Does Spring Boot’s connectionTimeout Affect Request Time? The Surprising Truth

Environment: Spring Boot 2.6.12

application.yml configuration

server:
  port: 8081
  tomcat:
    maxThreads: 10
    maxConnections: 10
    acceptCount: 1
    connectionTimeout: 3000

Test 1: Controller sleeps longer than connectionTimeout

@RestController
@RequestMapping("/test")
public class TestController {

  @GetMapping("/index")
  public Object index() {
    try {
      System.out.println(Thread.currentThread().getName());
      TimeUnit.SECONDS.sleep(10);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    return "success";
  }
}

The program responds normally, showing that the connectionTimeout parameter is unrelated to the actual request processing time.

Test 2: HttpURLConnection request

public class HttpURLConnectionDemo {
  public static void main(String[] args) throws Exception {
    HttpURLConnection con = (HttpURLConnection) new URL("http://localhost:8081/test/index").openConnection();
    con.setDoInput(true);
    con.setDoOutput(true);
    long start = System.currentTimeMillis();
    InputStream is = con.getInputStream();
    Scanner scan = new Scanner(is);
    while (scan.hasNext()) {
      System.out.println("Received: " + scan.next() + "
Time: " + (System.currentTimeMillis() - start));
    }
    scan.close();
    con.disconnect();
    con = null;
  }
}

Result:

The conclusion is that connectionTimeout does not depend on the type of client making the request.

Test 3: Socket connection

public class TomcatConnectionTimeoutDemo {
  public static void main(String[] args) throws Exception {
    Socket socket = new Socket("127.0.0.1", 8081);
    long start = System.currentTimeMillis();
    InputStream is = socket.getInputStream();
    is.read();
    System.out.println(System.currentTimeMillis() - start);
  }
}

Result:

After about 3 seconds the program ends, indicating the connection was closed. The next test adds continuous console input:

public class TomcatConnectionTimeoutDemo {
  public static void main(String[] args) throws Exception {
    Socket socket = new Socket("127.0.0.1", 8081);
    long start = System.currentTimeMillis();
    final OutputStream os = socket.getOutputStream();
    new Thread(() -> {
      Scanner scan = new Scanner(System.in);
      while (scan.hasNext()) {
        String content = scan.next();
        System.out.println("Preparing to send: " + content);
        try {
          os.write(content.getBytes());
          os.flush();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }).start();
    InputStream is = socket.getInputStream();
    is.read();
    System.out.println(System.currentTimeMillis() - start);
  }
}

Result without any input:

Result with continuous console input:

Conclusion: the connectionTimeout parameter defines the maximum idle time after a client has established a connection; if the client sends no data for longer than the configured timeout, the server closes the connection.