Jonathan Lalou's Blog

Case

Sometimes, you need test the Log4J’s loggers are called with the right parameters. How to perform these tests from with JUnit?

Let’s take an example: how to test these simple class and method?

[java]public class ClassWithLogger {
private static final Logger LOGGER = Logger.getLogger(ClassWithLogger.class);

public void printMessage(Integer foo){
LOGGER.warn("this is the message#" + foo);
}
}[/java]

Example

Define an almost empty Log4J appender, such as:

[java]public class TestAimedAppender extends ArrayList<String> implements Appender {
private final Class clazz;

public TestAimedAppender(Class clazz) {
super();
this.clazz = clazz;
}

@Override
public void addFilter(Filter newFilter) {
}

@Override
public Filter getFilter() {
return null;
}

@Override
public void clearFilters() {
}

public void close() {
}

@Override
public void doAppend(LoggingEvent event) {
add(event.getRenderedMessage());
}

@Override
public String getName() {
return "TestAppender for " + clazz.getSimpleName();
}

@Override
public void setErrorHandler(ErrorHandler errorHandler) {
}

@Override
public ErrorHandler getErrorHandler() {
return null;
}

@Override
public void setLayout(Layout layout) {
}

@Override
public Layout getLayout() {
return null;
}

@Override
public void setName(String name) {
}

public boolean requiresLayout() {
return false;
}
}[/java]

Then create a TestCase with two fields:

[java]public class ClassWithLoggerUnitTest {
private ClassWithLogger classWithLogger;
private TestAimedAppender appender;
…
}
[/java]

In the setup, remove all appenders, create an instance of our appender, and then add it to the logger related to the class which we want to test:

[java] @Before
public void setUp() throws Exception {
final Logger classWithLoggerLogger = Logger.getLogger(ClassWithLogger.class);
classWithLoggerLogger.removeAllAppenders();
appender = new TestAimedAppender(ClassWithLogger.class);
classWithLoggerLogger.addAppender(appender);
appender.clear();

classWithLogger = new ClassWithLogger();
}[/java]

Then write the following test. The code is documented:

[java] @Test
public void testPrintMessage() throws Exception {
final String expectedMessage = "this is the message#18";

// empty the appender
appender.clear();
// check it is actually empty before any call to the tested class
assertTrue(appender.isEmpty());

// call to the tested class
classWithLogger.printMessage(18);

// check the appender is no more empty
assertFalse(appender.isEmpty());
assertEquals(1, appender.size());
// check the content of the appender
assertEquals(expectedMessage, appender.get(0));

}[/java]

Conclusion

This basic example shows how to perform tests on logger, without overriding the original code or using mocks. Of course, you can improve this basic example, for instance in discriminating owing to the log level (INFO, WARN, ERROR, etc.), use generics, and even any other fantasy ;-).