mybatis如何根据mapper接口生成其实现类

• SpringBoot集成mybatis

• mybatis的statement的解析与加载

• mybatis如何根据mapper接口生成其实现类

• mybatis的mapper返回map结果集

• mybatis结果的组装

序

mybatis里头给sqlSession指定执行哪条sql的时候，有两种方式，一种是写mapper的xml的namespace+statementId，如下：

public Student findStudentById(Integer studId) {
logger.debug("Select Student By ID :{}", studId);
SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
try {
return sqlSession.selectOne("com.mybatis3.StudentMapper.findStudentById", studId);
} finally {
sqlSession.close();
}
}

另外一种方法是指定mapper的接口：

public Student findStudentById(Integer studId) {
logger.debug("Select Student By ID :{}", studId);
SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
try {
StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
return studentMapper.findStudentById(studId);
} finally {
sqlSession.close();
}
}

一般的话，比较推荐第二种方法，因为手工写namespace和statementId极大增加了犯错误的概率，而且也降低了开发的效率。

问题

mapper的实现类如何生成

如果使用mapper接口的方式，问题来了，这个是个接口，通过sqlSession对象get出来的一定是个实现类，问题是，我们并没有手工去写 实现类，那么谁去干了这件事情呢？答案是mybatis通过JDK的动态代理方式，在启动加载配置文件时，根据配置mapper的xml去生成。

mybatis-spring帮忙做了什么

自动open和close session

一、mapper代理类是如何生成的

启动时加载解析mapper的xml

如果不是集成spring的，会去读取<mappers>节点，去加载mapper的xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<settings>
<setting name="cacheEnabled" value="true"/>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="multipleResultSetsEnabled" value="true"/>
<setting name="useColumnLabel" value="true"/>
<setting name="useGeneratedKeys" value="false"/>
<setting name="defaultExecutorType" value="SIMPLE"/>
<setting name="defaultStatementTimeout" value="2"/>
</settings>
<typeAliases>
<typeAlias alias="CommentInfo" type="com.xixicat.domain.CommentInfo"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/demo"/>
<property name="username" value="root"/>
<property name="password" value=""/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/xixicat/dao/CommentMapper.xml"/>
</mappers>
</configuration>

如果是集成spring的，会去读spring的sqlSessionFactory的xml配置中的mapperLocations，然后去解析mapper的xml

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource"/>
<!-- 配置mybatis配置文件的位置 -->
<property name="configLocation" value="classpath:mybatis-config.xml"/>
<property name="typeAliasesPackage" value="com.xixicat.domain"/>
<!-- 配置扫描Mapper XML的位置 -->
<property name="mapperLocations" value="classpath:com/xixicat/dao/*.xml"/>
</bean>

然后绑定namespace(

XMLMapperBuilder

)

private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}

这里先去判断该namespace能不能找到对应的class，若可以则调用

configuration.addMapper(boundType);

configuration委托给MapperRegistry:

public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}

生成该mapper的代理工厂(

MapperRegistry

)

public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MapperProxyFactory<T>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}

这里的重点就是MapperProxyFactory类：

public class MapperProxyFactory<T> {

private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}

public Class<T> getMapperInterface() {
return mapperInterface;
}

public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}

}

getMapper的时候生成mapper代理类

@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}

new出来MapperProxy

public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

这里给代理类注入了sqlSession

MapperProxy实现InvocationHandler接口进行拦截代理

public class MapperProxy<T> implements InvocationHandler, Serializable {

private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache;

public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}

private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}

这里的代理拦截，主要是寻找到MapperMethod，通过它去执行SQL。

MapperMethod委托给SqlSession去执行sql

public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} else {
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}

其实这里就回到了第一种模式，该模式是直接指定了statement的Id（这里是command.getName()），而通过mapper的接口方式，则多了这么步骤，最后通过MapperMethod，给sqlSession传入statement的id。

sqlSession其实自己也不执行sql，它只是mybatis对外公布的一个api入口，具体它委托给了executor去执行sql。

什么时候去getMapper

• 手工get，比如

  public void createStudent(Student student) {
  SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
  try {
  StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
  studentMapper.insertStudent(student);
  sqlSession.commit();
  } finally {
  sqlSession.close();
  }
  }

• 集成spring的话
  在spring容器给指定的bean注入mapper的时候get出来(

  见MapperFactoryBean的getObject方法

  )

二、mybatis-spring帮忙做了什么

通过MapperScannerConfigurer将mapper适配成spring bean

<!-- 配置扫描Mapper接口的包路径 -->
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
<property name="basePackage" value="com.xixicat.dao"/>
</bean>

这里使用 MapperFactoryBean将Mapper接口配置成 Spring bean 实体同时注入sqlSessionFactory。

MapperScannerConfigurer给每个mapper生成对应的MapperFactoryBean

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
if (this.processPropertyPlaceHolders) {
processPropertyPlaceHolders();
}

ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
scanner.setAddToConfig(this.addToConfig);
scanner.setAnnotationClass(this.annotationClass);
scanner.setMarkerInterface(this.markerInterface);
scanner.setSqlSessionFactory(this.sqlSessionFactory);
scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
scanner.setResourceLoader(this.applicationContext);
scanner.setBeanNameGenerator(this.nameGenerator);
scanner.registerFilters();
scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

委托给ClassPathMapperScanner去scan

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);

if (beanDefinitions.isEmpty()) {
logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
} else {
for (BeanDefinitionHolder holder : beanDefinitions) {
GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();

if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
+ "' and '" + definition.getBeanClassName() + "' mapperInterface");
}

// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean
definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
definition.setBeanClass(MapperFactoryBean.class);

definition.getPropertyValues().add("addToConfig", this.addToConfig);

boolean explicitFactoryUsed = false;
if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionFactory != null) {
definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
explicitFactoryUsed = true;
}

if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionTemplate != null) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
explicitFactoryUsed = true;
}

if (!explicitFactoryUsed) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}

return beanDefinitions;
}

这里出现了MapperFactoryBean的身影，然后判断配置文件是指定注入sqlSessionFactory，还是 sqlTemplate(二者不能同时指定，这里是指定了sqlSessionFactory)。这里通过 sqlSessionFactoryBeanName暂时先注入引用，因为此时还在给spring托管的bean进行create，不确定 sqlSessionFactory是否已经被创建。

关于MapperFactoryBean

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

private Class<T> mapperInterface;

private boolean addToConfig = true;

/**
* Sets the mapper interface of the MyBatis mapper
*
* @param mapperInterface class of the interface
*/
public void setMapperInterface(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}

/**
* If addToConfig is false the mapper will not be added to MyBatis. This means
* it must have been included in mybatis-config.xml.
* <p>
* If it is true, the mapper will be added to MyBatis in the case it is not already
* registered.
* <p>
* By default addToCofig is true.
*
* @param addToConfig
*/
public void setAddToConfig(boolean addToConfig) {
this.addToConfig = addToConfig;
}

/**
* {@inheritDoc}
*/
@Override
protected void checkDaoConfig() {
super.checkDaoConfig();

notNull(this.mapperInterface, "Property 'mapperInterface' is required");

Configuration configuration = getSqlSession().getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
try {
configuration.addMapper(this.mapperInterface);
} catch (Throwable t) {
logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", t);
throw new IllegalArgumentException(t);
} finally {
ErrorContext.instance().reset();
}
}
}

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

/**
* {@inheritDoc}
*/
public Class<T> getObjectType() {
return this.mapperInterface;
}

/**
* {@inheritDoc}
*/
public boolean isSingleton() {
return true;
}

}

注意这里继承了SqlSessionDaoSupport，在spring把sqlSessionFactory创建出来后，会去把之前注入的引用改为真的实例，调用SqlSessionDaoSupport的setSqlSessionFactory方法。

public abstract class SqlSessionDaoSupport extends DaoSupport {

private SqlSession sqlSession;

private boolean externalSqlSession;

public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
if (!this.externalSqlSession) {
this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
}
}

public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
this.sqlSession = sqlSessionTemplate;
this.externalSqlSession = true;
}

/**
* Users should use this method to get a SqlSession to call its statement methods
* This is SqlSession is managed by spring. Users should not commit/rollback/close it
* because it will be automatically done.
*
* @return Spring managed thread safe SqlSession
*/
public SqlSession getSqlSession() {
return this.sqlSession;
}

/**
* {@inheritDoc}
*/
protected void checkDaoConfig() {
notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
}

}

这里值得注意的是setSqlSessionFactory方法new了一个SqlSessionTemplate。

SqlSessionTemplate

它的一个重要的构造器为

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator) {

notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
notNull(executorType, "Property 'executorType' is required");

this.sqlSessionFactory = sqlSessionFactory;
this.executorType = executorType;
this.exceptionTranslator = exceptionTranslator;
this.sqlSessionProxy = (SqlSession) newProxyInstance(
SqlSessionFactory.class.getClassLoader(),
new Class[] { SqlSession.class },
new SqlSessionInterceptor());
}

mybatis-srping比传统mybatis方法多做的事情就在于此，生成了一个sqlSessionProxy。这里static import了java.lang.reflect.Proxy.newProxyInstance;也就是使用使用jdk代理进行了 SqlSessionInterceptor拦截。

SqlSessionInterceptor

private class SqlSessionInterceptor implements InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}

到了这里就明白了mybatis-spring帮忙做了session的open和close。

关于一级缓存

最后留个问题给大家，如果是使用mybatis-spring，那么mybatis的一级缓存默认开启的话，那么有达到一级缓存的设计初衷的效果么？