Java：如何使主线程等待新线程终止

Question

我有一个Java类，使用ProcessBuilder创建了一个名为child的进程。子进程生成大量输出，我正在另一个线程中进行处理以避免主线程被阻塞。但是稍后我需要等待输出线程完成/终止才能继续执行，我不确定如何做到这一点。我认为join()是通常的方法，但我不确定在这种情况下该如何使用。以下是Java代码的相关部分。

    // Capture output from process called child on a separate thread

    final StringBuffer outtext = new StringBuffer("");
    new Thread(new Runnable() {
        public void run() {
            InputStream in = null;
            in = child.getInputStream();
            try {
                if (in != null) {
                    BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                    String line = reader.readLine();
                    while ((line != null)) {
                        outtext.append(line).append("\n");
                        ServerFile.appendUserOpTextFile(userName, opname, outfile, line+"\n");
                        line = reader.readLine();
                    }
                }
            } catch (IOException iox) {
                throw new RuntimeException(iox);
            }
        }
    }).start();

    // Write input to for the child process on this main thread
    //
    String intext = ServerFile.readUserOpTextFile(userName, opname, infile);
    OutputStream out = child.getOutputStream();
    try {
        out.write(intext.getBytes());
        out.close();
    } catch (IOException iox) {
        throw new RuntimeException(iox);
    }

    //  ***HERE IS WHERE I NEED TO WAIT FOR THE THREAD TO FINISH ***

    // Other code goes here that needs to wait for outtext to get all
    // of the output from the process

    // Then, finally, when all the remaining code is finished, I return
    // the contents of outtext

    return outtext.toString();

- Jeff Bullard

Answer 1

你可以加入那个线程。你将获得该线程的实例，需要等待时调用其join方法。

 Thread th = new Thread(new Runnable() {  ... } );
 th.start();
 //do work 
 //when need to wait for it to finish
 th.join();
 //th has now finished

其他人可能会建议使用CountdownLatch、CyclicBarrier或者Future，但我发现在非常低的级别上实现这个是最容易的。

- John Vint

Answer 2

你需要将线程分配给一个变量，稍后在该变量上调用join()。

- tangens

Answer 3

final StringBuffer outtext = new StringBuffer("");  
Thread outputDrainThread = new Thread(new Runnable() {
    public void run() {
        // ... 
    }
}).start();

// ...

//  ***HERE IS WHERE I NEED TO WAIT FOR THE THREAD TO FINISH ***
outputDrainThread.join();    

// ...
return outtext.toString();

- Binil Thomas