aws-lambda-java-integration

# AWS Lambda Java Integration

Patterns for creating high-performance AWS Lambda functions in Java with optimized cold starts.

## Overview

This skill provides complete patterns for AWS Lambda Java development, covering two main approaches:

1. **Micronaut Framework** - Full-featured framework with AOT compilation, dependency injection, and cold start < 1s
2. **Raw Java** - Minimal overhead approach with cold start < 500ms

Both approaches support API Gateway and ALB integration with production-ready configurations.

## When to Use

- Deploying Java functions to AWS Lambda
- Optimizing cold starts below 1 second
- Choosing between Micronaut and Raw Java approaches
- Configuring API Gateway or ALB integration
- Setting up CI/CD pipelines for Java Lambda

## Instructions

### 1. Choose Your Approach

| Approach | Cold Start | Best For | Complexity |
|----------|------------|----------|------------|
| Micronaut | < 1s | Complex apps, DI needed, enterprise | Medium |
| Raw Java | < 500ms | Simple handlers, minimal overhead | Low |

**Validate**: Confirm the approach fits your use case before proceeding.

### 2. Project Structure

```
my-lambda-function/
├── build.gradle (or pom.xml)
├── src/main/java/com/example/Handler.java
└── serverless.yml (or template.yaml)
```

**Validate**: Verify project structure matches the template.

### 3. Implementation Examples

#### Micronaut Handler

```java
@FunctionBean("my-function")
public class MyFunction implements Function<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    private final MyService service;

    public MyFunction(MyService service) {
        this.service = service;
    }

    @Override
    public APIGatewayProxyResponseEvent apply(APIGatewayProxyRequestEvent request) {
        // Process request
        return new APIGatewayProxyResponseEvent()
            .withStatusCode(200)
            .withBody("{\"message\": \"Success\"}");
    }
}
```

#### Raw Java Handler

```java
public class MyHandler implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    private static final MyService service = new MyService();

    @Override
    public APIGatewayProxyResponseEvent handleRequest(APIGatewayProxyRequestEvent request, Context context) {
        return new APIGatewayProxyResponseEvent()
            .withStatusCode(200)
            .withBody("{\"message\": \"Success\"}");
    }
}
```

**Validate**: Run `sam local invoke` to verify handler works before deployment.

## Core Patterns

### Connection Management

```java
// Initialize once, reuse across invocations
private static final DynamoDbClient dynamoDb = DynamoDbClient.builder()
    .region(Region.US_EAST_1)
    .build();

// Avoid: Creating clients in handler (slow on every invocation)
```

### Error Handling

```java
@Override
public APIGatewayProxyResponseEvent handleRequest(APIGatewayProxyRequestEvent request, Context context) {
    try {
        return successResponse(process(request));
    } catch (ValidationException e) {
        return errorResponse(400, e.getMessage());
    } catch (Exception e) {
        context.getLogger().log("Error: " + e.getMessage());
        return errorResponse(500, "Internal error");
    }
}
```

## Best Practices

### Configuration

- **Memory**: Start with 512MB, adjust based on profiling
- **Timeout**: Micronaut 10-30s, Raw Java 5-10s
- **Runtime**: Java 17 or 21 for best performance

### Packaging

- Use Gradle Shadow Plugin or Maven Shade Plugin
- Exclude unnecessary dependencies

### Monitoring

- Enable X-Ray tracing for performance analysis
- Use CloudWatch Insights to track cold vs warm starts

## Deployment Options

### Serverless Framework

```yaml
service: my-java-lambda
provider:
  name: aws
  runtime: java21
  memorySize: 512
  timeout: 10
package:
  artifact: build/libs/function.jar
functions:
  api:
    handler: com.example.Handler
    events:
      - http:
          path: /{proxy+}
          method: ANY
```

**Validate**: Run `serverless deploy` with `--stage dev` first.

### AWS SAM

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Resources:
  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: build/libs/function.jar
      Handler: com.example.Handler
      Runtime: java21
      MemorySize: 512
      Timeout: 10
      Events:
        ApiEvent:
          Type: Api
          Properties:
            Path: /{proxy+}
            Method: ANY
```

**Validate**: Run `sam validate` before deploying.

## Constraints and Warnings

### Java-Specific Constraints

- **Reflection**: Minimize use; prefer AOT compilation (Micronaut)
- **Classpath scanning**: Slows cold start; use explicit configuration
- **Large frameworks**: Spring Boot adds significant cold start overhead

### Common Pitfalls

1. **Initialization in handler** - Causes repeated work on warm invocations
2. **Oversized JARs** - Include only required dependencies
3. **Insufficient memory** - Java needs more memory than Node.js/Python
4. **No timeout handling** - Always set appropriate timeouts

## References

For detailed guidance on specific topics:

- **[Micronaut Lambda](references/micronaut-lambda.md)** - Complete Micronaut setup, AOT configuration, DI optimization
- **[Raw Java Lambda](references/raw-java-lambda.md)** - Minimal handler patterns, singleton caching, JAR packaging
- **[Serverless Deployment](references/serverless-deployment.md)** - Serverless Framework, SAM, CI/CD pipelines, provisioned concurrency
- **[Testing Lambda](references/testing-lambda.md)** - JUnit 5, SAM Local, integration testing, performance measurement

## Examples

### Example 1: Create a Micronaut Lambda Function

**Input:**
```
Create a Java Lambda function using Micronaut to handle user REST API
```

**Process:**
1. Configure Gradle project with Micronaut plugin
2. Create Handler class extending MicronautRequestHandler
3. Implement methods for GET/POST/PUT/DELETE
4. Configure application.yml w