9.1 Develop Javascript code that creates and uses Java members such as Java objects, methods, JavaBeans, Arrays, Collections, Interfaces.

Nashorn is the only JavaScript engine included in the JDK. However, you can use any script engine compliant with JSR 223

To get an instance of the Nashorn engine, pass in “nashorn”. Alternatively, you can use any of the following: “Nashorn”, “javascript”, “JavaScript”, “js”, “JS”, “ecmascript”, “ECMAScript”.

Command line tools:

  
jrunscript : Invokes any available script engine
jjs        : To evaluate a script file using Nashorn, 
             pass the name of the script file to the jjs tool.

 
jjs> java.lang
[JavaPackage java.lang]
jjs> typeof java.lang
object
jjs> java.lang.System
[JavaClass java.lang.System]
jjs> typeof java.lang.System
function

Nashorn interprets Java packages as JavaPackage objects, and Java classes as JavaClass function objects, which can be used as constructors for the classes.

The Java.type() function takes a string with the fully qualified Java class name, and returns the corresponding JavaClass function object.

 
jjs> var HashMap = Java.type("java.util.HashMap")
jjs> var mapDef = new HashMap()
jjs> mapDef.put('a', 'val');
jjs> mapDef.get('a');
val
jjs> mapDef.get('b');
null
jjs> mapDef.put('b', 'val2');
null
jjs> mapDef.get('b');
val2

Accessing Class and Instance Members

 
jjs> Java.type("java.lang.Math").PI
3.141592653589793

jjs> Java.type("java.lang.System").currentTimeMillis()
1429378149988

Extending Java Classes/Interfaces

You can extend a class using the Java.extend() function that takes a Java type as the first argument and method implementations (in the form of JavaScript functions) as the other arguments. Below shows a script that extends the java.lang.Runnable interface and uses it to construct a new java.lang.Thread object.

 
var Run = Java.type("java.lang.Runnable");
var MyRun = Java.extend(Run, {
    run: function() {
        print("Run in separate thread");
    }
});
var Thread = Java.type("java.lang.Thread");
var th = new Thread(new MyRun());

Nashorn can automatically extend single abstract method (SAM) classes if you provide the function for implementing the method as the argument to the constructor.

 
var Thread = Java.type("java.lang.Thread")
var th = new Thread(function() print("Run in a separate thread"))

Restricting Script Access to Specified Java Classes

The jdk.nashorn.api.scripting.ClassFilter interface enables you to restrict access to specified Java classes from scripts run by a Nashorn script engine

Code examples

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