设计模式-观察者（Observer）模式

主要角色

• 观察者
• 消息

职责

观察者模式有点象MQ中的消费者。始终监听者某一个事件，一旦发生就通知其他类。

类图

类关系示意图

代码

public class Course extends Observable{
    private String courseName;

    public Course(String courseName) {
        this.courseName = courseName;
    }

    public String getCourseName() {
        return courseName;
    }

    public void produceQuestion(Course course, Question question){
        System.out.println(question.getUserName()+"在"+course.courseName+"提交了一个问题");
        setChanged();
        notifyObservers(question);

    }
}
---
public class Teacher implements Observer{
    private String teacherName;

    public Teacher(String teacherName) {
        this.teacherName = teacherName;
    }


    @Override
    public void update(Observable o, Object arg) {
        Course course = (Course)o;
        Question question = (Question)arg;
        System.out.println(teacherName+"老师的"+course.getCourseName()+"课程接收到一个"+question.getUserName()+"提交的问答:"+question.getQuestionContent());

    }
}
---
public class Question {
    private String userName;
    private String questionContent;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getQuestionContent() {
        return questionContent;
    }

    public void setQuestionContent(String questionContent) {
        this.questionContent = questionContent;
    }
}

使用

public class Test {
    public static void main(String[] args) {
        Course course = new Course("Java设计模式精讲");
        Teacher teacher1 = new Teacher("Alpha");
        Teacher teacher2 = new Teacher("Beta");


        course.addObserver(teacher1);
        course.addObserver(teacher2);

        //业务逻辑代码
        Question question = new Question();
        question.setUserName("Geely");
        question.setQuestionContent("Java的主函数如何编写");

        course.produceQuestion(course,question);
    }
}

技巧

• 观察者模式的并发是重点，需要考虑并发大的设计。