Function
Specifies the system directory, which is the directory that
contains subdirectories holding databases that you create and the text file derby.properties.
If the system directory that you specify with derby.system.home does not exist at startup, creates the directory automatically.
Default
Current directory (the value of the JVM system property user.dir).
If you do not explicitly set the derby.system.home property when starting , the default is the directory in
which was started.
You should always explicitly
set the value of derby.system.home.
Example
-Dderby.system.home=C:\
Dynamic or static
This property is static; if you change it while is running,
the change does not take effect until you reboot.