Where is the Java scripting engine used? [closed]

Answer 1

Here are some cases where I've used it.

1) Java wants to call scripting language, example 1. I have a Java app that accepts user comments via the WMD JavaScript widget. (Same widget that StackOverflow uses, actually.) User enters comments in the Markdown format and a JavaScript library called Showdown converts it to HTML in two places: (1) on the client, to support real-time previews; and (2) on the server, since I want the client to send pure Markdown to the server and store that there so the user can edit the Markdown later (instead of having to somehow reverse the HTML into Markdown). When storing the comment on the server, I do run the conversion there as well, and I store the HTML alongside the Markdown so I don't have to dynamically convert the Markdown when displaying comment lists. To ensure that the HTML on the server matches the HTML on the client, I want to use the exact same Showdown library. So I run Showdown server-side inside the Rhino JavaScript engine.

2) Java wants to call scripting language, example 2. I'm working on a deployment automation application that involves stakeholders across different roles, such as developers, sysadmins and release engineers. The overall app (workflow and UI) is a Java app, but at various locations it calls various scripts (e.g. Ruby, bash), such as for pushing packages, verifying configuration, installing packages, smoke testing, etc. This is partly because script is better/more economical for expressing directory creation, copying, moving, wgetting, etc., and partly because the people who own that particular piece of the pie know how to work with scripting languages but not Java. So we invoke scripts here using Java's Scripting API. Admittedly in this case we could just execute the scripts outside of Java but see #3 below.

3) Scripting language wants to call Java. In the aforementioned deployment application, we have web-based deployment logs, and we put a lot of effort into making the deployment logs as easy to read and understand as possible, because a large developer/SQA/release engineer population consumes the logs, and not everybody understands all the details of what exactly goes on with a deployment. Pretty-printing and color-coding are part of the approach. We implemented a pretty-printing deployment log API in Java but we want to be able to have the scripts call that. So for example when the Ruby push script runs, we want it to log its progress to the pretty-printer. Running Ruby inside JRuby allows the Ruby script to see the Java pretty-printer API.

Answer 2

In Java 6, scripting engine support is built in. For example,

    // Create a script engine manager
    ScriptEngineManager factory = new ScriptEngineManager();

    // Create a JavaScript engine
    ScriptEngine engine = factory.getEngineByName("JavaScript");

    // Evaluate JavaScript code from String
    engine.eval("print('Hello, World')");

Why would you use one? Some reasons:

1. you have a library in a scripting language that you want to use in Java (e.g. a Python library that you could run via Jython)
2. You want to provide a configurable programming mechanism for customers, such that they can provide short code snippets. For example, I've done this in the past allowing customers to write filters using JavaScript (e.g. is x < 2 and y > 5 and z > 10 ?).
3. You can implement more complex logic in tools like Ant by scripting directly in the configuration file
4. You can implement solutions in a language more suited to that domain (e.g. using lambdas via Clojure), but maintain your reliance on the JVM.

Implementations include Rhino (a Java implementation of Javascript), Jython (a Java Python) and many more.

Answer 3

"What are all the right use-case to use scripting engine?" This is a pretty vague question. There are many use cases. Here are just a few I can think of right away:

1. Plugin/extension system
2. IDE
3. Programming tutorial with live demos

I am assuming you are referring to JSR 223 in particular. If so, you should look at scripting.dev.java.net

Answer 4

I haven't used JavaScript specifically, but I've integrated Groovy into my application framework to provide a domain specific language (DSL). I've created functions and classes that hook into my application.

The user is allowed to script common operations within the application (macros) as well as implement lightweight processing to avoid the much heavier code-compiler-jar-deploy solution. If the user has an idea for a plugin to my processing framework they can prototype via Groovy in realtime and move back to Java (maybe even native) when there is time (or when speed is needed). Keep in mind that scripting is typically orders of magnitude slower than Java/C#/C/C++