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.

Example:

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('util').promisify // Needs Node.js version 8 or greater, see comment below
};
java.classpath.push("commons-lang3-3.1.jar");
java.classpath.push("commons-io.jar");

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

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

NOTES:

• 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.
• For promisify implementation, if you are using Node.js version 8.0.0 or newer then promisify: require('util').promisify will work out of the box. If you need to support and older Node.js version then an implementation needs to be provided, for example, promisify: require("when/node").lift
• 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.

java

classpath

java.classpath*

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.

Example

java.classpath.push('commons.io.jar');
java.classpath.push('src');

options

java.options*

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.

Example

java.options.push('-Djava.awt.headless=true');
java.options.push('-Xmx1024m');

asyncOptions

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('util').promisify // Needs Node.js version 8 or greater, see comment below
  ifReadOnlySuffix: "_alt"
};

java.import(className)*

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

Arguments

• className - The name of the class to create. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).

Example

var Test = java.import('Test');
Test.someStaticMethodSync(5);
console.log(Test.someStaticField);

var value1 = Test.NestedEnum.Value1;

var test = new Test();
list.instanceMethodSync('item1');

newInstance

java.newInstance(className, [args...], callback)*

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 occurs, otherwise it will be the first argument in the callback.

Arguments

• className - The name of the class to create. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).
• callback(err, item) - Callback to be called when the class is created.

Example

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

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

instanceOf

java.instanceOf(javaObject, className)*

Determines of a javaObject is an instance of a class.

Arguments

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

Example

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

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

callStaticMethod

java.callStaticMethod(className, methodName, [args...], callback)*

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 occurs, otherwise it will be the first argument in the callback.

Arguments

java.callMethod(instance, methodName, [args...], callback)*

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 occurs, otherwise it will be the first argument in the callback.

Arguments

java.getStaticFieldValue(className, fieldName)*

Gets a static field value from the specified class.

Arguments

• className - The name of the class to get the value from. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).
• fieldName - The name of the field to get the value from.

Example

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

setStaticFieldValue

java.setStaticFieldValue(className, fieldName, newValue)*

Sets a static field value on the specified class.

Arguments

• className - The name of the class to set the value on. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).
• fieldName - The name of the field to set the value on.
• newValue - The new value to assign to the field.

Example

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

newArray

java.newArray(className, values[])*

Creates a new java array of given glass type. To create array of primitive types like char, byte, etc, pass the primitive type name (eg. java.newArray("char", "hello world\n".split(''))).

Arguments

• className - The name of the type of array elements. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).
• values - A JavaScript array of values to assign to the java array.

Example

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

newByte

java.newByte(val)*

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

Arguments

• val - The value of the java byte.

Example

var b = java.newByte(12);

newShort

java.newShort(val)*

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

Arguments

• val - The value of the java short.

Example

var s = java.newShort(12);

newLong

java.newLong(val)*

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

Arguments

• val - The value of the java long.

Example

var s = java.newLong(12);

newChar

java.newChar(val)*

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

Arguments

• val - The value of the java char.

Example

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

newDouble

java.newDouble(val)*

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

Arguments

• val - The value of the java double.

Example

var d = java.newDouble(3.14);

newFloat

java.newFloat(val)*

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

Arguments

• val - The value of the java float.

Example

var f = java.newFloat(3.14);

newProxy

java.newProxy(interfaceName, functions)*

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.

Arguments

• interfaceName - The name of the interface to proxy. Separate nested classes using '$' (eg. com.nearinfinty.MyClass$NestedClass).
• functions - A hash of functions matching the function in the interface.

Example

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);
thread.start();

isJvmCreated

java.isJvmCreated()*

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

registerClient

java.registerClient(before, after)*

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.

registerClientP

java.registerClientP(before, after)*

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

ensureJvm

java.ensureJvm(callback)*

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.

java object

Call Method

obj.methodName([args...], callback)*

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.

Arguments

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

Example

var list = java.newInstanceSync("java.util.ArrayList");
list.addSync("item1");
list.add("item2", function(err, result) {
  if(err) { console.error(err); return; }
});

Field Access

obj.fieldName = val*

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.

Example

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

Getting the Full Method Signature

Run javap -s -classpath . 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:

java.options.push('-Xrs');

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() {
      @Override
      public void run() {
        r.run();
      }
    });
  }
}

Compile ShutdownHookHelper and then use it as follows.

var java = require('./');
java.classpath.push('.');
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 conflicts ('name', 'arguments', 'caller')

The JavaScript 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 simply 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

Troubleshooting

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.

Debugging

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

License

(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.