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Discover how to effectively pass Java objects into your GraalJS functions and access their properties without encountering errors.
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This video is based on the question https://stackoverflow.com/q/70689954/ asked by the user 'Lenny Boi' ( https://stackoverflow.com/u/17919219/ ) and on the answer https://stackoverflow.com/a/70690539/ provided by the user 'Mike Clark' ( https://stackoverflow.com/u/312407/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Using a Java Object as an Argument in GraalJS: A Comprehensive Guide

If you're working with GraalJS and trying to integrate Java objects into your JavaScript functions, you may run into some hurdles due to tight access control settings. One common issue arises when attempting to access methods of a Java object from within a JavaScript function. In this guide, we will walk you through the problem and explore effective solutions.

The Problem

Imagine you have a Java object, such as a Train, and you want to invoke a JavaScript function that uses this object as an argument. Here's a quick look at some Java code to set up your GraalJS environment and run the function:

[[See Video to Reveal this Text or Code Snippet]]

You also have a simple Train class:

[[See Video to Reveal this Text or Code Snippet]]

In your JavaScript code (Test.js), you attempt to access the getSpeed method:

[[See Video to Reveal this Text or Code Snippet]]

However, you encounter the following error:

[[See Video to Reveal this Text or Code Snippet]]

This indicates that the JavaScript environment is unable to access the getSpeed() method of the Train object.

The Solution

Understanding GraalJS Security

GraalJS, in its default setting, limits access to Java methods and fields for security reasons. This means that methods like getSpeed() won't be accessible by default when invoking them from JavaScript unless configured properly.

Access Configurations

To resolve this issue, there are two main approaches you can take:

1. Enable Host Access Globally

If you want to allow access to all host fields and methods globally, you can use the following configuration:

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Adding this line will permit GraalJS to access all Java methods and properties on the Java objects.

2. Allow Access on a Per-Method Basis

For more granular security management, you can allow access for specific methods. This is done by using the -HostAccess.Export annotation on the method within your Java class. Here’s how you'd modify your Train class:

[[See Video to Reveal this Text or Code Snippet]]

With this annotation, only the getSpeed() method will be accessible from JavaScript, maintaining a tighter security model while still allowing necessary interactions.

Final Thoughts

Incorporating Java objects in GraalJS can enhance the functionality of your applications, but understanding and managing the security model is crucial. Whether you choose to enable broad access or select specific methods to expose, make sure it aligns with your application's security requirements.

With this guide, you should now be able to effectively pass Java objects to your JavaScript functions in GraalVM without running into access issues. Enjoy coding!