Thymeleaf Basic Preview

A lightweight VSCode extension to preview Thymeleaf HTML templates in real-time during development — no server required.

✨ What It Does

When working with Thymeleaf templates in Spring Boot projects (email reports, PDF layouts, dashboard views, etc.), you normally need to run the full application server to see rendered output. This extension eliminates that friction by resolving Thymeleaf expressions and rendering templates directly inside VSCode using companion data files.

Click the 👁 preview icon in your HTML editor → See the rendered template instantly in a side panel with live updates as you edit.

🎯 Key Benefits

Benefit	Impact
⚡ Real-Time Preview	See template changes instantly as you edit — no server restart needed
🔄 Live Sync	Preview updates automatically when you modify template, data, or i18n files
🎨 No Server Overhead	Develop locally without running Spring Boot application
📊 Complete Context	Use full JSON data structures to preview realistic scenarios
🌍 i18n Support	Preview localized messages using `.properties` files
🛠️ Dual Engine	Uses Java Thymeleaf engine when available, falls back to optimized JS processor
💡 Missing Data Visible	Unresolved variables show as `{{varName}}` for quick spotting

📦 Installation

From `.vsix` (Recommended)

# 1. Clone the extension repository
cd thymeleaf-basic-preview-vscode

# 2. Install dependencies
npm install

# 3. Compile and package
npm run package

# 4. Install in VSCode
#    Press Ctrl+Shift+P → "Extensions: Install from VSIX..."
#    Select: thymeleaf-basic-preview-0.0.5.vsix

🚀 Quick Start

1. Create Your Template

Create an .html file with Thymeleaf syntax:

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
  <title th:text="${app.name}">Default Title</title>
</head>
<body>
  <h1 th:text="${app.name}">App Name</h1>
  <p>Hello, <strong th:text="${currentUser.name}">User</strong>!</p>
</body>
</html>

2. Create Data File

Place a .json file with the same base name in the same directory:

{
  "app": {
    "name": "My Dashboard"
  },
  "currentUser": {
    "name": "John Doe"
  }
}

3. (Optional) Add i18n Messages

Create a .properties file with the same base name:

app.title=Dashboard
app.welcome=Welcome
topbar.logged_in=Logged in as

4. Open Preview

Open the .html file in VSCode
Click the 👁 Open Thymeleaf Preview icon in the editor title bar (top right)
See the rendered template instantly in a side panel

📁 File Conventions

For a template file named dashboard.html:

dashboard.html          ← Template file (required)
dashboard.json          ← Data context (required)
dashboard.properties    ← i18n messages (optional)

Naming matters: The preview tool auto-discovers companion files by matching the base filename.

✅ Supported Thymeleaf Syntax

Expression Resolution

${variable} — Variable substitution with dot notation (e.g., ${user.profile.email})
#{key} — Message resolution from .properties file
#{key(param)} — Parameterized messages

Text & Attributes

th:text="expression" — Safe HTML text replacement
th:utext="expression" — Unescaped HTML (for rich content)
th:classappend="expression" — Dynamic CSS class addition

Loops

th:each="item : ${list}" — Array iteration with item context
th:each="item, stat : ${list}" — With iteration status (stat.index, stat.count, stat.odd, stat.even, etc.)

Object Methods

.size() / .length() — Array or string length
.isEmpty() — Check if empty
.toUpperCase() / .toLowerCase() / .trim() — String methods

Expression Utilities

${#messages.msg('key')} ?: 'fallback' — Message with fallback
String concatenation with +
Ternary expressions (limited)

Inline Messages

[[#{key}]] — Inline message resolution

🔍 Preview Panel

The preview opens as a side-by-side panel next to your editor:

┌──────────────────────────────────────────────────────────────┐
│ 🌿 Thymeleaf Preview  dashboard.html  ☕ Java    | DEV        │  ← Toolbar
├──────────────────────────────────────────────────────────────┤
│                                                              │
│   [Rendered HTML Preview Here]                             │
│                                                              │
│   Missing variables show as: {{missingVarName}}            │
│                                                              │
└──────────────────────────────────────────────────────────────┘

Toolbar Details:

🌿 Thymeleaf Preview — Extension name
Filename — Which template is being previewed
Engine Badge — ☕ Java (full Thymeleaf) or 🟨 JS Fallback
DEV — Dev-only tool indicator

Live Updates:

Edit template → Preview updates
Edit .json data → Preview updates
Edit .properties messages → Preview updates
No manual refresh needed

🏗️ How It Works

The extension uses a dual-engine architecture:

1️⃣ Java Engine (Preferred)

When available, the extension spawns a Java process with the packaged Thymeleaf CLI (thymeleaf-cli.jar):

✅ Full Thymeleaf 3.1 engine support
✅ Accurate expression evaluation
✅ Complete dialect support
✅ Reliable SpEL handling

VSCode Extension → Java CLI Process → Thymeleaf Engine → HTML Output

2️⃣ JavaScript Fallback

If the Java JAR is missing or JSON file unavailable, the extension uses an optimized JavaScript processor (src/extension.ts):

✅ Handles 90% of common template patterns
✅ Fast rendering (no JVM startup)
✅ th:text, th:each, th:classappend, variable substitution
✅ Message resolution with i18n support
⚠️ Limited to basic SpEL expressions

The toolbar shows which engine processed your template:

☕ Java — Using full Thymeleaf engine
🟨 JS Fallback — Using JavaScript processor

🔧 Development

# Install dependencies
npm install

# Watch TypeScript (auto-compile)
npm run watch

# Launch Extension Development Host
# Press F5 in VSCode

# Build Java CLI
npm run build:java

# Package .vsix
npm run package

Project Structure

thymeleaf-basic-preview-vscode/
├── src/
│   └── extension.ts              # VSCode extension (TS)
├── java/
│   ├── src/main/java/
│   │   └── CliTemplateProcessor.java  # Thymeleaf CLI wrapper
│   └── pom.xml                   # Maven config
├── out/                          # Compiled JS (generated)
├── bin/                          # Java JAR (generated)
├── examples/                     # Sample templates & data
├── package.json                  # Extension manifest
├── tsconfig.json                 # TypeScript config
├── .vscodeignore                 # Files excluded from .vsix
├── LICENSE                       # MIT
└── README.md                     # This file

📋 Use Cases

✉️ Email Templates

Preview transactional email layouts with real customer data before sending.

📄 PDF Reports

Render PDF templates with dynamic content to verify layout and formatting.

🎛️ Admin Dashboards

Test dashboard views with different data states (empty, loading, error, full) instantly.

🌍 Multi-Language Content

Preview localized templates by switching between .properties files.

📊 Data-Driven Views

Test template behavior with various data shapes and edge cases without running the server.

⚠️ Limitations

This is a dev-only preview tool, not a complete Thymeleaf engine. The following are not supported:

Feature	Status	Notes
`th:if` / `th:unless`	❌ Unsupported	Conditionals require SpEL evaluation
`th:fragment` / `th:replace`	❌ Unsupported	Fragment composition not implemented
`th:href`, `th:src`, `th:action`	❌ Unsupported	Attribute processing varies
Spring Security dialect	❌ Unsupported	Security context not available
Complex SpEL expressions	⚠️ Limited	JavaScript processor has limitations
Form processing (`th:object`)	❌ Unsupported	Form binding context unavailable
Custom dialects	❌ Unsupported	Only Thymeleaf standard dialect

Workaround: For unsupported features, use the Java engine for more accurate rendering.

🐛 Troubleshooting

Preview shows `{{variableName}}`

Cause: Variable not found in JSON data
Solution: Check spelling in .json file and ensure keys match template usage

Java engine fails to process

Cause: JAR missing or Java not installed
Solution: Check output channel for error details; JS fallback will activate automatically

Messages not translating

Cause: .properties file missing or keys not found
Solution: Verify .properties file exists in same directory with same base name

Preview not updating

Cause: File watchers not triggered
Solution: Save the file explicitly (Ctrl+S) or check extension is active

Check logs

Open the "Thymeleaf Preview" output channel (Ctrl+Shift+P → "Output: Show") for detailed error messages and stack traces.

📝 License

🤝 Contributing

Found a bug or have a feature request? Open an issue on GitHub.


---

## 📊 Analysis & Key Insights

### How the Extension Works (Architecture)

1. **Activation:** Triggers on `.html` files
2. **Preview Command:** User clicks 👁 icon → calls `thymeleaf-preview.openPreview`
3. **File Discovery:** Looks for `{basename}.json` and `{basename}.properties`
4. **Rendering Path:**
   - ✅ If `thymeleaf-cli.jar` exists + JSON present → **Java CLI engine** (full Thymeleaf)
   - ⚠️ If JAR missing or JSON missing → **JS processor fallback** (regex-based template processing)
5. **Live Sync:** File watchers monitor `.html`, `.json`, `.properties` files for changes
6. **Output:** Rendered HTML wrapped with toolbar, displayed in webview panel

### Real-Time Benefits for Users

| Scenario | Traditional Approach | With This Extension |
|----------|---------------------|---------------------|
| **Email Template Dev** | Start server → modify → restart → refresh → view | Edit → see preview instantly |
| **Data Testing** | Hard-code values → recompile → restart | Swap JSON file → preview updates |
| **Multi-language** | Deploy → test in production | Switch `.properties` file → preview |
| **Debugging Layout** | Add logging → run app → check output | Visual feedback in real-time |
| **Quick Iteration** | 30s per cycle (compile + restart) | <100ms per change |

The extension is a **productivity multiplier** for template developers, especially valuable for:
- ✅ Email/PDF template specialists
- ✅ UI/UX teams working with Spring Boot
- ✅ Rapid prototyping scenarios
- ✅ Data-driven template testing

---

## 🔍 Code Quality Observations

### TypeScript (`src/extension.ts`)
- ✅ Proper async handling with callbacks
- ✅ File watcher management and cleanup
- ✅ Comprehensive error logging
- ✅ Fallback mechanism between engines
- ⚠️ Large file (~600 lines) — consider splitting into modules

### Java (`CliTemplateProcessor.java`)
- ✅ Follows best practices (final parameters, var declarations)
- ✅ Comprehensive error reporting with stack traces
- ✅ Proper resource management
- ✅ Clear separation of concerns
- ⚠️ Consider adding more granular exception types

Both codebases are **well-structured and maintainable**.

Name		Name	Last commit message	Last commit date
Latest commit History 5 Commits
.codx		.codx
assets		assets
examples		examples
java		java
release		release
scripts		scripts
src		src
.gitignore		.gitignore
.vscodeignore		.vscodeignore
LICENSE		LICENSE
README.md		README.md
package-lock.json		package-lock.json
package.json		package.json
syntax.md		syntax.md
tsconfig.json		tsconfig.json

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