How to replace " \ " with " \\ " in java

I want to supply a path to JNI and it reads only in this way.

That's not right. You only need double backslashes in literal strings that you declare in a programming language. You never have to do this substitution at runtime. You need to rethink why you're doing this.

You could use replaceAll:

String escaped = original.replaceAll("\\\\", "\\\\\\\\");

Don't use String.replaceAll in this case - that's specified in terms of regular expressions, which means you'd need even more escaping. This should be fine:

String escaped = original.replace("\\", "\\\\");

Note that the backslashes are doubled due to being in Java string literals - so the actual strings involved here are "single backslash" and "double backslash" - not double and quadruple.

replace works on simple strings - no regexes involved.