How to intercept SLF4J (with logback) logging via a JUnit test?

java

logging

junit

test-environment

byAlex Kataev·Dec 24, 2024

Quick and easy way to intercept SLF4J logs in JUnit tests, is to plug in a Logback Test Appender. Here's an illustrative code snippet:

import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.AppenderBase;
import ch.qos.logback.classic.Logger;
import org.slf4j.LoggerFactory;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

// Custom appender, kind of a stenographer for logs
class CapturingAppender extends AppenderBase<ILoggingEvent> {
    List<ILoggingEvent> capturedEvents = new CopyOnWriteArrayList<>();

    @Override
    protected void append(ILoggingEvent eventObject) {
        capturedEvents.add(eventObject); // Gotcha. You ain't going nowhere, log!
    }
    // Getter omitted for brevity
}

// In your test
CapturingAppender captor = new CapturingAppender(); // Your secret agent
Logger rootLogger = (Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME); // The unsuspecting target
rootLogger.addAppender(captor); // Deployed agent!
captor.start(); // Agent, you may start the capture.

// After executing actions known to trigger logging
assertThat(captor.capturedEvents, hasItem(hasProperty("message", is("Expected message"))));

How to analyse the captor? Simple. Invoke captor.capturedEvents.

Diving deeper into SLF4J logging interception

Setting the stage for testing

Before getting your test spy to work, make sure to setup a consistent test environment. Use logback-test.xml to define configurations specific to your test environment, like limiting logging levels.

Concurrent tests - a special case

With potential concurrent modifications in multi-threaded tests, a thread-safe implementation like CopyOnWriteArrayList ensures data integrity.

Cleaning up post-test

In the @AfterEach method, stops and detaches appenders for a clean slate for the next test, preventing unexpected side effects.

Let's talk about logging events

The ILoggingEvent interface contains various context information like the message, logger name, logging level, which can be very helpful for precise data analysis.