public interface Subject {
  //注册一个observer
  public void register(Observer observer);
  //移除一个observer
  public void remove(Observer observer);
  //通知所有观察者
  public void notifyObservers();
  //获取主题类要发布的消息
  public String getMessage();
}
ConcerteSubject:
public class MySubject implements Subject {
  private List<Observer> observers;
  private boolean changed;
  private String message;
  //对象锁， 用于同步更新观察者列表
  private final Object mutex = new Object();
  public MySubject() {
    observers = new ArrayList<Observer>();
    changed = false;
  }
  @Override
  public void register(Observer observer) {
    if (observer == null)
      throw new NullPointerException();
      //保证不重复
    if (!observers.contains(observer))
      observers.add(observer);
  }
  @Override
  public void remove(Observer observer) {
    observers.remove(observer);
  }
  @Override
  public void notifyObservers() {
    // temp list
    List<Observer> tempObservers = null;
    synchronized (mutex) {
      if (!changed)
        return;
      tempObservers = new ArrayList<>(this.observers);
      this.changed = false;
    }
    for(Observer obj : tempObservers) {
      obj.update();
    }
  }
  //主题类发布新消息
  public void makeChanged(String message) {
    System.out.println("The Subject make a change: " + message);
    this.message = message;
    this.changed = true;
    notifyObservers();
  }
  @Override
  public String getMessage() {
    return this.message;
  }
}

public class SubjectManagement {
  //一个记录 名字——主题类 的Map
  private Map<String, Subject> subjectList = new HashMap<String, Subject>();
  public void addSubject(String name, Subject subject) {
    subjectList.put(name, subject);
  }
  public void addSubject(Subject subject) {
    subjectList.put(subject.getClass().getName(), subject);
  }
  public Subject getSubject(String subjectName) {
    return subjectList.get(subjectName);
  }
  public void removeSubject(String name, Subject subject) {
  }
  public void removeSubject(Subject subject) {
  }
  //singleton
  private SubjectManagement() {}
  public static SubjectManagement getInstance() {
    return SubjectManagementInstance.instance;
  }
  private static class SubjectManagementInstance {
    static final SubjectManagement instance = new SubjectManagement();
  }
}

public interface Observer {
  public void update();
  public void setSubject(Subject subject);
}
ConcerteObserver:
public class MyObserver implements Observer {
  private Subject subject;
  // get the notify message from Concentrate Subject
  @Override
  public void update() {
    String message = subject.getMessage();
    System.out.println("From Subject " + subject.getClass().getName()
        + " message: " + message);
  }
  @Override
  public void setSubject(Subject subject) {
    this.subject = subject;
  }
  // subcirbe some Subject
  public void subscribe(String subjectName) {
    SubjectManagement.getInstance().getSubject(subjectName).register(this);
  }
  // cancel subcribe
  public void cancelSubcribe(String subjectName) {
    SubjectManagement.getInstance().getSubject(subjectName).remove(this);
  }
}

public class ObserverTest {
  private static MySubject writer;
  @BeforeClass
  public static void setUpBeforeClass() throws Exception {
    writer = new MySubject();
    //添加一个名为Linus的作家
    SubjectManagement.getInstance().addSubject("Linus",writer);
  }
  @Test
  public void test() {
    //定义几个读者
    MyObserver reader1 = new MyObserver();
    MyObserver reader2 = new MyObserver();
    MyObserver reader3 = new MyObserver();
    reader1.setSubject(writer);
    reader2.setSubject(writer);
    reader3.setSubject(writer);
    reader1.subscribe("Linus");
    reader2.subscribe("Linus");
    reader3.subscribe("Linus");
    writer.makeChanged("I have a new Changed");
    reader1.update();
  }
}

/**
 * A tagging interface that all event listener interfaces must extend.
 * @since JDK1.1
 */
public interface EventListener {
}

public class EventObject implements java.io.Serializable {
  private static final long serialVersionUID = 5516075349620653480L;
  /**
   * The object on which the Event initially occurred.
   */
  protected transient Object source;
  /**
   * Constructs a prototypical Event.
   *
   * @param  source  The object on which the Event initially occurred.
   * @exception IllegalArgumentException if source is null.
   */
  public EventObject(Object source) {
    if (source == null)
      throw new IllegalArgumentException("null source");
    this.source = source;
  }
  /**
   * The object on which the Event initially occurred.
   *
   * @return  The object on which the Event initially occurred.
   */
  public Object getSource() {
    return source;
  }
  /**
   * Returns a String representation of this EventObject.
   *
   * @return A a String representation of this EventObject.
   */
  public String toString() {
    return getClass().getName() + "[source=" + source + "]";
  }
}

public void setAttribute(String name, Object value) {
    ...
    //上面的逻辑代码已省略
    // 此处即通知监听者
    notifyAttributeAssigned(name, value, oldValue);
  }

private void notifyAttributeAssigned(String name, Object value,
      Object oldValue) {
    //从容器中获取webAPP中定义的Listener的实例对象
    Object listeners[] = context.getApplicationEventListeners();
    if ((listeners == null) || (listeners.length == 0)) {
      return;
    }
    boolean replaced = (oldValue != null);
    //创建相关事件对象
    ServletRequestAttributeEvent event = null;
    if (replaced) {
      event = new ServletRequestAttributeEvent(
          context.getServletContext(), getRequest(), name, oldValue);
    } else {
      event = new ServletRequestAttributeEvent(
          context.getServletContext(), getRequest(), name, value);
    }
    //遍历所有监听器列表， 找到对应事件的监听器
    for (int i = 0; i < listeners.length; i++) {
      if (!(listeners[i] instanceof ServletRequestAttributeListener)) {
        continue;
      }
      //调用监听器的方法， 实现监听操作
      ServletRequestAttributeListener listener =
        (ServletRequestAttributeListener) listeners[i];
      try {
        if (replaced) {
          listener.attributeReplaced(event);
        } else {
          listener.attributeAdded(event);
        }
      } catch (Throwable t) {
        ExceptionUtils.handleThrowable(t);
        context.getLogger().error(sm.getString("coyoteRequest.attributeEvent"), t);
        // Error valve will pick this exception up and display it to user
        attributes.put(RequestDispatcher.ERROR_EXCEPTION, t);
      }
    }
  }

package observer; 
 
import java.util.List; 
import java.util.Observable; 
import java.util.Observer; 
/** 
 *描述：警察张三 
 */ 
public class Police extends Observable { 
 
  private String time ; 
  public Police(List<Observer> list) { 
    super(); 
    for (Observer o:list) { 
      addObserver(o); 
    } 
  } 
  public void change(String time){ 
    this.time = time; 
    setChanged(); 
    notifyObservers(this.time); 
  } 
} 

package observer; 
 
import java.util.Observable; 
import java.util.Observer; 
/** 
 *描述：卧底A 
 */ 
public class UndercoverA implements Observer { 
 
  private String time; 
  @Override 
  public void update(Observable o, Object arg) { 
    time = (String) arg; 
    System.out.println("卧底A接到消息，行动时间为："+time); 
  } 
 
 
} 

package observer; 
 
import java.util.Observable; 
import java.util.Observer; 
/** 
 *描述：卧底B 
 */ 
public class UndercoverB implements Observer { 
  private String time; 
  @Override 
  public void update(Observable o, Object arg) { 
    time = (String) arg; 
    System.out.println("卧底B接到消息，行动时间为："+time); 
  } 
 
 
 
} 

package observer; 
 
import java.util.ArrayList; 
import java.util.List; 
import java.util.Observer; 
/** 
 *描述：测试 
 */ 
public class Client { 
 
  /** 
   * @param args 
   */ 
  public static void main(String[] args) { 
    UndercoverA o1 = new UndercoverA(); 
    UndercoverB o2 = new UndercoverB(); 
    List<Observer> list = new ArrayList<>(); 
    list.add(o1); 
    list.add(o2); 
    Police subject = new Police(list); 
    subject.change("02:25"); 
    System.out.println("===========由于消息败露，行动时间提前========="); 
    subject.change("01:05"); 
     
  } 
 
} 

卧底B接到消息，行动时间为：02:25
卧底A接到消息，行动时间为：02:25
===========由于消息败露，行动时间提前=========
卧底B接到消息，行动时间为：01:05
卧底A接到消息，行动时间为：01:05