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Capturing and Testing Logs in Java with SLF4J and Logback: A Simple Guide

Published at

1/8/2025

Setting Up the Example

We’ll create a basic service that logs an error when an exception occurs, and a corresponding test to verify the log message.

Step 1: Add Logback Test Dependency

...
<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-core</artifactId>
    <scope>test</scope>
</dependency>
...

Step 2: Implementing the Service

Here’s a simple service that catches exceptions and logs an error:

package com.example.loggingdemo;

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

public class SimpleService {
    private static final Logger logger = LoggerFactory.getLogger(SimpleService.class);

    public void performTask() {
        try {
            // Simulate some task that fails
            throw new IllegalArgumentException("Simulated exception");
        } catch (IllegalArgumentException e) {
            logger.error("An error occurred: {}", e.getMessage());
        }
    }
}

Step 3: Creating a Custom TestLogAppender

This appender captures log messages during tests, enabling assertions on their content.

package com.example.loggingdemo;

import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.AppenderBase;

public class TestLogAppender extends AppenderBase<ILoggingEvent> {
    private final StringBuilder logs = new StringBuilder();

    @Override
    protected void append(ILoggingEvent eventObject) {
        logs.append(eventObject.getFormattedMessage()).append("
");
    }

    public String getLogs() {
        return logs.toString();
    }
}

Step 4: Writing the Test

Now, let’s write a test to verify the log output.

package com.example.loggingdemo;

import static org.assertj.core.api.Assertions.assertThat;

import ch.qos.logback.classic.Logger;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.slf4j.LoggerFactory;

public class SimpleServiceTest {
    private TestLogAppender logAppender;
    private SimpleService simpleService;

    @BeforeEach
    public void setup() {
        // Attach TestLogAppender to the logger
        logAppender = new TestLogAppender();
        logAppender.start();
        Logger logger = (Logger) LoggerFactory.getLogger(SimpleService.class);
        logger.addAppender(logAppender);

        // Initialize the service
        simpleService = new SimpleService();
    }

    @Test
    public void testPerformTaskLogsError() {
        // Act
        simpleService.performTask();

        // Assert
        String logs = logAppender.getLogs();
        assertThat(logs).contains("An error occurred: Simulated exception");
    }
}

Important
Don't forget to start your log appender !

Step 5: Running the Test

When you run this test, it will confirm that the SimpleService logs the expected error message when an exception occurs.

How It Works

Custom Appender: The TestLogAppender captures log messages by overriding the append method of Logback’s AppenderBase.
Logger Configuration: During tests, we dynamically attach the custom appender to the target logger.
Assertions on Logs: Use the captured logs for assertions in your tests, verifying specific messages or patterns.

Conclusion

With this approach, you can confidently test your logging behavior, ensuring your application logs important events and errors as expected. This setup is versatile and can be adapted to more complex scenarios, such as testing log levels or message formatting.

Feel free to try this out and adapt it to your own projects!

Happy coding!

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