How Can I Safely Compile Database-Retrieved Strings into Executable Java Code?

String Conversion to Compilable Java Code

Querying a database often provides stored expressions that require evaluation within conditional structures. The ability to convert these strings into Java compilable code enables dynamic execution of custom expressions. This article explores the potential solution using Java's Compiler API, but highlights the potential risks involved in compiling arbitrary code.

Using Java Compiler API

For Java 6 and above, the Java Compiler API offers a means to programmatically compile Java code. The core component, JavaCompiler, allows you to define source code in memory, offering the ability to create a Comparator object dynamically.

String comparatorClassName = ...; // Random name for comparison class
String expression = ...; // Retrieved database expression
String source = "public class " + comparatorClassName + " implements Comparable<" + comparableClassName + "> {" +
                "    public int compare(" + comparableClassName + " a, " + comparableClassName + " b) {" +
                "        return " + expression + ";" +
                "    }" +
                "}";

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
Writer out = null; // Output writer
Iterable<? extends JavaFileObject> compilationUnits = new ArrayList<>();
compilationUnits.add(new SimpleJavaFileObject() { /* Details for loading source String */ });
compiler.getTask(out, null, null, null, null, compilationUnits).call();

Once compiled, you can instantiate the Comparator and execute the dynamic expression:

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

Cautions

• Compiling arbitrary Java code poses security risks. Only compile trusted code.
• This solution has not been thoroughly tested due to limitations on the compiler object.

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