该项目源码地址：https://github.com/ggb2312/JavaNotes/tree/master/design-pattern（设计模式相关代码与笔记）

1. 定义

允许一个对象在其内部状态改变时，改变它的行为

2. 适用场景

一个对象存在多个状态(不同状态下行为不同)，且状态可相互转换
比如：电商订单状态扭转等

3. 类图与角色

Context：它就是那个含有状态的对象，它可以处理一些请求，这些请求最终产生的响应会与状态相关。

State：状态接口，它定义了每一个状态的行为集合，这些行为会在Context中得以使用。

ConcreteState：具体状态，实现相关行为的具体状态类。

4. 相关设计模式

状态模式和享元模式
状态模式和享元模式可以组合使用

5. 模式实例

背景：视频的播放状态切换

（1）视频状态抽象类

/*** Create by lastwhisper on 2019/2/11*/
public abstract class CourseVideoState {protected CourseVideoContext courseVideoContext;public void setCourseVideoContext(CourseVideoContext courseVideoContext) {this.courseVideoContext = courseVideoContext;}public abstract void play();public abstract void speed();public abstract void pause();public abstract void stop();
}

（2）视频上下文

/*** Create by lastwhisper on 2019/2/11*/
public class CourseVideoContext {private CourseVideoState courseVideoState;public final static PlayState PLAY_STATE = new PlayState();public final static StopState STOP_STATE = new StopState();public final static PauseState PAUSE_STATE = new PauseState();public final static SpeedState SPEED_STATE = new SpeedState();public CourseVideoState getCourseVideoState() {return courseVideoState;}public void setCourseVideoState(CourseVideoState courseVideoState) {this.courseVideoState = courseVideoState;this.courseVideoState.setCourseVideoContext(this);}public void play() {this.courseVideoState.play();}public void speed() {this.courseVideoState.speed();}public void pause() {this.courseVideoState.pause();}public void stop() {this.courseVideoState.stop();}
}

（3）视频的各种具体状态类

PlayState 正常播放课程视频状态

/*** Create by lastwhisper on 2019/2/11*/
public class PlayState extends CourseVideoState {@Overridepublic void play() {System.out.println("正常播放课程视频状态");}@Overridepublic void speed() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);}@Overridepublic void pause() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);}@Overridepublic void stop() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);}
}

SpeedState 快进播放课程视频状态

/*** Create by lastwhisper on 2019/2/11*/
public class SpeedState extends CourseVideoState  {@Overridepublic void play() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);}@Overridepublic void speed() {System.out.println("快进播放课程视频状态");}@Overridepublic void pause() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);}@Overridepublic void stop() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);}
}

PauseState 暂停播放课程视频状态

/*** Create by lastwhisper on 2019/2/11*/
public class PauseState extends CourseVideoState  {@Overridepublic void play() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);}@Overridepublic void speed() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);}@Overridepublic void pause() {System.out.println("暂停播放课程视频状态");}@Overridepublic void stop() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);}
}

StopState 停止播放课程视频状态

/*** Create by lastwhisper on 2019/2/11*/
public class StopState extends CourseVideoState  {@Overridepublic void play() {super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);}@Overridepublic void speed() {System.out.println("ERROR 停止状态不能快进！！");}@Overridepublic void pause() {System.out.println("ERROR 停止状态不能暂停！！");}@Overridepublic void stop() {System.out.println("停止播放课程视频状态");}
}

（4）测试

/*** Create by lastwhisper on 2019/2/11*/
public class Test {public static void main(String[] args){CourseVideoContext courseVideoContext = new CourseVideoContext();courseVideoContext.setCourseVideoState(new PlayState());System.out.println("当前状态"+ courseVideoContext.getCourseVideoState().getClass().getSimpleName());courseVideoContext.pause();System.out.println("当前状态"+ courseVideoContext.getCourseVideoState().getClass().getSimpleName());courseVideoContext.speed();System.out.println("当前状态"+ courseVideoContext.getCourseVideoState().getClass().getSimpleName());courseVideoContext.stop();System.out.println("当前状态"+ courseVideoContext.getCourseVideoState().getClass().getSimpleName());courseVideoContext.speed();}
}

测试结果：

（5）类图

6. 优缺点

优点：

• 将不同的状态隔离
• 把各种状态的转换逻辑，分布到State的子类中，减少相互间依赖
• 增加新的状态非常简单

缺点：

• 状态多的业务场景导致类数目增加，系统变复杂

7. 扩展-JDK1.7源码以及框架中的状态模式

javax.faces.lifecycle.Lifecycle、javax.faces.webapp.FacesServlet