如果您正在使用log4j2，来自https://www.dontpanicblog.co.uk/2018/04/29/test-log4j2-with-junit/的解决方案允许我断言消息已被记录。

解决方案是这样的:

Define a log4j appender as an ExternalResource rule public class LogAppenderResource extends ExternalResource { private static final String APPENDER_NAME = "log4jRuleAppender"; /** * Logged messages contains level and message only. * This allows us to test that level and message are set. */ private static final String PATTERN = "%-5level %msg"; private Logger logger; private Appender appender; private final CharArrayWriter outContent = new CharArrayWriter(); public LogAppenderResource(org.apache.logging.log4j.Logger logger) { this.logger = (org.apache.logging.log4j.core.Logger)logger; } @Override protected void before() { StringLayout layout = PatternLayout.newBuilder().withPattern(PATTERN).build(); appender = WriterAppender.newBuilder() .setTarget(outContent) .setLayout(layout) .setName(APPENDER_NAME).build(); appender.start(); logger.addAppender(appender); } @Override protected void after() { logger.removeAppender(appender); } public String getOutput() { return outContent.toString(); } } Define a test that use your ExternalResource rule public class LoggingTextListenerTest { @Rule public LogAppenderResource appender = new LogAppenderResource(LogManager.getLogger(LoggingTextListener.class)); private LoggingTextListener listener = new LoggingTextListener(); // Class under test @Test public void startedEvent_isLogged() { listener.started(); assertThat(appender.getOutput(), containsString("started")); } }

不要忘记将log4j2.xml作为src/test/resources的一部分

实际上，您是在测试依赖类的副作用。对于单元测试，您只需要验证这一点

logger.info（）

使用正确的参数调用。因此，使用一个模拟框架来模拟记录器，这将允许您测试自己类的行为。

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

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。

对于Junit 5 (Jupiter)， Spring的OutputCaptureExtension非常有用。它从Spring Boot 2.2开始就可以使用，并且可以在Spring - Boot -test构件中使用。

示例(取自javadoc):

@ExtendWith(OutputCaptureExtension.class)
class MyTest {
    @Test
    void test(CapturedOutput output) {
        System.out.println("ok");
        assertThat(output).contains("ok");
        System.err.println("error");
    }

    @AfterEach
    void after(CapturedOutput output) {
        assertThat(output.getOut()).contains("ok");
        assertThat(output.getErr()).contains("error");
    }
}

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")));
}