@markusjx/java NPM

A bridge between Node.js programs and Java APIs written in Rust using napi-rs to provide a fast and memory-safe interface between the two languages.

The pre-compiled binaries will be provided with the package, the only thing you need to do on your machine is install a Java Runtime Environment (JRE) for this package to use. In contrast to other node.js <-> java interfaces, the binary is not hard linked to the JDK it has been compiled with but rather loads the jvm native library dynamically when the program first starts up.

The full documentation of this package is available here.

Installation

npm i @markusjx/java

Usage

Create the JVM

Create a new Java VM using the ensureJvm method. Calling this after the jvm has already been created will do nothing. Destroying the jvm manually is not (yet) supported.

Create the JVM with no extra options

This will first search for a suitable jvm native library on the system and then start the jvm with no extra options. This is also called when any call to the jvm is made but the jvm is not yet started.

import { ensureJvm } from '@markusjx/java';

ensureJvm();

Create the JVM with extra options

You can pass extra options to the jvm when creating it, for example requesting a specific jvm version, specifying the location of the jvm native library or passing additional arguments to the jvm.

import { ensureJvm, JavaVersion } from '@markusjx/java';

ensureJvm({
    libPath: 'path/to/jvm.dll',
    version: JavaVersion.VER_9,
    opts: '-Xms512m -Xmx512m',
});

Use daemon threads

By default, new threads will not be attached as daemon threads, meaning any thread attached to the jvm will be automatically detached once it is not required anymore. This slows down asynchronous calls but will prevent the jvm from being terminated if any thread is still running.

If you want to change this behaviour, you can pass the useDaemonThreads option to the ensureJvm function. This will make the jvm attach threads as daemon threads causing those threads to not be detached once not required anymore.

ensureJvm({
    useDaemonThreads: true,
});

Inject a JAR into the class path

In order to import your own classes into the node environment, you need to add the JAR file to the class path. You can do that with the appendClasspath or classpath.append methods. After loading a JAR, you can import classes from it like any other class from the JVM using importClass or importClassAsync.

import { appendClasspath } from '@markusjx/java';

// Append a single jar to the class path
appendClasspath('/path/to/jar.jar');

// Append multiple jars to the class path
appendClasspath(['/path/to/jar1.jar', '/path/to/jar2.jar']);

import { classpath } from '@markusjx/java';

// Append a single jar to the class path
classpath.append('/path/to/jar.jar');

Synchronous calls

If you want to use Java APIs in a synchronous way, you can use the synchronous API of this module. Any call to the Java API will be executed in the same thread as your node process so this may cause your program to hang until the execution is finished. But - in contrast to the asynchronous API - these calls are a lot faster as no extra threads need to be created/attached to the JVM.

All synchronous java methods are proceeded with the postfix Sync. This means, all methods of a class (static and non-static) are generated twice, once as a synchronous call and once as an asynchronous call.

If you are looking for asynchronous calls, take a look at the next section. In order to import a class synchronously, you can use the importClass function. Using this method does not affect your ability to call any method of the class asynchronously.

import { importClass } from '@markusjx/java';

// Import a class
const JString = importClass('java.lang.String');

// Create a new instance of the class
const str = new JString('Hello World');

// Call a method on the instance
str.lengthSync(); // 11

// Supported native types will be automatically converted
// to the corresponding type in the other language
str.toStringSync(); // 'Hello World'

Asynchronous calls

If you want to use Java APIs in an asynchronous way, you can use the asynchronous API of this module. Any call to the Java API will be executed in a separate thread and the execution will not block your program. This is in general a lot slower as the synchronous API but allows the program to run more smoothly.

If you want to improve the performance of the asynchronous API, you can force the module to attach any thread as a daemon thread to the JVM. This allows the program to not constantly attach new threads to the JVM as the old ones can be reused and thus improves the performance.

In order to import a class asynchronously, you can use the importClassAsync function.

import { importClassAsync } from '@markusjx/java';

const JString = await importClassAsync('java.lang.String');

// Create a new instance asynchrnously using 'newInstanceAsync'
const str = await JString.newInstanceAsync('Hello World');

// Call methods asynchronously
await str.length(); // 11
await str.toString(); // 'Hello World'

Implement a Java interface

You can also implement a Java interface in node.js using the newProxy method. Please note that when calling a java method that uses an interface defined by this method, you must call that method using the interface asynchronously as Node.js is single threaded and can't wait for the java method to return while calling the proxy method at the same time.

import { newProxy } from '@markusjx/java';

const proxy = newProxy('path.to.MyInterface', {
    // Define methods...
});

// Do something with the proxy
instance.someMethod(proxy);

// Destroy the proxy
proxy.reset();

Redirect the stdout and stderr from the java process

If you want to redirect the stdout and/or stderr from the java process to the node.js process, you can use the enableRedirect method.

import { stdout } from '@markusjx/java';

const guard = stdout.enableRedirect(
    (_, data) => {
        console.log('Stdout:', data);
    },
    (_, data) => {
        console.error('Stderr:', data);
    }
);

Value conversion rules

Any basic value such as string, number, boolean or BigInt may be passed to methods accepting matching types
string values will always be converted to java.lang.String
string values with just one character may be converted to char or java.lang.Char if required
Thus, in order to pass a char to a java method, use a string containing just one character
number values will be converted to int, long, double, float, java.lang.Integer, java.lang.Long, java.lang.Double or java.lang.Float depending on the type the java function to call requires
boolean values will be converted to either boolean or java.lang.Boolean
BigInt values will be converted to either long or java.lang.Long
Arrays will be converted to java arrays. Java arrays may only contain a single value type, therefore the type of the first element in the array will be chosen as the array type, empty arrays need no conversions.
java.lang.String values will be converted to string
int, double, float, java.lang.Integer, java.lang.Double or java.lang.Float values will be converted to number
long or java.lang.Long values will always be converted to BigInt
boolean or java.lang.Boolean values will be converted to boolean
char or java.lang.Character values will be converted to string
Java arrays will be converted to javascript arrays, applying the rules mentioned above except
Byte arrays will be converted to Buffer and vice-versa

Command line interface

This module also ships with a command line interface called java-ts-gen for creating typescript definitions for Java classes. The command line interface will create typescript definitions for all specified classes and their dependencies.

Installation

npm install -g @markusjx/java

Usage

java-ts-gen <output> <classnames..>

Positionals:
  classnames  The fully qualified class name(s) to convert              [string]
  output      The output file                                           [string]

Options:
  --help             Show help                                         [boolean]
  --version          Show version number                               [boolean]
  --classpath, --cp  The classpath to use                               [string]

Notes

The classpath argument can be supplied multiple times to add multiple jars to the classpath
Multiple class names can be supplied to generate definitions for multiple classes
The generated typescript files will automatically import all classes once the module is loaded.