languagetool-org/languagetool · critical · IOException

No free port in range ( - ) found.

Error message

No free port in range ( - ) found.

What it means

HTTPServer.getPortFromRange scans the configured minPort-maxPort range trying to open each port; if none can be bound it throws this IOException. It means every port in the range was already taken (or blocked) when the server started. The server cannot serve HTTP without a listening socket, so startup aborts.

Source

Thrown at languagetool-server/src/main/java/org/languagetool/server/HTTPServer.java:147

    }
  }
  
  private int getPortFromRange(int minPort, int maxPort) throws IOException {
    if (minPort > 0 && minPort < maxPort) {
         log.info("Try to find a free Port for Server in range {}-{}", minPort, maxPort);
         for (int p = minPort; p <= maxPort; p++) {
           try {
             log.info("Check port {}", p);
             ServerSocket serverSocket = new ServerSocket(p);
             port = serverSocket.getLocalPort();
             serverSocket.close();
             log.info("Port {} is available.", p);
             return p;
           } catch (IOException ex) {
             log.debug("Port {} is not available.", p);
           }
         }
         throw new IOException("No free port in range ("+ minPort + " - " + maxPort + ") found.");
    } else {
      throw new IOException("Invalid port configuration found. " +
                            "The value for '--port' need to be greater than 0 or if set to 0 you need to specify a minPort and maxPort in your properties file (minPort must be lower that maxPort).");
    }
  }

  @Override
  public void stop() {
    super.stop();
    if (executorService != null) {
      executorService.shutdownNow();
    }
  }

  public static void main(String[] args) {
    if (usageRequested(args)) {
      System.out.println("Usage: " + HTTPServer.class.getSimpleName() + " [--config propertyFile] [--port|-p port] [--public]");
      System.out.println("  --config FILE  a Java property file (one key=value entry per line) with values for:");

View on GitHub (pinned to 2e990059ce)

Solutions

1. Check which process occupies the range (ss -ltnp or netstat -ano) and stop the conflicting process
2. Widen the minPort/maxPort range in the server properties file or pick an unoccupied range
3. Use a fixed free port by setting --port to a specific available value instead of port 0 with a range
4. Verify no firewall/SELinux/container policy prevents binding to that range

Example fix

# before (server.properties)
minPort=8081
maxPort=8081
# after
minPort=8081
maxPort=8090

// before starting with port 0 + range
int free = -1;
for (int p = minPort; p <= maxPort; p++) {
  try (java.net.ServerSocket s = new java.net.ServerSocket(p)) { free = p; break; }
  catch (java.io.IOException ignored) {}
}
if (free == -1) throw new IllegalStateException("No free port in " + minPort + "-" + maxPort);

try { server.start(); } catch (IOException e) {
  if (e.getMessage().contains("No free port")) { widenRangeOrPickStaticPort(); } else throw e;
}

When it happens

Trigger: Starting the server with --port 0 (or port=0 in the properties file) plus minPort/maxPort properties set, while all ports in [minPort, maxPort] are already bound by other processes or unreachable due to OS restrictions.

Common situations: Running multiple LanguageTool instances on the same machine with overlapping port ranges; a stale process still holding the ports; running in a container/pod whose network policy blocks binding to that range; minPort==maxPort that is occupied.

Related errors

• Could not connect to server at ${serverBaseUrl}
• https_server_start_failed
• http_server_start_failed
• Invalid port configuration found. The value for '--port' nee
• Could not start LanguageTool HTTP server on , port

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/084d5221b49625e5. Report an issue: GitHub.