MyBatis+Spring基于接口编程的原理分析 -优快云博客

本文链接：https://blog.youkuaiyun.com/feiyeguohai1/article/details/84018708

本文详细解析了Spring与MyBatis的整合过程，重点介绍了如何利用MapperScannerConfigurer自动扫描和注册Mapper接口，并通过MapperProxy实现动态代理，最终实现无需Mapper实现类即可调用Mapper方法。

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整合Spring3及MyBatis3
对于整合Spring及Mybatis不作详细介绍，可以参考： MyBatis 3 User Guide Simplified Chinese.pdf，贴出我的主要代码如下：
UserMapper Interface:

Java代码

package org.denger.mapper;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
import org.denger.po.User;
public interface UserMapper {
@Select("select * from tab_uc_account where id=#{userId}")
User getUser(@Param("userId") Long userId);
}

package org.denger.mapper;

import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
import org.denger.po.User;

public interface UserMapper {

	@Select("select * from tab_uc_account where id=#{userId}")
	User getUser(@Param("userId") Long userId);
}

application-context.xml:

Xml代码

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd">
<context:annotation-config/>
<bean id="dataSourceProxy" class="org.springframework.jdbc.datasource.TransactionAwareDataSourceProxy">
<constructor-arg>
<ref bean="dataSource" />
</constructor-arg>
</bean>
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
<property name="driverClass" value="com.mysql.jdbc.Driver" />
<property name="jdbcUrl" value="jdbc:mysql://127.0.0.1:3306/noah" />
<property name="user" value="root" />
<property name="password" value="123456" />
</bean>
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
</bean>
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="sqlSessionFactory" ref="sqlSessionFactory"/>
<property name="basePackage" value="org.denger.mapper"></property>
</bean>
</beans>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
	xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
	xsi:schemaLocation="http://www.springframework.org/schema/beans	http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd">


	<!-- Provided by annotation-based configuration  -->
	<context:annotation-config/>

	<!--JDBC Transaction  Manage -->
	<bean id="dataSourceProxy" class="org.springframework.jdbc.datasource.TransactionAwareDataSourceProxy">
		<constructor-arg>
			<ref bean="dataSource" />
		</constructor-arg>
	</bean>

	<!-- The JDBC c3p0 dataSource bean-->
	<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
		<property name="driverClass" value="com.mysql.jdbc.Driver" />
		<property name="jdbcUrl" value="jdbc:mysql://127.0.0.1:3306/noah" />
		<property name="user" value="root" />
		<property name="password" value="123456" />
	</bean>

	<!--MyBatis integration with Spring as define sqlSessionFactory  -->
	<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
		<property name="dataSource" ref="dataSource" />
	</bean>

	<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
		<property name="sqlSessionFactory"  ref="sqlSessionFactory"/>
		<property name="basePackage" value="org.denger.mapper"></property>
	</bean>
</beans>

test Class:

Java代码

package org.denger.mapper;
import org.junit.Assert;
import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.AbstractJUnit4SpringContextTests;
@ContextConfiguration(locations = { "/application-context.xml"})
public class UserMapperTest extends AbstractJUnit4SpringContextTests{
@Autowired
public UserMapper userMapper;
@Test
public void testGetUser(){
Assert.assertNotNull(userMapper.getUser(300L));
}
}

package org.denger.mapper;

import org.junit.Assert;
import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.AbstractJUnit4SpringContextTests;

@ContextConfiguration(locations = { "/application-context.xml"})
public class UserMapperTest extends AbstractJUnit4SpringContextTests{

	@Autowired
	public UserMapper userMapper;
	
	@Test
	public void testGetUser(){
		Assert.assertNotNull(userMapper.getUser(300L));
	}
}

实现原理分析
     对于以上极其简单代码看上去并无特殊之处，主要亮点在于 UserMapper 居然不用实现类，而且在调用 getUser 的时候，也是使用直接调用了UserMapper实现类，那么Mybatis是如何去实现 UserMapper的接口的呢？
    可能你马上能想到的实现机制就是通过动态代理方式，好吧，看看MyBatis整个的代理过程吧。

    首先在Spring的配置文件中看到下面的Bean：

Xml代码

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="sqlSessionFactory" ref="sqlSessionFactory"/>
<property name="basePackage" value="org.denger.mapper"></property>
</bean>

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
		<property name="sqlSessionFactory"  ref="sqlSessionFactory"/>
		<property name="basePackage" value="org.denger.mapper"></property>
	</bean>

   以上的MapperScannerConfigurer class的注释中描述道：
   从base 包中搜索所有下面所有 interface，并将其注册到 Spring Bean容器中，其注册的class bean是MapperFactoryBean。
   好吧，看看它的注册过程，下面方法来从　MapperScannerConfigurer中的Scanner类中抽取，下面方法在初始化以上application-content.xml文件时就会进行调用。　主要用于是搜索 base packages 下的所有mapper class,并将其注册至 spring 的 benfinitionHolder中。

Java代码

/**
* Calls the parent search that will search and register all the candidates. Then the
* registered objects are post processed to set them as MapperFactoryBeans
*/
@Override
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
//#1
Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
if (beanDefinitions.isEmpty()) {
logger.warn("No MyBatis mapper was found in '" + MapperScannerConfigurer.this.basePackage
+ "' package. Please check your configuration.");
} else {
//#2
for (BeanDefinitionHolder holder : beanDefinitions) {
GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '"+ definition.getBeanClassName() + "' mapperInterface");
}
//#3
definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
definition.setBeanClass(MapperFactoryBean.class);
}
return beanDefinitions;
}

/**
* Calls the parent search that will search and register all the candidates. Then the
* registered objects are post processed to set them as MapperFactoryBeans
*/
@Override
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
        //#1
	Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
	if (beanDefinitions.isEmpty()) {
     		logger.warn("No MyBatis mapper was found in '" + MapperScannerConfigurer.this.basePackage
                        + "' package. Please check your configuration.");
	} else {
         //#2
         for (BeanDefinitionHolder holder : beanDefinitions) {
          		GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
			if (logger.isDebugEnabled()) {
				logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '"+ definition.getBeanClassName() + "' mapperInterface");
			}
           //#3
          definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
          definition.setBeanClass(MapperFactoryBean.class);
     }
	return beanDefinitions;
}

    #1: 了解Spring初始化Bean过程的人可能都知道，Spring 首先会将需要初始化的所有class先通过BeanDefinitionRegistry进行注册，并且将该Bean的一些属性信息(如scope、className、beanName等)保存至BeanDefinitionHolder中；Mybatis这里首先会调用Spring中的ClassPathBeanDefinitionScanner.doScan方法，将所有Mapper接口的class注册至BeanDefinitionHolder实例中，然后返回一个Set<BeanDefinitionHolder>，其它包含了所有搜索到的Mapper class BeanDefinitionHolder对象。

    #2: 首先，由于 #1: 　中注册的都是接口class，可以肯定的是接口是不能直接初始化的；实际 #2: 中for循环中替换当前所有 holder的 className为 MapperFactoryBean.class，并且将 mapper interface的class name setter 至 MapperFactoryBean 属性为 mapperInterface 中，也就是 #3: 代码所看到的。
　再看看 MapperFactoryBean，它是直接实现了 Spring 的　FactoryBean及InitializingBean　接口。其实既使不看这两个接口，当看MapperFactoryBean的classname就知道它是一个专门用于创建 Mapper 实例Bean的工厂。

　至此，已经完成了Spring与mybatis的整合的初始化配置的过程。

    接着，当我在以上的 Test类中，需要注入 UserMapper接口实例时，由于mybatis给所有的Mapper 实例注册都是一个MapperFactory的工厂，所以产生UserMapper实现仍需要 MapperFactoryBean来进行创建。接下来看看 MapperFactoryBean的处理过程。
    先需要创建Mapper实例时，首先在 MapperFactoryBean中执行的方法是：

Java代码

/**
* {@inheritDoc}
*/
public void afterPropertiesSet() throws Exception {
Assert.notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
Assert.notNull(this.mapperInterface, "Property 'mapperInterface' is required");
Configuration configuration = this.sqlSession.getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
configuration.addMapper(this.mapperInterface);
}
}

/**
     * {@inheritDoc}
     */
    public void afterPropertiesSet() throws Exception {
        Assert.notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
        Assert.notNull(this.mapperInterface, "Property 'mapperInterface' is required");

        Configuration configuration = this.sqlSession.getConfiguration();
        if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
            configuration.addMapper(this.mapperInterface);
        }
    }

上面方法中先会检测当前需要创建的 mapperInterface在全局的 Configuration中是否存在，如果不存在则添加。呆会再说他为什么要将 mapperInterface 添加至 Configuration中。该方法调用完成之后，就开始调用 getObject方法来产生mapper实例，看到MapperFactoryBean.getObject的该方法代码如下：

Java代码

/**
* {@inheritDoc}
*/
public T getObject() throws Exception {
return this.sqlSession.getMapper(this.mapperInterface);
}

 /**
     * {@inheritDoc}
     */
    public T getObject() throws Exception {
        return this.sqlSession.getMapper(this.mapperInterface);
    }

通过Debug可以看到 sqlSession及mapperInterface对象：

到目前为止我们的 UserMapper实例实际上还并未产生；　再进入org.mybatis.spring.SqlSessionTemplate中的getMapper方法，该方法将 this及mapper interface class 作为参数传入 org.apache.ibatis.session.Configuration的getMapper() 方法中，代码如下：

Java代码

/**
* {@inheritDoc}
*/
public <T> T getMapper(Class<T> type) {
return getConfiguration().getMapper(type, this);
}

/**
     * {@inheritDoc}
     */
    public <T> T getMapper(Class<T> type) {
        return getConfiguration().getMapper(type, this);
    }

于是，再进入 org.apache.ibatis.session.Configuration.getMapper中代码如下：

Java代码

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

其中 mapperRegistry保存着当前所有的 mapperInterface class.　那么它在什么时候将 mapperinterface class 保存进入的呢？其实就是在上面的 afterPropertiesSet 中通过 configuration.addMapper(this.mapperInterface) 添加进入的。
再进入　org.apache.ibatis.binding.MapperRegistry.getMapper方法，代码如下：

Java代码

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
　　//首先判断当前knownMappers是否存在mapper interface class.因为前面说到 afterPropertiesSet 中已经将当前的 mapperinterfaceclass 添加进入了。
if (!knownMappers.contains(type))
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
try {
return MapperProxy.newMapperProxy(type, sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
　　//首先判断当前knownMappers是否存在mapper interface class.因为前面说到 afterPropertiesSet 中已经将当前的 mapperinterfaceclass 添加进入了。
    if (!knownMappers.contains(type))
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    try {
      return MapperProxy.newMapperProxy(type, sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

　嗯，没错，看到　MapperProxy.newMapperProxy后可以肯定的是它确实采用的代理模式，再进入一看究竟吧：
　　

Java代码

public static <T> T newMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) {
ClassLoader classLoader = mapperInterface.getClassLoader();
Class[] interfaces = new Class[]{mapperInterface};
MapperProxy proxy = new MapperProxy(sqlSession);
return (T) Proxy.newProxyInstance(classLoader, interfaces, proxy);
}

public static <T> T newMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) {
    ClassLoader classLoader = mapperInterface.getClassLoader();
    Class[] interfaces = new Class[]{mapperInterface};
    MapperProxy proxy = new MapperProxy(sqlSession);
    return (T) Proxy.newProxyInstance(classLoader, interfaces, proxy);
  }

JDK的动态代理就不说了，至此，基本Mybatis已经完成了Mapper实例的整个创建过程，也就是你在具体使用 UserMapper.getUser 时，它实际上调用的是 MapperProxy，因为此时所返回的 MapperProxy是实现了 UserMapper接口的。只不过 MapperProxy拦截了所有对userMapper中方法的调用，如下：
　

Java代码

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//如果调用不是 object 中默认的方法(如equals之类的)
if (!OBJECT_METHODS.contains(method.getName())) {
//因为当前MapperProxy代理了所有 Mapper，所以他需要根据当前拦截到的方法及代理对象获取 MapperInterface class，也就是我这里的 UserMapper.class
final Class declaringInterface = findDeclaringInterface(proxy, method);
//然后再根据UserMapper.class、及当前调用的Method(也就是getUser)及SqlSession构造一个 MapperMethod，在这里面会获取到 getUser方法上的 @Select() 的SQL，然后再通过 sqlSession来进行执行
final MapperMethod mapperMethod = new MapperMethod(declaringInterface, method, sqlSession);
//execute执行数据库操作，并返回结果集
final Object result = mapperMethod.execute(args);
if (result == null && method.getReturnType().isPrimitive()) {
throw new BindingException("Mapper method '" + method.getName() + "' (" + method.getDeclaringClass() + ") attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
} catch (SQLException e) {
e.printStackTrace();
}
return null;
}

 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
     //如果调用不是 object 中默认的方法(如equals之类的)
      if (!OBJECT_METHODS.contains(method.getName())) {
        //因为当前MapperProxy代理了所有 Mapper，所以他需要根据当前拦截到的方法及代理对象获取 MapperInterface  class，也就是我这里的 UserMapper.class
        final Class declaringInterface = findDeclaringInterface(proxy, method);
        //然后再根据UserMapper.class、及当前调用的Method(也就是getUser)及SqlSession构造一个 MapperMethod，在这里面会获取到 getUser方法上的 @Select() 的SQL，然后再通过 sqlSession来进行执行
        final MapperMethod mapperMethod = new MapperMethod(declaringInterface, method, sqlSession);
        //execute执行数据库操作，并返回结果集
        final Object result = mapperMethod.execute(args);
        if (result == null && method.getReturnType().isPrimitive()) {
          throw new BindingException("Mapper method '" + method.getName() + "' (" + method.getDeclaringClass() + ") attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
        }
        return result;
      }
    } catch (SQLException e) {
      e.printStackTrace();
    }
    return null;
  }

为什么说它拦截了呢？可以看到, 它并没有调用 method.invoke(object)方法，因为实际上 MapperProxy只是动态的 implement 了UserMapper接口，但它没有真正实现 UserMapper中的任何方法。至于结果的返回，它也是通过 MapperMethod.execute 中进行数据库操作来返回结果的。　说白了，就是一个实现了 MapperInterface 的 MapperProxy 实例被MapperProxy代理了，可以debug看看　userMapper实例就是 MapperProxy。