Can Java Compile and Execute Code from a String?

Compiling Dynamic Code from a String

Question:

Can a String containing code be converted into a format that can be compiled and executed in Java?

Answer:

Yes, using the Java Compiler API. Here's how:

In Java 6 or later, utilize the JavaCompiler class to compile code dynamically.

Code:

String comparableClassName = ...;
String comparatorClassName = ...;
String source = "public class " + comparatorClassName + " implements Comparable<" + comparableClassName + "> {" +
                "    public int compare(" + comparableClassName + " a, " + comparableClassName + " b) {" +
                "        return " + expression + ";" +
                "    }" +
                "}";

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

/*
 * Refer to the JavaCompiler JavaDoc page for examples of the following objects (most 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<>();
compilationUnits.add(
    new SimpleJavaFileObject() {
        // See 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();

Note:

• Compiling arbitrary code can have security implications.
• Store the Java expression in the database field with appropriate references to variables a and b.

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