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SLF4JTesting is a library that provides facilities for log testing that are simple to use and optimised for use in environments with concurrent test execution and/or where console logging is desired.

SLF4JTesting provides a "Testing Double" of SLF4J LoggerFactory as a utility to conveniently configure logging on a test by test basis. It is useful in both unit and integration tests cases and also as a general purpose logging implementation that does not rely on singletons or other constructs that might cause cross-test interference.

SLF4JTesting has been designed to:

  • support unit and integration testing
  • support concurrent test execution such as TestNG parallel suites or Scalatest parallel test execution
  • promote dependency injection of the logging implementation
  • support testing of multi-threaded components
  • support testing of assemblies
  • provide a simple console logging facility
  • allow selective suppression of individual console logging messages (eg to hide any expected errors)
  • support integration with mocking libraries (eg Mockito)

SLF4JTesting differs from some other solutions because it:

  • does not pollute the classpath with any StaticLoggerBinder stuff
  • does not compete with other SLF4J implementations resident on the classpath
  • does not rely on cluttering the classpath with special logback.xml/logback-test.xml files
  • does not use statics
  • does not rely on singletons anywhere
  • does not need thread-locals etc


Setting up

Include the latest version of the jar as a test dependency from maven central

Basic Example

Here we use constructor injection using the SLF4J logger factory interface.

import org.slf4j.ILoggerFactory;
import org.slf4j.Logger;

public class Example1 {

    private final Logger logger;

    public Example1(ILoggerFactory lf) {
        this.logger = lf.getLogger(Example1.class.getName());

    public void aMethodThatLogs() {"Hello World!");

The .getName() is a tiny bit of boilerplate that's only necessary because SLF4J's' ILoggerFactory interface does not provide a by-class convenience method like LoggerFactory does.

A sample program might be ...

class RunExample {
  public static void main(String[] args) {
    Example1 ex = new Example1(LogFactory.getILoggerFactory());

This constructor injection is not that arduous, even less so if you use a dependency injection framework, but this injection provides the seam that allows us to inject our testing double.

Testing might look like this ...

import org.junit.Test;
import static org.junit.Assert.assertTrue;

public class Example1UnitTest {

    public void testAMethodThatLogs() throws Exception {
        TestLoggerFactory loggerFactory = new TestLoggerFactory();

        Example1 sut = new Example1(loggerFactory);

        TestLogger logger = loggerFactory.getLogger(Example1.class.getName());

See Also

Further examples and explanation can be found in the examples. These are phrased as tests and are a good place to look for ideas.


Isolation of one test from another and support for concurrent test execution

A common complaint with other approaches is that their reliance on globals and static config means that tests interfere with each other. This can be dealt with perhaps by having a "reset" function on the log testing framework that is executed before each test case. However these other approaches do not facilitate concurrent test execution because by their nature they rely in singletons that would be mutated by the concurrent test cases making any kind of assertions difficult or impossible. At best you would end up with tests the blink.

Improved construction patterns and testability

I strongly believe in dependency injection (DI) so anything that looks like a static or singleton offends me, statics and singletons are an impediment to testable code.

We can't conveniently substitute statics for during testing and in the case of SLF4J we end up resorting to nasties like classpath manipulation to get over this.

Anything global or static leaks information between tests and typically requires before and after setup and tear down which is more test boilerplate. This pattern means that running tests classes or class methods concurrently leads to interference as all threads interact with the shared mutable global.

These problems can be avoided if we do not permit this share state between tests and instead use POJO's and DI; this library attempts to make it easy to use a POJO/DI pattern with SLF4J and write tests for that code.

Console logging during tests

Typically we don't want logging going to the console in tests. A possible exception to this is that it's useful if any unexpected errors are logged to the console. Sometimes however we are performing negative tests and we expect certain error messages but we do not want to see these on the console.

SLF4JTesting supports optional console logging but also a regex based scheme to suppress certain expected log lines. This helps us keep our CI console clean without losing the benefits of logging of unexpected conditions that would help us diagnose failing tests more easily.

Multithreaded code

Sometimes we have an integration test where the test subject performs logging from another thread than the foreground test thread. SLF4JTesting collects all logging occurring in the test subject regardless of thread.

Assembly integration testing

Sometimes we want to test an assembly where the assembly constructs objects internally. If each parent object is injected with the logger factory and any objects the parent creates are also injected with the logger factory then this pattern allows the entire logging implementation to be substituted in tests.

import org.slf4j.ILoggerFactory;
import org.slf4j.Logger;

public class AssemblyExample {

    private final ChildObject child;

    public AssemblyExample(ILoggerFactory lf) {
        child = new ChildObject(lf);

class ChildObject {
    private final Logger logger;
    private final GrandchildObject grandchild;

    ChildObject(ILoggerFactory lf) {
        logger = lf.getLogger(ChildObject.class.getName());
        grandchild = new GrandchildObject(lf);

    public void aMethodThatLogs() {"Hello World!");

class GrandchildObject {
    private final Logger logger;

    GrandchildObject (ILoggerFactory lf) {
        logger = lf.getLogger(GrandchildObject.class.getName());

    public void aMethodThatLogs() {"Bye World!");

Making assertions

It is possible to make some assertions about what was logged by using methods provided by TestLogger.

TestLogger logger = loggerFactory.getLogger(Example1.class.getName());


You can make further assertions by using a mocking framework.

SLF4JLogging allows you to hook a mock (eg Mockito or other) up to the logging implementation so that you can use the power of your chosen mocking framework directly to test the logging.

SLF4J logging does not impose any particular mocking framework.

public class MockingExampleUnitTest {
    public void testAMethodThatLogsWithAMock() throws Exception {
        Logger mockLogger = Mockito.mock(Logger.class);

        TestLoggerFactory loggerFactory = Settings.instance().
                delegate(BasicExample.class.getName(), mockLogger).

        BasicExample sut = new BasicExample(loggerFactory);

        Mockito.verify(mockLogger).info("Hello World!");

Locally configurable console output & logging suppression

Logging in tests can be controlled on a test by test basis without classpath fun and games.

It is also possible to selectively suppress the console logging using regular expressions. This is particularly useful on those occasions where you want console logging turned on in a test (eg for diagnostics) but you do not want error logging caused by deliberate failures to clog the build console. In my experience this is most often around integration testing.

TestLoggerFactory loggerFactory = Settings.instance().
            suppressPrinting(".*Pattern to suppress.*").buildLogging();

Logger logger = loggerFactory.getLogger("name");

logger.error("Pattern to suppress - will not be printed");
logger.error("This will be printed");

Scala note

I do mostly scala programming these days so I thought a scala usage example might be interesting

import org.slf4j.ILoggerFactory
import org.slf4j.Logger

class Example1(lf:ILoggerFactory = LoggerFactory.getIloggerFactory()) {
    private val logger = lf.getLogger(classOf[Example1].getName())

    def aMethodThatLogs(): Unit ="Hello World!")

The above example provices a default impl for the logger factory if the caller doesn't provide one.

Or a scala translation of the assembly example above ...

import org.slf4j.ILoggerFactory
import org.slf4j.Logger

class AssemblyExample(lf: ILoggerFactory) {
     private val ChildObject child = new ChildObject(lf)

class ChildObject(lf: ILoggerFactory) {
     private val logger = lf.getLogger(classOf[ChildObject].getName())
     private val grandchild = new GrandchildObject(lf)

     def aMethodThatLogs(): Unit ="Hello World!")

 class GrandchildObject(lf: ILoggerFactory) {
     private val logger = lf.getLogger(classOf[GrandchildObject].getName())
     def aMethodThatLogs(): Unit ="Bye World!")

Scala-Logging example

Simple - As before, inject the logging implementation on the constructor then wrap the SLF4J logger with scala-logging.

class MyService(loggerFactory: ILoggerFactory) {
  private val logger = com.typesafe.scalalogging.Logger(loggerFactory.getLogger(classOf[MyService].getName))"scala optimised logging !!")