Closing infinite InputStream when reading in separate thread causes deadlock

Question

I have a thread that's reading from an input stream, and (just for simplicity) printing it back out to the console. See below for the code snippet.

I want to be able to terminate this thread at some point, so I should be able to do this by closing the input stream. This should cause an exception to be thrown, interrupting the BufferedReader::readLine method, closing the thread.

For some reason, the close() invocation just hangs.

What's the reason that this is happening? Is there a way I can close down the thread without requiring the user to input any further text?

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

public class Test {

  public static void main(String[] args) throws Exception {
    ReaderTest readerTest = new ReaderTest(System.in);
    new Thread(readerTest).start();
    Thread.sleep(1000L);
    System.out.println("About to close...");
    System.in.close(); // Should cause thread to throw exception and terminate, but just hangs
    System.out.println("All closed...");
  }

  private static class ReaderTest implements Runnable {
    private final InputStream inputStream;

    public ReaderTest(InputStream inputStream) {
      this.inputStream = inputStream;
    }

    @Override
    public void run() {
      try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
        String line;
        while ((line = reader.readLine()) != null) {
          System.out.println(line);
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

calling 'System.in.available()' also hangs for me, for some reason. — Most Noble Rabbit, Apr 15 '21 at 15:19

adarsh · Accepted Answer · 2021-04-15T17:25:03.583

For some reason, the close() invocation just hangs.

Your thread is waiting for readLine() to release lock: You are using a blocking IO.

public ReaderTest(InputStream inputStream) {
   this.inputStream = inputStream; // This inputStream (in from System.in) is a monitor
}

while ((line = reader.readLine()) != null) {
       System.out.println(line);
}

The readLine() method proceeds in following steps:

Take lock on inputStream - Here, it's System.in
Go in an infinite loop (Blocking) waiting for an input
When you enter a line (and hit return or enter) on your console, it reads them and returns the output, releasing the lock momentarily
It then goes into the same infinite (Blocking) loop again (You're calling readLine in a while loop)

If you provide some input, the main thread gets to execute System.in.close(); (because the reader thread released lock momentarily)

So, if you provide an input after 1 second (Thread.sleep(1000L);), the readLine() will find that stream is closed and it's this time java.io.IOException: Stream closed is thrown.

You can see the last line you entered (before IOException is thrown) by updating read block as:

@Override
public void run() {
    String line = null; // make line reachable inside catch block
    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));

        while ((line = reader.readLine()) != null) {
            System.out.println(line);
        }
    } catch (IOException e) {
        System.out.println("The last line I read before stream was closed : " + line);
        e.printStackTrace();
    }
}

Your explanation makes sense, thank you. That is what I observed -- entering some text in the terminal after the sleep has concluded does indeed allow the thread to terminate. In this case, I'd like the thread to close *without* requiring the user to input any further text. Is this possible? I'll amend my question to make the desired answer more clear. — Ekos IV, Apr 15 '21 at 17:22
@EkosIV You can *attempt* to make it non-blocking as suggested [here](https://stackoverflow.com/questions/7872846/how-to-read-from-standard-input-non-blocking). But, the Streams are designed to be [blocking](https://stackoverflow.com/questions/5049319/how-to-create-a-java-non-blocking-inputstream-from-a-httpsurlconnection) — adarsh, Apr 15 '21 at 17:38

score 0 · Answer 2 · answered Apr 15 '21 at 17:33

It is probably related to the System.in, changing your main in:

public static void main(String[] args) throws Exception {
        ServerSocket socket = new ServerSocket(8883);
        InputStream is = socket.accept().getInputStream();
        ReaderTest readerTest = new ReaderTest(is);
        new Thread(readerTest).start();
        Thread.sleep(1000L);
        System.out.println("About to close...");
        is.close(); // cause thread to throw exception        System.out.println("All closed...");
    }

It does what you expect.

(watch out you need to telnet or nc to localhost 8883 to let the code pass the .accept() method of the socket)

Closing infinite InputStream when reading in separate thread causes deadlock

2 Answers2