设计模式之观察者模式

1、设计思路

观察者模式又称为发布/订阅(Publish/Subscribe)模式。观察者模式提供了一种对象设计，让主题和观察者之间松耦合。主题只知道观察者实现了某个接口（也就是Observer接口）。主题不需要知道观察者具体类是谁、做了些什么或其他任何细节。主题唯一依赖的东西是一个实现Observer接口的对象列表，所以我们可以随时增加观察者和删除观察者。有新类型的观察者出现时，主题的代码不需要修改。

2、UML类关系图

3、代码实现

最形象的栗子是：杂志订阅，杂志是主题，观察者是订阅者。当出版新杂志时候，这个事件会自动通知所有的订阅者。根据OO基本原则，应该针对接口编程，主题和订阅者一般都作为接口。

/** * 出版社主题接口对象   * */public interface Subject {public void registerObserver(Observer observer);public void removeObserver(Observer observer);public void notifyObservers();}

/** * 具体的出版社对象 * */public class ConcreteSubject implements Subject {private Boolean changeflag;private List<Observer> obsList = new ArrayList<>();private String bookname;private String author;@Overridepublic void registerObserver(Observer observer) {obsList.add(observer);}@Overridepublic void removeObserver(Observer observer) {if(obsList.contains(observer)){obsList.remove(observer);}}@Overridepublic void notifyObservers() {if(changeflag){for (Observer observer : obsList) {observer.update(bookname,author);}}}public void publishBook(String bookname,String author,boolean changeflag){this.bookname = bookname;this.author = author;this.changeflag = changeflag;notifyObservers();}public Boolean getChangeflag() {return changeflag;}public void setChangeflag(Boolean changeflag) {this.changeflag = changeflag;}public List<Observer> getObsList() {return obsList;}public void setObsList(List<Observer> obsList) {this.obsList = obsList;}public String getBookname() {return bookname;}public void setBookname(String bookname) {this.bookname = bookname;}public String getAuthor() {return author;}public void setAuthor(String author) {this.author = author;}}

/** * 观察者 * */public interface Observer {public void update(String bookname, String author);}


/** * 具体的观察者 * */public class ConcreteObserver implements Observer{private ConcreteSubject concreteSubject;private String readerName;public ConcreteObserver(ConcreteSubject subject, String reader) {super();this.concreteSubject = subject;this.readerName = reader;subject.registerObserver(this);}@Overridepublic void update(String bookname,String author) {System.out.println("订阅者获得一本新书《"+bookname+"》，作者是："+author);}public ConcreteSubject getConcreteSubject() {return concreteSubject;}public void setConcreteSubject(ConcreteSubject concreteSubject) {this.concreteSubject = concreteSubject;}public String getReaderName() {return readerName;}public void setReaderName(String readerName) {this.readerName = readerName;}}
public static void main(String[] args) {ConcreteSubject subject = new ConcreteSubject();ConcreteObserver observer1 = new ConcreteObserver(subject,"小明");ConcreteObserver observer2 = new ConcreteObserver(subject,"小星");ConcreteObserver observer3 = new ConcreteObserver(subject,"小宇");ConcreteObserver observer4 = new ConcreteObserver(subject,"小志");subject.publishBook("追风筝的人", "啊啊", true);subject.publishBook("深入理解JVM", "嗯嗯", true);}
测试结果：