Java Cookbook

Resolving Port Conflicts

Problem

Somebody else is using the port you want, and they won’t let go!

Solution

Use a PortOwnershipListener .

Discussion

If you run the CommPortOpen program and select a port that is opened by a native program such as HyperTerminal on MS-Windows, you will get a PortInUseException after the timeout period is up:

C:\javasrc\commport>java CommPortOpen
Exception in thread "main" javax.comm.PortInUseException: Port currently owned 
by Unknown Windows Application
        at javax.comm.CommPortIdentifier.open(CommPortIdentifier.java:337)
        at CommPortOpen.main(CommPortOpen.java:41)

If, on the other hand, you run two copies of CommPortOpen at the same time for the same port, you will see something like the following:

C:\javasrc\commport>java CommPortOpen
Exception in thread "main" javax.comm.PortInUseException: Port currently owned 
by DarwinSys DataComm
        at javax.comm.CommPortIdentifier.open(CommPortIdentifier.java:337)
        at CommPortOpen.main(CommPortOpen.java:41)

C:\javasrc\commport>

To resolve conflicts over port ownership, you can register a PortOwnershipListener so that you will be told if another application wants to use the port. Then you can either close the port and the other application will get it, or ignore the request and the other program will get a PortInUseException, as we did here.

What is this “listener”? The Event Listener model is used in many places in Java. It may be best known for its uses in GUIs (see Section 13.5). The basic form is that you have to register ...