观察者模式

当对象间存在多对一的依赖关系时，可以使用观察者模式（Observer Pattern）。观察者模式属于行为型模式。

概况的说就是：一个类（被观察者Observable）管理着所有依赖于它的类（观察者Observer），并且它状态变化时会主动给这些依赖它的类发出通知。

---

自定义观察者模式的demo：

观察者接口：

public interface Observer {    void update(Observable o);}

具体的观察者类：

public class ObserverMomo implements Observer {    @Override    public void update(Observable observable) {        System.out.println("观察者1知道了" + observable.getClass().getSimpleName() + "发生了变化");    }}

被观察者：

public class Observable {    List<Observer> observers = new ArrayList<>();    public void addObserver(Observer observer) {        observers.add(observer);    }    public void changed() {        notifyObservers();    }    private void notifyObservers() {        for (Observer observer : observers) {            observer.update(this);        }    }}

调用：

Observable observable = new Observable();observable.addObserver(new ObserverMomo());observable.changed();

---

使用JDk提供的java.util.Observable和java.util.Observer写的的读者订阅作者的demo:

读者类：

public class Reader implements Observer {    private String readerName;    public Reader(String readerName) {        this.readerName = readerName;    }    // 订阅作者，也就是通过作者姓名在WriterManager中获取到作者对象，然后把自己添加到这个作者的观察者列表中    public void subscribe(String writerName) {        WriterManager.getInstance().getWriter(writerName).addObserver(this);    }    // 取消订阅    public void unSubscribe(String writerName) {        WriterManager.getInstance().getWriter(writerName).deleteObserver(this);    }    @Override    public void update(Observable observable, Object arg) {        if (observable instanceof Writer) {            Writer writer = (Writer) observable;            System.out.println(readerName + "知道了作者" + writer.getWriterName() + "的新书的信息是" + writer.getNewBook());        }    }}

作者类：

public class Writer extends Observable {    private String writerName;    private String newBook;    public Writer(String writerName) {        super();        this.writerName = writerName;        WriterManager.getInstance().add(this);    }    public String getWriterName() {        return writerName;    }    public String getNewBook() {        return newBook;    }    // 发布新书，通知读者    public void PublishNewBook(String newBook) {        this.newBook = newBook;        // 一定要调用setChanged方法，然后再调用notifyObservers方法        setChanged();        notifyObservers();    }}

管理作者列表：

public class WriterManager {    // 作者列表    private Map<String, Writer> writerMap = new HashMap<>();    // 添加作者    public void add(Writer writer) {        writerMap.put(writer.getWriterName(), writer);    }    //  根据作者姓名获取作者对象    public Writer getWriter(String writerName) {        return writerMap.get(writerName);    }    // 单例    private WriterManager() {    }    public static WriterManager getInstance() {        return WriterManagerInstance.instance;    }    private static class WriterManagerInstance {        private static WriterManager instance = new WriterManager();    }}

---

源码分析：

观察者接口

package java.util;public interface Observer {    // arg变量是可以选择调用有参的notifyObservers传过来的附加参数    void update(Observable o, Object arg);}

被观察者类

package java.util;public class Observable {    private boolean changed = false;    private final ArrayList<Observer> observers;    public Observable() {        observers = new ArrayList<>();    }    public synchronized void addObserver(Observer o) {        if (o == null)            throw new NullPointerException();        if (!observers.contains(o)) {            observers.add(o);        }    }    public synchronized void deleteObserver(Observer o) {        observers.remove(o);    }    public void notifyObservers() {        notifyObservers(null);    }    // 通知观察者    public void notifyObservers(Object arg) {        Observer[] arrLocal;        synchronized (this) {               // 如果没有变化，直接return就不会通知观察者了            if (!hasChanged())                return;            arrLocal = observers.toArray(new Observer[observers.size()]);            clearChanged();        }        for (int i = arrLocal.length-1; i>=0; i--)            arrLocal[i].update(this, arg);    }    public synchronized void deleteObservers() {        observers.clear();    }    protected synchronized void setChanged() {        changed = true;    }    protected synchronized void clearChanged() {        changed = false;    }    public synchronized boolean hasChanged() {        return changed;    }    public synchronized int countObservers() {        return observers.size();    }}