Unit Testing Log Statements

Sometimes asserting on a log statement might be the only way of validating a method call. One way of asserting log statements is to create a custom appender and attaching it to the logger to capture the logs. log4j-api provides the framework to create a custom appender.

import org.apache.logging.log4j.core.Filter;
import org.apache.logging.log4j.core.Layout;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.appender.AbstractAppender;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

public class CustomAppender extends AbstractAppender {

    private List messages = new ArrayList<>();

    public CustomAppender(String name, Filter filter, Layout layout) {
        super(name, filter, layout);
    }

    public CustomAppender(String name, Filter filter, Layout layout, boolean ignoreExceptions) {
        super(name, filter, layout, ignoreExceptions);
    }

    @Override
    public void append(LogEvent event) {
        byte[] data = getLayout().toByteArray(event);
        messages.add(new String(data).trim()); // optional trim
    }

    @Override
    public void stop() {
        
    }

    public List getMessages() {
        return messages;
    }

}

Following method adds the custom appender to the logger:

private void setUpLogHandler() {
        final LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
        final AbstractConfiguration config = (AbstractConfiguration) ctx.getConfiguration();

        // Create and add the appender
        customAppender = new CustomAppender("Custom", null, PatternLayout.createDefaultLayout());
        customAppender.start();
        config.addAppender(customAppender);

        // Create and add the logger
        AppenderRef[] refs = new AppenderRef[]{AppenderRef.createAppenderRef("Custom", null, null)};
        LoggerConfig loggerConfig = LoggerConfig.createLogger("false", Level.INFO, ClassUnderTest.class.getCanonicalName(), "true", refs, null, config, null);
        loggerConfig.addAppender(customAppender, null, null);
        config.addLogger(ClassUnderTest.class.getCanonicalName(), loggerConfig);
        ctx.updateLoggers();
    }

Below is an example test method which asserts on a method that logs:

    @Test
    public void logTest() {
        setUpLogHandler();
        ClassUnderTest ct = new ClassUnderTest();
        ct.methodThatLogs();
        assertThat(customAppender.getMessages(), hasItem("new log message"));
    }