private int hungry;

    private String name;


    public String getName() {
        return name;
    }

    public Baby(String name, int hungry) {
        this.hungry = hungry;
        this.name = name;
        addObserver(new Parents());//添加观察者对象，需要家长监管
    }

    /**
     * 婴儿开始哭泣
     */
    public void cry() {
        if (hungry < 100) {
            System.out.printf("baby%s饿了，开始哭泣...%n", name);
            setChanged();//饥饿值过低，触发变化，此方法必须被调用
            notifyObservers();//通知观察者
        }
    }
}

/**
 * 家长，观察者
 */
class Parents implements Observer {
    @Override
    public void update(Observable o, Object arg) {

        if (o instanceof Baby) {
            Baby baby = (Baby) o;
            System.out.println(baby.getName()+"开始哭泣，赶紧哄宝宝啦");
        }
    }

}

public class Main {

    public static void main(String[] args) {
        Baby baby = new Baby("豆豆",9);
        baby.cry();
    }
}

/*
输出结果
baby豆豆饿了，开始哭泣...
豆豆开始哭泣，赶紧哄宝宝啦
*/

 

二 关于动态代理模式

1）代理模式是设计模式中非常常见的一种模式，这种模式可以实现对原有方法的扩展，举个例子经纪人可以替明星们办理一些事情，那么此时经纪人可以视为明星的代理。

2）代理模式可以分为静态代理和动态代理，在这里我们只对JDK提供的动态代理进行讨论。

3）JDK提供的代理模式一定是针对接口进行的代理，针对类的代理可以自行参考cglib框架 

4）InvocationHandler:是代理实例的调用处理程序 实现的接口。 每个代理实例都具有一个关联的调用处理程序。对代理实例调用方法时，将对方法调用进行编码并将其指派到它的调用处理程序的 invoke 方法。

//proxy:代理类，method：代理执行的方法 args:方法参数
public Object invoke(Object proxy, Method method, Object[] args);

5）Proxy:该类主要是获取或者新创建动态代理对象

//该方法主要用于获取代理对象，注意一定是针对接口进行代理 
public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException

6）针对上述例子进行改造：添加保姆类并改造Baby类的构造方法：

package com.bdqn.s2.javaoop.study.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Observer;

/**
 * 保姆类
 */
public class Nanny implements InvocationHandler {

    private Observer parents;

    public Nanny(){
        parents = new Parents();
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("保姆开始照顾孩子");
        Object object = method.invoke(parents, args);
        return object;
    }
}

Baby类构造函数改造：

public Baby(String name, int hungry) {
        this.hungry = hungry;
        this.name = name;
        addObserver((Observer) Proxy.newProxyInstance(Baby.class.getClassLoader(),new Class[]{Observer.class},new Nanny()));
    }

输出结果：

baby豆豆饿了，开始哭泣...
保姆开始照顾孩子
豆豆开始哭泣，赶紧哄宝宝啦