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Migration Guide from Rhino to GraalVM JavaScript

This document serves as a migration guide for code previously targeted to the Rhino engine. See the Java Interoperability guide for an overview of supported features.

Both Rhino and GraalVM JavaScript support a similar set of syntax and semantics for Java interoperability. The most important differences relevant for migration are listed here.

Java.type(typename) instead of java.a.b.c.typename #

GraalVM JavaScript does not put available Java classes in the JavaScript scope. You have to explicitly load the classes using Java.type(typename). GraalVM JavaScript supports the Packages global object, but loading the classes explicitly is still encouraged. The following Java package globals are available in Nashorn compatibility mode (js.nashorn-compat option): java, javafx, javax, com, org, edu.

Console Output of Java Classes and Java Objects #

GraalVM JavaScript provides a print builtin function. It tries to special-case its behavior on Java classes and Java objects to provide the most useful output.

Note that GraalVM JavaScript also provides a console.log function. This is an alias for print in pure JavaScript mode, but uses an implementation provided by Node.js when in Node mode. The behavior around interop objects differs for console.log in Node mode as it does not implement special treatment for such objects.

JavaScript vs Java Strings #

GraalVM JavaScript uses Java strings internally to represent JavaScript strings. This makes it impossible to differentiate whether a specific string was created by JavaScript or by Java code. In GraalVM JavaScript, the JavaScript properties take precedence over Java fields or methods. For instance, you can query the length property (of JavaScript) but you cannot call the length function (of Java) on JavaScript strings - length behaves like a data property, not like a function.

JavaImporter #

The JavaImporter feature is available only in Nashorn compatibility mode (js.nashorn-compat option).

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