Feel free to use this as a reference to tinkering with Java at runtime.
Choosing your JRE
To choose your JRE, use
sudo update-alternatives --config java
This will give something like the following output.
Selection Path Priority Status
0 /usr/lib/jvm/java-6-openjdk/jre/bin/java 1061 auto mode
* 1 /usr/lib/jvm/java-6-openjdk/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-6-sun/jre/bin/java 63 manual mode
3 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1051 manual mode
You can then select which Java runtime you want through the number selection.
Choosing your JVM
Sun/Oracle have two JVM choices, -server and -client.
If you select the OpenJDK as your Java runtime environment you have additional options.
When you type
java into the terminal with no other parameters, the help lists several optional VMs. I'm not sure which ones come with OpenJDK but 3 popular ones are JamVM, Zero and Cacao
To use these, simply type
java -jamvm 'your other parameters here'
java -cacao 'your other parameters here'
java -zero 'your other parameters here'
java -server 'your other parameters here
The -server VM is normally the default. You can also specify
-client but in 64-bit IcedTea6 it appears to run the same version as -server.
There are most likely others but I find the default option to be the most responsive.
Setting your Memory
Finally, how to set the memory of Java (just because)
java -Xmx1024m -Xms128m 'your other parameters here'
This limits the memory allowed for the Java program to a maximum of 1024 MB, and sets its initial memory size to 128 MB. This is a great way of defining minimum system requirements. The Java 6 man page for the
java command describes these options and others.
That's all. If anyone has additional Java tweaks for Ubuntu then leave them in the comments and I'll add them.