Roedy Green <>
Thu, 25 Oct 2007 14:51:27 GMT
On Thu, 25 Oct 2007 11:36:33 GMT, Roedy Green
<> wrote, quoted or indirectly quoted
someone who said :

Has anyone found either sample code or a tutorial or even notes on how
to use to compile generated code on the fly?

Here is some code I wrote trying to understand how this works.
It seem to compile my on-the-fly generated in RAM source code ok,
but I have not figured out how to direct the class file output
somewhere that I can load the code. I will figure this out
eventually, but I thought some of you might enjoy a challenge.
This is one of Sun's more inscrutable bits of code.

 * Demonstrate generating Java source code on the fly, compiling it
 * executing it.<br>
package com.mindprod.example;

import java.util.Arrays;

 * Demonstrate generating Java source code on the fly, compiling it
 * with the JavaCompiler class and executing it.
 * Note JavaCompiler is quite different from JavaCompilerTool that was
 * released with JDK 1.6 beta, now withdrawn.
 * The source code is generated in RAM and never written to disk.
 * @author Roedy Green
 * @version 1.0 2007-10-25
public final class TestJavaCompiler {
// -------------------------- STATIC METHODS

     * Compile from within this JVM without spawning javac.exe or a
separate JVM.
     * @param source points to source, possibly in RAM.
     * @return status of the compile, true all went perfectly without
     * @throws if trouble writing class files.
    private static boolean compile(JavaFileObject... source) {
        final JavaCompiler compiler =

        final JavaCompiler.CompilationTask task =
compiler.getTask(null /* default System.err */,
                null /* standard file manager */,
                null /* standard DiagnosticListener */,
                null /* no options */,
                null /* no annotation classes */,
                // we must convert JavaFileObject... to Iterable<?
extends JavaFileObject>
                Arrays.asList(source) /* source code */);

     * Compose source for sample Java program on the fly.
     * @param className name of class you want to generate
     * @param expression a an expression involving a and/or b you want
     * calculate.
     * @return text of an on-the-fly composed Java class.
    private static String composeAProgram(String className, String
expression) {
        final StringBuilder sb = new StringBuilder(1000);
        sb.append("package com.mindprod.example;\n");
        sb.append("import java.util.Date;\n");
        sb.append("public final class ").append(className).append("
implements Calculator\n");
        sb.append("public double calc( double a, double b )\n");
        sb.append(" {\n");
        sb.append(" return ");
        sb.append(" }\n");
        sb.append("public Date whenCompiled()\n");
        sb.append(" {\n");
        sb.append(" return new Date(");
        sb.append(" }\n");
        return sb.toString();

// --------------------------- main() method

     * Compose a program on the fly, compile it, and execute it.
     * @param args not used
     * @throws if problems writing class
     * @throws ClassNotFoundException if generated class cannot
be found.
     * @throws IllegalAccessException if try to instantiate a
class we are not permitted to access.
     * @throws InstantiationException if cant intstantiate class
     * @throws if malformed class name.
    public static void main(String[] args) throws URISyntaxException,
            InstantiationException {
        // compose text of Java program on the fly.
        final String programText =
                composeAProgram("Hypotenuse", "Math.sqrt( a*a + b*b
        System.out.println("PROGRAM TO BE COMPILED");
        // compile it
        final boolean status = compile(new
RamJavaFileObject("Hypotenuse", programText));
        System.out.println("status of compile: " + status);
        // Load class and create an instance.
        final Calculator calculator =
        // execute its methods:
                .println("Hypotenuse.calc( 3, 4 ) is : "
                        + calculator.calc(3.0, 4.0));
        System.out.println("compiled on: " +

 * Represents the source text of a Java program in RAM.
class RamJavaFileObject extends SimpleJavaFileObject {
     * source text of the program to be compiled
    private final String programText;

    // -------------------------- PUBLIC INSTANCE METHODS
     * constructor
     * @param className class name, without package
     * @param programText text of the program.
     * @throws if malformed class name.
    public RamJavaFileObject(String className, String programText)
throws URISyntaxException {
        super(new URI(className + ".java"), Kind.SOURCE);
        this.programText = programText;

     * Get the text of the java program
     * @param ignoreEncodingErrors ignored.
    public CharSequence getCharContent(boolean ignoreEncodingErrors)
throws IOException {
        return programText;
Roedy Green Canadian Mind Products
The Java Glossary

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