how could I request Java garbage collection externally, starting the program from JAR (Windows BAT used)?
System.gc()But... When I'm running the jar from command-line / bat the java console doesn't seem to open. Couldn't find help with a brief googling, maybe somebody here?
You can use jconsole to connect to a JVM that is running locally - This provides a "Perform GC" button on the GUI.
You'll need to specify -Dcom.sun.management.jmxremote when you kick off your java process.
For a purely command-line approach, you should be able to use jcmd. I believe jcmd has been part of the standard JDK install since at least Java 1.7. The command would be something like this:
jcmd <process-id> GC.run
You can get a list of available diagnostic commands that jcmd provides for a particular process like this:
jcmd <process-id> help
If you use the very latest java 6 you also have jvisualvm which complements and extend the jconsole functionality. They are both very useful tools.
You normally should not have any reason to force garbage collection. Doing so, messes up the garbage collector algorithms (mainly their performance). It will also slow down the program while garbage is being collected. If there is a memory issue, you should use memory tracing tools to find out where references are being held? (Are listeners unregistered?)
As in the other answers, you can use jconsole or jvisualvm. You can also use jmx to do it programmatically.
