CODEWITHSUNDEEP
javamultithreadingsockets
HUA Di
HUA Di

Reputation: 901

Why socketRead0 make a thread in RUNNABLE state

I've written a simple ECHO protocol server in Java use Thread.

It's so simple:

ServerSocket serverSocket = new ServerSocket(7000, 0, InetAddress.getByName("127.0.0.1"));
while (true) {
    Socket socket = serverSocket.accept();
    new Thread(() -> {
        try (Socket s = socket;
             BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream()));
             BufferedWriter out = new BufferedWriter(new OutputStreamWriter(s.getOutputStream()))) {
            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                out.write(inputLine + '\n');
                out.flush();
            }
        } catch (IOException e) {
        }
    }).start();
}

The thread is waiting client's input on inputLine = in.readLine(). But I found all the worker thread are in RUNNABLE state through jstack which I think they may be in WAITING state.

The thread is blocked on waiting data arriving, but why here it is RUNNABLE?

Here's the jstack output:

"Thread-0" #10 prio=5 os_prio=31 tid=0x00007f9f8282e800 nid=0x1307 runnable [0x000070000623a000]
 java.lang.Thread.State: RUNNABLE
  at java.net.SocketInputStream.socketRead0(Native Method)
  at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
  at java.net.SocketInputStream.read(SocketInputStream.java:170)
  at java.net.SocketInputStream.read(SocketInputStream.java:141)
  at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
  at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
  at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
  - locked <0x000000076b167a18> (a java.io.InputStreamReader)
  at java.io.InputStreamReader.read(InputStreamReader.java:184)
  at java.io.BufferedReader.fill(BufferedReader.java:161)
  at java.io.BufferedReader.readLine(BufferedReader.java:324)
  - locked <0x000000076b167a18> (a java.io.InputStreamReader)
  at java.io.BufferedReader.readLine(BufferedReader.java:389)
  at EchoServer3.lambda$main$0(EchoServer3.java:20)

Upvotes: 3

Views: 816

Answers (1)

user207421
user207421

Reputation: 311031

WAITING means that a Java thread is waiting for another Java thread to release a Java object lock.

Java doesn't know about operating system blocks or locks or scheduling. So the fact that a Java thread may be blocked waiting for input doesn't affect Java's view of its state.

Upvotes: 4

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