how can I debug a jar at runtime?

Even though it is a runnable jar, you can still run it from a console -- open a terminal window, navigate to the directory containing the jar, and enter "java -jar yourJar.jar". It will run in that terminal window, and sysout and syserr output will appear there, including stack traces from uncaught exceptions. Be sure to have your debug set to true when you compile. And good luck.

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Just thought of something else -- if you're on Win7, it often has permission problems with user applications writing files to specific directories. Make sure the directory to which you are writing your output file is one for which you have permissions.

In a future project, if it's big enough, you can use one of the standard logging facilities for 'debug' output; then it will be easy(ier) to redirect it to a file instead of depending on having a console. But for a smaller job like this, this should be fine.

http://www.eclipsezone.com/eclipse/forums/t53459.html

Basically run it with:

-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=1044

The application, at launch, will wait until you connect from another source.

so the CLI command will be:

java -jar yourJarFileName.jar -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=1044

You can activate JVM's debugging capability when starting up the java command with a special option:

java -agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=y -jar path/to/some/war/or/jar.jar

Starting up jar.jar like that on the command line will:

• put this JVM instance in the role of a server (server=y) listening on port 8000 (address=8000)
• write Listening for transport dt_socket at address: 8000 to stdout and
• then pause the application (suspend=y) until some debugger connects. The debugger acts as the client in this scenario.

Common options for selecting a debugger are:

• Eclipse Debugger: Under Run -> Debug Configurations... -> select Remote Java Application -> click the New launch configuration button. Provide an arbitrary Name for this debug configuration, Connection Type: Standard (Socket Attach) and as Connection Properties the entries Host: localhost, Port: 8000. Apply the Changes and click Debug. At the moment the Eclipse Debugger has successfully connected to the JVM, jar.jar should begin executing.
• jdb command-line tool: Start it up with jdb -connect com.sun.jdi.SocketAttach:port=8000