AccessController.doPrivileged

It is just getting a system property. Retrieving system properties requires permissions which the calling code may not have. The doPrivileged asserts the privileges of the calling class irrespective of how it was called. Clearly, doPrivileged is something you need to be careful with.

The code quoted is the equivalent of:

String lineSeparator = java.security.AccessController.doPrivileged(
    new java.security.PrivilegedAction<String>() {
        public String run() {
            return System.getProperty("line.separator");
        }
    }
 );

(Don’t you just love the conciseness of Java’s syntax?)

Without asserting privileges, this can be rewritten as:

String lineSeparator = System.getProperty("line.separator");