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devGuide/EXCEPTION_HANDLING_GUIDE.md

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+# Exception Handling Guide
+
+This guide outlines the common error handling patterns used within Stirling-PDF and provides tips for internationalising error messages. The examples cover the main languages found in the project: Java, JavaScript, HTML/CSS, and a small amount of Python.
+
+## General Principles
+
+- **Fail fast and log clearly.** Exceptions should provide enough information for debugging without exposing sensitive data.
+- **Use consistent user messages.** Text shown to users must be pulled from the localisation files so that translations are centrally managed.
+- **Avoid silent failures.** Always log unexpected errors and provide the user with a helpful message.
+
+## Java
+
+Java forms the core of Stirling-PDF. When adding new features or handling errors:
+
+1. **Create custom exceptions** to represent specific failure cases. This keeps the code self-documenting and easier to handle at higher levels.
+2. **Use `try-with-resources`** when working with streams or other closable resources to ensure clean-up even on failure.
+3. **Return meaningful HTTP status codes** in controllers by throwing `ResponseStatusException` or using `@ExceptionHandler` methods.
+4. **Log with context** using the project’s logging framework. Include identifiers or IDs that help trace the issue.
+5. **Internationalise messages** by placing user-facing text in `messages_en_US.properties` and referencing them with message keys.
+
+## JavaScript
+
+On the client side, JavaScript handles form validation and user interactions.
+
+- Use `try`/`catch` around asynchronous operations (e.g., `fetch`) and display a translated error notice if the call fails.
+- Validate input before sending it to the server and provide inline feedback with messages from the translation files.
+- Log unexpected errors to the browser console for easier debugging, but avoid revealing sensitive information.
+
+## HTML & CSS
+
+HTML templates should reserve a space for displaying error messages. Example pattern:
+
+```html
+<div class="error" role="alert" th:text="${errorMessage}"></div>
+```
+
+Use CSS classes (e.g., `.error`) to style the message so it is clearly visible and accessible. Keep the markup simple to ensure screen readers can announce the error correctly.
+
+## Python
+
+Python scripts in this project are mainly for utility tasks. Follow these guidelines:
+
+- Wrap file operations or external calls in `try`/`except` blocks.
+- Print or log errors in a consistent format. If the script outputs messages to end users, ensure they are translatable.
+
+Example:
+
+```python
+try:
+  perform_task()
+except Exception as err:
+  logger.error("Task failed: %s", err)
+  print(gettext("task.error"))
+```
+
+## Internationalisation (i18n)
+
+All user-visible error strings should be defined in the main translation file (`messages_en_US.properties`). Other language files will use the same keys. Refer to messages in code rather than hard-coding text.
+
+When creating new messages:
+
+1. Add the English phrase to `messages_en_US.properties`.
+2. Reference the message key in your Java, JavaScript, or Python code.
+3. Update other localisation files as needed.
+
+Following these patterns helps keep Stirling-PDF stable, easier to debug, and friendly to users in all supported languages.