how can I debug a jar at runtime?

I'm into a quite strange position (from my java-newbie point of view):

1. using Eclipse I wrote a "java program" (some .java files with classes into) which essentially (batch) reads a text *.csv file, "evaluates" its contents, and writes out results into an *_out.csv text file. To locate the input file it uses a "file chooser" (got sample from here: http://docs.oracle.com/javase/tutorial/uiswing/components/filechooser.html)

2. I debugged all the code and, using the debugger, it works.

3. I ran the code (the main class, which calls all the other in sequence) and it works, while in Eclipse.

4. I exported all the project's contents into a "runnable jar" file.

Notice that, file chooser apart, this is mainly a "batch" which reads and writes: nearly no User Interface at all. While into Eclipse I shown some internal results using something like "if(debug) System.out.print("something to print");" providing to set "debug" TRUE while debugging and FALSE while in production environment.

ALL of the above worked!

Now, starting the runnable jar (double-click on the jar file, in Win/XP), I can see the file chooser and I can use it but, after choosing the input file... nothing more: (having no user interface) I don't know if the file was read, I don't see any generated output file, and I've even no "console" to list any intermediate debug message, to see if the jar is working, even if I re-export it with the debug variable set to TRUE.

Is there a way to "runtime debug" the running jar (like VB's MsgBox, or something other)? some kind of "log file" I can "enable" or look into? (obviously, as my jar is not writing the result file, I just can't try writing a *.log too) I have also to say I just can't install other than Eclipse on my machine (and just been lucky it ran), so no usual developer's tools, utilities and other useful things.

Solution 1:

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

Solution 2:

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