Future模式

一、定义
Future模式用来获取线程的执行结果。在Thread-Per-Message模式中,如果调用一个线程异步执行任务,没有办法获取到返回值,就像:
host.request(10,'A');
而Future模式送出请求后,马上就要获取返回值,就像:
Data data=host.request(10,'A');
但是上述的返回值并不是程序的执行结果,因为线程是异步的,主线程调用该该方法时,异步线程可能才刚刚启动。需要一段时间后像下面这样获取执行结果:
data.getContent();

二、模式案例

Data接口/实现:

public interface Data {
    public abstract String getContent();
}
public class RealData implements Data {
    private final String content;
    public RealData(int count, char c) {
        System.out.println("        making RealData(" + count + ", " + c + ") BEGIN");
        char[] buffer = new char[count];
        for (int i = 0; i < count; i++) {
            buffer[i] = c;
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
            }
        }
        System.out.println("        making RealData(" + count + ", " + c + ") END");
        this.content = new String(buffer);
    }
    public String getContent() {
        return content;
    }
}
public class FutureData implements Data {
    private RealData realdata = null;
    private boolean ready = false;
    public synchronized void setRealData(RealData realdata) {
        if (ready) {
            return;
        }
        this.realdata = realdata;
        this.ready = true;
        notifyAll();
    }
    public synchronized String getContent() {
        while (!ready) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
        return realdata.getContent();
    }
}

Host类:

public class Host {
    public Data request(final int count, final char c) {
        System.out.println("    request(" + count + ", " + c + ") BEGIN");
        final FutureData future = new FutureData();
        new Thread() {
            public void run() {
                RealData realdata = new RealData(count, c);
                future.setRealData(realdata);
            }
        }.start();
        System.out.println("    request(" + count + ", " + c + ") END");
        return future;
    }
}

执行:

public class Main {
    public static void main(String[] args) {
        System.out.println("main BEGIN");
        Host host = new Host();
        Data data1 = host.request(10, 'A');
        Data data2 = host.request(20, 'B');
        Data data3 = host.request(30, 'C');
 
        System.out.println("main otherJob BEGIN");
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        System.out.println("main otherJob END");
        System.out.println("data1 = " + data1.getContent());
        System.out.println("data2 = " + data2.getContent());
        System.out.println("data3 = " + data3.getContent());
        System.out.println("main END");
    }
}

三、模式讲解
Future模式的角色如下:

  • Client(委托人)参与者
    Client参与者会向Host参与者送出请求(request),Client参与者会马上得到VirtualData,作为请求结果的返回值。(案例中的Main类就是Client)
  • Host参与者
    Host参与者接受请求(request),然后创建线程进行异步处理。Host参与者会立即返回Future(以VirturalData的形式)。
  • VirtualData(虚拟数据)参与者
    VirtualData是用来统一代表Future参与者与RealData参与者。(案例中Data接口就是VirtualData参与者)
  • RealData(实际数据)参与者
    RealData表示实际的数据。
  • Future参与者
    Future参与者包含获取实际的数据和设置实际数据的方法。Host类会创建该对象,当异步线程处理完成时,会调用Future的设置数据的方法。