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>2.2.0</version>
</dependency>

You can find the latest version here maven search for net.openhft:compiler

Simple example

You needa CachedCompiler and access to you JDK's tools.jar.

 private final CachedCompiler cc = new CachedCompiler(null, null);
 
 // dynamically you can call
 Class aClass = cc.loadFromJava(className, javaCode);
 KnownInterface o = (KnownInterface) aClass.newInstance();

I suggest making you class implement a "KnownInterface" 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.