LogCaptor

Install library with:

Install with maven

<dependency>
    <groupId>io.github.hakky54</groupId>
    <artifactId>logcaptor</artifactId>
    <version>2.7.7</version>
    <scope>test</scope>
</dependency>

Install with Gradle

testImplementation 'io.github.hakky54:logcaptor:2.7.7'

Install with Scala SBT

libraryDependencies += "io.github.hakky54" % "logcaptor" % "2.7.7" % Test

Install with Apache Ivy

<dependency org="io.github.hakky54" name="logcaptor" rev="2.7.7" />

Introduction
Usage
Known issues
- Using Log Captor alongside with other logging libraries
- Capturing logs of static inner classes
Contributing
Contributors
License

Introduction

Hey, hello there 👋 Welcome, you are I hope you will like this library ❤️

LogCaptor is a library which will enable you to easily capture logging entries for unit testing purposes.

Do you want to capture the console output? Please have a look at ConsoleCaptor.

Advantages

No mocking required
No custom JUnit extension required
Plug & play

Supported Java versions

Java 8
Java 11+

Tested Logging libraries

SLFJ4
Logback
Java Util Logging
Apache Log4j
Apache Log4j2
Log4j with Lombok
Log4j2 with Lombok
SLFJ4 with Lombok
Java Util Logging with Lombok

See the unit test LogCaptorShould for all the scenario's or checkout this project Java Tutorials which contains more isolated examples of the individual logging frameworks

Usage

Capture logs

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class FooService {

    private static final Logger LOGGER = LogManager.getLogger(FooService.class);

    public void sayHello() {
        LOGGER.info("Keyboard not responding. Press any key to continue...");
        LOGGER.warn("Congratulations, you are pregnant!");
    }

}

Unit test:

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

import nl.altindag.log.LogCaptor;
import org.junit.jupiter.api.Test;

public class FooServiceShould {

    @Test
    public void logInfoAndWarnMessages() {
        LogCaptor logCaptor = LogCaptor.forClass(FooService.class);

        FooService fooService = new FooService();
        fooService.sayHello();

        // Get logs based on level
        assertThat(logCaptor.getInfoLogs()).containsExactly("Keyboard not responding. Press any key to continue...");
        assertThat(logCaptor.getWarnLogs()).containsExactly("Congratulations, you are pregnant!");

        // Get all logs
        assertThat(logCaptor.getLogs())
                .hasSize(2)
                .contains(
                    "Keyboard not responding. Press any key to continue...",
                    "Congratulations, you are pregnant!"
                );
    }
}

Initialize LogCaptor once and reuse it during multiple tests with clearLogs method within the afterEach method:

import nl.altindag.log.LogCaptor;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeAll;

public class FooServiceShould {

    private static LogCaptor logCaptor;
    private static final String EXPECTED_INFO_MESSAGE = "Keyboard not responding. Press any key to continue...";
    private static final String EXPECTED_WARN_MESSAGE = "Congratulations, you are pregnant!";
    
    @BeforeAll
    public static setupLogCaptor() {
        logCaptor = LogCaptor.forClass(FooService.class);
    }

    @AfterEach
    public void clearLogs() {
        logCaptor.clearLogs();
    }
    
    @AfterAll
    public static void tearDown() {
        logCaptor.close();
    }

    @Test
    public void logInfoAndWarnMessagesAndGetWithEnum() {
        FooService service = new FooService();
        service.sayHello();

        assertThat(logCaptor.getInfoLogs()).containsExactly(EXPECTED_INFO_MESSAGE);
        assertThat(logCaptor.getWarnLogs()).containsExactly(EXPECTED_WARN_MESSAGE);

        assertThat(logCaptor.getLogs()).hasSize(2);
    }

    @Test
    public void logInfoAndWarnMessagesAndGetWithString() {
        FooService service = new FooService();
        service.sayHello();

        assertThat(logCaptor.getInfoLogs()).containsExactly(EXPECTED_INFO_MESSAGE);
        assertThat(logCaptor.getWarnLogs()).containsExactly(EXPECTED_WARN_MESSAGE);

        assertThat(logCaptor.getLogs()).hasSize(2);
    }

}

Class which will log events if specific log level has been set

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class FooService {

    private static final Logger LOGGER = LogManager.getLogger(FooService.class);

    public void sayHello() {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Keyboard not responding. Press any key to continue...");
        }
        LOGGER.info("Congratulations, you are pregnant!");
    }

}

Unit test:

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

import nl.altindag.log.LogCaptor;
import org.junit.jupiter.api.Test;

public class FooServiceShould {

    @Test
    public void logInfoAndWarnMessages() {
        LogCaptor logCaptor = LogCaptor.forClass(FooService.class);
        logCaptor.setLogLevelToInfo();

        FooService fooService = new FooService();
        fooService.sayHello();

        assertThat(logCaptor.getInfoLogs()).contains("Congratulations, you are pregnant!");
        assertThat(logCaptor.getDebugLogs())
            .doesNotContain("Keyboard not responding. Press any key to continue...")
            .isEmpty();
    }
}

Class which will also log an exception

import nl.altindag.log.service.Service;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;

public class FooService {

    private static final Logger LOGGER = LoggerFactory.getLogger(ZooService.class);

    @Override
    public void sayHello() {
        try {
            tryToSpeak();
        } catch (IOException e) {
            LOGGER.error("Caught unexpected exception", e);
        }
    }

    private void tryToSpeak() throws IOException {
        throw new IOException("KABOOM!");
    }
}

Unit test:

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

import nl.altindag.log.LogCaptor;
import nl.altindag.log.model.LogEvent;
import org.junit.jupiter.api.Test;

public class FooServiceShould {

    @Test
    void captureLoggingEventsContainingException() {
        LogCaptor logCaptor = LogCaptor.forClass(ZooService.class);

        FooService service = new FooService();
        service.sayHello();

        List<LogEvent> logEvents = logCaptor.getLogEvents();
        assertThat(logEvents).hasSize(1);

        LogEvent logEvent = logEvents.get(0);
        assertThat(logEvent.getMessage()).isEqualTo("Caught unexpected exception");
        assertThat(logEvent.getLevel()).isEqualTo("ERROR");
        assertThat(logEvent.getThrowable()).isPresent();

        assertThat(logEvent.getThrowable().get())
                .hasMessage("KABOOM!")
                .isInstanceOf(IOException.class);
    }
}

Capture Managed Diagnostic Context (MDC)

import nl.altindag.log.service.LogMessage;
import nl.altindag.log.service.Service;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

public class FooService {

    private static final Logger LOGGER = LoggerFactory.getLogger(ServiceWithSlf4jAndMdcHeaders.class);

    public void sayHello() {
        try {
            MDC.put("my-mdc-key", "my-mdc-value");
            LOGGER.info(LogMessage.INFO.getMessage());
        } finally {
            MDC.clear();
        }

        LOGGER.info("Hello there!");
    }

}

Unit test:

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

import nl.altindag.log.LogCaptor;
import nl.altindag.log.model.LogEvent;
import org.junit.jupiter.api.Test;

public class FooServiceShould {

   @Test
   void captureLoggingEventsContainingMdc() {
      LogCaptor logCaptor = LogCaptor.forClass(FooService.class);

      FooService service = new FooService();
      service.sayHello();

      List<LogEvent> logEvents = logCaptor.getLogEvents();

      assertThat(logEvents).hasSize(2);

      assertThat(logEvents.get(0).getDiagnosticContext())
              .hasSize(1)
              .extractingByKey("my-mdc-key")
              .isEqualTo("my-mdc-value");

      assertThat(logEvents.get(1).getDiagnosticContext()).isEmpty();
   }
}

Disable any logs for a specific class

In some use cases a unit test can generate too many logs by another class. This could be annoying as it will cause noise in your build logs. LogCaptor can disable those log messages with the following snippet:

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

import nl.altindag.log.LogCaptor;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;

public class FooServiceShould {

    private static LogCaptor logCaptorForSomeOtherService = LogCaptor.forClass(SomeService.class);

    @BeforeAll
    static void disableLogs() {
        logCaptorForSomeOtherService.disableLogs();
    }

    @AfterAll
    static void resetLogLevel() {
        logCaptorForSomeOtherService.resetLogLevel();
    }

    @Test
    void logInfoAndWarnMessages() {
        LogCaptor logCaptor = LogCaptor.forClass(FooService.class);

       FooService service = new FooService();
        service.sayHello();

        assertThat(logCaptor.getLogs())
                .hasSize(2)
                .contains(
                    "Keyboard not responding. Press any key to continue...",
                    "Congratulations, you are pregnant!"
                );
    }
}

Disable all logs

Add logback-test.xml to your test resources with the following content:

<configuration>
    <statusListener class="ch.qos.logback.core.status.NopStatusListener" />
</configuration>

Returnable values from LogCaptor

import nl.altindag.log.LogCaptor;
import nl.altindag.log.model.LogEvent;

import java.time.ZonedDateTime;
import java.util.List;
import java.util.Map;
import java.util.Optional;

import org.junit.jupiter.api.Test;

class FooServiceShould {

   @Test
   void showCaseAllReturnableValues() {
        LogCaptor logCaptor = LogCaptor.forClass(FooService.class);

        List<String> logs = logCaptor.getLogs();
        List<String> infoLogs = logCaptor.getInfoLogs();
        List<String> debugLogs = logCaptor.getDebugLogs();
        List<String> warnLogs = logCaptor.getWarnLogs();
        List<String> errorLogs = logCaptor.getErrorLogs();
        List<String> traceLogs = logCaptor.getTraceLogs();

        LogEvent logEvent = logCaptor.getLogEvents().get(0);
        String message = logEvent.getMessage();
        String formattedMessage = logEvent.getFormattedMessage();
        String level = logEvent.getLevel();
        List<Object> arguments = logEvent.getArguments();
        String loggerName = logEvent.getLoggerName();
        String threadName = logEvent.getThreadName();
        ZonedDateTime timeStamp = logEvent.getTimeStamp();
        Map<String, String> diagnosticContext = logEvent.getDiagnosticContext();
        Optional<Throwable> throwable = logEvent.getThrowable();
    }
    
}

Known issues

Using Log Captor alongside with other logging libraries

When building your maven or gradle project it can complain that you are using multiple SLF4J implementations. Log Captor is using logback as SLF4J implementation and SLF4J doesn't allow you to use multiple implementations, therefore you need to explicitly specify which to use during which build phase. You can fix that by excluding your main logging framework during the unit/integration test phase. Below is an example for Maven Failsafe and Maven Surefire:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-surefire-plugin</artifactId>
            <configuration>
                <classpathDependencyExcludes>
                    <classpathDependencyExclude>org.apache.logging.log4j:log4j-slf4j-impl</classpathDependencyExclude>
                </classpathDependencyExcludes>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-failsafe-plugin</artifactId>
            <configuration>
                <classpathDependencyExcludes>
                    <classpathDependencyExclude>org.apache.logging.log4j:log4j-slf4j-impl</classpathDependencyExclude>
                </classpathDependencyExcludes>
            </configuration>
        </plugin>
    </plugins>
</build>

And for gradle:

configurations {
    testImplementation {
        exclude group: 'org.apache.logging.log4j', module: 'log4j-slf4j-impl'
    }
}

Capturing logs of static inner classes

LogCaptor successfully catches logs of static inner classes with LogCaptor.forClass(StaticInnerClass.class) construction when SLF4J, Log4J, JUL is used. This doesn't apply to Log4J2 by default it uses different method to for initializing the logger. It used Class.getCanonicalName() under the covers instead of Class.getName(). You should use LogCaptor.forName(StaticInnerClass.class.getCanonicalName()) for successful test execution with Log4j2.