受到@RonaldBlaschke的解决方案的启发，我想到了这个:

public class Log4JTester extends ExternalResource {
    TestAppender appender;

    @Override
    protected void before() {
        appender = new TestAppender();
        final Logger rootLogger = Logger.getRootLogger();
        rootLogger.addAppender(appender);
    }

    @Override
    protected void after() {
        final Logger rootLogger = Logger.getRootLogger();
        rootLogger.removeAppender(appender);
    }

    public void assertLogged(Matcher<String> matcher) {
        for(LoggingEvent event : appender.events) {
            if(matcher.matches(event.getMessage())) {
                return;
            }
        }
        fail("No event matches " + matcher);
    }

    private static class TestAppender extends AppenderSkeleton {

        List<LoggingEvent> events = new ArrayList<LoggingEvent>();

        @Override
        protected void append(LoggingEvent event) {
            events.add(event);
        }

        @Override
        public void close() {

        }

        @Override
        public boolean requiresLayout() {
            return false;
        }
    }

}

．.． 这允许你做:

@Rule public Log4JTester logTest = new Log4JTester();

@Test
public void testFoo() {
     user.setStatus(Status.PREMIUM);
     logTest.assertLogged(
        stringContains("Note added to account: premium customer"));
}

你也许可以用更聪明的方式来使用hamcrest，但我就讲到这里。

在这里，模拟是一个选项，尽管这很困难，因为记录器通常是私有的静态final -所以设置模拟记录器不是小意思，或者需要修改被测试的类。

你可以创建一个自定义的Appender(或者其他什么名字)，然后注册它——要么通过一个只测试的配置文件，要么通过运行时(在某种程度上，依赖于日志框架)。 然后，您可以获得该appender(如果在配置文件中声明，可以是静态的，如果在运行时插入它，则可以通过它的当前引用)，并验证其内容。

另一个选项是模拟Appender并验证消息是否已记录到此Appender。Log4j 1.2的示例。X和mockito:

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;

import org.apache.log4j.Appender;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.spi.LoggingEvent;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.mockito.ArgumentCaptor;

public class MyTest {

    private final Appender appender = mock(Appender.class);
    private final Logger logger = Logger.getRootLogger();

    @Before
    public void setup() {
        logger.addAppender(appender);
    }

    @Test
    public void test() {
        // when
        Logger.getLogger(MyTest.class).info("Test");

        // then
        ArgumentCaptor<LoggingEvent> argument = ArgumentCaptor.forClass(LoggingEvent.class);
        verify(appender).doAppend(argument.capture());
        assertEquals(Level.INFO, argument.getValue().getLevel());
        assertEquals("Test", argument.getValue().getMessage());
        assertEquals("MyTest", argument.getValue().getLoggerName());
    }

    @After
    public void cleanup() {
        logger.removeAppender(appender);
    }
}

Log4J2的API略有不同。你也可以使用它的async appender。我为此创建了一个锁定的appender:

    public static class LatchedAppender extends AbstractAppender implements AutoCloseable {

    private final List<LogEvent> messages = new ArrayList<>();
    private final CountDownLatch latch;
    private final LoggerConfig loggerConfig;

    public LatchedAppender(Class<?> classThatLogs, int expectedMessages) {
        this(classThatLogs, null, null, expectedMessages);
    }
    public LatchedAppender(Class<?> classThatLogs, Filter filter, Layout<? extends Serializable> layout, int expectedMessages) {
        super(classThatLogs.getName()+"."+"LatchedAppender", filter, layout);
        latch = new CountDownLatch(expectedMessages);
        final LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
        final Configuration config = ctx.getConfiguration();
        loggerConfig = config.getLoggerConfig(LogManager.getLogger(classThatLogs).getName());
        loggerConfig.addAppender(this, Level.ALL, ThresholdFilter.createFilter(Level.ALL, null, null));
        start();
    }

    @Override
    public void append(LogEvent event) {
        messages.add(event);
        latch.countDown();
    }

    public List<LogEvent> awaitMessages() throws InterruptedException {
        assertTrue(latch.await(10, TimeUnit.SECONDS));
        return messages;
    }

    @Override
    public void close() {
        stop();
        loggerConfig.removeAppender(this.getName());
    }
}

像这样使用它:

        try (LatchedAppender appender = new LatchedAppender(ClassUnderTest.class, 1)) {

        ClassUnderTest.methodThatLogs();
        List<LogEvent> events = appender.awaitMessages();
        assertEquals(1, events.size());
        //more assertions here

    }//appender removed

正如前面提到的，您可以使用mock框架。为此，您必须在类中公开记录器(尽管我可能更倾向于使其包私有而不是创建公共setter)。

另一种解决方案是手工创建一个假记录器。您必须编写伪记录器(更多的fixture代码)，但在这种情况下，我更喜欢使用模拟框架中保存的代码来增强测试的可读性。

我会这样做:

class FakeLogger implements ILogger {
    public List<String> infos = new ArrayList<String>();
    public List<String> errors = new ArrayList<String>();

    public void info(String message) {
        infos.add(message);
    }

    public void error(String message) {
        errors.add(message);
    }
}

class TestMyClass {
    private MyClass myClass;        
    private FakeLogger logger;        

    @Before
    public void setUp() throws Exception {
        myClass = new MyClass();
        logger = new FakeLogger();
        myClass.logger = logger;
    }

    @Test
    public void testMyMethod() {
        myClass.myMethod(true);

        assertEquals(1, logger.infos.size());
    }
}

这里有一个简单有效的Logback解决方案。 它不需要添加/创建任何新类。 它依赖于ListAppender:一个白盒回logback appender，其中将日志条目添加到公共List字段中，我们可以使用该字段来进行断言。

这里有一个简单的例子。

Foo类:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class Foo {

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

    public void doThat() {
        LOGGER.info("start");
        //...
        LOGGER.info("finish");
    }
}

FooTest类:

import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.read.ListAppender;

public class FooTest {

    @Test
    void doThat() throws Exception {
        // get Logback Logger 
        Logger fooLogger = (Logger) LoggerFactory.getLogger(Foo.class);

        // create and start a ListAppender
        ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
        listAppender.start();

        // add the appender to the logger
        // addAppender is outdated now
        fooLogger.addAppender(listAppender);

        // call method under test
        Foo foo = new Foo();
        foo.doThat();

        // JUnit assertions
        List<ILoggingEvent> logsList = listAppender.list;
        assertEquals("start", logsList.get(0)
                                      .getMessage());
        assertEquals(Level.INFO, logsList.get(0)
                                         .getLevel());

        assertEquals("finish", logsList.get(1)
                                       .getMessage());
        assertEquals(Level.INFO, logsList.get(1)
                                         .getLevel());
    }
}

JUnit断言听起来不太适合断言列表元素的某些特定属性。 像AssertJ或Hamcrest这样的Matcher/断言库似乎更好:

使用AssertJ，它将是:

import org.assertj.core.api.Assertions;

Assertions.assertThat(listAppender.list)
          .extracting(ILoggingEvent::getMessage, ILoggingEvent::getLevel)
          .containsExactly(Tuple.tuple("start", Level.INFO), Tuple.tuple("finish", Level.INFO));