VS Code Integration

VS Code Integration

Use LeanJ directly in Visual Studio Code to see performance metrics in your code editor.

Prerequisites

• Visual Studio Code 1.80+
• LeanJ CLI installed and in PATH (or configured via settings)
• Java JDK 11+ for the profiled application
• Java Extension Pack installed (recommended)

Installation

Install the Extension

1. Open VS Code
2. Go to Extensions (Ctrl+Shift+X / Cmd+Shift+X)
3. Search for “LeanJ”
4. Click Install
5. Reload VS Code when prompted

Configure CLI Path

The extension needs to find the CLI. It searches in this order:

1. Extension settings: leanj.cliPath
2. PATH environment variable

Set extension setting:

1. Go to File → Preferences → Settings (or Code → Settings → Settings on macOS)
2. Search for “LeanJ”
3. Set LeanJ: Cli Path to your CLI executable path:
   • Windows: C:\path\to\leanj.exe
   • macOS/Linux: /path/to/leanj

Or add CLI to PATH (recommended):

• See Installation Guide for PATH setup

Usage

Method 1: Automatic (Recommended)

1. Start LeanJ Profiling:
   • Open Command Palette (Ctrl+Shift+P / Cmd+Shift+P)
   • Run: LeanJ: Start Profiling

The extension automatically:

• Calls the CLI
• Starts the controller (if not running)
• Discovers JVMs
• Attaches the agent

2. Run your application:

   • Use your normal run configuration
   • The agent is already attached

3. View metrics:

   • Open your Java files
   • See inline hints above methods
   • Hover for detailed popup
   • Check the LeanJ panel in the sidebar

Method 2: Manual CLI + IDE

1. Start controller manually:

   leanj controller start

2. Discover JVMs:

   leanj jvms

3. Attach agent:

   leanj attach --pid <pid>

4. Start profiling in VS Code:

   • Command Palette → LeanJ: Start Profiling

Viewing Metrics

Inline Hints

Metrics appear directly above method declarations:

╰─ Estimated avg execution time: 2.1ms
╰─ Max time last run: 4.8ms
╰─ Allocations per call: ~12KB
╰─ Call count: 150
╰─ Trend: +15%

public void processPayments() {
    // your code
}

Hover Popup

Hover over a method to see detailed metrics:

• Average execution time
• Maximum time
• Allocations
• Call count
• Trend (percentage change)
• Context information

Profiler Panel

1. Open the LeanJ panel in the sidebar
2. Or use Command Palette: LeanJ: Show Panel

The panel shows:

• All profiled methods
• Real-time statistics
• Filtering and sorting options
• JVM connection status

Metrics Explained

Average Execution Time

Cumulative average from profiling start. Shows typical performance.

Max Time

Maximum execution time in the last measurement window. Helps identify outliers.

Allocations

Memory allocated per call. Useful for finding memory-intensive operations.

Call Count

Total number of method invocations since profiling started.

Trend

Percentage change compared to previous measurement. Positive means slower, negative means faster.

Extension Settings

Configure the extension behavior:

1. File → Preferences → Settings
2. Search for “LeanJ”

Available settings:

• LeanJ: Cli Path: Path to CLI executable
• LeanJ: Auto Start Controller: Automatically start controller when profiling
• LeanJ: Refresh Interval: Metrics refresh interval in milliseconds
• LeanJ: Show Inline Hints: Enable/disable inline hints

Multiple JVM Support

You can profile multiple JVMs simultaneously:

1. Attach to multiple JVMs using CLI:

   leanj attach --pid 12345
   leanj attach --pid 67890

2. The extension shows metrics from all attached JVMs

3. Metrics are aggregated per method

4. Use the JVM selector in the panel to filter by JVM

Docker JVMs

Docker containers with the agent automatically appear in the JVM list:

1. Ensure agent is configured in Dockerfile:

   ENV JAVA_TOOL_OPTIONS="-javaagent:/agent/agent.jar"
   ENV LEANJ_PROJECT=my-service

2. Container starts and registers with controller

3. Appears in leanj jvms output

4. Extension can attach and show metrics

Troubleshooting

CLI Not Found

Error: CLI not found

Solution:

1. Verify CLI is installed: See Installation Guide
2. Check CLI is in PATH: leanj --version
3. Set extension setting: leanj.cliPath
4. Reload VS Code

Controller Not Running

Error: Controller not running

Solution:

1. Start controller manually: leanj controller start
2. Or enable auto-start in extension settings
3. Check logs: logs/profiler-controller.log

No Metrics Visible

Issue: Inline hints not showing

Solution:

1. Enable inline hints in extension settings
2. Ensure agent is attached: leanj status
3. Check controller logs for errors
4. Verify application is running and being profiled
5. Reload VS Code window

Metrics Not Updating

Issue: Metrics are stale

Solution:

1. Check controller is receiving data: leanj status
2. Verify agent is connected: check controller logs
3. Restart profiling: Command Palette → LeanJ: Start Profiling
4. Adjust refresh interval in settings

Advanced Configuration

Custom Controller Port

Set environment variable:

export LEANJ_CONTROLLER_PORT=8888

Or use CLI:

leanj controller start --port 8888

Workspace Matching

The CLI automatically matches JVMs to your workspace:

• Compares Git root
• Matches VS Code workspace path
• Uses user.dir comparison

This helps identify the correct JVM when multiple are running.

Next Steps

• Learn about Docker integration
• Explore CLI commands
• Configure advanced settings
• Read How It Works for architecture details