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Nashorn: Run JavaScript on the JVM

Nashorn

Nashorn is a high-performance JavaScript runtime written in Java for the JVM. It allows developers to embed JavaScript code inside their Java applications and even use Java classes and methods from their JavaScript code. You can think it as an alternative to Google's V8 JavaScript engine. It is a successor to Rhino JavaScript runtime which came bundled with earlier JDK versions. Nashorn is written from scratch using new language features like JSR 292(Supporting Dynamically Typed Languages) and invokedynamic.

From the Nashorn documentation:

Nashorn uses invokedynamic to implement all of its invocations. If an invocation has a Java object receiver, Nashorn attempts to bind the call to an appropriate Java method instead of a JavaScript function. Nashorn has full discretion about how it resolves methods. As an example, if it can't find a field in the receiver, it looks for an equivalent Java Bean method. The result is completely transparent for calls from JavaScript to Java.

Currently, Nashorn supports ECMAScript 5.1 specification and work is in progress to support ECMAScript 6 as well. Few ECMAScript 6 features like let and const are available in latest JDK 8 updates(40 and above) and we will cover them later in this chapter.

In this chapter, we will cover the following:

  • Working with Nashorn command-line
  • Accessing Java classes and methods
  • Using external JavaScript libraries
  • Writing scripts
  • Using Nashorn from Java code
  • Using Java 8 features like Streams and Lambdas inside JavaScript code
  • Turning off Java language access

Working with Nashorn command-line

JDK 8 comes bundled with two command-line tools that can be used to work with Nashorn engine. These two command-line tools are jrunscript and jjs. jjs is recommended to be used when working with Nashorn so we will only discuss it. To use jjs, you have to add jjs to the path. On *nix machines, you can do that adding a symbolic link as shown below.

$ cd /usr/bin
$ ln -s $JAVA_HOME/bin/jjs jjs

Windows users can add $JAVA_HOME/bin to the path for easy access.

Once you have set the symbolic link you can access jjs from your terminal. To check version of jjs, run the following command.

$ jjs -v
nashorn 1.8.0_60
jjs>

It will render the version and then show jjs> prompt. You can view the full version of jjs by using jjs -fv command.

To quit the jjs shell, you can use Ctrl-C.

Once you are inside jjs, you can execute any JavaScript code as shown below.

jjs> print("learning about Nashorn")
learning about Nashorn

You can define functions as shown below.

jjs> function add(a,b) {return a + b;}

You can call the add function as shown below.

jjs> add(5,10)
15

Accessing Java classes and methods

It is very easy to access Java classes from within Nashorn. Assuming you are inside the jjs shell, you can create an instance of HashMap as shown below.

jjs> var HashMap = Java.type("java.util.HashMap")
jjs> var userAndAge = new HashMap()
jjs> userAndAge.put("shekhar",32)
null
jjs> userAndAge.put("rahul",33)
null
jjs> userAndAge.get("shekhar")
32

In the code shown above we have used Java global object to create HashMap object. Java global object has type method that takes a string with the fully qualified Java class name, and returns the corresponding JavaClass function object.

jjs> HashMap
[JavaClass java.util.HashMap]

The var userAndAge = new HashMap() is used to instantiate java.util.HashMap class using the new keyword.

You can access values by either calling the get method or using the [] notation as shown below.

jjs> userAndAge["shekhar"]
32

Similarly, you can work with other Java collections. To use an ArrayList you will write code as shown below.

jjs> var List = Java.type("java.util.ArrayList")
jjs> var names = new List()
jjs> names.add("shekhar")
true
jjs> names.add("rahul")
true
jjs> names.add("sameer")
true
jjs> names.get(0)
shekhar
jjs> names[1]
rahul

Accessing static methods

To access static methods you have to first get the Java type using Java.type method and then calling method on JavaClass function object.

jjs> var UUID = Java.type("java.util.UUID")
jjs>
jjs> UUID.randomUUID().toString()
e4242b89-0e94-458e-b501-2fc4344d5498

You can sort list using Collections.sort method as shown below.

jjs> var Collections = Java.type("java.util.Collections")
jjs>
jjs> Collections.sort(names)
jjs> names
[rahul, sameer, shekhar]
jjs>

Using external JavaScript libraries

Let's suppose we want to use an external JavaScript library in our JavaScript code. Nashorn comes up with a built-in function -- load that loads and evaluates a script from a path, URL, or script object. To use lodash library we can write code as shown below.

jjs> load("https://raw.github.com/lodash/lodash/3.10.1/lodash.js")

jjs> _.map([1, 2, 3], function(n) { return n * 3; });
3,6,9

Writing scripts

You can use Nashorn extensions that enable users to write scripts that can use Unix shell scripting features. To enable shell scripting features, you have to start jjs with -scripting option as shown below.

jjs -scripting
jjs>

Now you have access to Nashorn shell scripting global objects.

$ARG: This global object can be used to access the arguments passed to the script

$ jjs -scripting -- hello hey
jjs>
jjs> $ARG
hello,hey

$ENV: A map containing all the current environment variables

jjs> $ENV["HOME"]
/Users/shekhargulati

jjs> $ENV.JAVA_HOME
/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home

$EXEC: launches processes to run commands

jjs> $EXEC("pwd")
/Users/shekhargulati/java8-the-missing-tutorial

Writing executable scripts

You can use shebang(#!) at the beginning of the script to make a script file run as shell executable. Let's write a simple script that reads content of a file. We will use Java's Files and Paths API.

#!/usr/bin/jjs

var Paths = Java.type("java.nio.file.Paths");
var Files = Java.type("java.nio.file.Files");

Files.lines(Paths.get($ARG[0])).forEach(function(line){print(line);})

We will invoke it as

$ jjs ch10/lines.js -- README.md

Using Nashorn from Java code

To use Nashorn from inside Java code, you have to create an instance of ScriptEngine from ScriptEngineManager as shown below. Once you have ScriptEngine you can evaluate expressions.

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class NashornExample1 {

    public static void main(String[] args) throws ScriptException {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine nashorn = manager.getEngineByName("nashorn");
        Integer eval = (Integer) nashorn.eval("10 + 20");
        System.out.println(eval);
    }
}

Using bindings

import javax.script.*;
import java.util.AbstractMap.SimpleEntry;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toMap;

public class NashornExample2 {

    public static void main(String[] args) throws ScriptException {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine nashorn = manager.getEngineByName("nashorn");

        Bindings bindings = new SimpleBindings(Stream.of(
                new SimpleEntry<>("a", 10),
                new SimpleEntry<>("b", 20))
                .collect(toMap(SimpleEntry::getKey, SimpleEntry::getValue)));
        Double eval = (Double) nashorn.eval("a + b", bindings);
        System.out.println(eval);
    }
}

Using Java 8 features like Streams and Lambdas inside JavaScript code

Java 8 supports lambdas and many API in JDK make use of them. Every collection in Java has forEach method that accepts a consumer. Consumer is an interface with one method. In Java, you can write following:

Arrays.asList("shekhar","rahul","sameer").forEach(name -> System.out.println(name));

// shekhar
// rahul
// sameer

In Nashorn, you can use them same API but you will pass JavaScript function instead as shown below.

jjs> var Arrays = Java.type("java.util.Arrays")
jjs> Arrays.asList("shekhar","rahul","sameer")
[shekhar, rahul, sameer]
jjs> var names = Arrays.asList("shekhar","rahul","sameer")
jjs> names.forEach(function(name){print(name);})
shekhar
rahul
sameer

You can also use Stream API with Nashorn as shown below.

jjs> names
    .stream().filter(function(name){return name.startsWith("s");})
    .forEach(function(name){print(name);})

shekhar
sameer

Turning off Java language access

In case you need to disallow Java usage, you can very easily turn off by passing --no-java option to jjs as shown below.

→ jjs --no-java
jjs>
jjs> var HashMap = Java.type("java.util.HashMap")
<shell>:1 TypeError: null has no such function "type"

Now when you will try to use java.util.HashMap you will get TypeError.

You can do the same with Java code as well.

import jdk.nashorn.api.scripting.ClassFilter;
import jdk.nashorn.api.scripting.NashornScriptEngineFactory;

import javax.script.ScriptEngine;
import javax.script.ScriptException;

public class NashornExample3 {

    public static void main(String[] args) throws ScriptException {
        NashornScriptEngineFactory factory = new NashornScriptEngineFactory();
        ScriptEngine nashorn = factory.getScriptEngine(new NoJavaFilter());
        Integer eval = (Integer) nashorn.eval("var HashMap = Java.type('java.util.HashMap')");
        System.out.println(eval);
    }

    private static class NoJavaFilter implements ClassFilter{

        @Override
        public boolean exposeToScripts(String s) {
            return false;
        }
    }
}

You will get following exception when you run this program.

Caused by: java.lang.ClassNotFoundException: java.util.HashMap
	at jdk.nashorn.internal.runtime.Context.findClass(Context.java:1032)
	at jdk.nashorn.internal.objects.NativeJava.simpleType(NativeJava.java:493)
	at jdk.nashorn.internal.objects.NativeJava.type(NativeJava.java:322)
	at jdk.nashorn.internal.objects.NativeJava.type(NativeJava.java:314)
	at jdk.nashorn.internal.objects.NativeJava.type(NativeJava.java:310)
	at jdk.nashorn.internal.scripts.Script$\^eval\_.:program(<eval>:1)
	at jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:640)
	at jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:228)
	at jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:393)

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