# Java — LSP setup (Hermes / LitHermes)

- **Recommended server:** `jdtls`
- **Extensions:** `.java`
- **Install hint:** `https://github.com/eclipse-jdtls/eclipse.jdt.ls`

## Install

- **macOS:** `brew install jdtls`
- **Linux:** Download from [eclipse-jdtls/eclipse.jdt.ls](https://github.com/eclipse-jdtls/eclipse.jdt.ls) releases, extract, and wrap the launcher as `jdtls` on PATH (some distros package it as `jdtls`/`jdt-language-server`).
- **Windows:** Download the release archive and add the `jdtls` launcher (`bin/jdtls.bat` or the Python wrapper) to PATH.

Requires a **JDK 17+** to run the language server itself (the project may target an older Java version).

Confirm it resolves:

```bash
command -v jdtls
```

## Configure

Add a `java` entry to the Hermes LSP config under `lsp.<language>`:

```json
{
  "lsp": {
    "java": {
      "command": [
        "jdtls"
      ],
      "extensions": [
        ".java"
      ]
    }
  }
}
```

Hermes routes `.java` edits to `jdtls` via the `extensions` list. jdtls maintains
a per-project workspace data directory and the first index is slow (it resolves
the full classpath and builds). If `jdtls` cannot find a runtime, export
`JAVA_HOME` for a JDK 17+ in the shell that launches Hermes:

```bash
export JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home
```

Most settings (runtimes, format, import order) come from `settings.java.*`
defaults and work for Maven/Gradle projects with a standard layout.

## Alternatives

**No mainstream alternative.** `jdtls` (Eclipse JDT Language Server) is the
de-facto standard and powers the official VS Code Java extension.

## Troubleshooting

- **PATH:** `jdtls` on PATH; reopen shell after install.
- **No JDK found:** server exits immediately — set `JAVA_HOME` to a JDK 17+.
- **Slow / no completions at first:** the initial classpath index can take a minute or more on large Maven/Gradle projects; wait for it to finish.
- **Stale state:** delete the jdtls workspace data dir to force a clean re-index if results go wrong after big dependency changes.
- **Build tool required:** keep `pom.xml` / `build.gradle` valid; a broken build descriptor breaks symbol resolution.

## Verify

```bash
node --experimental-strip-types ../../scripts/verify-lsp.ts path/to/File.java
# or: bun ../../scripts/verify-lsp.ts path/to/File.java
```