public boolean execute() throws BuildException 
{
        attributes.log("Using external javac compiler", Project.MSG_VERBOSE);
        Commandline cmd = new Commandline();
        cmd.setExecutable(getJavac().getJavacExecutable());
        if (!assumeJava11() && !assumeJava12()) {
            setupModernJavacCommandlineSwitches(cmd);
        } else {
            setupJavacCommandlineSwitches(cmd, true);
        }
        int firstFileName = assumeJava11() ? -1 : cmd.size();
        logAndAddFilesToCompile(cmd);
        //On VMS platform, we need to create a special java options file
        //containing the arguments and classpath for the javac command.
        //The special file is supported by the "-V" switch on the VMS JVM.
        if (Os.isFamily("openvms")) {
            return execOnVMS(cmd, firstFileName);
        }
        return
                executeExternalCompile(cmd.getCommandline(), firstFileName,
                        true)
                == 0;
}

Performs a compile using the Javac externally.