Convert String to Code

If you are using Java 6, you could try the Java Compiler API. At its core is the JavaCompiler class. You should be able to construct the source code for your Comparator object in memory.

Warning: I have not actually tried the code below as the JavaCompiler object is not available on my platform, for some odd reason…

Warning: Compiling arbitrary Java code can be hazardous to your health.

Consider yourself warned…

String comparableClassName = ...; // the class name of the objects you wish to compare
String comparatorClassName = ...; // something random to avoid class name conflicts
String source = "public class " + comparatorClassName + " implements Comparable<" + comparableClassName + "> {" +
                "    public int compare(" + comparableClassName + " a, " + comparableClassName + " b) {" +
                "        return " + expression + ";" +
                "    }" +
                "}";

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

/*
 * Please refer to the JavaCompiler JavaDoc page for examples of the following objects (most of which can remain null)
 */
Writer out = null;
JavaFileManager fileManager = null;
DiagnosticListener<? super JavaFileObject> diagnosticListener = null;
Iterable<String> options = null;
Iterable<String> classes = null;
Iterable<? extends JavaFileObject> compilationUnits = new ArrayList<? extends JavaFileObject>();
compilationUnits.add(
    new SimpleJavaFileObject() {
        // See the JavaDoc page for more details on loading the source String
    }
);

compiler.getTask(out, fileManager, diagnosticListener, options, classes, compilationUnits).call();

Comparator comparator = (Comparator) Class.forName(comparableClassName).newInstance();

After this, you just have to store the appropriate Java expression in your database field, referencing a and b.