Socket.getOutputStream must precede the socket.getInputStream?!? Why?

Question

This is server code.......
import java.io.*;
import java.net.*;
import java.util.*;
public class DateServer extends Thread {
   private ServerSocket dateServer;
   public static void main(String argv[]) throws Exception {
     new DateServer();
   public DateServer() throws Exception {
     dateServer = new ServerSocket(3000);
     System.out.println("Server listening on port 3000.");
     this.start();
   public void run() {
     while(true) {
       try {
        System.out.println("Waiting for connections.");
        Socket client = dateServer.accept();
        System.out.println("Accepted a connection from: "+
client.getInetAddress());
        Connect c = new Connect(client);
       } catch(Exception e) {}
class Connect extends Thread {
   private Socket client = null;
   private ObjectInputStream ois = null;
   private ObjectOutputStream oos = null;
   public Connect() {}
   public Connect(Socket clientSocket) {
     client = clientSocket;
     try {
      ois = new ObjectInputStream(client.getInputStream());
      oos = new ObjectOutputStream(client.getOutputStream());
     } catch(Exception e1) {
         try {
            client.close();
         }catch(Exception e) {
           System.out.println(e.getMessage());
         return;
     this.start();
   public void run() {
      try {
         oos.writeObject(new Date());
         oos.flush();
         // close streams and connections
         ois.close();
         oos.close();
         client.close();
      } catch(Exception e) {}
}This is client code....
import java.io.*;
import java.net.*;
import java.util.*;
public class DateClient {
   public static void main(String argv[]) {
      ObjectOutputStream oos = null;
      ObjectInputStream ois = null;
      Socket socket = null;
      Date date = null;
      try {
        // open a socket connection
        socket = new Socket("127.0.0.1", 3000);
        // open I/O streams for objects
        oos = new ObjectOutputStream(socket.getOutputStream());
        ois = new ObjectInputStream(socket.getInputStream());
          System.out.println( "BAU!" );
        // read an object from the server
        date = (Date) ois.readObject();
        System.out.print("The date is: " + date);
        oos.close();
        ois.close();
      } catch(Exception e) {
        System.out.println(e.getMessage());
}OK.
Those work just fine. Now if you put socket.getInputStream before the socket.getOutputStream then it hangs!
Any ideea why this happens and why is not documented... I just lost tow hour tring to figure it out why my program just hangs. After I have set the order as In the example above all works....
If anyone knows why please share your knowledge...
Hope this will save others a lot of lost time.

Answer 1

The order matters because the ObjectOutputStream constructor writes a stream header and the ObjectInputStream constructor reads the header.
If both ends of the conversation do "new ObjectInputStream()" they both forever wait for the other end to write the header. So one end needs to create the output stream first and the input stream second, and the other end in the other order (or do them in different threads.)
If it isn't documented it should be IMHO. Re-check the javadocs; if it isn't mentioned file a bug report to Sun. There is a bug parade link on http://java.sun.com/ somewhere.