write-script-java

---
name: write-script-java
description: MUST use when writing Java scripts.
---

## CLI Commands

Place scripts in a folder.

After writing, tell the user which command fits what they want to do:

- `wmill script preview <script_path>` — **default when iterating on a local script.** Runs the local file without deploying.
- `wmill script run <path>` — runs the script **already deployed** in the workspace. Use only when the user explicitly wants to test the deployed version, not local edits.
- `wmill generate-metadata` — regenerate the local `.script.yaml` (input schema) and `.lock` (resolved dependencies) for scripts you changed, and refresh their content hashes in `wmill-lock.yaml`. Local files only — **not** a deploy. See "Keep metadata in sync" below.
- Deploy local changes to the workspace — via `git push` or `wmill sync push` depending on how the repo is wired (see the **Deploying** section in `AGENTS.wmill.md`). Only suggest/run a deploy when the user explicitly asks to deploy/publish/push — not when they say "run", "try", or "test".

### Preview vs run — choose by intent, not habit

If the user says "run the script", "try it", "test it", "does it work" while there are **local edits to the script file**, use `script preview`. Do NOT push the script to then `script run` it — pushing is a deploy, and deploying just to test overwrites the workspace version with untested changes.

Only use `script run` when:
- The user explicitly says "run the deployed version" / "run what's on the server".
- There is no local script being edited (you're just invoking an existing script).

Only use `sync push` when:
- The user explicitly asks to deploy, publish, push, or ship.
- The preview has already validated the change and the user wants it in the workspace.

### Keep metadata in sync after editing

`wmill-lock.yaml` tracks a content hash for each item. Editing a script's content — most importantly **adding or removing an import** or **changing `main`'s arguments** — invalidates that hash and leaves the `.lock`, the `.script.yaml` input schema, and the hash row out of date. Run `wmill generate-metadata` (scoped to what you touched) after such edits so the resolved lock, the auto-generated args UI (driven by `.script.yaml`), and `wmill-lock.yaml` all match the code. Leaving them stale produces spurious diffs in git-sync and CI.

This only writes local files (it is **not** a deploy), but it re-resolves dependencies, so it can bump unpinned versions (the same as deploying from the UI; expected, not a bug). So by default offer it and run it once the user agrees, rather than running it silently after every edit — unless the project's `AGENTS.md` opts into running metadata automatically (see the "Keeping metadata in sync" preference there). Either way YOU run the command, not the user. After running it, diff the regenerated `.lock` / `.script.lock` files and tell the user which dependency versions changed (e.g. `requests 2.31.0 → 2.32.0`), so they can catch an unwanted bump before deploying — even under `Metadata: auto`, since it's information, not a confirmation gate. Pin versions in code to keep them fixed.

With no path argument, `generate-metadata` regenerates only the items whose content hash drifted — not everything. Imports propagate: editing a script that others import marks every importer stale too, so a one-line change to a shared module can regenerate many locks (by design — their locks must reflect the imported code). If it touches more than you expect, run `wmill generate-metadata --dry-run` — it lists each stale item with a reason (`content changed` or `depends on <path>`) without changing anything — then narrow with a path argument (`wmill generate-metadata f/foo`) or `--strict-folder-boundaries`.

If the on-disk `.lock` and `.script.yaml` are already correct and only `wmill-lock.yaml` needs its hashes refreshed (hash drift, or bootstrapping missing entries), use `wmill generate-metadata rehash` — it re-records hashes from disk with no backend round-trip and no dependency changes.

### After writing — offer to test, don't wait passively

If the user hasn't already told you to run/test/preview the script, offer it as a one-sentence next step (e.g. "Want me to run `wmill script preview` with sample args?"). Do not present a multi-option menu.

If the user already asked to test/run/try the script in their original request, skip the offer and just execute `wmill script preview <path> -d '<args>'` directly — pick plausible args from the script's declared parameters. The shape varies by language: `main(...)` for code languages, the SQL dialect's own placeholder syntax (`$1` for PostgreSQL, `?` for MySQL/Snowflake, `@P1` for MSSQL, `@name` for BigQuery, etc.), positional `$1`, `$2`, … for Bash, `param(...)` for PowerShell.

`wmill script preview` does not deploy, but it still executes script code and may cause side effects; run it yourself when the user asked to test/preview (or after confirming that execution is intended). `wmill generate-metadata` does not deploy either — it only writes local files (locks, schemas, hashes) — but offer it before running (or run automatically if the project's `AGENTS.md` opts in), per "Keep metadata in sync" above. Deploying to the workspace (`git push` or `wmill sync push` depending on how the repo is wired — see the **Deploying** section) is the only step that mutates remote state — do it only when the user explicitly asks to deploy/publish/push.

For a **visual** open-the-script-in-the-dev-page preview (rather than `script preview`'s run-and-print-result), use the `preview` skill.

Use `wmill resource-type list --schema` to discover available resource types.

# Windmill Script Writing Guide

## General Principles

- A script's inputs are its parameters. Credentials and configuration come in as resource-typed parameters, never hard-coded or read from the environment; the language section below shows how that language declares parameters
- When no resource type exists for a service, still take its connection as one resource-typed parameter, of a custom type holding the connection fields (base URL, account identifiers, credentials), its secret in a variable the resource references. A custom type is named `c_<name>`, which keeps it apart from a hub type synced later: the parameter takes `RT.C<Name>` in TypeScript, a `c_<name>` TypedDict in Python. A plain string parameter or a variable read inside the code only when the user asks for it: the resource keeps a connection's fields together, lets whoever runs the script pick an existing connection, and keeps the secret out of the arguments a run records. If the instance is reported as not having synced the Windmill Hub's resource types, mention that a superadmin can sync them, since the hub has types for most services. Otherwise, or when the user prefers it to syncing, propose the resource type and ask before creating it
- A Windmill Hub script takes a resource type the hub publishes, so on an instance that has not synced the hub's types that type is missing: say so, and that a superadmin can sync them, before asking for a resource of it. Hub scripts often declare the type inline (`type Stripe = { token: string }`), which is still the `stripe` resource type; in code you write, name it the way the language section shows (`RT.Stripe` in TypeScript) rather than copying the inline declaration
- Before relying on a resource type, a hub script's included, run `wmill resource-type list`: it shows whether the workspace has the type, and warns when the instance has not synced the hub's types
- Libraries are installed automatically - do not show installation instructions
- In a language with an entrypoint function (TypeScript, Python, Go, Rust, PHP, R, …), name it `main` (`Main` in C#) and do not call it; in TypeScript it must be async. SQL, GraphQL, Bash, PowerShell and Ansible scripts have no `main`: their language section shows how they take arguments
- Where the language has a Windmill client (`wmill`), use it to interact with the platform
- A script's input schema may carry a top-level `prompt_for_ai` string: its author's instructions to an AI choosing the inputs. Follow it when you pick arguments to run that script, and keep it when you rewrite the schema

## Return Values

- A script can return any JSON-serializable value; a SQL script returns the rows its query produces
- Return values become available to subsequent flow steps via `results.step_id`

## Preprocessor Scripts

Preprocessor scripts process raw trigger data from various sources (webhook, custom HTTP route, SQS, WebSocket, Kafka, NATS, MQTT, AMQP, Postgres, GCP Pub/Sub, Azure, or email) before passing it to the flow. This separates the trigger logic from the flow logic and keeps the auto-generated UI clean.

A preprocessor is written in TypeScript or Python: its function is named `preprocessor` instead of `main`, and it receives a single parameter called `event` (the language section gives its type).

The returned object determines the parameter values passed to the flow.
e.g., `{ b: 1, a: 2 }` calls the flow with `a = 2` and `b = 1`, assuming the flow has two inputs called `a` and `b`.

# Java

The script must contain a Main public class with a `public static main()` method:

```java
public class Main {
    public static Object main(String name, int count) {
        java.util.Map<String, Object> result = new java.util.HashMap<>();
        result.put("name", name);
        result.put("count", count);
        return result;
    }
}
```

**Important:**
- Class must be named `Main`
- Method must be `public static Object main(...)`
- Return type is `Object` or `void`

## Maven Dependencies

Add dependencies using comments at the top:

```java
//requirements:
//com.google.code.gson:gson:2.10.1
//org.apache.httpcomponents:httpclient:4.5.14

import com.google.gson.Gson;

public class Main {
    public static Object main(String input) {
        Gson gson = new Gson();
        return gson.fromJson(input, Object.class);
    }
}
```