Testing the presence of log messages with java.util.logging

Sometimes there is value in creating a unit test to assert that a specific log message actually gets printed. It might be for audit logs or making sure that system misconfigurations get logged properly. A couple of years ago my colleague Daniel blogged about how to create a custom Log4j appender and to use that in your unit tests to assert the presence of certain log messages. Read about it here.

Today I was resolving an issue in the Arquillian (the open source integration testing framework for Java) codebase. This involved in logging a warning in a certain use case. I obviously wanted to test my code by adding a test case for the different use cases, asserting that the log message got printed out correctly. I’ve used the approach of asserting log messages in unit tests many times in the past, but I’ve always used Log4j in those cases. This time around I was forced to solve the problem for plain java.util.logging (JUL) which Arquillian uses. Fun, as I’m always up for a challenge.

What I did was similar to the log4j approach. I need to add a custom log handler which I attach to the logger in the affected class. I create an outputstream, which I attach to a StreamHandler. I then attach the StreamHandler to the logger. As long as I have a reference to the output stream, I can then get the logged contents and use that in my assertions. Example below using JUnit 4:

private static Logger log = Logger.getLogger(AnnotationDeploymentScenarioGenerator.class.getName()); // matches the logger in the affected class
private static OutputStream logCapturingStream;
private static StreamHandler customLogHandler;

public void attachLogCapturer()
  logCapturingStream = new ByteArrayOutputStream();
  Handler[] handlers = log.getParent().getHandlers();
  customLogHandler = new StreamHandler(logCapturingStream, handlers[0].getFormatter());

public String getTestCapturedLog() throws IOException
  return logCapturingStream.toString();

… then I can use the above methods in my test case:

public void shouldLogWarningForMismatchingArchiveTypeAndFileExtension() throws Exception
  final String expectedLogPart = "unexpected file extension";

  new AnnotationDeploymentScenarioGenerator().generate(
        new TestClass(DeploymentWithMismatchingTypeAndFileExtension.class));

  String capturedLog = getTestCapturedLog();


Tommy Tynjä

2 thoughts on “Testing the presence of log messages with java.util.logging”

  1. Great post!

    Nothing is untestable!

    I think it’s perfectly valid case to want to unit test your logging. When you use format string (%s, {}, etc.) they can get quite complicate. Especially since there are different variants of format strings.

  2. One problem I’ve encountered with JUL though, is that the logging level seems to be locale specific. In some cases you might want to assert that the log message is outputted on the appropriate logging level, e.g. WARN. On my machine though, with Swedish locale, warnings are outputted as “VARNING”, which can be troublesome form a testing/CI perspective.

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