在 Java 中，BufferedReader readLine 在交互式 shell 上阻塞

Question

I have a jar file which keeps waiting for user input, process and print out result (in multiple lines) like this

>Input sentence 1
Result 1
Result 2
>Input sentence 2
Result 1
Result 2
Result 3
>

The only way to exit this jar program is by pressing Ctrl + C.

Now, I want to invoke this jar file from my java program. My code looks like this:

processBuilder = new ProcessBuilder(PATH_TO_BIN);
Process process = processBuilder.start();
writer = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
reader = new BufferedReader(new InputStreamReader(process.getInputStream()));

writer.write(inputSentence);
writer.newLine();
writer.flush();
String line;
while ((line = reader.readLine()) != null){ //<<<< it hangs here
    System.out.println(line);
}

My program can successfully print out the result returned by "jar" program, but it keep hanging at the next reader.readLine() and I can not provide the next input to "jar" program.

Is there any way to fix this issue?

Thanks in advance!

Answer 1

You could use one Thread to read and other to write, then it wouldn't block on read, like this:

    ProcessBuilder processBuilder = new ProcessBuilder(PATH_TO_BIN);
    Process process = processBuilder.start();
    BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));

      
    new Thread () {
      public void run() {
        BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
        String line = null;
        do {             
          line = reader.readLine();
          System.out.println(line);
        } while (line != null); // <<<< it waits here for next line of input 
      }
    }.start();
       

    // and here you can write something to the process InputSteram...
    writer.write(inputSentence);
    writer.newLine();
    writer.flush();

Answer 2

Method ready() from BufferedReader -Tells whether this stream is ready to be read.

You can use this method to check if stream is ready to be read and if yes you can do your work,

while (reader.ready()){
//Do the processing
}

See if this works.

Answer 3

I'm going to base off of @Krzystof's answer, however I'm assuming you're going to be using Java 7 or 8 (+).

In which case, you should be using Try-With-Resources, as follows:

public static void main (String[] args) throws java.lang.Exception {

    ProcessBuilder prBuilder = new ProcessBuilder("/path/to/bin");
    Process process = prBuilder.start();
    try (BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream())) | BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()))) {

        String line;

        while ((line = reader.readLine()) != null) {
            // Process input
        }

    } catch (IOException ex) {
        // Process IOException
    } finally {
        process.kill();
    }
}