浅谈Java日志框架slf4j作用及其实现原理

<!-- 原文：五月的仓颉http://www.cnblogs.com/xrq730/p/8619156.html -->
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>

   <groupId>org.xrq.log</groupId>
   <artifactId>log-test</artifactId>
   <version>1.0.0</version>
   <packaging>jar</packaging>

   <name>log-test</name>
   <url>http://maven.apache.org</url>

   <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
   </properties>

   <dependencies>
    <dependency>
      <groupId>junit</groupId>
       <artifactId>junit</artifactId>
       <version>4.11</version>
       <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.25</version>
    </dependency>
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <version>1.7.25</version>
    </dependency>
    <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.17</version>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.7.21</version>
    </dependency>
   </dependencies>
</project>

@Test
public void testSlf4j() {
  Logger logger = LoggerFactory.getLogger(Object.class);
  logger.error("123");
 }

public static Logger getLogger(Class<?> clazz) {
  Logger logger = getLogger(clazz.getName());
  if (DETECT_LOGGER_NAME_MISMATCH) {
    Class<?> autoComputedCallingClass = Util.getCallingClass();
    if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
      Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
              autoComputedCallingClass.getName()));
      Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
    }
  }
  return logger;
}

private final static void bind() {
  try {
    Set<URL> staticLoggerBinderPathSet = null;
    // skip check under android, see also
    // http://jira.qos.ch/browse/SLF4J-328
    if (!isAndroid()) {
      staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
      reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
    }
    // the next line does the binding
    StaticLoggerBinder.getSingleton();
    INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
    reportActualBinding(staticLoggerBinderPathSet);
    fixSubstituteLoggers();
    replayEvents();
    // release all resources in SUBST_FACTORY
    SUBST_FACTORY.clear();
  } catch (NoClassDefFoundError ncde) {
    String msg = ncde.getMessage();
    if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
      INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
      Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
      Util.report("Defaulting to no-operation (NOP) logger implementation");
      Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
    } else {
      failedBinding(ncde);
      throw ncde;
    }
  } catch (java.lang.NoSuchMethodError nsme) {
    String msg = nsme.getMessage();
    if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
      INITIALIZATION_STATE = FAILED_INITIALIZATION;
      Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
      Util.report("Your binding is version 1.5.5 or earlier.");
      Util.report("Upgrade your binding to version 1.6.x.");
    }
    throw nsme;
  } catch (Exception e) {
    failedBinding(e);
    throw new IllegalStateException("Unexpected initialization failure", e);
  }
}

static Set<URL> findPossibleStaticLoggerBinderPathSet() {
  // use Set instead of list in order to deal with bug #138
  // LinkedHashSet appropriate here because it preserves insertion order
  // during iteration
  Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
  try {
    ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
    Enumeration<URL> paths;
    if (loggerFactoryClassLoader == null) {
      paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
    } else {
      paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
    }
    while (paths.hasMoreElements()) {
      URL path = paths.nextElement();
      staticLoggerBinderPathSet.add(path);
    }
  } catch (IOException ioe) {
    Util.report("Error getting resources from path", ioe);
  }
  return staticLoggerBinderPathSet;
}

private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) {
  if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
    Util.report("Class path contains multiple SLF4J bindings.");
    for (URL path : binderPathSet) {
      Util.report("Found binding in [" + path + "]");
    }
    Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
  }
}