java - 我怎样才能在 stdout/stderr 准备就绪时进行回调而不是忙于轮询？

Question

我发现所有关于从 Process stdout 读取的讨论都是以阻塞方式进行的，如下所示:

Process process=Runtime.getRuntime().exec(cmd);
BufferedReader in=new BufferedReader(
    new InputStreamReader(process.getInputStream()));
for(String line;(line=in.readLine())!=null;)
{
    useText(line);
}

如果我试图避免阻塞，我的选择是使用 in.ready() 轮询流。

但我真正需要的是让进程继续运行并做一些有用的事情，并且只在数据实际可用时才尝试读取任何内容。所以我正在寻找类似 Qt 的 QProcess::readyReadStandardOutput 信号的东西。请注意，BufferedReader 的方法ready 不是我要找的:它只是一种我必须反复检查的方法，而不是任何类型的信号。

Java 中有类似的东西吗？

Answer 1

如果你有一个事件循环，你可以启动一个后台线程来向那个事件循环添加事件。

例如

ProcessBuilder pb = new ProcessBuilder(cmd);
pb.redirectErrorStream(true); // capture errors as well.
Process process = pb.start();
BufferedReader in = new BufferedReader(
        new InputStreamReader(process.getInputStream()));
Thread t = new Thread(() -> {
    try {
        for (String line; (line = in.readLine()) != null; ) {
            Activity.runOnUiThread(() -> useText(line));
        }
    } catch (Exception e) {
        logger.log(e);
    } finally {
        Activity.runOnUiThread(() -> noMoreText());
    }
});
t.setDaemon(true);
t.start();

---

你可以有一个线程只执行 queue.add(in.readLine()) 然后你的主线程可以轮询这个队列。这样所有多线程问题都变得简单且包含在内。

static final String EOF = new String(); // use for == comparison later

ProcessBuilder pb = new ProcessBuilder(cmd);
pb.redirectErrorStream(true); // capture errors as well.
Process process = pb.start();
BufferedReader in = new BufferedReader(
        new InputStreamReader(process.getInputStream()));
int capacity = 1024;
BlockingQueue<String> lines = new ArrayBlockingQueue<>(capacity);
Thread t = new Thread(() -> {
    try {
        for (String line; (line = in.readLine()) != null; ) {
            lines.add(line);
            while (lines.remainingCapacity() < 2) // don't run out of memory if too much.
                Thread.sleep(50);
        }
    } catch (Exception e) {
        logger.log(e);
    } finally {
        lines.offer(EOF);
    }
});
t.setDaemon(true);
t.start();

// later in a loop
String line = lines.poll();
if (line == null) // no data yet.

else if (line == EOF) // we have the EOF marker.