你的位置:首页 > Java教程

[Java教程]mybatis如何根据mapper接口生成其实现类


mybatis系列

  • 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

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

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

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

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

<?<!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.</mappers></configuration>

如果是集成spring的,会去读spring的sqlSessionFactory的

<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.<property name="typeAliasesPackage" value="com.xixicat.domain"/>    <!-- 配置扫描Mapper <property name="mapperLocations" value="classpath:com/xixicat/dao/*.</bean>

然后绑定namespace()

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#load"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. @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?