java中的观察者模式
今天来简单介绍下java中的观察者模式，所谓观察者模式通俗理解就是，订阅关系。那么就存在订阅者和发布者两个实体，首先订阅者关注发布者，发布者可以发布消息通知所有订阅者。下面分两步解析java中的观察者模式：1.源码解读 2.demo展示
1.UML解释
1.1 Observable(java class): 被观察者（发布者Publisher）
源码：
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.
*
* An observable object can have one or more observers. An observer
* may be any object that implements interface Observer. After an
* observable instance changes, an application calling the
* Observable's notifyObservers method
* causes all of its observers to be notified of the change by a call
* to their update method.
*
* 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.
*
* Note that this notification mechanism has nothing to do with threads
* and is completely separate from the wait and notify
* mechanism of class Object.
*
* When an observable object is newly created, its set of observers is
* empty. Two observers are considered the same if and only if the
* equals 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 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 null 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
* hasChanged method, then notify all of its observers
* and then call the clearChanged method to
* indicate that this object has no longer changed.
*
* Each observer has its update method called with two
* arguments: this observable object and null. In other
* words, this method is equivalent to:
*
* notifyObservers(null)
*
* @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
* hasChanged method, then notify all of its observers
* and then call the clearChanged method to indicate
* that this object has no longer changed.
*
* Each observer has its update method called with two
* arguments: this observable object and the arg 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 Observable object as having been changed; the
* hasChanged method will now return true.
*/
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 hasChanged method will now return false.
* This method is called automatically by the
* notifyObservers 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 true if and only if the setChanged
* method has been called more recently than the
* clearChanged method on this object;
* false otherwise.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#setChanged()
*/
public synchronized boolean hasChanged() {
return changed;
}
/**
* Returns the number of observers of this Observable object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
}
1.2 Observer(java interface)：观察者（订阅者Subscriber）
源码：
package java.util;
/**
* A class can implement the Observer 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 Observable object's
* notifyObservers method to have all the object's
* observers notified of the change.
*
* @param o the observable object.
* @param arg an argument passed to the notifyObservers
* method.
*/
void update(Observable o, Object arg);
}
2.demo：
Publisher:
package com.lin.DesignPatterns.Observer;
import java.util.Observable;
/**
* @Description:
* @Author: lingx
* @Date: Created in 2019/7/21 11:54
* @Modified By:
**/
public class Publisher extends Observable {
private String data="";
public String getData(){
return data;
}
public void setData(String data){
if(!this.data.equals(data)){
this.data=data;
setChanged(); //改变通知这状态
}
notifyObservers();
}
}
Subscriber:
package com.lin.DesignPatterns.Observer;
import java.util.Observable;
import java.util.Observer;
/**
* @Description:
* @Author: lingx
* @Date: Created in 2019/7/21 11:51
* @Modified By:
**/
public class Subscriber implements Observer {
public Subscriber(Observable o){
o.addObserver(this);
}
@Override
public void update(Observable o, Object arg) {
System.out.println("收到通知:"+((Publisher)o).getData());
}
}
Test:
package com.lin.DesignPatterns.Observer;
/**
* @Description:
* @Author: lingx
* @Date: Created in 2019/7/21 11:57
* @Modified By:
**/
public class Test {
public static void main(String[] args) {
Publisher publisher = new Publisher();
Subscriber subscriber = new Subscriber(publisher);
publisher.setData("hello world");
}
}