Java设计模式之状态模式详解

// 定义抽象状态接口
interface State {
    void handle();
}

// 定义具体状态类
class ConcreteState1 implements State {
    @Override
    public void handle() {
        System.out.println("当前状态为 State1.");
    }
}

class ConcreteState2 implements State {
    @Override
    public void handle() {
        System.out.println("当前状态为 State2.");
    }
}

// 定义环境类
class Context {
    private State state;

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

    public void request() {
        state.handle();
    }
}

// 示例程序
public class StatePatternDemo {
    public static void main(String[] args) {
        // 创建状态对象
        State state1 = new ConcreteState1();
        State state2 = new ConcreteState2();

        // 创建环境对象
        Context context = new Context();
        context.setState(state1);
        context.request();

        context.setState(state2);
        context.request();
    }
}

public abstract class DownloadState {
    protected DownloadContext context;

    public void setContext(DownloadContext context) {
        this.context = context;
    }

    public abstract void download(String url, String filePath);
}

public class DownloadingState extends DownloadState {
    private FutureTask<Integer> futureTask;

    @Override
    public void download(String url, String filePath) {
        System.out.println("开始下载文件：" + filePath);

        // 开始下载
        DownloadTask task = new DownloadTask(url, filePath);
        futureTask = new FutureTask<>(task);
        ThreadPool.getInstance().execute(futureTask);

        // 状态转换
        try {
            int result = futureTask.get();
            if (result == 0) {
                context.setState(new FinishedState());
            } else {
                context.setState(new ErrorState());
            }
        } catch (Exception e) {
            e.printStackTrace();
            context.setState(new ErrorState());
        }
    }
}

public class FinishedState extends DownloadState {
    @Override
    public void download(String url, String filePath) {
        System.out.println("文件已下载完成，无需重复下载！");
        context.closeDownloadThread(filePath);
    }
}

public class ErrorState extends DownloadState {
    @Override
    public void download(String url, String filePath) {
        System.out.println("下载文件出错，无法继续下载！");
        context.closeDownloadThread(filePath);
    }
}

public class DownloadContext {
    private DownloadState currentState;
    private Map<String, FutureTask<Integer>> taskMap;

    public DownloadContext() {
        this.currentState = new FinishedState();
        this.taskMap = new HashMap<>();
      }

      public void setState(DownloadState state) {
        this.currentState = state;
        this.currentState.setContext(this);
      }

      public void download(String url, String filePath) {
        FutureTask<Integer> task = this.taskMap.get(filePath);

        if (task == null || task.isDone() || task.isCancelled()) {
          this.taskMap.remove(filePath);
          this.currentState.download(url, filePath);
        } else {
          System.out.println("文件 " + filePath + " 正在下载中，无需重复下载！");
        }
      }

      public void closeDownloadThread(String filePath) {
        FutureTask<Integer> task = this.taskMap.get(filePath);

        if (task != null) {
          task.cancel(true);
          this.taskMap.remove(filePath);
          System.out.println("已关闭文件 " + filePath + " 的下载线程。");
        }
      }
}

public class ThreadPool {
    private ExecutorService executor;

    private ThreadPool() {
        this.executor = Executors.newFixedThreadPool(5);
    }

    private static class Singleton {
        private static final ThreadPool INSTANCE = new ThreadPool();
    }

    public static ThreadPool getInstance() {
        return Singleton.INSTANCE;
    }

    public void execute(Runnable task) {
        this.executor.execute(task);
    }
}

public class Main {
    public static void main(String[] args) {
        DownloadContext context = new DownloadContext();

        String url1 = "https://cdn.pixabay.com/photo/2018/10/30/16/06/water-lily-3784022__340.jpg";
        String filePath1 = "water-lily.jpg";

        String url2 = "https://cdn.pixabay.com/photo/2020/07/14/13/10/excursion-5407227__340.jpg";
        String filePath2 = "excursion.jpg";

        context.download(url1, filePath1);
        context.download(url2, filePath2);

        System.out.println("------------------------------------");

        context.download(url1, filePath1);
        context.download(url2, filePath2);
    }
}