MyBatis源码分析之日志logging详解

private void loadCustomLogImpl(Properties props) {
 // 从 MyBatis 的 TypeAliasRegistry 中查找 logImpl 键所对应值的类对象
 // 这里 logImpl 对应的 value 值可以从 org.apache.ibatis.session.Configuration 的构造方法中找到
 // 注意 Log 类，这是 MyBatis 内部对日志对象的抽象
 Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
 // 将查找到的 Class 对象设置到 Configuration 对象中
 configuration.setLogImpl(logImpl);
}

// 注意这个类型的方法都是同步方法
public synchronized static useXxxLogging(...);

public static Log getLog(...);

private static tryImplementation(Runnable);

private static setImplementation(Class);

private static void setImplementation(Class<? extends Log> implClass) {
 try {
  // 获取 Log实现类的构造方法，它只有一个字符串作为参数
  Constructor<? extends Log> candidate = implClass.getConstructor(String.class);

  // 创建一个 Log 对象，打印 debug 日志
  Log log = candidate.newInstance(LogFactory.class.getName());
  if (log.isDebugEnabled()) {
   log.debug("Logging initialized using '" + implClass + "' adapter.");
  }

  // ...
  // 把 candidate 对象设置到 LogFactory 的静态变量 logConstructor，这个静态变量在 getLog() 方法
  // 中被用到
  logConstructor = candidate;
 } catch (Throwable t) {
  throw new LogException("Error setting Log implementation. Cause: " + t, t);
 }
}

public interface Log {

 boolean isDebugEnabled();

 boolean isTraceEnabled();

 void error(String s, Throwable e);

 void error(String s);

 void debug(String s);

 void trace(String s);

 void warn(String s);

}

public class Log4jImpl implements Log {

 private static final String FQCN = Log4jImpl.class.getName();

 // 这里包装了 Log4j 框架的日志对象，从而实现适配
 private final Logger log;

 public Log4jImpl(String clazz) {
 log = Logger.getLogger(clazz);
 }

 @Override
 public boolean isDebugEnabled() {
 return log.isDebugEnabled();
 }

 @Override
 public boolean isTraceEnabled() {
 return log.isTraceEnabled();
 }

 @Override
 public void error(String s, Throwable e) {
 log.log(FQCN, Level.ERROR, s, e);
 }

 @Override
 public void error(String s) {
 log.log(FQCN, Level.ERROR, s, null);
 }

 @Override
 public void debug(String s) {
 log.log(FQCN, Level.DEBUG, s, null);
 }

 @Override
 public void trace(String s) {
 log.log(FQCN, Level.TRACE, s, null);
 }

 @Override
 public void warn(String s) {
 log.log(FQCN, Level.WARN, s, null);
 }

}

static {
 // 依次执行如下代码，当没有该类会抛 ClassNotFoundException ，然后继续执行
 tryImplementation(LogFactory::useSlf4jLogging);
 tryImplementation(LogFactory::useCommonsLogging);
 tryImplementation(LogFactory::useLog4J2Logging);
 tryImplementation(LogFactory::useLog4JLogging);
 tryImplementation(LogFactory::useJdkLogging);
 tryImplementation(LogFactory::useNoLogging);
}