一般來說，日志是程序相當(dāng)次要的副作用輸出，很少需要專門的單元測試來保證它的行為。不過也不排除在某些情況下需要在單元測試中驗(yàn)證日志，比如：

• 某個(gè)場景下日志輸出是下游模塊的關(guān)鍵信息。一般出現(xiàn)在錯(cuò)誤處理分支中。
• 某個(gè)模塊除了日志沒有顯著的輸出來檢驗(yàn)它的行為。紅燈，模塊可測試性差的表現(xiàn)。

這里介紹以比較常用的logback和log4j2為例介紹驗(yàn)證日志的方法，以備不時(shí)之需。

被測試代碼

public class SampleService {
    private static Logger LOG = LoggerFactory.getLogger(SampleService.class);
    @Autowired SampleDependency dependency;

    public String foo() {
        LOG.info("starting foo");
        String bar = "";
        try {
            bar = dependency.getExternalValue("bar");
        } catch (Exception e) {
            System.out.println(LOG.getClass().getTypeName());
            LOG.error("calling foo error", e);
        }
        return bar;
    }
}

因?yàn)閮煞N框架外加slf4j中有很多同名的類，示例代碼中全部帶上完整包名以免混淆。

Logback

@Test
public void testLogback() {
    //given
    ch.qos.logback.core.Appender<ch.qos.logback.classic.spi.ILoggingEvent> appender = mock(ch.qos.logback.core.Appender.class);
    ((ch.qos.logback.classic.Logger) LoggerFactory.getLogger(SampleService.class)).addAppender(appender);
    doThrow(new RuntimeException("something wrong...")).when(dependency).getExternalValue(anyString());

    //when
    service.foo();

    //then
    ArgumentCaptor<ch.qos.logback.classic.spi.ILoggingEvent> logCaptor = ArgumentCaptor.forClass(ch.qos.logback.classic.spi.ILoggingEvent.class);
    //通過ArgumentCaptor捕獲所有l(wèi)og,
    //verify默認(rèn)是一次調(diào)用，改為atLeast(0)讓任意次數(shù)記錄log都能驗(yàn)證通過
    verify(appender, atLeast(0)).doAppend(logCaptor.capture()); 
    logCaptor.getAllValues().stream()
            .filter(event -> event.getFormattedMessage().equals("calling foo error"))
            .findFirst().orElseThrow(AssertionError::new);
}

Log4j2

@Test
public void testLog4J2() {
    //given
    org.apache.logging.log4j.core.Appender appender = mock(org.apache.logging.log4j.core.Appender.class);
    when(appender.getName()).thenReturn("mocked");  //加入appender時(shí)需要
    when(appender.isStarted()).thenReturn(true);  //使appender生效時(shí)需要
    ((org.apache.logging.log4j.core.Logger) org.apache.logging.log4j.LogManager.getRootLogger() ).addAppender(appender);
    doThrow(new RuntimeException("something wrong...")).when(dependency).getExternalValue(anyString());

    //when
    service.foo();

    //then
    ArgumentCaptor<org.apache.logging.log4j.core.LogEvent> logCaptor = ArgumentCaptor.forClass(org.apache.logging.log4j.core.LogEvent.class);
    verify(appender, atLeast(0)).append(logCaptor.capture());
    logCaptor.getAllValues().stream()
            .filter(event -> event.getMessage().getFormattedMessage().equals("calling foo error"))
            .findFirst().orElseThrow(AssertionError::new);
}