mybatis如何根據mapper定位sql並執行

一.mybatis給sqlSession指定sql方式：　

mybatis里頭給sqlSession指定執行哪條sql的時候，有兩種方式，一種是寫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(); } }

另外一種方法是指定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(); } }

一般的話，比較推薦第二種方法，因為手工寫namespace和statementId極大增加了犯錯誤的概率，而且也降低了開發的效率。

 

二.mapper的實現類如何生成

如果使用mapper接口的方式，問題來了，這個是個接口，通過sqlSession對象get出來的一定是個實現類，問題是，我們並沒有手工去寫 實現類，那么誰去干了這件事情呢？答案是mybatis通過JDK的動態代理方式，在啟動加載配置文件時，根據配置mapper的xml去生成。

啟動時加載解析mapper的xml

如果不是集成spring的，會去讀取<mappers>節點，去加載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，會去讀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"/> <!-- 配置掃描Mapper XML的位置 --> <property name="mapperLocations" value="classpath:com/xixicat/dao/*.xml"/> </bean>

然后綁定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); } } } }

這里先去判斷該namespace能不能找到對應的class，若可以則調用

configuration.addMapper(boundType);

configuration委托給MapperRegistry:

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

生成該mapper的代理工廠(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); } } } }

這里的重點就是MapperProxyFactory類：

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的時候生成mapper代理類

@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出來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); }

這里給代理類注入了sqlSession

MapperProxy實現InvocationHandler接口進行攔截代理

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; } }

這里的代理攔截，主要是尋找到MapperMethod，通過它去執行SQL。

MapperMethod委托給SqlSession去執行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; }

其實這里就回到了第一種模式，該模式是直接指定了statement的Id（這里是command.getName()），而通過mapper的接口方式，則多了這么步驟，最后通過MapperMethod，給sqlSession傳入statement的id。

sqlSession其實自己也不執行sql，它只是mybatis對外公布的一個api入口，具體它委托給了executor去執行sql。

什么時候去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的話
  在spring容器給指定的bean注入mapper的時候get出來(見MapperFactoryBean的getObject方法)