Java：return 来自 Runnable 的结果

Question

假设以下简化示例。令 B 代表 class 处理一些光栅数据：

import java.awt.image.BufferedImage;

public class B implements Runnable{
    private boolean c;
    private Runnable f;

    public B (boolean c_, Runnable f_) { c = c_; f = f_;}

    public BufferedImage process() {
            //Some operations
            BufferedImage output = null;
            if (c) output = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
            return output;
    }

    public void run() { process();}
}

process() 方法可能会但可能不会创建输出评分器。由于计算成本，过程运行s 在一个单独的线程中。

让 A 代表一个 class，其中的过程将是运行。它还包含一些 post 等待线程完成的处理步骤：

import java.awt.image.BufferedImage;

public class A {
    public A(){}
    public void compute() {
            boolean c = true;
            B b = new B( c, new Runnable() {
                    public void run() {
                            //Post process, after the thread has been finished
                            BufferedImage img = ??? //Get resulting raster, how?
                            if (img != null) {
                                    //Perform some steps
                            }
                    }
            });

            Thread t = new Thread(b);
            t.start ();  //Run  procedure
    }
}

但是，如何获得使用 B process() 方法创建的结果栅格 "inside" 运行()A中的方法？

避免模型，当输出图像与

一起表示 B 的数据成员时

b.getImage();

我阅读了一篇关于回调的 post

但是这里怎么实现呢？感谢您的帮助和一个简短的例子。

Answer 1

使用ExecutorService, specifically it submit(Callable) method which returns a Future which get() or isDone()方法可以调用检索结果：

public class B implements Callable<BufferedImage> {
    private boolean c;

    public B (boolean c) { this.c = c; }

    public BufferedImage call() {
        //Some operations
        if (!c)
            return null;
        return new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
    }
}

// Somewhere, e.g. in your A.compute() method

ExecutorService exe = Executors.newFixedThreadPool(2); // Two threads in parallel
B b = new B(true);
Future<BufferedImage> res = exe.submit(b); // Will return immediately. Processing will run in a thread of 'exe' executor
// ... do things
System.out.println("Finished: "+res.isDone());
BufferedImage img = res.get(); // Will block until image is available (i.e. b.call() returns)

您可以使用不同风格的 ExecutorService，您可以在其中对可能 (submit(Callable)) 或可能 (execute(Runnable)) return 结果的处理进行排队。您要使用的执行器类型取决于您需要的处理类型和订单。

Answer 2

您可以尝试这样做：

public class B{
    private boolean c;
    volatile boolean finished = false; // it can be shared among threads
    BufferedImage output;

    public B (boolean c_) { c = c_;}

    public void process() {
       Thread t = new Thread(new Runnable(){
            @Override
            public void run() {
                if (c) output = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
                finished = true;
            }       
       });
       t.start();         
   }
}

public class A {
   public void compute() {
      B b = new B(true);
      b.process();
      while(!b.finished){System.out.println("Processing");}
      // when finished check if output is not null
      // and then do some stuff
      if(b.output!=null){System.out.println(b.output);}
   }
}

Java：return 来自 Runnable 的结果

Java: return results from Runnable

java

multithreading

return

runnable