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Editorial Team
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Asked: 2026-05-25T20:59:24+00:00

I am using an ObjectInputStream and ObjectOutputStream for my client to talk to a

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I am using an ObjectInputStream and ObjectOutputStream for my client to talk to a server over TCP. The client is not able to receive a response back from the server… when I step through in debug mode the response goes through fine. However, if I run the program without a break point it will throw a NullPointerException exception.

Initialization:

ObjectOutputStream oos = null;
ObjectInputStream ois = null;
Socket socket = null;

socket = new Socket(server, port);
oos = new ObjectOutputStream(socket.getOutputStream());
oos.flush();
ois = new ObjectInputStream(socket.getInputStream());

Code that breaks:

          try
            {
                oos.writeObject(request);   
                serverResponse = (Message) ois.readObject();
                output.append(serverResponse.data + "\n");
            } 
            catch(Exception ex) 
            {
                System.err.println("Error adding car to server: " + ex.getMessage());
                return;
            }

The code above is throwing the NullPointerException. If I use a break point and step through, I get a server response just fine. I have confirmed that in every instance the server is reading the

Any help would be greatly appreciated!

EDIT STACK TRACE AT EX:

java.lang.NullPointerException
    at CarInventoryClient.actionPerformed(CarInventoryClient.java:251)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
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1 Answer

  1. Editorial Team

    This was a race condition.

    ois = new ObjectInputStream(socket.getInputStream());

    The above line was causing the thread to block in my class’s constructor. Java’s documentation states that it will block until input headers are received. The action handler was sending then receiving a packet from the server – this woke the constructor code back up and finished the initialization.

    When there was a thread sleep, or the debugger was attached, the constructor code would complete before the data was received. However, in real time execution, readObject was being called before the unblocked initialization could finish. This is why ois was being seen as null.

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