Log4j2 - Change log level at runtime

為了能夠在系統執行期間蒐集發生問題的資訊,有時我們會在執行功能之前,去動態調整log level,儘可能最大化地去蒐集必要資訊。在Log4j 1.x時,我們採用了以下寫法去做到動態切換log level:

Logger logger = LogManager.getLogger(packName);
logger.setLevel(Level.TRACE);

本篇文章主要分享在Log4j2可行的做法。範例程式可參考: link

SUT

public class TestLogger {
    private static final Logger LOGGER = LoggerFactory.getLogger(TestLogger.class);
 
    public static void debug(String message) {
        LOGGER.debug(message);   
    }
 
    public static void error(String message) {
        LOGGER.error(message);   
    }
}

Configuration properties

預設啟用了ConsoleAppender,而SUT的level是設為error;另外appender.console.follow設為true,讓Log4j可以反應system.out或system.err的變更,這主要和我們測試時會去攔截console有關:

rootLogger.level = info
rootLogger.appenderRef.stdout.ref = STDOUT

status = error
dest = err
name = PropertiesConfig

filter.threshold.type = ThresholdFilter
filter.threshold.level = debug
 
appender.console.type = Console
appender.console.name = STDOUT
appender.console.follow = true
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %d %p %C{1.} [%t] %m%n
appender.console.filter.threshold.type = ThresholdFilter
appender.console.filter.threshold.level = trace

logger.console.name = org.tonylin.practice.log4j2.example1
logger.console.level = error
logger.console.additivity = false
logger.console.appenderRef.rolling.ref = STDOUT

Unit Test

測試程式我使用了console-captor library讓我可以很容易去捕捉console內容;測試後我會透過Configurator.reconfigure去重置設置內容:

    private ConsoleCaptor captor;
 
    @Before
    public void setup() {
        captor = new ConsoleCaptor();
        Configurator.reconfigure();
    }
 
    @After
    public void teardown() {
      captor.close();
    }

首先讓我們先測試debug level不會有console,而error level會有console且內容符合預期:

    @Test
    public void Should_NotSystemOutToConsole_When_LogDebugWithDefaultConfiguration() {   
        TestLogger.debug("test debug");
        assertEquals(0, captor.getStandardOutput().size());
    }
 
    @Test
    public void Should_SystemOutToConsole_When_LogErrorWithDefaultConfiguration() {
        TestLogger.error("test error");
        assertEquals(1, captor.getStandardOutput().size());
        assertTrue(captor.getStandardOutput().get(0).contains("test error"));
    }

接著就是本篇主角了,第一個方法是直接用Configurator去設定,雖然簡單,但這不是一個public的API,不建議在production code使用:

    @Test
    public void Should_SystemOutToConsole_When_LogDebugAfterChangingLogLevelWithConfigurator() {
        Configurator.setLevel(TestLogger.class.getName(), Level.DEBUG);
 
        TestLogger.debug("test debug");
        assertEquals(1, captor.getStandardOutput().size());
        assertTrue(captor.getStandardOutput().get(0).contains("test debug"));
    }

另外一個方法參考自這裡,其實就是Configurator實作的方法(Configurator像是一個Utility的Class):

    @Test
    public void Should_SystemOutToConsole_When_LogDebugAfterChangingLogLevel() {
        LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
        Configuration config = ctx.getConfiguration();
        LoggerConfig loggerConfig = config.getLoggerConfig(TestLogger.class.getName());
        loggerConfig.setLevel(Level.DEBUG);
        ctx.updateLoggers();
 
        TestLogger.debug("test debug");
        assertEquals(1, captor.getStandardOutput().size());
        assertTrue(captor.getStandardOutput().get(0).contains("test debug"));
    }

註: LogManager.getContext(true)的真正作用情境還不曉得為何,有弄清楚後再分享。