Start the JVM with the "-D" switch to pass properties to the application and read them with the System.getProperty() method.
SET myvar = Hello world
Set myothervar = nothing
java -Dmyvar="%myvar%" -Dmyothervar="%myothervar%" myClass
Then, in myClass:
String myvar = System.getProperty("myvar");
String myothervar = System.getProperty("myothervar");
If you don't know the name of the variable to be passed to the JVM, then there is no 100% Java way to retrieve them.