在 Java 开发中常用的日志框架有 Log4j、Log4j2、Apache Common Log、java.util.logging、slf4j 等，这些日志框架对外提供的接口各不相同。本章详细描述 MyBatis 是如何通过适配器的方式集成和复用这些第三方框架的。
日志适配器
MyBatis 的日志模块位于 org.apache.ibatis.logging 包中，该模块中 Log 接口定义了日志模块的功能，然后分别为不同的日志框架定义不同的日志适配器，这些日志适配器都继承 Log 接口，LogFactory 工厂负责创建对应的日志框架适配器。
下面来看 jdk14 日志适配器模式的类图：
在 LogFactory 类加载时会执行其静态代码快，按照顺序加载并实例化对应日志框架的适配器，然后用 logConstructor 字段记录当前使用的第三方日志框架的适配器。
tryImplementation() 方法使用 try cache 捕获了加载日志框架过程中产生的异常信息，且在 cache 中没做任何操作，所以不会有异常信息抛出，正常执行。如果没有引入任何日志框架，会使用 useJdkLogging ，这是 JDK 自带的日志工具。
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框架的适配器的构造方法
private static Constructor<? extends Log> logConstructor;
// 尝试加载每种日志框架，调用顺序是：
// useSlf4jLogging --> useCommonsLogging --> useLog4J2Logging -->
// useLog4JLogging --> useJdkLogging --> useNoLogging
static {
tryImplementation(new Runnable() {
@Override
public void run() {
useSlf4jLogging();
}
});
tryImplementation(new Runnable() {
@Override
public void run() {
useCommonsLogging();
}
});
tryImplementation(new Runnable() {
@Override
public void run() {
useLog4J2Logging();
}
});
tryImplementation(new Runnable() {
@Override
public void run() {
useLog4JLogging();
}
});
tryImplementation(new Runnable() {
@Override
public void run() {
useJdkLogging();
}
});
tryImplementation(new Runnable() {
@Override
public void run() {
useNoLogging();
}
});
}
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);
}
}
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);
}
// 尝试加载日志框架
private static void tryImplementation(Runnable runnable) {
if (logConstructor == null) {
try {
runnable.run();
} catch (Throwable t) {
// 加载异常被忽略了
// ignore
}
}
}
private static void setImplementation(Class<? extends Log> implClass) {
try {
//获取指定适配器的构造方法
Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
// 实例化适配器
Log log = candidate.newInstance(LogFactory.class.getName());
// 输出日志
if (log.isDebugEnabled()) {
log.debug("Logging initialized using '" + implClass + "' adapter.");
}
// 初始化logConstructor字段
logConstructor = candidate;
} catch (Throwable t) {
throw new LogException("Error setting Log implementation. Cause: " + t, t);
}
}
}
引入 SLF4J 日志框架
SLF4J 只是日志框架统一的接口定义（参考：http://www.slf4j.org/manual.html），还需要引入实现，pom.xml 文件增加如下内容：
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.28</version>
</dependency> <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
<version>1.7.28</version>
<scope>test</scope>
</dependency>
在 LogFactory 类加载的时候，其静态代码块会将 logConstructor 初始化为 SLF4J 适配器的构造函数。
通过 simplelogger.properties 属性文件配置日志级别为 debug，属性文件的命名是固定的，参考 org.slf4j.impl.SimpleLoggerConfiguration 类：
执行一个简单的查询操作，会输出如下日志信息：
[main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.transaction.jdbc.JdbcTransaction - Opening JDBC Connection
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - Created connection 1426329391.
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - ==> Preparing: select id, name, sex, selfcard_no, note from t_student where id = ?
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - ==> Parameters: 1(Long)
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - <== Total: 1
Slf4jImpl 类实现了 org.apache.ibatis.logging.Log 接口，并封装了 SLF4J 的日志接口，Log 接口的功能全部通过调用 SLF4J 的日志接口实现。
Slf4jImpl 构造函数中实现了 SLF4J 的版本区分，根据不同的版本使用不同的接口实现日志功能，源码如下：
public class Slf4jImpl implements Log {
private Log log;
public Slf4jImpl(String clazz) {
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);
}
@Override
public boolean isDebugEnabled() {
return log.isDebugEnabled();
}
@Override
public boolean isTraceEnabled() {
return log.isTraceEnabled();
}
@Override
public void error(String s, Throwable e) {
log.error(s, e);
}
@Override
public void error(String s) {
log.error(s);
}
@Override
public void debug(String s) {
log.debug(s);
}
@Override
public void trace(String s) {
log.trace(s);
}
@Override
public void warn(String s) {
log.warn(s);
}
}
MyBatis 源码篇