场景复现
你知道MyBatis是怎么实现日志的?额,这个简单,我知道啊!不就是在mybatis-config.xml文件中配置一下吗?
<!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>
看,就是上面这么配置就行了,这样还可以实现日志切换呢!
看官方文档介绍,有如下日志实现可以切换:
你知道具体的原理吗?MyBatis是怎样实现日志切换的呢?
额,这个,这个...(挠头,源码还没看过呢,😓)
源码撸起来~
对于这个不确定的事,作为程序员的我,怎能轻易说不知道呢?!源码走起来~
找到一个入口
首先,先从GitHub上将MyBatis的源码给clone到本地.老大们撸了那么多行代码,从哪开始下手呢???
大佬们,也是很规范的,写完代码有不少的Junit测试类,从test/java中找到了logging包.那就从这开始吧~
@Testpublic 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());}
SqlSessionFactoryBuilder类
//根据配置文件IO构建SqlSessionFactorypublic 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. } }}
XMLConfigBuilder XML配置构建器
构造函数
//reader是XML配置文件IOpublic 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;}
parse解析方法
/** * 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;}
parseConfiguration 解析配置方法
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); }}
其中Properties settings = settingsAsProperties(root.evalNode("settings"));是解析setttings节点,将settings节点的内容解析到 Properties 对象,其中涉及到很多操作,不在本文分析之列.
settingsElement(settings);将settings中的配置设置到Configuration对象.
settingsElement settings节点的配置
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")));}
Configuration 配置类
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); }}
从上面的Configuration构造函数中可以看到,类型别名中定义了日志实现的名称与,实现类.class(这些实现类都是MyBatis实现的,稍后再说)
//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); }}
BaseBuilder
构造函数
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.classprotected <T> Class<? extends T> resolveAlias(String alias) { //这个地方的typeAliasRegistry是从configuration中获取的 return typeAliasRegistry.resolveAlias(alias);}
TypeAliasRegistry 类型别名
类型别名注册
//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classpublic <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); }}
Slf4jImpl Slf4j实现类
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); } ...}
可见,最终还是调用具体的日志API实现!
LogFactory 日志工厂
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); } }}
可以看到其中有静态代码块,随着类的加载,尝试加载日志的实现!
总结
通过以上的一步一步分析,可以看到MyBatis是分别通过对日志实现进行进行包装,最终还是调用具体的日志API实现.
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本文标题: MyBatis是如何实现日志模块的详解
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