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Using java-web under Node

The package's Node entry runs Java on Node 23.6 or newer, against the same packaged JDK image (dist/boot.pack, dist/jdk/) that pages use. The host file system is the VFS root, and Java sees the host's environment.

Command

npx @project516/java-web [options] Main [args]
npx @project516/java-web [options] -jar app.jar [args]
Option Meaning
-cp, -classpath, --class-path class path, :-separated (default .)
-Dkey=value a system property
-ea, -da, -esa and the rest assertions, as with java
--enable-preview preview features
--add-exports, --add-opens, --add-reads, --add-modules, --module-path, -p module options, as with java
--locale-data locales other than English (see support.md)
-Xmx the largest array, and what Runtime.maxMemory() reports (default and at most 1g, rounded up to 2 MB steps, below 2m an error); the total heap is not counted
--max-rss MB exit when resident memory passes this (default 2048)
--jdk DIR a java-web JDK image instead of the packaged one
--debug log VM events

Other -X and HotSpot-only options are ignored.

From JS

import { runNode } from "@project516/java-web";

const out = [];
const code = await runNode({
  classpath: ["classes"],
  mainClass: "Main",        // or jar: "app.jar"
  args: ["a", "b"],
  stdout: (bytes) => out.push(bytes),
});

The package ships TypeScript declarations (dist/node.d.ts). runNode resolves to the exit code. Its options:

  • classpath, mainClass or jar, args, props (system properties)

  • stdout, stderr (byte callbacks; default the process's), input (fixed stdin bytes)

  • env (what System.getenv() returns; default the host's environment)

  • natives: Java native methods in JS, as in the browser (browser.md)

  • assertions (the flags, for example ["-ea"]), enablePreview, localeData

  • hostName (what InetAddress.getLocalHost() reports), maxRssMb, jdk, debug

  • fetch: sockets stay inside the VM, and http: requests go to this fetch, as in the browser

  • snapshotDir: where boot snapshots are kept, or null for none (see below)

startLibrary(options) starts a VM in library mode, and libraryOf(vm) is its lib, used as cheerpjRunLibrary's result is in the browser. vm.halt(0) stops it.

Boot snapshots

Most of a start is the JDK's own boot. The second time a setup boots, java-web saves the VM as it is after boot, and later starts with the same setup load it instead, which is about three times faster. A setup is the java-web build, the working directory, the class path, system properties and VM options: boot reads all of them. The snapshots, about 1 MB each, are kept in $XDG_CACHE_HOME/java-web/snapshots (~/.cache/java-web/snapshots), the 16 most recently used. Set JAVA_WEB_SNAPSHOT_DIR to keep them elsewhere, or to an empty value to turn them off.