MyBatis是怎样实现日志模块的详解

<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
  <settings>
  	<setting name="logImpl" value="SLF4J"/>
  </settings>
</configuration>

@Test
public void shouldReadLogImplFromSettings() throws Exception {
//try-with-resources语句,目的就是为了读取配置文件IO
//mybatis-config.xml配置文件内容如上,<setting name="logImpl" value="SLF4J"/>
try (Reader reader = Resources.getResourceAsReader("org/apache/ibatis/logging/mybatis-config.xml")) {
 //解析配置文件,解析配置文件的代码就在这了~
 new SqlSessionFactoryBuilder().build(reader);
}

Log log = LogFactory.getLog(Object.class);
log.debug("Debug message.");
assertEquals(log.getClass().getName(), NoLoggingImpl.class.getName());
}

//根据配置文件IO构建SqlSessionFactory
public SqlSessionFactory build(Reader reader) {
  return build(reader, null, null);
}


public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
  try {
    //构建XML配置构建器
    XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
    //解析XML配置文件
    return build(parser.parse());
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error building SqlSession.", e);
  } finally {
    ErrorContext.instance().reset();
    try {
      reader.close();
    } catch (IOException e) {
      // Intentionally ignore. Prefer previous error.
    }
  }
}

//reader是XML配置文件IO
public XMLConfigBuilder(Reader reader, String environment, Properties props) {
  this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
}

//实际调用的XML配置文件构建器构建函数
private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
  //注意了,这是重点,在这地方调用了父类的构造函数,新建了一个Configuration对象,下面看看Configuration构造函数做了什么
  super(new Configuration());
  ErrorContext.instance().resource("SQL Mapper Configuration");
  this.configuration.setVariables(props);
  this.parsed = false;
  this.environment = environment;
  this.parser = parser;
}

/**
 * 解析 mybatis-config.xml
 * @return
 */
public Configuration parse() {
  // 只能解析一次
  if (parsed) {
    throw new BuilderException("Each XMLConfigBuilder can only be used once.");
  }
  parsed = true;
  //根据XPATH获取configuration节点
  parseConfiguration(parser.evalNode("/configuration"));
  return configuration;
}

private void parseConfiguration(XNode root) {
  try {
    //issue #117 read properties first
    // 解析 properties 节点
    propertiesElement(root.evalNode("properties"));
    // 解析 settings 节点
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);
    // 解析 typeAliases 节点
    typeAliasesElement(root.evalNode("typeAliases"));
    // 解析 plugins 节点
    pluginElement(root.evalNode("plugins"));
    // 解析 objectFactory 节点
    objectFactoryElement(root.evalNode("objectFactory"));
    // 解析 objectWrapperFactory 节点
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
    // 解析 reflectorFactory 节点
    reflectorFactoryElement(root.evalNode("reflectorFactory"));
    settingsElement(settings);
    // 解析 environments 节点， 需要在 objectFactory 和 objectWrapperFactory才能读取
    environmentsElement(root.evalNode("environments"));
    // 解析 databaseIdProvider 节点
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
    // 解析 typeHandlers 节点
    typeHandlerElement(root.evalNode("typeHandlers"));
    // 解析 mappers 节点
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

private void settingsElement(Properties props) throws Exception {
  ...
  configuration.setLogPrefix(props.getProperty("logPrefix"));
  @SuppressWarnings("unchecked")
  // 这个不就是从settings中解析日志的实现吗?快看看~
  // resovleClass是父类BaseBuilder中的方法
  Class<? extends Log> logImpl = (Class<? extends Log>) resolveClass(props.getProperty("logImpl"));
  //将日志的实现类设置到configuration配置类中
  configuration.setLogImpl(logImpl);
  configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
}

public class Configuration {
  ...
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  ...
  public Configuration() {
    ...
  
    //日志类型别名
    typeAliasRegistry.registerAlias("SLF4J", Slf4jImpl.class);
    typeAliasRegistry.registerAlias("COMMONS_LOGGING", JakartaCommonsLoggingImpl.class);
    typeAliasRegistry.registerAlias("LOG4J", Log4jImpl.class);
    typeAliasRegistry.registerAlias("LOG4J2", Log4j2Impl.class);
    typeAliasRegistry.registerAlias("JDK_LOGGING", Jdk14LoggingImpl.class);
    typeAliasRegistry.registerAlias("STDOUT_LOGGING", StdOutImpl.class);
    typeAliasRegistry.registerAlias("NO_LOGGING", NoLoggingImpl.class);
  
    typeAliasRegistry.registerAlias("CGLIB", CglibProxyFactory.class);
    typeAliasRegistry.registerAlias("JAVASSIST", JavassistProxyFactory.class);
  
    languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);
    languageRegistry.register(RawLanguageDriver.class);
  }
}

//设置日志实现类
public void setLogImpl(Class<? extends Log> logImpl) {
  if (logImpl != null) {
    this.logImpl = logImpl;
    //调用日志工厂,设置日志实现
    LogFactory.useCustomLogging(this.logImpl);
  }
}

public BaseBuilder(Configuration configuration) {
  this.configuration = configuration;
  //可以看到这个地方的类型别名是从configuration中获取的
  this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();
  this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();
}

protected <T> Class<? extends T> resolveClass(String alias) {
  if (alias == null) {
   return null;
  }
  try {
   //解析别名
   return resolveAlias(alias);
  } catch (Exception e) {
   throw new BuilderException("Error resolving class. Cause: " + e, e);
  }
}

//这个不就是从别名中获取对应的实现吗??!
//配置SLF4J,返回Slf4jImpl.class
protected <T> Class<? extends T> resolveAlias(String alias) {
  //这个地方的typeAliasRegistry是从configuration中获取的
  return typeAliasRegistry.resolveAlias(alias);
}

//就是将key转换位小写,存入HashMap中,key是别名,value是Class类对象
public void registerAlias(String alias, Class<?> value) {
  if (alias == null) {
    throw new TypeException("The parameter alias cannot be null");
  }
  // issue #748
  String key = alias.toLowerCase(Locale.ENGLISH);
  if (TYPE_ALIASES.containsKey(key) && TYPE_ALIASES.get(key) != null && !TYPE_ALIASES.get(key).equals(value)) {
    throw new TypeException("The alias '" + alias + "' is already mapped to the value '" + TYPE_ALIASES.get(key).getName() + "'.");
  }
  TYPE_ALIASES.put(key, value);
}

//根据对应的别名key,获取实现类class
public <T> Class<T> resolveAlias(String string) {
  try {
    if (string == null) {
      return null;
    }
    // issue #748
    String key = string.toLowerCase(Locale.ENGLISH);
    Class<T> value;
    if (TYPE_ALIASES.containsKey(key)) {
      value = (Class<T>) TYPE_ALIASES.get(key);
    } else {
      value = (Class<T>) Resources.classForName(string);
    }
    return value;
  } catch (ClassNotFoundException e) {
    throw new TypeException("Could not resolve type alias '" + string + "'. Cause: " + e, e);
  }
}

public class Slf4jImpl implements Log {

 private Log log;

 public Slf4jImpl(String clazz) {
  //使用Slf4j的API获取日志器
  Logger logger = LoggerFactory.getLogger(clazz);

  if (logger instanceof LocationAwareLogger) {
   try {
    // check for slf4j >= 1.6 method signature
    logger.getClass().getMethod("log", Marker.class, String.class, int.class, String.class, Object[].class, Throwable.class);
    log = new Slf4jLocationAwareLoggerImpl((LocationAwareLogger) logger);
    return;
   } catch (SecurityException e) {
    // fail-back to Slf4jLoggerImpl
   } catch (NoSuchMethodException e) {
    // fail-back to Slf4jLoggerImpl
   }
  }

  // Logger is not LocationAwareLogger or slf4j version < 1.6
  log = new Slf4jLoggerImpl(logger);
 }

 ...
}

package org.apache.ibatis.logging;

import java.lang.reflect.Constructor;

/**
 * 日志工厂
 */
public final class LogFactory {

 /**
  * Marker to be used by logging implementations that support markers
  */
 public static final String MARKER = "MYBATIS";

 // 记录当前使用的第三方日志库组件所对应的适配器的方法
 private static Constructor<? extends Log> logConstructor;

 // tryImplementation 进行尝试加载
 static {
  tryImplementation(LogFactory::useSlf4jLogging);
  tryImplementation(LogFactory::useCommonsLogging);
  tryImplementation(LogFactory::useLog4J2Logging);
  tryImplementation(LogFactory::useLog4JLogging);
  tryImplementation(LogFactory::useJdkLogging);
  tryImplementation(LogFactory::useNoLogging);
 }

 // 私有化
 private LogFactory() {
  // disable construction
 }

 public static Log getLog(Class<?> aClass) {
  return getLog(aClass.getName());
 }

 public static Log getLog(String logger) {
  try {
   return logConstructor.newInstance(logger);
  } catch (Throwable t) {
   throw new LogException("Error creating logger for logger " + logger + ". Cause: " + t, t);
  }
 }

 /**
  * 以下的 useXXXogging 的方法都是尝试加载日志的实现
  * 最终的实现都是 setImplementation
  */


 public static synchronized void useCustomLogging(Class<? extends Log> clazz) {
  setImplementation(clazz);
 }

 public static synchronized void useSlf4jLogging() {
  setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class);
 }

 public static synchronized void useCommonsLogging() {
  setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class);
 }

 public static synchronized void useLog4JLogging() {
  setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class);
 }

 public static synchronized void useLog4J2Logging() {
  setImplementation(org.apache.ibatis.logging.log4j2.Log4j2Impl.class);
 }

 public static synchronized void useJdkLogging() {
  setImplementation(org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl.class);
 }

 public static synchronized void useStdOutLogging() {
  setImplementation(org.apache.ibatis.logging.stdout.StdOutImpl.class);
 }

 public static synchronized void useNoLogging() {
  setImplementation(org.apache.ibatis.logging.nologging.NoLoggingImpl.class);
 }

 /**
  * 尝试加载
  * @param runnable
  */
 private static void tryImplementation(Runnable runnable) {
  if (logConstructor == null) {
   try {
    // 会调用 useSlf4jLogging 类似的方法
    runnable.run();
   } catch (Throwable t) {
    // ignore
   }
  }
 }

 /**
  * 设计日志的实现类
  * @param implClass Log 的子类
  */
 private static void setImplementation(Class<? extends Log> implClass) {
  try {
   // 通过反射获取构造方法
   Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
   Log log = candidate.newInstance(LogFactory.class.getName());
   if (log.isDebugEnabled()) {
    log.debug("Logging initialized using '" + implClass + "' adapter.");
   }
   //设置日志实现的有参构造函数
   logConstructor = candidate;
  } catch (Throwable t) {
   throw new LogException("Error setting Log implementation. Cause: " + t, t);
  }
 }

}