Is it possible to create an Object from a source code String?

[Rob Micah]

Hello, moose-people. I come in peace. This is my first post here and I have a question about dynamically creating objects. Let's say I have a String containing something like "new Date()".
Is there a way to evaluate that string and return an Object out of it? I have looked at the javax.tools package but I can't see any way to retrieve an object.

[Stephan van Hulst]

Sure there is. It depends on the format of the String.

If you describe exactly how the String format generally looks, we might suggest some ideas.

And welcome to CodeRanch

[Joydeep Ghatak]

Hi Rob,

As correctly mentioned by Stephan, yes it is very much possible, but at the same time we would like to know more on two points,

a String containing something like "new Date()"

Need some more information on that. (Exactly how are going to do that.)

Is there a way to evaluate that string

What kind of evaluation (I aasume you mean String manupulation) is going to perform ?

Also I think the following link will be helpful for you,
https://coderanch.com/t/385084/java/java/creating-simple-java-objects-dynamically

Thanks
Joydeep

[Jesper de Jong]

In dynamic programming languages, for example JavaScript or Python, you can easily do such a thing. But Java is a compiled language, it's not dynamically interpreted from source code, so it is not easy, and there is no built-in way, to evaluate arbitrary Java statements in a string at runtime.

You might be able to do something like what you want using BeanShell.

[Rob Micah]

I'm trying to create a collection of Objects that are created dynamically based on a set of Strings that contain Java expressions. These objects will be represented by a select list in a web interface. I'm sort of playing the middle man in this operation. Does that make sense or should I go into more detail?

[Stephan van Hulst]

Maybe if you tell us the purpose of it.

This seems like a very complicated and probably unnecessary way of doing things. Like Jesper hinted, there are languages better suited for this sort of thing.

[Bear Bibeault]

If there are a finite number of possibilities that will be presented in a dropdown, it might just make sense to have all of them pre-evaluated (and perhaps stored in a Map) and just choose the one that matches the dropdown selection.

[Rob Micah]

Here's my situation in detail. I am using JasperReports in a web application. Jasper lets users set default values for its parameters using Java expressions. Unfortunately it has no built-in support for providing a pre-defined list of values for a parameter. That's where I'm trying to apply this technique; by letting users specify in a list of properties Java expressions just like they can do for the default value.

So the flow looks like this:

A developer creates a report, compiles it, and places it in a pre-defined directory for the web application to read.

A user sends a request through a browser for a certain report

My web application reads the compiled report parameters and dynamically generates a page for the user to enter parameters.

The web app runs the report and returns its output

So step 3 is where I need this support. Granted this is ambitious and I have somewhat accomplished this by now using the ScriptEngine class with Javascript but it would be nice to keep things consistent for the people who have to design these reports.