模拟Appender可以帮助捕获日志行。 在http://clearqa.blogspot.co.uk/2016/12/test-log-lines.html上找到示例

// Fully working test at: https://github.com/njaiswal/logLineTester/blob/master/src/test/java/com/nj/Utils/UtilsTest.java

@Test
public void testUtilsLog() throws InterruptedException {

    Logger utilsLogger = (Logger) LoggerFactory.getLogger("com.nj.utils");

    final Appender mockAppender = mock(Appender.class);
    when(mockAppender.getName()).thenReturn("MOCK");
    utilsLogger.addAppender(mockAppender);

    final List<String> capturedLogs = Collections.synchronizedList(new ArrayList<>());
    final CountDownLatch latch = new CountDownLatch(3);

    //Capture logs
    doAnswer((invocation) -> {
        LoggingEvent loggingEvent = invocation.getArgumentAt(0, LoggingEvent.class);
        capturedLogs.add(loggingEvent.getFormattedMessage());
        latch.countDown();
        return null;
    }).when(mockAppender).doAppend(any());

    //Call method which will do logging to be tested
    Application.main(null);

    //Wait 5 seconds for latch to be true. That means 3 log lines were logged
    assertThat(latch.await(5L, TimeUnit.SECONDS), is(true));

    //Now assert the captured logs
    assertThat(capturedLogs, hasItem(containsString("One")));
    assertThat(capturedLogs, hasItem(containsString("Two")));
    assertThat(capturedLogs, hasItem(containsString("Three")));
}

这里有一个很好的和优雅的方法来解决这个问题: https://www.baeldung.com/junit-asserting-logs

哇。我不知道为什么这么难。我发现我无法使用上面的任何代码示例，因为我使用的是log4j2而不是slf4j。这是我的解决方案:

public class SpecialLogServiceTest {

  @Mock
  private Appender appender;

  @Captor
  private ArgumentCaptor<LogEvent> captor;

  @InjectMocks
  private SpecialLogService specialLogService;

  private LoggerConfig loggerConfig;

  @Before
  public void setUp() {
    // prepare the appender so Log4j likes it
    when(appender.getName()).thenReturn("MockAppender");
    when(appender.isStarted()).thenReturn(true);
    when(appender.isStopped()).thenReturn(false);

    final LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
    final Configuration config = ctx.getConfiguration();
    loggerConfig = config.getLoggerConfig("org.example.SpecialLogService");
    loggerConfig.addAppender(appender, AuditLogCRUDService.LEVEL_AUDIT, null);
  }

  @After
  public void tearDown() {
    loggerConfig.removeAppender("MockAppender");
  }

  @Test
  public void writeLog_shouldCreateCorrectLogMessage() throws Exception {
    SpecialLog specialLog = new SpecialLogBuilder().build();
    String expectedLog = "this is my log message";

    specialLogService.writeLog(specialLog);

    verify(appender).append(captor.capture());
    assertThat(captor.getAllValues().size(), is(1));
    assertThat(captor.getAllValues().get(0).getMessage().toString(), is(expectedLog));
  }
}

请注意，在Log4J 2中。X，公共接口org.apache.logging.log4j。Logger不包括setAppender()和removeAppender()方法。

但是，如果您没有做太花哨的事情，您应该能够将它强制转换为实现类org.apache.logging.log4j.core。记录器，它公开了这些方法。

这里有一个Mockito和AssertJ的例子:

// Import the implementation class rather than the API interface
import org.apache.logging.log4j.core.Logger;

// Cast logger to implementation class to get access to setAppender/removeAppender
Logger log = (Logger) LogManager.getLogger(MyClassUnderTest.class);

// Set up the mock appender, stubbing some methods Log4J needs internally
Appender appender = mock(Appender.class);
when(appender.getName()).thenReturn("Mock Appender");
when(appender.isStarted()).thenReturn(true);

log.addAppender(appender);
try {
    new MyClassUnderTest().doSomethingThatShouldLogAnError();
} finally {
    log.removeAppender(appender);
}

// Verify that we got an error with the expected message
ArgumentCaptor<LogEvent> logEventCaptor = ArgumentCaptor.forClass(LogEvent.class);
verify(appender).append(logEventCaptor.capture());
LogEvent logEvent = logEventCaptor.getValue();
assertThat(logEvent.getLevel()).isEqualTo(Level.ERROR);
assertThat(logEvent.getMessage().getFormattedMessage()).contains(expectedErrorMessage);

这是我为logback所做的。

我创建了一个TestAppender类:

public class TestAppender extends AppenderBase<ILoggingEvent> {

    private Stack<ILoggingEvent> events = new Stack<ILoggingEvent>();

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

    public void clear() {
        events.clear();
    }

    public ILoggingEvent getLastEvent() {
        return events.pop();
    }
}

然后在我的testng单元测试类的父类中创建了一个方法:

protected TestAppender testAppender;

@BeforeClass
public void setupLogsForTesting() {
    Logger root = (Logger)LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
    testAppender = (TestAppender)root.getAppender("TEST");
    if (testAppender != null) {
        testAppender.clear();
    }
}

我在src/test/resources中定义了一个logback-test.xml文件，并添加了一个测试appender:

<appender name="TEST" class="com.intuit.icn.TestAppender">
    <encoder>
        <pattern>%m%n</pattern>
    </encoder>
</appender>

并将这个appender添加到根appender:

<root>
    <level value="error" />
    <appender-ref ref="STDOUT" />
    <appender-ref ref="TEST" />
</root>

现在，在从父测试类扩展而来的测试类中，我可以获得appender并记录最后一条消息，并验证消息、级别和throwable。

ILoggingEvent lastEvent = testAppender.getLastEvent();
assertEquals(lastEvent.getMessage(), "...");
assertEquals(lastEvent.getLevel(), Level.WARN);
assertEquals(lastEvent.getThrowableProxy().getMessage(), "...");

对于log4j2，解决方案略有不同，因为AppenderSkeleton不再可用。此外，使用Mockito或类似的库来创建带有ArgumentCaptor的Appender将无法工作，因为MutableLogEvent在多个日志消息上被重用。我为log4j2找到的最佳解决方案是:

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.Logger;
import org.apache.logging.log4j.core.appender.AbstractAppender;

private static MockedAppender mockedAppender;
private static Logger logger;

@Before
public void setup() {
    mockedAppender.message.clear();
}

/**
 * For some reason mvn test will not work if this is @Before, but in eclipse it works! As a
 * result, we use @BeforeClass.
 */
@BeforeClass
public static void setupClass() {
    mockedAppender = new MockedAppender();
    logger = (Logger)LogManager.getLogger(MatchingMetricsLogger.class);
    logger.addAppender(mockedAppender);
    logger.setLevel(Level.INFO);
}

@AfterClass
public static void teardown() {
    logger.removeAppender(mockedAppender);
}

@Test
public void test() {
    // do something that causes logs
    for (String e : mockedAppender.message) {
        // add asserts for the log messages
    }
}

private static class MockedAppender extends AbstractAppender {

    List<String> message = new ArrayList<>();

    protected MockedAppender() {
        super("MockedAppender", null, null);
    }

    @Override
    public void append(LogEvent event) {
        message.add(event.getMessage().getFormattedMessage());
    }
}