如何在日志记录器中对消息进行JUnit断言

请使用下面的代码。我在我的Spring集成测试中也使用了同样的代码，用于记录日志。使用assertJobIsScheduled方法来断言在日志中打印的文本。

import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.spi.LoggingEvent;
import ch.qos.logback.core.Appender;

private Logger rootLogger;
final Appender mockAppender = mock(Appender.class);

@Before
public void setUp() throws Exception {
    initMocks(this);
    when(mockAppender.getName()).thenReturn("MOCK");
    rootLogger = (Logger) LoggerFactory.getLogger(ch.qos.logback.classic.Logger.ROOT_LOGGER_NAME);
    rootLogger.addAppender(mockAppender);
}

private void assertJobIsScheduled(final String matcherText) {
    verify(mockAppender).doAppend(argThat(new ArgumentMatcher() {
        @Override
        public boolean matches(final Object argument) {
            return ((LoggingEvent)argument).getFormattedMessage().contains(matcherText);
        }
    }));
}

对于使用Google的Flogger日志记录API的任何人来说，以下是您可以编写的用于测试的日志后端的方法：

import com.google.common.flogger.backend.LogData
import com.google.common.flogger.backend.LoggerBackend
import com.google.common.flogger.backend.system.BackendFactory
import java.lang.RuntimeException
import java.util.logging.Level
import java.util.logging.Level.SEVERE
import java.util.logging.Level.WARNING

/** Stores information about logs that occurred during test execution. */
class LogCollection {
  val messages = mutableListOf<String>()
}

class TestLoggingBackendFactory : BackendFactory() {
  /** Gets a singleton logging backend. */
  override fun create(p0: String?) = TestLoggingBackend
}

/** Flogger logging backend that listens to new logs during the test. */
object TestLoggingBackend : LoggerBackend() {
  val logs = LogCollection()

  override fun getLoggerName(): String = "TestLoggingBackend"

  override fun isLoggable(level: Level): Boolean = level in setOf(SEVERE, WARNING)

  override fun log(logData: LogData) {
    handleError(e = null, logData)
  }

  override fun handleError(e: RuntimeException?, logData: LogData) {
    logs.messages.add(logData.literalArgument.toString())
  }
}

然后你需要在某个地方设置系统属性

systemProperty(
  "flogger.backend_factory",
  "my.package.here.TestLoggingBackendFactory#getInstance"
)

现在，您可以通过TestLoggingBackend.logs访问所有发生的日志。
我进一步将其转化为了一个带有Truth断言的JUnit规则，您可以在这里看到：LoggingRule.kt。

如果您正在使用log4j2，则https://www.dontpanicblog.co.uk/2018/04/29/test-log4j2-with-junit/中的解决方案可以让我断言消息已被记录。
解决方案如下：

• 将log4j appender定义为ExternalResource规则

  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();
      }
  }
  

• 定义一个使用您的ExternalResource规则的测试

  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。