Many languages, and in the runtime bind them

While working on App Engine's new Java runtime, we realized that we were missing something important. Something the Python runtime already had. Something sleek, yet powerful ... and approximately 80 characters wide and 25 characters tall. Yes, we're talking about the Python runtime's nifty shell demo app.

Naturally, we wanted to take the existing shell demo and extend its functionality for our new Java environment. Our idea was that we'd create one REPL to rule them all: a shell demo that supports several different JVM-based languages simultaneously. It was an easy decision to use GWT to build the user-interface, making the terminal interface a cinch. Once we had the basic shell framework, we then added some languages: Beanshell, Clojure, Groovy, JavaScript, Python, Ruby, Scala, and Scheme.

This helps us underscore two important compatibility features of Java App Engine:

1. 
   Google App Engine for Java accepts Java bytecode, not Java source code. It even supports loading bytecode generated at run time. That means that our new Java runtime can support any language with a compiler that targets the JVM. In fact, many advanced scenarios work as well - such as libraries that rely on runtime bytecode generation (such as dependency injection frameworks, AOP libraries, and expression-language runtimes).
   


2. 
   The security sandbox in Google App Engine for Java is flexible. Other techniques for sandboxing Java can be restrictive about the permissions they grant to untrusted code, limiting the types of constructs you can use. For example, many secure Java environments don't allow you to create custom ClassLoaders or use tricks for reflection like setAccessible. Java App Engine attempts to provide this type of functionality in a secure fashion, making it possible to run more types of code (and more languages).
   

Now, it must be said: please note that this is a demonstration app we developed to show off the power and flexibility of our new Java runtime; it's not a production service, and you shouldn't rely on it for anything important. Further, since this is a single shared instance of this application, running server code code from many users at once, you should exercise caution in entering any sensitive data like passwords into this demo. Finally, though we've worked to ensure rich functionality for as many languages as possible, this is still a proof of concept, and there is surely some language functionality missing from our environment.

But, without further ado, may we present to you, the Lord of the REPLS.

-- Google Software Engineers, Toby Reyelts and James Robinson.

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