Instead of redirecting like this:

java Foo > log

do it like this:

java Foo >> log

(That's assuming that's how you're redirecting the output. If it's not, please give more details.)

Instead of using ">" to redirect like this:

java Foo > log

use ">>" to append normal "stdout" output to a new or existing file:

java Foo >> log

However, if you also want to capture "stderr" errors (such as why the Java program couldn't be started), you should also use the "2>&1" tag which redirects "stderr" (the "2") to "stdout" (the "1"). For example:

java Foo >> log 2>&1