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Java Standard Edition (SE)

The Nashorn Java API


Java Platform, Standard Edition Nashorn User's Guide
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2 The Nashorn Java API

This chapter describes how to access Java from a script interpreted by the Nashorn engine.

The sections in this chapter contain examples of script statements interpreted by the Nashorn engine in interactive language shell mode. This interactive shell is started by running the jjs command without any scripts passed to it. This is useful for trying things out, but the main purpose of the Nashorn Java API is to write Java applications as scripts that can be interpreted by the Nashorn engine.

2.1 Accessing Java Classes

There are two approaches to access packages and classes using Nashorn: the traditional approach is to use the Packages global object, and the recommended approach is to use the Java global object. This section describes both approaches.

The predefined top-level Packages object enables you to access Java packages and classes using their fully qualified names, as if they are properties of the Packages object. The following example shows how you can access the MyPackage package and its MyClass class if MyPackage.jar is in your class path:

jjs> Packages.MyPackage
[JavaPackage MyPackage]
jjs> Packages.MyPackage.MyClass
[JavaClass MyPackage.MyClass]

Accessing standard Java packages and classes is more straightforward than accessing custom packages and classes. For your convenience, there are global objects defined for each of the standard Java packages: com, edu, java, javafx, javax, and org. They have aliases that correspond to properties of the Packages object. The following example shows how you can access the java.lang package and the java.lang.System class:

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

As you can see from the previous example, Nashorn interprets Java packages as JavaPackage objects, and Java classes as JavaClass function objects, which can be used as constructors for the classes. For more information about instantiating a class, see Section 2.2, "Creating Java Objects".

The traditional approach for accessing Java packages and classes is intuitive and straightforward, but at the same time, it can be inefficient, limited, and error-prone for the following reasons:

  • Each property access has a cost, so accessing a package or class in a deep hierarchy can be slow.

  • There is no special syntax for creating Java arrays. You must use the java.lang.reflect.Array class as a workaround.

  • If you misspell a class name, Nashorn assumes that you provided a package name, and interprets it as a JavaPackage object instead of a JavaClass function object. You might not be aware of this until an error is thrown when you attempt to use it as a class. To avoid this, use the typeof operator to conditionally test that the construct you are trying to access is interpreted as a function object. The following example shows how this conditional check works:

    jjs> typeof java.lang.System == "function"
    true
    jjs> typeof java.lang.Zyztem == "function"
    false
    

To avoid the disadvantages of the approach previously described, Nashorn defines the Java global object that has several functions for working with Java classes. The Java.type() function takes a string with the fully qualified Java class name, and returns the corresponding JavaClass function object. The following example shows how you can access the java.lang.System class:

jjs> Java.type("java.lang.System")
[JavaClass java.lang.System]

Similar to importing classes in Java, it is a good practice to declare variables of JavaClass type at the beginning of a script. The following example shows how you can declare the System variable and give it a value of the java.lang.System class:

jjs> var System = Java.type("java.lang.System")
jjs> System
[JavaClass java.lang.System]

2.2 Creating Java Objects

To instantiate a class, pass the JavaClass function object to the new operator. Nashorn invokes the corresponding constructor based on the arguments passed to the function. The following example shows how you can instantiate the java.util.HashMap class with the default initial capacity and with the initial capacity set to 100:

jjs> var HashMap = Java.type("java.util.HashMap")
jjs> var mapDef = new HashMap()
jjs> var map100 = new HashMap(100)

2.3 Accessing Class and Instance Members

You can use the standard dot notation to access static fields, methods, and inner classes as follows:

jjs> Java.type("java.lang.Math").PI
3.141592653589793
jjs> Java.type("java.lang.System").currentTimeMillis()
1375813353330
jjs> Java.type("java.util.Map").Entry
[JavaClass java.util.Map$Entry]

An inner class can also be accessed using internal representation with the dollar sign ($) as the separator, or a dot, which is consistent with Java:

jjs> Java.type("java.util.Map$Entry")
[JavaClass java.util.Map$Entry]
jjs> Java.type("java.util.Map.Entry")
[JavaClass java.util.Map$Entry]

To invoke an instance method or access an instance field of an object, use the dot operator, similar to how it is done in Java. The following example shows how you can call the toUpperCase() method on a String object:

jjs> var String = Java.type("java.lang.String")
jjs> var str = new String("Hello")
jjs> str
Hello
jjs> var upper = str.toUpperCase()
jjs> upper
HELLO

Nashorn also supports member access using the bracket notation, where you specify the name of the member as a string between brackets ([]) that immediately follow the class (in case of a static member) or object (in case of an instance member). This method is defined by the ECMAScript as an alternative to the dot notation, and is not intuitive for Java developers. However, it can be used to resolve method overload ambiguity. By default, Nashorn uses the overloaded method that best matches the arguments, and this is not always what you expect. For example, if you want to print a double value, you must use the java.lang.System.out.println(double) method overload, as shown in the following example:

jjs> Java.type("java.lang.System").out.println(10)
10
jjs> Java.type("java.lang.System").out["println(double)"](10)
10.0

2.4 Using JavaBeans

Nashorn enables you to treat accessor and mutator methods in JavaBeans as equivalent JavaScript properties. The name of the property is the name of the JavaBean method without the get or set suffix, and starts with a lowecase letter. For example you can call the getYear() and setYear() methods in a java.util.Date object using the year property as follows:

jjs> var Date = Java.type("java.util.Date")
jjs> var date = new Date()
jjs> date.year + 1900
2013
jjs> date.year = 2014 - 1900
114
jjs> date.year + 1900
2014

2.5 Working with Java Arrays

To access a Java array class, pass to the Java.type() function the type of objects that comprise the array followed by a pair of brackets (similar to Java syntax). The following example shows how you can access a Java array of integers and a Java array of String objects:

jjs> Java.type("int[]")
[JavaClass [I]
jjs> Java.type("java.lang.String[]")
[JavaClass [Ljava.lang.String;]

After you have the array type object, you can use it to instantiate an array as you do any other class. You can access array entries by their indexes, and use the dot or bracket notation to access members (similar to Java syntax), as shown in the following example:

jjs> var IntArrayType = Java.type("int[]")
jjs> var arr = new IntArrayType(10)
jjs> arr[1] = 123
123
jjs> arr[2] = 321
321
jjs> arr[1] + arr[2]
444
jjs> arr[10]
java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 10
jjs> arr.length
10

If you have an existing JavaScript array, you can convert it to a Java array using the Java.to() function. The following example shows how you can convert a JavaScript array of strings "a", "b", and "c", to a java.lang.String[] array with the same values:

jjs> var jsArr = ["a","b","c"]
jjs> var strArrType = Java.type("java.lang.String[]")
jjs> var javaArr = Java.to(jsArr, strArrType)
jjs> javaArr.class
class [Ljava.lang.String;
jjs> javaArr[0]
a

You can iterate through a Java array's indexes and values using the for and for each statements as follows:

jjs> for (var i in javaArr) print(i)
0
1
2
jjs> for each (var i in javaArr) print(i)
a
b
c

2.6 Working with Java Strings

Strings are interpreted by Nashorn as java.lang.String objects. However, if you concatenate two strings you get an instance of the jdk.nashorn.internal.runtime.ConsString class. This is not a problem for most scripts, because both classes implement the java.lang.CharSequence interface, but in some cases, Nashorn passes the ConsString object when a method expects a java.lang.Object argument. To avoid this, use the String() function to ensure that the string is a Java String object, as shown in the following example:

jjs> var a = "abc"
jjs> a.class
class java.lang.String
jjs> var b = a + "def"
jjs> b.class
class jdk.nashorn.internal.runtime.ConsString
jjs> var c = String(b)
jjs> c.class
class java.lang.String

2.7 Working with Java Numbers

Nashorn interprets numbers as java.lang.Double, java.lang.Long, or java.lang.Integer objects, depending on the computation performed. You can use the Number() function to force a number to be a Double object, as shown in the following example:

jjs> var intNum = 10
jjs> intNum.class
class java.lang.Integer
jjs> var dblNum = Number(intNum)
jjs> dblNum.class
class java.lang.Double

2.8 Working with Java Collections

Nashorn interprets Java collections as arrays. You can access collection elements using the index in brackets ([]) and iterate over the values of a collection using the for each statement, as shown in the following example:

jjs> var ArrayList = Java.type("java.util.ArrayList")
jjs> var alist = new ArrayList()
jjs> alist.add("a")
true
jjs> alist.add("b")
true
jjs> alist.add("c")
true
jjs> alist[1]
b
jjs> for each (var i in alist) print(i)
a
b
c

Unlike other collections, to iterate over keys and values in a map, you must use the keySet() and values() methods. The following example shows how you can create a HashMap object and iterate over its keys and values:

jjs> var HashMap = Java.type("java.util.HashMap")
jjs> var hm = new HashMap()
jjs> hm.put("name", "Bob")
jjs> hm.put("age", 40)
jjs> hm.put("weight", 180)
jjs> for each (var i in hm.keySet()) print(i)
weight
age
name
jjs> for each (var i in hm.values()) print(i)
180
40
Bob

2.9 Extending Java Classes

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. Example 2-1 shows a script that extends the java.lang.Runnable interface and uses it to construct a new java.lang.Thread object.

Example 2-1 Extending a Java Class

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. Example 2-2 shows a script that extends the java.lang.Runnable interface and uses it to construct a new java.lang.Thread object, but it uses fewer lines of code than in Example 2-1, because the Java.extend() function is called automatically for a SAM class.

Example 2-2 Extending a Java SAM Class

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

For more information about the capabilities of the Java.extend() function, see Java Scripting Programmer's Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/scripting/programmer_guide/index.html

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