How to grep a String in all the java files in subdirectories?

Use recursive grep:

grep -r "String" --include=*.java .

I guess the unxUtils provide the find utility. So you could try to do:

find . -name "*.java" -exec grep "String" {} \;

or

find . -name "*.java" -exec grep "String" {} \+

The first version will execute one grep command for each file found. The latter version will only execute as many grep commands as necessary but not every find version supports the + argument.