Java:如何让这个主线程等待新线程终止

Question

我有一个 java 类，它使用 ProcessBuilder 创建一个名为 child 的进程。子进程生成大量输出，我在单独的线程上排出这些输出以防止主线程被阻塞。但是，稍后我需要等待输出线程完成/终止才能继续，但我不确定该怎么做。我认为 join() 是执行此操作的常用方法，但我不确定在这种情况下该怎么做。这是 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();

Answer 1

您可以加入该线程。您将获得线程的实例，并在需要等待时调用其连接方法。

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

其他人会建议使用 CountdownLatch、CyclicBarrier 甚至 Future，但我发现这是在非常低的级别上最容易实现的。