Implementing Logging
1. Understanding Spring Boot Logging Defaults (Logback)
| Aspect | Default |
|---|---|
| Framework | SLF4J + Logback |
| Pattern | %d %5p [%t] %c{1.} : %m%n |
| Output | Console only |
| Bridges | JCL, JUL, Log4j → SLF4J |
2. Configuring Log Levels
Example: Log level per package
logging:
level:
root: INFO
com.example: DEBUG
org.hibernate.SQL: DEBUG
org.hibernate.type.descriptor.sql.BasicBinder: TRACE
org.springframework.security: DEBUG| Level | Use |
|---|---|
| ERROR | Failures requiring attention |
| WARN | Anomalies, recoverable issues |
| INFO | Lifecycle, key business events |
| DEBUG | Diagnostic detail |
| TRACE | Very fine-grained |
3. Using SLF4J Logger Interface
Example: SLF4J parameterised logging
private static final Logger log = LoggerFactory.getLogger(OrderService.class);
log.info("Order placed id={} total={}", order.getId(), order.getTotal());
log.error("DB failure for order {}", id, ex); // exception last
if (log.isDebugEnabled()) log.debug("Heavy: {}", expensive());4. Logging with Different Levels
| Method | When |
|---|---|
log.error(msg, ex) |
Caught critical exception |
log.warn(msg) |
Retry triggered, deprecated path |
log.info(msg) |
Service started, request handled |
log.debug(msg) |
Method entry/exit, payloads (no PII) |
5. Configuring Logback XML
Example: Logback JSON encoder for production
<configuration>
<include resource="org/springframework/boot/logging/logback/defaults.xml"/>
<springProperty name="appName" source="spring.application.name"/>
<appender name="JSON" class="ch.qos.logback.core.ConsoleAppender">
<encoder class="net.logstash.logback.encoder.LogstashEncoder"/>
</appender>
<springProfile name="prod">
<root level="INFO"><appender-ref ref="JSON"/></root>
</springProfile>
</configuration>6. Using Log4j2 as Alternative
Example: Switch logging to Log4j2
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>7. Logging to Files
Example: Log to file with rolling pattern
logging:
file:
name: /var/log/app.log
path: /var/log
pattern:
file: "%d{ISO8601} %-5level [%thread] %logger{36} - %msg%n"8. Configuring Log Rotation and Archiving
| Property | Use |
|---|---|
logging.logback.rollingpolicy.max-file-size |
10MB |
logging.logback.rollingpolicy.max-history |
30 (days) |
logging.logback.rollingpolicy.total-size-cap |
1GB |
logging.logback.rollingpolicy.file-name-pattern |
app-%d{yyyy-MM-dd}.%i.log.gz |
9. Using MDC for Contextual Logging
Example: Propagate trace ID with MDC filter
public class TraceFilter extends OncePerRequestFilter {
@Override protected void doFilterInternal(HttpServletRequest req, HttpServletResponse res, FilterChain chain)
throws IOException, ServletException {
String tid = Optional.ofNullable(req.getHeader("X-Trace-Id")).orElse(UUID.randomUUID().toString());
MDC.put("traceId", tid);
try { chain.doFilter(req, res); }
finally { MDC.clear(); }
}
}
// pattern: %X{traceId}10. Logging HTTP Requests and Responses
| Tool | Description |
|---|---|
CommonsRequestLoggingFilter |
Built-in filter |
logging.level.org.springframework.web=DEBUG |
Built-in MVC trace |
Custom HandlerInterceptor |
Per-route logging |
| WebFilter (WebFlux) | Reactive equivalent |
11. Integrating with ELK Stack
| Component | Role |
|---|---|
| Logstash/Filebeat | Ship logs |
| Elasticsearch | Index + search |
| Kibana | Visualize |
12. Using Structured Logging
Example: Logstash encoder dependency
<dependency>
<groupId>net.logstash.logback</groupId>
<artifactId>logstash-logback-encoder</artifactId>
<version>7.4</version>
</dependency>Example: Structured key-value log entry
log.atInfo()
.setMessage("Order placed")
.addKeyValue("orderId", id)
.addKeyValue("total", total)
.log();Note: Spring Boot 3.4+ supports built-in structured logging via
logging.structured.format.console=ecs|logstash|gelf.