Question

I know this has probably something to do with class loaders, however I couldn’t find an example (it might be I’m google-ing for the wrong keywords.

I am trying to load a class (or a method) form a string. The string doesn’t contain the name of a class, but the code for a class, e.g.

class MyClass implements IMath {
    public int add(int x, int y) {
         return x + y;
    }
}

and then do something like this:

String s = "class MyClass implements IMath { public int add(int x, int y) { return x + y; }}";
IMath loadedClass = someThing.loadAndInitialize(string);
int result = loadedClass.add(5,6);

Now obviously, the someThing.loadAndInitialize(string) – part is the one I don’t know how to achieve. Is this even possible? Or would it be easier to run JavaScripts and somehow “give” the variables / objects (like x and y)?

Thank you for any hints.

Answer 1

Use Java Compiler API. Here is a blog post that shows you how to do it.

You can use temporary files for this, as this requires input/output file, or you can create custom implementation of JavaFileObject that reads source from string. From the javadoc:

   /**
    * A file object used to represent source coming from a string.
    */
   public class JavaSourceFromString extends SimpleJavaFileObject {
       /**
        * The source code of this "file".
        */
       final String code;

       /**
        * Constructs a new JavaSourceFromString.
        * @param name the name of the compilation unit represented by this file object
        * @param code the source code for the compilation unit represented by this file object
        */
       JavaSourceFromString(String name, String code) {
           super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),
                 Kind.SOURCE);
           this.code = code;
       }

       @Override
       public CharSequence getCharContent(boolean ignoreEncodingErrors) {
           return code;
       }
   }

Once you have the output file (which is a compiled .class file), you can load it using URLClassLoader as follows:

    ClassLoader loader = new URLClassLoader(new URL[] {myClassFile.toURL());
    Class myClass = loader.loadClass("my.package.MyClass");

and then instantiate it, using:

    myClass.newInstance();

or using a Constructor.