/logging-patterns

name: logging-patterns
description: Java logging best practices with SLF4J, structured logging (JSON), and MDC for request tracing. Includes AI-friendly log formats for Claude Code debugging. Use when user asks about logging, debugging application flow, or analyzing logs.

Logging Patterns Skill

Effective logging for Java applications with focus on structured, AI-parsable formats.

When to Use

• User says "add logging" / "improve logs" / "debug this"
• Analyzing application flow from logs
• Setting up structured logging (JSON)
• Request tracing with correlation IDs
• AI/Claude Code needs to analyze application behavior

---

AI-Friendly Logging

> **Key insight:** JSON logs are better for AI analysis - faster parsing, fewer tokens, direct field access.

Why JSON for AI/Claude Code?

# Text format - AI must "interpret" the string
2026-01-29 10:15:30 INFO OrderService - Order 12345 created for user-789, total: 99.99

# JSON format - AI extracts fields directly
{"timestamp":"2026-01-29T10:15:30Z","level":"INFO","orderId":12345,"userId":"user-789","total":99.99}

| Aspect | Text | JSON | |--------|------|------| | Parsing | Regex/interpretation | Direct field access | | Token usage | Higher (repeated patterns) | Lower (structured) | | Error extraction | Parse stack trace text | `exception` field | | Filtering | grep patterns | `jq` queries |

Recommended Setup for AI-Assisted Development

# application.yml - JSON by default
logging:
  structured:
    format:
      console: logstash  # Spring Boot 3.4+

# When YOU need to read logs manually:
# Option 1: Use jq
# tail -f app.log | jq .

# Option 2: Switch profile temporarily
# java -jar app.jar --spring.profiles.active=human-logs

Log Format Optimized for AI Analysis

{
  "timestamp": "2026-01-29T10:15:30.123Z",
  "level": "INFO",
  "logger": "com.example.OrderService",
  "message": "Order created",
  "requestId": "req-abc123",
  "traceId": "trace-xyz",
  "orderId": 12345,
  "userId": "user-789",
  "duration_ms": 45,
  "step": "payment_completed"
}

**Key fields for AI debugging:**

• `requestId` - group all logs from same request
• `step` - track progress through flow
• `duration_ms` - identify slow operations
• `level` - quick filter for errors

Reading Logs with AI/Claude Code

When asking AI to analyze logs:

# Get recent errors
cat app.log | jq 'select(.level == "ERROR")' | tail -20

# Follow specific request
cat app.log | jq 'select(.requestId == "req-abc123")'

# Find slow operations
cat app.log | jq 'select(.duration_ms > 1000)'

AI can then: 1. Parse JSON directly (no guessing) 2. Follow request flow via requestId 3. Identify exactly where errors occurred 4. Measure timing between steps

---

Quick Setup (Spring Boot 3.4+)

Native Structured Logging

Spring Boot 3.4+ has built-in support - no extra dependencies!

# application.yml
logging:
  structured:
    format:
      console: logstash    # or "ecs" for Elastic Common Schema

# Supported formats: logstash, ecs, gelf

Profile-Based Switching

# application.yml (default - JSON for AI/prod)
spring:
  profiles:
    default: json-logs

---
spring:
  config:
    activate:
      on-profile: json-logs
logging:
  structured:
    format:
      console: logstash

---
spring:
  config:
    activate:
      on-profile: human-logs
# No structured format = human-readable default
logging:
  pattern:
    console: "%d{HH:mm:ss.SSS} %-5level [%thread] %logger{36} - %msg%n"

**Usage:**

# Default: JSON (for AI, CI/CD, production)
./mvnw spring-boot:run

# Human-readable when needed
./mvnw spring-boot:run -Dspring.profiles.active=human-logs

---

Setup for Spring Boot < 3.4

Logstash Logback Encoder

**pom.xml:**

<dependency>
    <groupId>net.logstash.logback</groupId>
    <artifactId>logstash-logback-encoder</artifactId>
    <version>7.4</version>
</dependency>

**logback-spring.xml:**

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <!-- JSON (default) -->
    <springProfile name="!human-logs">
        <appender name="JSON" class="ch.qos.logback.core.ConsoleAppender">
            <encoder class="net.logstash.logback.encoder.LogstashEncoder">
                <includeMdcKeyName>requestId</includeMdcKeyName>
                <includeMdcKeyName>userId</includeMdcKeyName>
            </encoder>
        </appender>
        <root level="INFO">
            <appender-ref ref="JSON"/>
        </root>
    </springProfile>

    <!-- Human-readable (optional) -->
    <springProfile name="human-logs">
        <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
            <encoder>
                <pattern>%d{HH:mm:ss.SSS} %-5level [%thread] %logger{36} - %msg%n</pattern>
            </encoder>
        </appender>
        <root level="INFO">
            <appender-ref ref="CONSOLE"/>
        </root>
    </springProfile>

</configuration>

Adding Custom Fields (Logstash Encoder)

import static net.logstash.logback.argument.StructuredArguments.kv;

// Fields appear as separate JSON keys
log.info("Order created",
    kv("orderId", order.getId()),
    kv("userId", user.getId()),
    kv("total", order.getTotal()),
    kv("step", "order_created")
);

// Output:
// {"message":"Order created","orderId":123,"userId":"u-456","total":99.99,"step":"order_created"}

---

SLF4J Basics

Logger Declaration

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Service
public class OrderService {
    private static final Logger log = LoggerFactory.getLogger(OrderService.class);
}

// Or with Lombok
@Slf4j
@Service
public class OrderService {
    // use `log` directly
}

Parameterized Logging

// ✅ GOOD: Evaluated only if level enabled
log.debug("Processing order {} for user {}", orderId, userId);

// ❌ BAD: Always concatenates
log.debug("Processing order " + orderId + " for user " + userId);

// ✅ For expe