Does a way exist to instruct the Java compiler to compile code only
if a certain version of Java is supported? For example, I may want to
measure with milliseconds in Java 1.4 or nanoseconds if I'm running in
Java 5.

import java.io.IOException;

public class JVMLoadTimeRunner {

    public static void main(String[] args) {

        long start = System.currentTimeMillis(); // only compile this line
in Java 1.4
        //long start = System.nanoTime(); // only compile this line in Java
5

        try {
            Process newP = Runtime.getRuntime().exec( cmd );
            newP.waitFor();
            long end = System.currentTimeMillis();
            System.out.println( "Process Execution took " + (float)(end-start)/
1000F + " seconds" );
// Java 5
            // System.out.println( "Process Execution took " + (float)(end-
start)/1000000000F + " seconds" );
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            System.out.println( "Process wait threw an Exception" );
            e.printStackTrace();
        }
    }
}