Structured Logging in Spring Boot 3.4: JSON, ECS, and Custom Formats

Log recording is a core part of application troubleshooting and system observability. Structured logging outputs logs in a predefined, machine‑readable format (commonly JSON), enabling powerful search, filtering, and analysis.

Spring Boot 3.4 provides out‑of‑the‑box support for structured logging. It includes built‑in formats such as Elastic Common Schema (ECS) and Logstash, and can be extended for custom needs.

Enable Structured Logging on the Console

Add the following property to application.properties to output console logs in ECS format:

logging.structured.format.console=ecs

When the application starts, logs appear as JSON, e.g.:

{"@timestamp":"2024-07-30T08:41:10.561295200Z","log.level":"INFO","process.pid":67455,"process.thread.name":"main","service.name":"structured-logging-demo","log.logger":"com.example.structured_logging_demo.StructuredLoggingDemoApplication","message":"Started StructuredLoggingDemoApplication in 0.329 seconds (process running for 0.486)","ecs.version":"8.11"}

Write Structured Logs to a File

Configure file output by adding to application.properties :

logging.structured.format.file=ecs
logging.file.name=log.json

The console shows human‑readable logs, while log.json contains the structured JSON entries.

Adding Extra Fields

Use MDC to attach additional data (e.g., userId) to each log event:

@Component
class MyLogger implements CommandLineRunner {
private static final Logger LOGGER = LoggerFactory.getLogger(MyLogger.class);
@Override
public void run(String... args) {
MDC.put("userId", "1");
LOGGER.info("Hello structured logging!");
MDC.remove("userId");
}
}

The resulting JSON includes the extra field:

{..., "message":"Hello structured logging!", "userId":"1", ...}

Alternatively, use the fluent API without MDC:

@Component
class MyLogger implements CommandLineRunner {
private static final Logger LOGGER = LoggerFactory.getLogger(MyLogger.class);
@Override
public void run(String... args) {
LOGGER.atInfo().setMessage("Hello structured logging!")
.addKeyValue("userId", "1")
.log();
}
}

Custom Log Formatter

Create a class implementing StructuredLogFormatter<ILoggingEvent> and register it in application.properties :

class MyStructuredLoggingFormatter implements StructuredLogFormatter<ILoggingEvent> {
@Override
public String format(ILoggingEvent event) {
return "time=" + event.getTimeStamp() + " level=" + event.getLevel() + " message=" + event.getMessage() + "\n";
}
}

logging.structured.format.console=com.example.structured_logging_demo.MyStructuredLoggingFormatter

Running the app now prints logs like:

time=1722330118045 level=INFO message=Hello structured logging!

For JSON output, Spring Boot 3.4 introduces JsonWriter :

class MyStructuredLoggingFormatter implements StructuredLogFormatter<ILoggingEvent> {
private final JsonWriter<ILoggingEvent> writer = JsonWriter.<ILoggingEvent>of(members -> {
members.add("time", e -> e.getInstant());
members.add("level", e -> e.getLevel());
members.add("thread", e -> e.getThreadName());
members.add("message", e -> e.getFormattedMessage());
members.add("application").usingMembers(app -> {
app.add("name", "StructuredLoggingDemo");
app.add("version", "1.0.0-SNAPSHOT");
});
members.add("node").usingMembers(node -> {
node.add("hostname", "node-1");
node.add("ip", "10.0.0.7");
});
}).withNewLineAtEnd();
@Override
public String format(ILoggingEvent event) {
return writer.writeToString(event);
}
}

logging.structured.format.console=com.example.structured_logging_demo.MyStructuredLoggingFormatter

The generated log looks like:

{"time":"2024-07-30T09:14:49.377308361Z","level":"INFO","thread":"main","message":"Hello structured logging!","application":{"name":"StructuredLoggingDemo","version":"1.0.0-SNAPSHOT"},"node":{"hostname":"node-1","ip":"10.0.0.7"}}

Note: Spring Boot 3.0/3.1 lifecycle has ended; upgrade to the latest version.