Java-Runtime-Compiler

This takes a String, compiles it and loads it returning you a class from what you built.
By default it uses the current ClassLoader. It supports nested classes, but otherwise builds one class at a time.

On maven central

You can include in your project with

<dependency>
    <groupId>net.openhft</groupId>
    <artifactId>compiler</artifactId>
    <version><!-- The latest version (see above) --></version>
</dependency>

Simple example

You need a CachedCompiler and access to your JDK's tools.jar.

 // dynamically you can call
 String className = "mypackage.MyClass";
 String javaCode = "package mypackage;\n" +
                  "public class MyClass implements Runnable {\n" +
                  "    public void run() {\n" +
                  "        System.out.println(\"Hello World\");\n" +
                  "    }\n" +
                  "}\n";
 Class aClass = CompilerUtils.CACHED_COMPILER.loadFromJava(className, javaCode);
 Runnable runner = (Runnable) aClass.newInstance();
 runner.run();

I suggest making you class implement a KnownInterface of your choice as this will allow you to call/manipulate instances of you generated class.

Another more hacky way is to use this to override a class, provided it hasn't been loaded already.
This means you can redefine an existing class and provided the methods and fields used match, you have compiler redefine a class and code already compiled to use the class will still work.

Using the CachedCompiler.

In this example, you can configure the compiler to write the files to a specific directory when you are in debug mode.

private static final CachedCompiler JCC = CompilerUtils.DEBUGGING ?
                                                   new CachedCompiler(new File(parent, "src/test/java"), new File(parent, "target/compiled")) :
                                                   CompilerUtils.CACHED_COMPILER;

By selecting the src directory to match where your IDE looks for those files, it will allow your debugger to set into code you have generated at runtime.

Note: you may need to delete these files if you want to regenerate them.