浅谈java中内置的观察者模式与动态代理的实现

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：

/*
 * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
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 */
package java.util;

/**
 * A class can implement the <code>Observer</code> interface when it
 * wants to be informed of changes in observable objects.
 *
 * @author  Chris Warth
 * @see     java.util.Observable
 * @since   JDK1.0
 */
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：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：

/*
 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
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 *
 */

package java.util;

/**
 * This class represents an observable object, or "data"
 * in the model-view paradigm. It can be subclassed to represent an
 * object that the application wants to have observed.
 * <p>
 * An observable object can have one or more observers. An observer
 * may be any object that implements interface <tt>Observer</tt>. After an
 * observable instance changes, an application calling the
 * <code>Observable</code>'s <code>notifyObservers</code> method
 * causes all of its observers to be notified of the change by a call
 * to their <code>update</code> method.
 * <p>
 * The order in which notifications will be delivered is unspecified.
 * The default implementation provided in the Observable class will
 * notify Observers in the order in which they registered interest, but
 * subclasses may change this order, use no guaranteed order, deliver
 * notifications on separate threads, or may guarantee that their
 * subclass follows this order, as they choose.
 * <p>
 * Note that this notification mechanism has nothing to do with threads
 * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
 * mechanism of class <tt>Object</tt>.
 * <p>
 * When an observable object is newly created, its set of observers is
 * empty. Two observers are considered the same if and only if the
 * <tt>equals</tt> method returns true for them.
 *
 * @author  Chris Warth
 * @see     java.util.Observable#notifyObservers()
 * @see     java.util.Observable#notifyObservers(java.lang.Object)
 * @see     java.util.Observer
 * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
 * @since   JDK1.0
 */
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();
    }
}

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）由于JDK提供的代理模式所代理的类继承了Proxy，因此我们只能接口进行代理，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