Java 状态模式讲解和代码示例

状态是一种行为设计模式，让你能在一个对象的内部状态变化时改变其行为。

该模式将与状态相关的行为抽取到独立的状态类中，让原对象将工作委派给这些类的实例，而不是自行进行处理。

进一步了解状态模式

复杂度：

流行度：

使用示例： 在 Java 语言中，状态模式通常被用于将基于 switch语句的大型状态机转换为对象。

这里是核心 Java 程序库中一些状态模式的示例：

javax.faces.lifecycle.LifeCycle#execute() （由FacesServlet控制：行为依赖于当前 JSF 生命周期的阶段（状态））

识别方法： 状态模式可通过受外部控制且能根据对象状态改变行为的方法来识别。

媒体播放器的接口

在本例中，状态模式允许媒体播放器根据当前的回放状态进行不同的控制行为。播放器主类包含一个指向状态对象的引用，它将完成播放器的绝大部分工作。某些行为可能会用一个状态对象替换另一个状态对象，改变播放器对用户交互的回应方式。

states

states/State.java: 通用状态接口

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * Common interface for all states.
 */
public abstract class State {
    Player player;

    /**
     * Context passes itself through the state constructor. This may help a
     * state to fetch some useful context data if needed.
     */
    State(Player player) {
        this.player = player;
    }

    public abstract String onLock();
    public abstract String onPlay();
    public abstract String onNext();
    public abstract String onPrevious();
}

states/LockedState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * Concrete states provide the special implementation for all interface methods.
 */
public class LockedState extends State {

    LockedState(Player player) {
        super(player);
        player.setPlaying(false);
    }

    @Override
    public String onLock() {
        if (player.isPlaying()) {
            player.changeState(new ReadyState(player));
            return "Stop playing";
        } else {
            return "Locked...";
        }
    }

    @Override
    public String onPlay() {
        player.changeState(new ReadyState(player));
        return "Ready";
    }

    @Override
    public String onNext() {
        return "Locked...";
    }

    @Override
    public String onPrevious() {
        return "Locked...";
    }
}

states/ReadyState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * They can also trigger state transitions in the context.
 */
public class ReadyState extends State {

    public ReadyState(Player player) {
        super(player);
    }

    @Override
    public String onLock() {
        player.changeState(new LockedState(player));
        return "Locked...";
    }

    @Override
    public String onPlay() {
        String action = player.startPlayback();
        player.changeState(new PlayingState(player));
        return action;
    }

    @Override
    public String onNext() {
        return "Locked...";
    }

    @Override
    public String onPrevious() {
        return "Locked...";
    }
}

states/PlayingState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

public class PlayingState extends State {

    PlayingState(Player player) {
        super(player);
    }

    @Override
    public String onLock() {
        player.changeState(new LockedState(player));
        player.setCurrentTrackAfterStop();
        return "Stop playing";
    }

    @Override
    public String onPlay() {
        player.changeState(new ReadyState(player));
        return "Paused...";
    }

    @Override
    public String onNext() {
        return player.nextTrack();
    }

    @Override
    public String onPrevious() {
        return player.previousTrack();
    }
}

ui

ui/Player.java: 播放器的主要代码

package refactoring_guru.state.example.ui;

import refactoring_guru.state.example.states.ReadyState;
import refactoring_guru.state.example.states.State;

import java.util.ArrayList;
import java.util.List;

public class Player {
    private State state;
    private boolean playing = false;
    private List<String> playlist = new ArrayList<>();
    private int currentTrack = 0;

    public Player() {
        this.state = new ReadyState(this);
        setPlaying(true);
        for (int i = 1; i <= 12; i++) {
            playlist.add("Track " + i);
        }
    }

    public void changeState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }

    public void setPlaying(boolean playing) {
        this.playing = playing;
    }

    public boolean isPlaying() {
        return playing;
    }

    public String startPlayback() {
        return "Playing " + playlist.get(currentTrack);
    }

    public String nextTrack() {
        currentTrack++;
        if (currentTrack > playlist.size() - 1) {
            currentTrack = 0;
        }
        return "Playing " + playlist.get(currentTrack);
    }

    public String previousTrack() {
        currentTrack--;
        if (currentTrack < 0) {
            currentTrack = playlist.size() - 1;
        }
        return "Playing " + playlist.get(currentTrack);
    }

    public void setCurrentTrackAfterStop() {
        this.currentTrack = 0;
    }
}

ui/UI.java: 播放器的 GUI

package refactoring_guru.state.example.ui;

import javax.swing.*;
import java.awt.*;

public class UI {
    private Player player;
    private static JTextField textField = new JTextField();

    public UI(Player player) {
        this.player = player;
    }

    public void init() {
        JFrame frame = new JFrame("Test player");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel context = new JPanel();
        context.setLayout(new BoxLayout(context, BoxLayout.Y_AXIS));
        frame.getContentPane().add(context);
        JPanel buttons = new JPanel(new FlowLayout(FlowLayout.CENTER));
        context.add(textField);
        context.add(buttons);

        // Context delegates handling user's input to a state object. Naturally,
        // the outcome will depend on what state is currently active, since all
        // states can handle the input differently.
        JButton play = new JButton("Play");
        play.addActionListener(e -> textField.setText(player.getState().onPlay()));
        JButton stop = new JButton("Stop");
        stop.addActionListener(e -> textField.setText(player.getState().onLock()));
        JButton next = new JButton("Next");
        next.addActionListener(e -> textField.setText(player.getState().onNext()));
        JButton prev = new JButton("Prev");
        prev.addActionListener(e -> textField.setText(player.getState().onPrevious()));
        frame.setVisible(true);
        frame.setSize(300, 100);
        buttons.add(play);
        buttons.add(stop);
        buttons.add(next);
        buttons.add(prev);
    }
}

Demo.java: 初始化代码

package refactoring_guru.state.example;

import refactoring_guru.state.example.ui.Player;
import refactoring_guru.state.example.ui.UI;

/**
 * Demo class. Everything comes together here.
 */
public class Demo {
    public static void main(String[] args) {
        Player player = new Player();
        UI ui = new UI(player);
        ui.init();
    }
}