0.6.2-pre • Published 8 years ago

@jozias/java v0.6.2-pre

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Bridge API to connect with existing Java APIs.

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###Other projects that might be helpful

  • node-java-maven - manages your node-java classpath by using maven dependency mangement.


$ npm install java


  • node-gyp requires python 2.x not python 3.x. See https://github.com/TooTallNate/node-gyp/issues/155 for more details.
  • If you see an error such as "Call to 'node findJavaHome.js' returned exit status 1" Try running node findJavaHome.js in the node-java directory to see the full failure message.
  • If you are having problems finding 'jni.h'. Make sure you have the JDK installed not just the JRE. If you are using OpenJDK you want the openjdk-7-jdk package, not openjdk-7-jre. Mavericks users see Issue #86 if you run into this.

Installation OSX

  • If you run into strange runtime issues, it could be because the Oracle JDK does not advertise itself as available for JNI. See Issue 90 for more details and manual workarounds. If this does occur for you, please update the issue.

Installation Windows

For 64 bit installs with 32 bit node:

If you get ENOENT errors looking for <nodepath>\node_modules\node-gyp\.., ensure you have node-gyp installed as a global nodule:

npm install -g node-gyp

If you get D9025 warnings and C1083 errors when looking for .sln or .h files, be sure you've got the node-gyp's dependencies, as explained here.

Installation ARM (Raspberry Pi)

GYP_DEFINES="armv7=0" CCFLAGS='-march=armv6' CXXFLAGS='-march=armv6' npm install java

Manual compile (Using node-gyp)

node-gyp configure build
npm test

NOTE: You will need node-gyp installed using "npm install -g node-gyp"


If you want to play with node-java but don't want to setup the build environment you can run it in docker.

docker run -it joeferner/node-java bash

Then inside the docker container create a directory and run

npm install java

Then create a file called test.js with the following contents

var java = require('java');
var javaLangSystem = java.import('java.lang.System');

javaLangSystem.out.printlnSync('Hello World');

Then run

node test.js

Java 1.8 support

Manual compilation for Java 1.8 support requires additional steps:

node-gyp configure build
npm test

Java 1.8 language features can be used in Java classes only if a Java 1.8 JRE is available. The script compile-java8-code.sh is used only to compile java classes used in the 'test8' unit tests, but these classes are checked into the test8/ directory. Note that unit tests in the test8/ directory will pass (by design) if run against a Java 1.7 JRE, provided that a java.lang.UnsupportedClassVersionError is caught with the message 'Unsupported major.minor version 52.0' (the expected behavior when Java 1.8 language features are used in an older JRE).

Installation node-webkit

npm install -g nw-gyp
npm install java
cd node_modules/java
nw-gyp configure --target=0.10.5
nw-gyp build

See testIntegration/webkit for a working example

Using node-java in existing maven projects

When using node-java in existing maven projects, all the dependencies and the class files of the project have to be pushed to the classpath.

One possible solution would be:

Issue the command:

mvn dependency:copy-dependencies

Then create the following module javaInit:

"use strict";
var fs = require("fs");
var java = require("java");
var baseDir = "./target/dependency";
var dependencies = fs.readdirSync(baseDir);

    java.classpath.push(baseDir + "/" + dependency);


exports.getJavaInstance = function() {
    return java;

and then in the consuming class write:

var javaInit = require('./javaInit');
var java = javaInit.getJavaInstance();

//your code goes here

Quick Examples

var java = require("java");

var list1 = java.newInstanceSync("java.util.ArrayList");
console.log(list1.sizeSync()); // 0
console.log(list1.sizeSync()); // 1

java.newInstance("java.util.ArrayList", function(err, list2) {
  console.log(list2.toStringSync()); // [item1, item2]

var ArrayList = java.import('java.util.ArrayList');
var list3 = new ArrayList();
list3.equalsSync(list1); // true

Create a char array

var charArray = java.newArray("char", "hello world\n".split(''));

Create a byte array

var byteArray = java.newArray(
  "hello world\n"
    .map(function(c) { return java.newByte(String.prototype.charCodeAt(c)); }));

Using java.lang.Long and long

JavaScript only supports 32-bit integers. Because of this java longs must be treated specially. When getting a long result the value may be truncated. If you need the original value there is a property off of the result called "longValue" which contains the un-truncated value as a string. If you are calling a method that takes a long you must create it using java.newInstance.

var javaLong = java.newInstanceSync("java.lang.Long", 5);
console.log('Possibly truncated long value: ' + javaLong);
console.log('Original long value (as a string): ' + javaLong.longValue);
java.callStaticMethodSync("Test", "staticMethodThatTakesALong", javaLong);


Exceptions from calling methods either caught using JavaScript try/catch block or passed to a callback as the first parameter may have a property named "cause" which has a reference to the Java Exception object which caused the error.

try {
} catch(ex) {

AsyncOptions: control over the generation of sync, async & promise method variants.

As of release 0.4.5 it became possible to create async methods that return promises by setting the asyncOptions property of the java object. With release 0.4.7 this feature is extended to allow changing the suffix assigned for sync and async method variants, and to further configure this module to optionally omit generation of any of these variants.


var java = require("java");
java.asyncOptions = {
  asyncSuffix: undefined,     // Don't generate node-style methods taking callbacks
  syncSuffix: "",              // Sync methods use the base name(!!)
  promiseSuffix: "Promise",   // Generate methods returning promises, using the suffix Promise.
  promisify: require("when/node").lift

java.import("java.util.ArrayList"); // see NOTE below

    .then(function(list) { return list.addPromise("item1"); })
    .then(function(list) { return list.addPromise("item2"); })
    .catch(function(err) { /* handle error */ });


  • If you want the defacto standard behavior, simply don't set java.asyncOptions.
  • If you do provide asyncOptions, be aware that this module will not generate method variants of a given flavor if you don't provide a string value for the corresponding suffix (asyncSuffix, syncSuffix, promiseSuffix). In the example above, the application is configured to omit the method variants using node-style async callback functions.
  • If you provide asyncOptions.promiseSuffix then you must also set asyncOptions.promisify to a function that promisifies a node-style async function. I.e. the provided function must take as input a function whose last argument is a node callback function, and it must return an equivalent promise-returning function. Several Promises/A+ libraries provide such functions, but it may be necessary to provide a wrapper function. See testHelpers.js for an example.
  • If you provide asyncOptions.promisify then you must provide a non-empty string for asyncOptions.promiseSuffix.
  • Either (but not both) asyncSuffix or syncSuffix can be the empty string. If you want the defacto standard behavior for no suffix on async methods, you must provide an empty string for asyncSuffix.
  • We've tested promises with five Promises/A+ implementations. See testHelpers.js for more information.
  • NOTE: Due to specifics of initialization order, the methods java.newInstancePromise, java.callMethodPromise, and java.callStaticMethodPromise are not available until the JVM has been created. You may need to call some other java method such as java.import() to finalize java initialization, or even better, the function java.ensureJvm().
Special note about the exported module functions newInstance, callMethod, and callStaticMethod.

These methods come in both async and sync variants. If you provide the promisify and promiseSuffix attributes in asyncOptions then you'll also get the Promises/A+ variant for these three functions. However, if you change the defacto conventions for the syncSuffix (i.e. 'Sync') and/or asyncSuffix (i.e. '') it will not affect the naming for these three functions. I.e. no matter what you specify in asyncOptions, the async variants are named newInstance, callMethod, and callStaticMethod, and the sync variants are named newInstanceSync, callMethodSync, and callStaticMethodSync.

Varargs support

With v0.5.0 node-java now supports methods with variadic arguments (varargs). Prior to v0.5.0, a javascript call to a Java varargs method had to construct an array of the variadic arguments using java.newArray(). With v0.5.0 javascript applications can simply use the variadic style.

In most cases it is still acceptable to use java.newArray(). But it is now possible to pass a plain javascript array, or use the variadic style. For example, consider these snippets from the unit test file test/varargs-test.js:

    test.equal(Test.staticVarargsSync(5, 'a', 'b', 'c'), '5abc');
    test.equal(Test.staticVarargsSync(5, ['a', 'b', 'c']), '5abc');
    test.equal(Test.staticVarargsSync(5, java.newArray('java.lang.String', ['a', 'b', 'c'])), '5abc');

Note that when passing a Javascript array (e.g. ['a', 'b', 'c']) for a varargs parameter, node-java must infer the Java type of the array. If all of the elements are of the same javascript primitive type (string in this example) then node-java will create a Java array of the corresponding type (e.g. java.lang.String). The Java types that node-java can infer are: java.lang.String, java.lang.Boolean, java.lang.Integer, java.lang.Long, and java.lang.Double. If an array has a mix of Integer, Long, and Double, then the inferred type will be java.lang.Number. Any other mix will result in an inferred type of java.lang.Object.

Methods accepting varargs of a generic type are also problematic. You will need to fall back to using java.newArray(). See Issue #285.

JVM Creation

With v0.5.1 a new API is available to make it easier for a complex application to have full control over JVM creation. In particular, it is now easier to compose an application from several modules, each of which must add to the Java classpath and possibly do other operations just before or just after the JVM has been created. See the methods ensureJvm and registerClient. See also several of the tests in the testAsyncOptions directory.

Release Notes


  • Support for varargs. This change is not 100% backwards compatible, but the fix is generally easy and results in more natural code.




java objects

API Documentation

Array of paths or jars to pass to the creation of the JVM.

All items must be added to the classpath before calling any other node-java methods.



Array of options to pass to the creation of the JVM.

All items must be added to the options before calling any other node-java methods.



Loads the class given by className such that it acts and feels like a javascript object.


  • className - The name of the class to create. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)


var Test = java.import('Test');

var value1 = Test.NestedEnum.Value1;

var test = new Test();

java.newInstanceSync(className, args...) : result

Creates an instance of the specified class. If you are using the sync method an exception will be throw if an error occures, otherwise it will be the first argument in the callback.


  • className - The name of the class to create. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • callback(err, item) - Callback to be called when the class is created.


var list = java.newInstanceSync("java.util.ArrayList");

java.newInstance("java.util.ArrayList", function(err, list) {
  if(err) { console.error(err); return; }
  // new list

Determines of a javaObject is an instance of a class.


  • javaObject - Instance of a java object returned from a method or from newInstance.
  • className - A string class name.


var obj = java.newInstanceSync("my.package.SubClass");

if(java.instanceOf(obj, "my.package.SuperClass")) {
  console.log("obj is an instance of SuperClass");

java.callStaticMethodSync(className, methodName, args...) : result

Calls a static method on the specified class. If you are using the sync method an exception will be throw if an error occures, otherwise it will be the first argument in the callback.


  • className - The name of the class to call the method on. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • methodName - The name of the method to call. The method name can include the full signature (see Getting the full method signature).
  • callback(err, item) - Callback to be called when the class is created.


var result = java.callStaticMethodSync("com.nearinfinty.MyClass", "doSomething", 42, "test");

java.callStaticMethod("com.nearinfinty.MyClass", "doSomething", 42, "test", function(err, results) {
  if(err) { console.error(err); return; }
  // results from doSomething

java.callMethodSync(instance, methodName, args...) : result

Calls a method on the specified instance. If you are using the sync method an exception will be throw if an error occures, otherwise it will be the first argument in the callback.


  • instance - An instance of the class from newInstance.
  • methodName - The name of the method to call. The method name can include the full signature (see Getting the full method signature).
  • callback(err, item) - Callback to be called when the class is created.


var instance = java.newInstanceSync("com.nearinfinty.MyClass");

var result = java.callMethodSync("com.nearinfinty.MyClass", "doSomething", 42, "test");

java.callMethodSync(instance, "doSomething", 42, "test", function(err, results) {
  if(err) { console.error(err); return; }
  // results from doSomething

Gets a static field value from the specified class.


  • className - The name of the class to get the value from. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • fieldName - The name of the field to get the value from.


var data = java.getStaticFieldValue("com.nearinfinty.MyClass", "data");

Sets a static field value on the specified class.


  • className - The name of the class to set the value on. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • fieldName - The name of the field to set the value on.
  • newValue - The new value to assign to the field.


java.setStaticFieldValue("com.nearinfinty.MyClass", "data", "Hello World");

Creates a new java array of type class.


  • className - The name of the type of array elements. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • values - A javascript array of values to assign to the java array.


var newArray = java.newArray("java.lang.String", ["item1", "item2", "item3"]);

Creates a new java byte. This is needed because javascript does not have the concept of a byte.


  • val - The value of the java byte.


var b = java.newByte(12);

Creates a new java short. This is needed because javascript does not have the concept of a short.


  • val - The value of the java short.


var s = java.newShort(12);

Creates a new java long. This is needed because javascript does not have the concept of a long.


  • val - The value of the java long.


var s = java.newLong(12);

Creates a new java char. This is needed because javascript does not have the concept of a char.


  • val - The value of the java char.


var ch = java.newChar('a');

Creates a new java double. This is needed to force javascript's number to a double to call some methods.


  • val - The value of the java double.


var d = java.newDouble(3.14);

Creates a new java float. This is needed to force javascript's number to a float to call some methods.


  • val - The value of the java float.


var f = java.newFloat(3.14);

Creates a new java Proxy for the given interface. Functions passed in will run on the v8 main thread and not a new thread.

The returned object has a method unref() which you can use to free the object for garbage collection.


  • interfaceName - The name of the interface to proxy. For subclasses seperate using a '$' (eg. com.nearinfinty.MyClass$SubClass)
  • functions - A hash of functions matching the function in the interface.


var myProxy = java.newProxy('java.lang.Runnable', {
  run: function () {
    // This is actually run on the v8 thread and not the new java thread
    console.log("hello from thread");

var thread = java.newInstanceSync("java.lang.Thread", myProxy);

Returns true if the JVM has been created. The JVM can only be created once.

Register that a client wants to be called back immediately before and/or immediately after the JVM is created. If used, this function must be called before the JVM has been created. The before function is typically used to add to the classpath. The function may execute asynchronous operations (such as a async glob function). The after function is sometimes useful for doing one-time initialization that requires the JVM to first be initialized. If either function is unnecessary, use null or undefined. See also registerClientP and ensureJvm. See the unit tests in testAsyncOptions for examples.

Like java.registerClient, but before and after are assumed to be functions returning promises.

If the JVM has not yet been created, execute the full JVM initialization process, then call callback function when initialization is complete. If the JVM has been created, just call the callback. Note that the full initialization process includes: 1) executing all registered client before hooks, 2) creating the JVM, then 3) executing all registered client after hooks.

obj.methodNameSync(args...) : result

Once you have a java object either by creating a new instance or as a result of a method call you can then call methods on that object. All public, non-static methods are exposed in synchronous and asynchronous flavors.


  • args - The arguments to pass to the method.
  • callback(err, item) - Callback to be called when the method has completed.


var list = java.newInstanceSync("java.util.ArrayList");
list.add("item2", function(err, result) {
  if(err) { console.error(err); return; }

val = obj.fieldName

Once you have a java object either by creating a new instance or as a result of a method call you can get instance field values.


var list = java.newInstanceSync("com.nearinfinty.MyClass");
list.data = "test";
var data = list.data;

Run javap -s -classpath <your-class-path> <your-class-name>. Find the method name you are looking for. For example:

public int methodAmbiguous(java.lang.Double);
  Signature: (Ljava/lang/Double;)I

The full method signature would be methodAmbiguous(Ljava/lang/Double;)I.

If you have grep, a shortcut is javap -s -classpath . my.company.MyClass | grep -A1 myMethodName.

Signal Handling

The JVM intercepts signals (Ctrl+C, etc.) before node/v8 gets to handle them. To fix this there are a couple options.

Signal Handling Option 1

One option to capture these events is to add the following flag:


As man java says, the -Xrs flag will “reduce usage of operating-system signals by the Java virtual machine (JVM)”, to avoid issues when developing “applications that embed the JVM”.

Signal Handling Option 2

Hook into the runtime shutdown hook.

First create a java wrapper around the Runtime.addShutdownHook method to allow using a proxy object.

public class ShutdownHookHelper {
  public static void setShutdownHook(final Runnable r) {
    Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {

Compile ShutdownHookHelper and then use it as follows.

var java = require('./');
var ShutdownHookHelper = java.import('ShutdownHookHelper');

ShutdownHookHelper.setShutdownHookSync(java.newProxy('java.lang.Runnable', {
  run: function () {
    console.log("do shutdown stuff here instead.");

Object lifetime

When you call a Java method through node-java, any arguments (V8/JavaScript objects) will be converted to Java objects on the v8 main thread via a call to v8ToJava (found in utils.cpp). The JavaScript object is not held on to and can be garbage collected by v8. If this is an async call, the reference count on the Java objects will be incremented. The Java method will be invoked in a node.js async thread (see uv_queue_work). When the method returns, the resulting object will be returned to the main v8 thread and converted to JavaScript objects via a call to javaToV8 and the Java object's reference count will then be decremented to allow for garbage collection. The resulting v8 object will then be returned to the callers callback function.

Static member name conficts ('name', 'arguments', 'caller')

The Javscript object returned by java.import(classname) is a Javascript constructor Function, implemented such that you can create instances of the Java class. For example:

var Test = java.import('Test');
var test = new Test();

Test.someStaticMethod(function(err, result) { ... });

var value1 = Test.NestedEnum.Value1;

But Javascript reserves a few property names of Function objects: name, arguments, and caller. If your class has public static members (either methods or fields) with these names, node-java is unable to create the necessary property to implement the class's API. For example, suppose your class Test implements a static method named caller, or has a NestedEnum with a value name:

public class Test {
    public static String caller() { return "something"; }
    public enum NestedEnum { foo, name };

In Javascript, you would expect to be able to use those static members like this:

var Test = java.import('Test');
Test.caller(function(err, result) { ... });  // ERROR
var value = Test.NestedEnum.name;  // ERROR

Node-java can't create those properties, so the above code won't work. Instead, node-java appends a suffix to the name. The default suffix is simpy an underscore _, but you can change the suffix using asyncOptions:

var java = require('java');

java.asyncOptions = {
  asyncSuffix: "",
  syncSuffix: "Sync",
  ifReadOnlySuffix: "_alt"

var Test = java.import('Test');
Test.caller_alt(function(err, result) { ... });  // OK
var value = Test.NestedEnum.name_alt;  // OK


Error: Cannot find module '../build/jvm_dll_path.json'

Either postInstall.js didn't run or there was a problem detecting java. Try running postInstall.js manually.


    npm install
    node-gyp build --debug
    gdb --args `which node` ./node_modules/.bin/nodeunit test


(The MIT License)

Copyright (c) 2012 Near Infinity Corporation

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.