一、模式簡(jiǎn)介

定義：對(duì)有狀態(tài)的對(duì)象，把復(fù)雜的“判斷邏輯”提取到不同的狀態(tài)對(duì)象中，允許狀態(tài)對(duì)象在其內(nèi)部狀態(tài)發(fā)生改變時(shí)改變其行為。
場(chǎng)景：當(dāng)一個(gè)對(duì)象的行為取決于它的狀態(tài)，并且它必須在運(yùn)行時(shí)根據(jù)狀態(tài)改變它的行為時(shí)。當(dāng)一個(gè)操作中含有龐大的分支結(jié)構(gòu)，并且這些分支決定于對(duì)象的狀態(tài)時(shí)。

• 角色結(jié)構(gòu)：
• 環(huán)境（Context）角色：也稱為上下文，它定義了客戶感興趣的接口，維護(hù)一個(gè)當(dāng)前狀態(tài)，并將與狀態(tài)相關(guān)的操作委托給當(dāng)前狀態(tài)對(duì)象來(lái)處理。
• 抽象狀態(tài)（State）角色：定義一個(gè)接口，用以封裝環(huán)境對(duì)象中的特定狀態(tài)所對(duì)應(yīng)的行為。
• 具體狀態(tài)（Concrete State）角色：實(shí)現(xiàn)抽象狀態(tài)所對(duì)應(yīng)的行為。

二、模式實(shí)現(xiàn)

public abstract class AbstractThreadState { -> 抽象狀態(tài)
    protected String state;
    protected abstract String state();
}

public class NewConcreteThreadState extends AbstractThreadState { -> 具體狀態(tài)
    public void start(ThreadContext context){
        if("new".equals(context.getState().state())){
            System.out.println("線程處于new狀態(tài)，切換到runnable狀態(tài)");
            context.setState(new RunnableConcreteThreadState());
        }else{
            System.out.println("線程不處于new狀態(tài)，不能調(diào)用start方法");
        }
    }

    @Override
    protected String state() {
        super.state = "new";
        return this.state;
    }
}

public class RunnableConcreteThreadState extends AbstractThreadState { -> 具體狀態(tài)
    @Override
    protected String state() {
        super.state = "runnable";
        return this.state;
    }

    public void cpu(ThreadContext context){
        if("runnable".equals(context.getState().state())){
            System.out.println("線程處于runnable狀態(tài)，切換到running狀態(tài)");
            context.setState(new RunningConcreteThreadState());
        }else{
            System.out.println("線程不處于runnable狀態(tài)，不能調(diào)用cpu方法");
        }
    }
}

public class RunningConcreteThreadState extends AbstractThreadState { -> 具體狀態(tài)
    @Override
    protected String state() {
        super.state = "running";
        return this.state;
    }

    public void suspend(ThreadContext context){
        if("running".equals(context.getState().state())){
            context.setState(new BlockConcreteThreadState());
            System.out.println("線程處于runnable狀態(tài)，切換到block狀態(tài)");
        }else{
            System.out.println("線程不處于runnable狀態(tài)，不能調(diào)用suspend方法");
        }
    }

    public void stop(ThreadContext context){
        if("running".equals(context.getState().state())){
            context.setState(new DeadConcreteThreadState());
            System.out.println("線程處于runnable狀態(tài)，切換到deal狀態(tài)");
        }else{
            System.out.println("線程不處于runnable狀態(tài)，不能調(diào)用stop方法");
        }
    }
}

public class BlockConcreteThreadState extends AbstractThreadState { -> 具體狀態(tài)
    @Override
    protected String state() {
        super.state = "block";
        return this.state;
    }

    public void resume(ThreadContext context){
        if("block".equals(context.getState().state())){
            System.out.println("線程處于block狀態(tài)，切換到runnable狀態(tài)");
            context.setState(new RunningConcreteThreadState());
        }else{
            System.out.println("線程不處于block狀態(tài)，不能調(diào)用resume方法");
        }
    }
}

public class DeadConcreteThreadState extends AbstractThreadState { -> 具體狀態(tài)
    @Override
    protected String state() {
        super.state = "deal";
        return this.state;
    }

    public void dead(){
        System.out.println("線程處于deal狀態(tài)");
    }
}

public class ThreadContext { -> 環(huán)境
    private AbstractThreadState state;

    public ThreadContext(AbstractThreadState state) {
        this.state = state;
    }

    public void start() {
        if(state instanceof NewConcreteThreadState){
            ((NewConcreteThreadState) state).start(this);
        }else{
            System.out.println("線程正處于"+state.state()+"狀態(tài),無(wú)法start");
        }
    }

    public void cpu() {
        if(state instanceof RunnableConcreteThreadState){
            ((RunnableConcreteThreadState) state).cpu(this);
        }else{
            System.out.println("線程正處于"+state.state()+"狀態(tài),無(wú)法cpu");
        }
    }

    public void stop() {
        if(state instanceof RunningConcreteThreadState){
            ((RunningConcreteThreadState) state).stop(this);
        }else{
            System.out.println("線程正處于"+state.state()+"狀態(tài),無(wú)法stop");
        }
    }

    public void suspend() {
        if(state instanceof RunningConcreteThreadState){
            ((RunningConcreteThreadState) state).suspend(this);
        }else{
            System.out.println("線程正處于"+state.state()+"狀態(tài),無(wú)法suspend");
        }
    }

    public void resume() {
        if(state instanceof BlockConcreteThreadState){
            ((BlockConcreteThreadState) state).resume(this);
        }else{
            System.out.println("線程正處于"+state.state()+"狀態(tài),無(wú)法resume");
        }
    }

    public AbstractThreadState getState() {
        return state;
    }

    public void setState(AbstractThreadState state) {
        this.state = state;
    }
}

以Java線程狀態(tài)切換為例子，狀態(tài)包含new、runnable、running、block、dead，每種狀態(tài)有各自的行為，當(dāng)調(diào)用者與線程對(duì)象產(chǎn)生互動(dòng)后，線程對(duì)象內(nèi)部的狀態(tài)隨之發(fā)生改變，線程對(duì)象的行為同時(shí)發(fā)生改變。

ThreadContext context=new ThreadContext(new NewConcreteThreadState());
context.start();
context.cpu();
context.suspend();
context.resume();
context.cpu();
context.stop();