# The Java 21 error

> Why the Firebase CLI's emulators stop with "no longer supports Java version before 21", how to fix it with Java, and how Firenook runs with no Java at all.

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:

```console
$ 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:

```console
$ 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](https://firebase.google.com/support/release-notes/cli)
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:

```sh
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:

```sh
npm install --save-dev firenook
```

Then change one word in the command you run:

```diff
 "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](https://firenook.dev/docs/get-started/switch) covers the
whole move, including keeping data between runs.