The usual DocBook element for source code is the programlisting. One of the optional attributes for a programlisting is the language. If you set the language attribute to cpp or to java, then the source code will be highlighted accordingly.

Here, for example, is a programlisting with no language value set.

#include <iostream>

using namespace std;
/*  A simple
    program */
int main()
{
  cout << "Hello World!" << endl;
  return 0; // Zero denotes a normal termination
}

and here is the same listing with a language attribute of cpp:

#include <iostream>

using namespace std;
/*  A simple
    program */
int main()
{
  cout << "Hello World!" << endl;
  return 0; // Zero denotes a normal termination
}