一、传统的jdbc操作步骤
获取驱动获取jdbc连接创建参数化预编译的sql绑定参数发送sql到数据库执行将将获取到的结果集返回应用关闭连接传统的jdbc代码：
package com.zjp;import java.sql.Connection;import java.sql.DriverManager;import java.sql.PreparedStatement;import java.sql.ResultSet;public class JDBCTest {public static void main(String[] args) {try {Connection con = null; //定义一个MYSQL链接对象Class.forName("com.mysql.jdbc.Driver").newInstance(); //MYSQL驱动con = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mybatis?useUnicode=true", "root", "root"); //链接本地MYSQL//更新一条数据String updateSql = "UPDATE user_t SET user_name = 'test' WHERE id = ?";PreparedStatement pstmt = con.prepareStatement(updateSql);pstmt.setString(1, "1");long updateRes = pstmt.executeUpdate();System.out.print("UPDATE:" + updateRes);//查询数据并输出String sql = "select * from user_t where id = ?";PreparedStatement pstmt2 = con.prepareStatement(sql);pstmt2.setString(1, "1");ResultSet rs = pstmt2.executeQuery();while (rs.next()) { //循环输出结果集String id = rs.getString("id");String username = rs.getString("user_name");System.out.print("\r\n\r\n");System.out.print("id:" + id + ",username:" + username);}//关闭资源rs.close();pstmt.close();pstmt2.close();con.close();} catch (Exception e) {e.printStackTrace();}}}
二、mybatis 执行sql
mybatis要执行sql，同样也需要获取到数据库连接，这个在mybatis里面就是sqlSession
// 使用MyBatis提供的Resources类加载mybatis的配置文件Reader reader = Resources.getResourceAsReader("mybatis-config.xml");// 构建sqlSession的工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);SqlSession session = sqlSessionFactory.openSession();
2.1 获取mapper对象
获取到了session对象之后就是获取mapper对象了，在mybatis中是使用动态代理的方式获取
@Overridepublic <T> T getMapper(Class<T> type) {//最后会去调用MapperRegistry.getMapperreturn configuration.<T>getMapper(type, this);}
/*** 返回代理类* @param type* @param sqlSession* @return*/@SuppressWarnings("unchecked")public <T> T getMapper(Class<T> type, SqlSession sqlSession) {// MapperProxyFactory去把代理类做出来final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {// 这里就是返回代理对象return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}
@SuppressWarnings("unchecked")protected T newInstance(MapperProxy<T> mapperProxy) {//用JDK自带的动态代理生成映射器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);}
通过session获取mapper，session通过MapperProxyFactory代理对象去获取mapper对象，最终在newInstance中我们看到了使用jdk自带的动态代理方式获取到了mapper对象。
2.2 执行sql语句
现在获取到了mapper对象，那么下一步就是执行sql语句了。
@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {//代理以后，所有Mapper的方法调用时，都会调用这个invoke方法//并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行if (Object.class.equals(method.getDeclaringClass())) {try {return method.invoke(this, args);} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}}//这里优化了，去缓存中找MapperMethodfinal MapperMethod mapperMethod = cachedMapperMethod(method);//执行 sqlreturn mapperMethod.execute(sqlSession, args);}
所有的sql语句都会调用invoke方法，会后都要调用execute来执行sql语句
public Object execute(SqlSession sqlSession, Object[] args) {Object result;//可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法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()) {//如果有结果处理器executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {//如果结果有多条记录result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {//如果结果是mapresult = executeForMap(sqlSession, args);} else {//否则就是一条记录Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);}} 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;}
这个方法也就是通过使用枚举的方式执行insert|update|delete|select，分别执行不同的方法。这里跟着select，这个也是最复杂的。在这里我们看到了很重要的一个方法
result = sqlSession.selectOne(command.getName(), param);
通过源码可知selectOne方法转而去调用selectList,很简单的，如果得到0条则返回null，得到1条则返回1条，得到多条报TooManyResultsException错，特别需要主要的是当没有查询到结果的时候就会返回null。因此一般建议在mapper中编写resultType的时候使用包装类型而不是基本类型，比如推荐使用Integer而不是int。这样就可以避免NPE
//核心selectOne@Overridepublic <T> T selectOne(String statement, Object parameter) {// Popular vote was to return null on 0 results and throw exception on too many.List<T> list = this.<T>selectList(statement, parameter);if (list.size() == 1) {return list.get(0);} else if (list.size() > 1) {throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());} else {return null;}}
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {//根据statement id找到对应的MappedStatementMappedStatement ms = configuration.getMappedStatement(statement);//转而用执行器来查询结果,注意这里传入的ResultHandler是nullreturn executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);} finally {ErrorContext.instance().reset();}}
从selectList方法中可以看出来，最终的查询还是交给了executor
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {//得到绑定sqlBoundSql boundSql = ms.getBoundSql(parameter);//创建缓存KeyCacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);//查询return query(ms, parameter, rowBounds, resultHandler, key, boundSql);}
从MappedStatement对象中获取到BoundSql对象，BoundSql对象包含了我们需要执行的sql语句。
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();//新建一个StatementHandler//这里看到ResultHandler传入了StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);//准备语句stmt = prepareStatement(handler, ms.getStatementLog());//StatementHandler.queryreturn handler.<E>query(stmt, resultHandler);} finally {closeStatement(stmt);}}
通过一些列的跟踪，定位到了doQuery方法，最终sql语句的执行交给了StatementHandler对象，这个对象也就是我们最常用的，封装的是PreparedStatement
@Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {PreparedStatement ps = (PreparedStatement) statement;ps.execute();// 结果交给了ResultSetHandler 去处理return resultSetHandler.<E> handleResultSets(ps);}
很明显这里就是使用的PreparedStatement进行处理。
public List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());final List<Object> multipleResults = new ArrayList<Object>();int resultSetCount = 0;ResultSetWrapper rsw = getFirstResultSet(stmt);List<ResultMap> resultMaps = mappedStatement.getResultMaps();//一般resultMaps里只有一个元素int resultMapCount = resultMaps.size();validateResultMapsCount(rsw, resultMapCount);while (rsw != null && resultMapCount > resultSetCount) {ResultMap resultMap = resultMaps.get(resultSetCount);handleResultSet(rsw, resultMap, multipleResults, null);rsw = getNextResultSet(stmt);cleanUpAfterHandlingResultSet();resultSetCount++;}String[] resultSets = mappedStatement.getResulSets();if (resultSets != null) {while (rsw != null && resultSetCount < resultSets.length) {ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);if (parentMapping != null) {String nestedResultMapId = parentMapping.getNestedResultMapId();ResultMap resultMap = configuration.getResultMap(nestedResultMapId);handleResultSet(rsw, resultMap, null, parentMapping);}rsw = getNextResultSet(stmt);cleanUpAfterHandlingResultSet();resultSetCount++;}}return collapseSingleResultList(multipleResults);}
最后就是返回结果集了。
