如何在stdout/stderr就绪状态下进行回调，而不是忙于轮询

Question

我发现的所有关于从Processstdout读取的讨论都是以阻塞的方式进行的，比如这里：

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流程继续运行，做一些有用的事情，只在数据实际可用时才尝试读取任何内容。所以我在寻找类似Qt的QProcess::readyReadStandardOutput信号。注意，BufferedReader的下一个ready并不是我想要的：它只是一个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

如果您有一个事件循环，您可以启动一个后台线程将事件添加到该事件循环中。

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();

---

您可以有一个只执行queue.add(in.readLine())的线程，然后您的主线程可以轮询此队列。这样，所有的多线程问题都很简单，而且都包含在内。

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.