Finding new Java class at runtime

Question

I have a functionality that I wish to provide to a customer for a software mockup that we are preparing - and I want to know if it's

1. possible
2. intelligent (a.k.a. not stupid)
3. the best thing

I want the customer to be able to write a java class that implements my Computable interface and stick it in some predetermined folder. This folder will contain the .java files rather than .class files. Then, at runtime, I want my program to search that folder and extract all of the Computables from that folder and store them in a map from the name of the Computable to the Computable object. The Computable should only have a default constructor and the it interface will only have one method called compute which maps an array of Object to an Object.

Answer 1

The Java Compiler API introduced in Java SE 6 should give you what you need.

Answer 2

You may find Google Reflections useful to find classes implementing/extending a certain interface/superclass in the classpath. It's then as straightforward as

Reflections reflections = new Reflections("my.project.prefix");
Set<Class<? extends SomeClassOrInterface>> subTypes = reflections.getSubTypesOf(SomeClassOrInterface.class);

Then, to test if it indeed has a no-arg default constructor, just check for each if Class#newInstance() doesn't throw any exception.

Answer 3

There are several suggestions provided as answers to this question.

Here too On-the-fly, in-memory java code compilation for Java 5 and Java 6

Answer 4

If it's easy enough to compile at runtime that would be fine.

You can use javax.tools to do the compilation as needed. Create dynamic applications with javax.tools may help, too. It's also possible to do it in memory.

One caveat: using the compiler creates a dependency on the JDK; the JRE alone is insufficient.