How can Socket.getInputStream() block

[Naveen Sampra]

Hi All,

I am trying to find an answer to the following weird condition.


I have written Dispatcher class that accepts client requests and forwards them to appropriate server instances. There are 3 server instances.

The code is some what as follows

//Dispatcher class
    ......
    ServerSocket ss = new ServerSocket(4000);
    while (true) {
      Socket inSoc = ss.accept();
      //spawn new thread to handle request passing "inSoc"
      new RequestHandlerThread(inSoc).start();      
    }  
 
//RequestHandlerThread
    ......
    ......
    public void run() 
    {
ObjectInputStream inClient = new ObjectInputStream(inSoc.getInputStream());
ObjectOutputStream outClient = new ObjectOutputStream(inSoc.getOutputStream());        
        //read the object passed in using "inClient" and based on info
        //in the object find out which server to dispath request
        Object obj = inClient.readObject();
        String sendToHost = //find out server instance name
        String sendToPort = //find out port
 
        Socket s = new Socket(sendToHost, sendToPort);
ObjectOutputStream outServer = new ObjectOutputStream(s.getOutputStream());
ObjectInputStream inServer = new ObjectInputStream(s.getInputStream());
        //use outServer to dispath "obj" to server
 
    }
 
      

However the program just hangs while trying to do s.getInputStream() above. The server is running and

Socket s = new Socket(sendToHost, sendToPort)

exeuctes fine.

Why does the program block at

ObjectInputStream inServer = new ObjectInputStream(s.getInputStream());

[ April 22, 2005: Message edited by: Naveen Sampra ]

[Joe Ess]

The ObjectInputStream constructor blocks until it completes reading the serialization stream header. Code which waits for an ObjectInputStream to be constructed before creating the corresponding ObjectOutputStream for that stream will deadlock, since the ObjectInputStream constructor will block until a header is written to the stream, and the header will not be written to the stream until the ObjectOutputStream constructor executes. This problem can be resolved by creating the ObjectOutputStream before the ObjectInputStream, or otherwise removing the timing dependency between completion of ObjectInputStream construction and the creation of the ObjectOutputStream.

Java Object Serialization Specification

[Naveen Sampra]

Thanks for your input. I'm really perplexed with this problem.

In my code above as suggested in the quote, ObjectOutputStream is created "before" the ObjectInputStream. So why does ObjectInputStream still block ?