JDK 封装好的原生的观察者模式，使用起来很简单，只需要

1. 被观察者 extends Observable，使用 addObserver(Observer observer) 添加观察者，使用 setChange() + notifityObservers(Object data) 通知更新
2. 观察者 implements Observer，实现 update(Observable observable, Object data) 方法即可

源码也很简单，我们通过源码了解其机制：

观察者：

package java.util;


/**
 * {@code Observer} is the interface to be implemented by objects that
 * receive notification of updates on an {@code Observable} object.
 *
 * @see Observable
 */
public interface Observer {

    /**
     * This method is called if the specified {@code Observable} object's
     * {@code notifyObservers} method is called (because the {@code Observable}
     * object has been updated.
     *
     * @param observable
     *            the {@link Observable} object.
     * @param data
     *            the data passed to {@link Observable#notifyObservers(Object)}.
     */
    void update(Observable observable, Object data);
}

是一个接口，有一个方法接收更新，更新的参数是产生更新的被观察者 Observable 和更新发出的信息 Object

被观察者：

package java.util;

/**
 * Observable is used to notify a group of Observer objects when a change
 * occurs. On creation, the set of observers is empty. After a change occurred,
 * the application can call the {@link #notifyObservers()} method. This will
 * cause the invocation of the {@code update()} method of all registered
 * Observers. The order of invocation is not specified. This implementation will
 * call the Observers in the order they registered. Subclasses are completely
 * free in what order they call the update methods.
 *
 * @see Observer
 */

上面意思是被观察者用于在变化发生时通知一组观察者对象。
在创建被观察者时，是没有观察者的，需要通过注册（addObserver(Observer observer)）。
通过 notifyObservers() 调用所有注册（获得了引用）的观察者的 update() 方法。
调用没有特定次序，会按照观察者的注册顺序调用。子类可以重写 notifyObservers() 按自定义次序更新。

public class Observable {

    List<Observer> observers = new ArrayList<Observer>();

    boolean changed = false;

    /**
     * Constructs a new {@code Observable} object.
     */
    public Observable() {
    }

    /**
     * Adds the specified observer to the list of observers. If it is already
     * registered, it is not added a second time.
     *
     * @param observer
     *            the Observer to add.
     */
    public void addObserver(Observer observer) {
        if (observer == null) {
            throw new NullPointerException("observer == null");
        }
        synchronized (this) {
            if (!observers.contains(observer))
                observers.add(observer);
        }
    }

observers：声明 Observer 类型的 List 用于存放注册的 Observer
changed：
无参构造函数用以生成被观察者
addObserver()：添加观察者，判断如果为空，抛出空对象异常；使用 synchronized(this) {} 上锁，使用同步方式添加，能保证不会二次添加；使用 contains() 方法判断对象是否已经注册过

    /**
     * Clears the changed flag for this {@code Observable}. After calling
     * {@code clearChanged()}, {@code hasChanged()} will return {@code false}.
     */
    protected void clearChanged() {
        changed = false;
    }

    /**
     * Returns the number of observers registered to this {@code Observable}.
     *
     * @return the number of observers.
     */
    public int countObservers() {
        return observers.size();
    }

    /**
     * Removes the specified observer from the list of observers. Passing null
     * won't do anything.
     *
     * @param observer
     *            the observer to remove.
     */
    public synchronized void deleteObserver(Observer observer) {
        observers.remove(observer);
    }

    /**
     * Removes all observers from the list of observers.
     */
    public synchronized void deleteObservers() {
        observers.clear();
    }

clearChanged()：设更新标志为 false，清除更新
countObservers()：返回 List 的长度
deleteObserver()：移除 List 中指定的观察者
deleteObservers()：清空 List 中所有观察者

    /**
     * Returns the changed flag for this {@code Observable}.
     *
     * @return {@code true} when the changed flag for this {@code Observable} is
     *         set, {@code false} otherwise.
     */
    public boolean hasChanged() {
        return changed;
    }

    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every observer in the list of observers using null as the
     * argument. Afterwards, calls {@code clearChanged()}.
     * <p>
     * Equivalent to calling {@code notifyObservers(null)}.
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every Observer in the list of observers using the specified
     * argument. Afterwards calls {@code clearChanged()}.
     *
     * @param data
     *            the argument passed to {@code update()}.
     */
    @SuppressWarnings("unchecked")
    public void notifyObservers(Object data) {
        int size = 0;
        Observer[] arrays = null;
        synchronized (this) {
            if (hasChanged()) {
                clearChanged();
                size = observers.size();
                arrays = new Observer[size];
                observers.toArray(arrays);
            }
        }
        if (arrays != null) {
            for (Observer observer : arrays) {
                observer.update(this, data);
            }
        }
    }

    /**
     * Sets the changed flag for this {@code Observable}. After calling
     * {@code setChanged()}, {@code hasChanged()} will return {@code true}.
     */
    protected void setChanged() {
        changed = true;
    }
}

hasChanged()：返回更新标志，用以保证更新时通知
notifyObservers()：调用 notifyObservers(null) 方法，没有数据传输
notifyObservers(Oject data)：当 hasChanged() 为 true 时，进入通知逻辑：一、使用 clearChanged() 清除这次更新；二、将 List 转化为 array 数组，这是为了使用 for(Oject object: arrays) 对注册的观察者进行遍历，效率比较高效；三、使用 update() 传递 Oject 或空值进行数据传输
setChanged()：通知时要使用 setChanged() 使更新标志生效