非常感谢这些(令人惊讶的)快速而有用的回答;他们让我找到了正确的解决方法。

我想要使用的代码库使用java.util.logging作为其记录器机制，我对这些代码感到不够熟悉，无法完全将其更改为log4j或记录器接口/facade。但基于这些建议，我“破解”了一个j.u.l handler扩展，这是一种享受。

下面是一个简短的总结。延长java.util.logging.Handler:

class LogHandler extends Handler
{
    Level lastLevel = Level.FINEST;

    public Level  checkLevel() {
        return lastLevel;
    }    

    public void publish(LogRecord record) {
        lastLevel = record.getLevel();
    }

    public void close(){}
    public void flush(){}
}

显然，您可以从LogRecord中存储您喜欢/想要/需要的任何数量，或者将它们全部推入堆栈，直到溢出。

在junit-test的准备过程中，你创建了一个java.util.logging.Logger，并添加这样一个新的LogHandler:

@Test tester() {
    Logger logger = Logger.getLogger("my junit-test logger");
    LogHandler handler = new LogHandler();
    handler.setLevel(Level.ALL);
    logger.setUseParentHandlers(false);
    logger.addHandler(handler);
    logger.setLevel(Level.ALL);

对setUseParentHandlers()的调用将使正常的处理程序静默，以便(对于这个junit-test运行)不会发生不必要的日志记录。做任何你的测试代码需要使用这个记录器，运行测试和assertEquality:

    libraryUnderTest.setLogger(logger);
    methodUnderTest(true);  // see original question.
    assertEquals("Log level as expected?", Level.INFO, handler.checkLevel() );
}

(当然，您可以将大部分工作移到@Before方法中，并进行各种其他改进，但这会使演示变得混乱。)

I've needed this several times as well. I've put together a small sample below, which you'd want to adjust to your needs. Basically, you create your own Appender and add it to the logger you want. If you'd want to collect everything, the root logger is a good place to start, but you can use a more specific if you'd like. Don't forget to remove the Appender when you're done, otherwise you might create a memory leak. Below I've done it within the test, but setUp or @Before and tearDown or @After might be better places, depending on your needs.

此外，下面的实现将所有内容收集到内存中的List中。如果您记录了很多日志，您可能会考虑添加一个过滤器来删除无聊的条目，或者将日志写入磁盘上的临时文件(提示:LoggingEvent是可序列化的，因此如果您的日志消息是可序列化的，那么您应该能够序列化事件对象)。

import org.apache.log4j.AppenderSkeleton;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.spi.LoggingEvent;
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

public class MyTest {
    @Test
    public void test() {
        final TestAppender appender = new TestAppender();
        final Logger logger = Logger.getRootLogger();
        logger.addAppender(appender);
        try {
            Logger.getLogger(MyTest.class).info("Test");
        }
        finally {
            logger.removeAppender(appender);
        }

        final List<LoggingEvent> log = appender.getLog();
        final LoggingEvent firstLogEntry = log.get(0);
        assertThat(firstLogEntry.getLevel(), is(Level.INFO));
        assertThat((String) firstLogEntry.getMessage(), is("Test"));
        assertThat(firstLogEntry.getLoggerName(), is("MyTest"));
    }
}

class TestAppender extends AppenderSkeleton {
    private final List<LoggingEvent> log = new ArrayList<LoggingEvent>();

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

    @Override
    protected void append(final LoggingEvent loggingEvent) {
        log.add(loggingEvent);
    }

    @Override
    public void close() {
    }

    public List<LoggingEvent> getLog() {
        return new ArrayList<LoggingEvent>(log);
    }
}

检查这个库https://github.com/Hakky54/log-captor

在maven文件中包含库的引用:

<dependency>
    <groupId>io.github.hakky54</groupId>
    <artifactId>logcaptor</artifactId>
    <version>2.5.0</version>
    <scope>test</scope>
</dependency>

在java代码测试方法中，你应该包括以下内容:

LogCaptor logCaptor = LogCaptor.forClass(MyClass.class);

 // do the test logic....

assertThat(logCaptor.getLogs()).contains("Some log to assert");

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")));
}

如果我想做的只是看到一些字符串被记录(而不是验证确切的日志语句，这太脆弱了)，我所做的是将StdOut重定向到缓冲区，做一个contains，然后重置StdOut:

PrintStream original = System.out;
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
System.setOut(new PrintStream(buffer));

// Do something that logs

assertTrue(buffer.toString().contains(myMessage));
System.setOut(original);

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));
  }
}