What is the correct order of ObjectInputStream ObjectOutputStream creation on the client and the server when sending objects over a network connection?

Tim Rohaly

According to the API documentation for ObjectInputStream's constructor:

The stream header containing the magic number and version number are read from the stream and verified. This method will block until the corresponding ObjectOutputStream has written and flushed the header.
This is a very important point to be aware of when trying to send objects in both directions over a socket because opening the streams in the wrong order will cause deadlock.

Consider for example what would happen if both client and server tried to construct an ObjectInputStream from a socket's input stream, prior to either constructing the corresponding ObjectOutputStream. The ObjectInputStream constructor on the client would block, waiting for the magic number and version number to arrive over the connection, while at the same time the ObjectInputStream constructor on the server side would also block for the same reason. Hence, deadlock.

Because of this, you should always make it a practice in your code to open the ObjectOutputStream and flush it first, before you open the ObjectInputStream. The ObjectOutputStream constructor will not block, and invoking flush() will force the magic number and version number to travel over the wire. If you follow this practice in both your client and server, you shouldn't have a problem with deadlock.