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~
@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());
}
SqlSessionFactoryBuilder类
//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.
}
}
}
XMLConfigBuilder XML配置构建器
构造函数
//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;
}
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.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);
}
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);
}
}
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.
好了，以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，谢谢大家对天达云的支持。