Java:在线程中启动进程并获取对创建进程的引用

Question

我有以下集成测试代码，在测试运行之前我启动了各种服务:

@BeforeClass
public static void setup() throws Exception {

  // Set some vars like javaBin location

  ProcessBuilder builder = new ProcessBuilder(javaBin, "-jar", "app.jar");
  Process process = launchProcess(builder);

}

private static Process launchProcess(ProcessBuilder builder) throws IOException {

  AtomicReference<Process> process = new AtomicReference<>();
  new Thread(() -> {
    try {
      process.set(builder.start());

      BufferedReader processStd =  new BufferedReader(new InputStreamReader(process.get().getInputStream());     
      BufferedReader processErr = new BufferedReader(new InputStreamReader(process.get().getErrorStream()));

      // To prevent deadlocks due to limited buffer size
      String s = "";
      while(processStd .readLine() != null) {}
      while((s = processErr .readLine()) != null) {
          System.err.println(s);
      }
    } 
    catch (IOException e) {
      e.printStackTrace();
    }
  }).start();

  return process.get();
}

进程启动正常，但我 process.get() 总是返回 null。我究竟做错了什么？如何使用上述方法获得对已启动进程的引用？如果我在 setup() 函数本身中设置进程，即 process = builder.start() ，那么它可以正常工作。但是，这也会导致大量重复，因为我需要在单独的 JVM 中启动大量单独的服务。

Answer 1

问题很可能是在您在 launchProcess 末尾使用 process.get() 返回时线程尚未真正启动。

在从方法返回之前，您需要等待线程实际启动并执行对 process.set(builder.start()) 的调用。

这可以通过像 CountDownLatch 这样简单的东西来完成。

private static Process launchProcess(ProcessBuilder builder) throws IOException {
  final CountDownLatch latch = new CountDownLatch(1);
  // ...
  new Thread(() -> {
    try {
      process.set(builder.start());
      latch.countDown();
      //...
    } catch (IOException e) {
      //...
    }
  }).start();

  latch.await();
  return process.get();
}