Nashorn is a runtime environment for programs written in ECMAScript 5.1.

# Usage

The recommended way to use Nashorn is through the JSR-223 "Scripting for the Java Platform" APIs found in the {@link javax.script} package. Usually, you'll obtain a {@link javax.script.ScriptEngine} instance for Nashorn using:

    import javax.script.*;
    ...
    ScriptEngine nashornEngine = new ScriptEngineManager().getEngineByName("nashorn");

and then use it just as you would any other JSR-223 script engine. See {@code jdk.nashorn.api.scripting} package for details.

# Compatibility

Nashorn is 100% compliant with the ECMA-262 Standard, Edition 5.1. It requires a Java Virtual Machine that implements the JSR-292 "Supporting Dynamically Typed Languages on the Java Platform" specification (often referred to as "invokedynamic"), as well as the already mentioned JSR-223.

# Interoperability with the Java platform

In addition to being a 100% ECMAScript 5.1 runtime, Nashorn provides features for interoperability of the ECMAScript programs with the Java platform. In general, any Java object put into the script engine's context will be visible from the script. In terms of the standard, such Java objects are not considered "native objects", but rather "host objects", as defined in section 4.3.8. This distinction allows certain semantical differences in handling them compared to native objects. For most purposes, Java objects behave just as native objects do: you can invoke their methods, get and set their properties. In most cases, though, you can't add arbitrary properties to them, nor can you remove existing properties.

## Java collection handling

Native Java arrays and {@link java.util.List}s support indexed access to their elements through the property accessors, and {@link java.util.Map}s support both property and element access through both dot and square-bracket property accessors, with the difference being that dot operator gives precedence to object properties (its fields and properties defined as {@code getXxx} and {@code setXxx} methods) while the square bracket operator gives precedence to map elements. Native Java arrays expose the {@code length} property.

## ECMAScript primitive types

ECMAScript primitive types for number, string, and boolean are represented with {@link java.lang.Number}, {@link java.lang.CharSequence}, and {@link java.lang.Boolean} objects. While the most often used number type is {@link java.lang.Double} and the most often used string type is {@link java.lang.String}, don't rely on it as various internal optimizations cause other subclasses of {@code Number} and internal implementations of {@code CharSequence} to be used.

## Type conversions

When a method on a Java object is invoked, the arguments are converted to the formal parameter types of the Java method using all allowed ECMAScript conversions. This can be surprising, as in general, conversions from string to number will succeed according to Standard's section 9.3 "ToNumber" and so on; string to boolean, number to boolean, Object to number, Object to string all work. Note that if the Java method's declared parameter type is {@code java.lang.Object}, Nashorn objects are passed without any conversion whatsoever; specifically if the JavaScript value being passed is of primitive string type, you can only rely on it being a {@code java.lang.CharSequence}, and if the value is a number, you can only rely on it being a {@code java.lang.Number}. If the Java method declared parameter type is more specific (e.g. {@code java.lang.String} or {@code java.lang.Double}), then Nashorn will of course ensure the required type is passed.

## SAM types

As a special extension when invoking Java methods, ECMAScript function objects can be passed in place of an argument whose Java type is so-called "single abstract method" or "SAM" type. While this name usually covers single-method interfaces, Nashorn is a bit more versatile, and it recognizes a type as a SAM type if all its abstract methods are overloads of the same name, and it is either an interface, or it is an abstract class with a no-arg constructor. The type itself must be public, while the constructor and the methods can be either public or protected. If there are multiple abstract overloads of the same name, the single function will serve as the shared implementation for all of them, and additionally it will also override any non-abstract methods of the same name. This is done to be consistent with the fact that ECMAScript does not have the concept of overloaded methods.

## The {@code Java} object

Nashorn exposes a non-standard global object named {@code Java} that is the primary API entry point into Java platform-specific functionality. You can use it to create instances of Java classes, convert from Java arrays to native arrays and back, and so on. The methods on the objects are directly implemented by public static methods on the class {@code NativeJava}, see that class for details on what functionality is available.

## Representations of Java types

The method {@code Java.type(typeName)} takes a name of a type, and returns an object representing a Java type. You can use that object to both create new instances of Java classes, as well as to access static fields and methods on them. The type object is distinct from the {@code java.lang.Class} object, which represents the reflective run-time type identity and doesn't carry i.e. static members. Again, see the link for {@code NativeJava} above for details.

## Other non-standard built-in objects

In addition to {@code Java}, Nashorn also exposes some other non-standard built-in objects: {@code JSAdapter}, {@code JavaImporter}, {@code Packages}.