The Java 21 error

This page is for you if firebase emulators:start stopped with an error about Java. It explains why the official emulators need Java 21, how to fix that, and how Firenook avoids Java altogether.

The error

With Java 17 on the PATH, the current Firebase CLI (firebase-tools 15.34.0) stops before any emulator starts:

$ firebase emulators:start --project demo-shop
i  emulators: Shutting down emulators.

Error: firebase-tools no longer supports Java version before 21. Please install a JDK at version 21 or above to get a compatible runtime.

With no Java at all, it stops with this instead:

$ firebase emulators:start --project demo-shop
i  emulators: Shutting down emulators.

Error: Could not spawn `java -version`. Please make sure Java is installed and on your system PATH.

Why it happens

The official Cloud Firestore, Realtime Database and Cloud Storage emulators are Java programs: .jar files the Firebase CLI downloads on first use (the Firestore emulator alone is about 137 MB in firebase-tools 15.34.0). Before it starts any of them, the CLI runs java -version and stops when the major version is below 21.

The minimum moved with the CLI’s releases. Version 14.19.0 deprecated Java versions below 21, and version 15.0.0 removed support for them. So the error usually appears right after an upgrade to firebase-tools 15, on a machine or CI image that still has an older Java.

Fix it with Java

Install a JDK at version 21 or later, then make sure the shell that runs the emulators finds it:

java -version

The first line must name version 21 or later. If it names an older one, another Java comes first on your PATH, or JAVA_HOME points at it. The same applies to CI images and dev containers: each needs its own Java 21.

Or run Firenook, which needs no Java

Firenook’s emulators are native code in one npm package, and Node runs your Functions, as in production. It never runs java: it starts the whole suite with no Java on the PATH, and its own CI checks a start with java absent. It needs Node 22, 24 or 26, and nothing else is downloaded at install or start.

Switching keeps your firebase.json, rules, Functions code and data. Install it next to firebase-tools, which you keep for deploying:

npm install --save-dev firenook

Then change one word in the command you run:

"scripts": {
  "emulators": "firebase emulators:start --import ./data"
  "emulators": "firenook emulators:start --import ./data"
}

Firenook reads the official export format, so --import takes the data the official emulator wrote, and your app connects on the same ports.

Next

Switch from the Firebase emulators covers the whole move, including keeping data between runs.