MyBatis源码分析-SQL语句执行的完整流程
MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以对配置和原生Map使用简单的 XML 或注解,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。如何新建MyBatis源码工程请点击MyBatis源码分析-IDEA新建MyBatis源码工程。
MyBatis框架主要完成的是以下2件事情:
- 根据JDBC规范建立与数据库的连接。
- 通过反射打通Java对象与数据库参数交互之间相互转换的关系。
MyBatis框架是一种典型的交互式框架,先准备好交互的必要条件,然后构建一个交互的环境,在交互环境中划分会话,在会话中与数据库进行交互数据。
1 MyBatis主要的类
- Configuration MyBatis所有的配置信息都维持在Configuration对象之中。
- SqlSession 作为MyBatis工作的主要顶层API,表示和数据库交互的会话,完成必要数据库增删改查功能
- Executor MyBatis执行器,是MyBatis 调度的核心,负责SQL语句的生成和查询缓存的维护
- StatementHandler 封装了JDBC Statement操作,负责对JDBC statement 的操作,如设置参数、将Statement结果集转换成List集合。
- ParameterHandler 负责对用户传递的参数转换成JDBC Statement 所需要的参数,
- ResultSetHandler 负责将JDBC返回的ResultSet结果集对象转换成List类型的集合;
- TypeHandler 负责java数据类型和jdbc数据类型之间的映射和转换
- MappedStatement MappedStatement维护了一条<select|update|delete|insert>节点的封装,
- SqlSource 负责根据用户传递的parameterObject,动态地生成SQL语句,将信息封装到BoundSql对象中,并返回
- BoundSql 表示动态生成的SQL语句以及相应的参数信息
以上几个类在SQL操作中都会涉及,在SQL操作中重点关注下SQL参数什么时候写入和结果集怎么转换为Java对象,这两个过程正好对应的类是PreparedStatementHandler和ResultSetHandler类。
(图片来自《深入理解mybatis原理》 MyBatis的架构设计以及实例分析)
2 SQL执行流程
MyBatis主要设计目的还是为了让我们在执行SQL时对输入输出的数据的管理更加方便,所以方便的让我们写出SQL和方便的获取SQL的执行结果是MyBatis的核心竞争力。下面就用一个例子来从源码角度看一下SQL的完整执行流程。
新建配置文件conf.xml:
<?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="false"/> <!--setting name="logImpl" value="STDOUT_LOGGING"/--> <!-- 日志 --> </settings> <typeAliases> <typeAlias type="com.luoxn28.dao.User" alias="User"/> </typeAliases> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <!-- 声明使用那种事务管理机制 JDBC/MANAGED --> <!-- 配置数据库连接信息 --> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://192.168.1.150:3306/xxx" /> <property name="username" value="xxx" /> <property name="password" value="xxx" /> </dataSource> </environment> </environments> <mappers> <mapper resource="userMapper.xml"/> </mappers> </configuration>
首先建立数据表,这里就以user表为例 :
DROP TABLE IF EXISTS user; CREATE TABLE user ( id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, name VARCHAR(32) NOT NULL, password VARCHAR(32) NOT NULL, sex int, email VARCHAR(32), phone VARCHAR(16), admin VARCHAR(16) );
然后新建与数据表对应的类User:
/** * User - 用户类 */ public class User { public static final int MAN = 0; // 男生 public static final int WOMAN = 1; // 女生 public static final int OTHER = 2; // 其他 private int id; // 用户id private String name; // 用户名 private String password; // 用户密码 private int sex; // 用户性别 private String email; // 用户邮箱 private String phone; // 用户手机 private String admin; // 用户是否是管理员,"admin"表示是管理员,其他为普通用户 public User() { } public User(String name, String password, int sex, String email, String phone) { this.name = name; this.password = password; this.sex = sex; this.email = email; this.phone = phone; this.admin = ""; } public User(String name, String password, String sex, String email, String phone) { this.name = name; this.password = password; setSex(sex); // this.sex = sex; this.email = email; this.phone = phone; this.admin = ""; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public int getSex() { return sex; } public void setSex(int sex) { this.sex = sex; } public void setSex(String sexStr) { int sex = Integer.valueOf(sexStr); switch (Integer.valueOf(sexStr)) { case 0: { this.sex = MAN; break; } case 1: { this.sex = WOMAN; break; } default: { this.sex = OTHER; break; } } } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getPhone() { return phone; } public void setPhone(String phone) { this.phone = phone; } public String getAdmin() { return admin; } public void setAdmin(String admin) { this.admin = admin; } @Override public String toString() { return "User{" + "id=" + id + ", name='" + name + '\'' + ", password='" + password + '\'' + ", sex=" + sex + ", email='" + email + '\'' + ", phone='" + phone + '\'' + ", admin='" + admin + '\'' + '}'; } }
再新建usre表的配置文件:
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.luoxn28.dao.UserDao"> <select id="getById" parameterType="int" resultType="User"> SELECT * FROM user WHERE id=#{id}; <!-- #{xxx} xxx为类中的数据域名称 --> </select> <select id="getAll" resultType="com.luoxn28.dao.User"> SELECT * FROM user; </select> </mapper>
最后新建测试类:
/** * MyBatis测试类 */ public class TestMain { public static void main(String[] args) throws IOException { String resouce = "conf.xml"; InputStream is = Resources.getResourceAsStream(resouce); // 构建sqlSession工厂 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is); // 获取sqlSession SqlSession session = sqlSessionFactory.openSession(); User user; try { /** * 第一种方式: 直接执行已映射的 SQL 语句 */ String statement = "com.luoxn28.dao.UserDao.getById"; user = session.selectOne(statement, 1); System.out.println(user); } finally { session.close(); } /** * 第二种方式: 执行更清晰和类型安全的代码 */ // UserDao userDao = session.getMapper(UserDao.class); // user = userDao.getById(1); // System.out.println(user); } }
由于我们分析的是SQL的执行流程,那就重点关注下 user = session.selectOne(statement, 1); 这行代码~ 注意,传进去的参数是1。
session是DefaultSqlSession类型的,因为sqlSessionFactory默认生成的SqlSession是DefaultSqlSession类型。selectOne()会调用selectList()。
// DefaultSqlSession类 public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); // CURD操作是交给Excetor去处理的 return 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(); } }
在DefaultSqlSession.selectList中的各种CURD操作都是通多Executor进行的,这里executor的类型是CachingExecutor,接着跳转到其中的query方法中。
// CachingExecutor 类 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { BoundSql boundSql = ms.getBoundSql(parameterObject); // 获取绑定的sql命令,比如"SELECT * FROM xxx" CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }
getBoundSql为了获取绑定的sql命令,在创建完cacheKey之后,就进入到CachingExecutor 类中的另一个query方法中。
// CachingExecutor 类 @Override public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, parameterObject, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }
这里真正执行query操作的是SimplyExecutor代理来完成的,接着就进入到了SimplyExecutor的父类BaseExecutor的query方法中。
// SimplyExecutor的父类BaseExecutor类 @SuppressWarnings("unchecked") @Override public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId()); if (closed) { throw new ExecutorException("Executor was closed."); } if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); } List<E> list; try { queryStack++; /** * localCache是一级缓存,如果找不到就调用queryFromDatabase从数据库中查找 */ list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) { deferredLoad.load(); } // issue #601 deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { // issue #482 clearLocalCache(); } } return list; }
因为是第一次SQL查询操作,所以会调用queryFromDatabase方法来执行查询。
// SimplyExecutor的父类BaseExecutor类 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { localCache.removeObject(key); } localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }
从数据库中查询数据,进入到SimplyExecutor中进行操作。
// SimplyExecutor类 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(); StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); // 子流程1:SQL查询参数的设置 stmt = prepareStatement(handler, ms.getStatementLog()); // StatementHandler封装了Statement // 子流程2:SQL查询操作和结果集的封装 return handler.<E>query(stmt); } finally { closeStatement(stmt); } }
注意,在prepareStatement方法中会进行SQL查询参数的设置,也就是咱们最开始传递进来的参数,其值为1。handler.<E>query(stmt)方法中会进行实际的SQL查询操作和结果集的封装(封装成Java对象)。当流程走到这里时,程序已经压栈有一定深度了,因为接下来程序分析会兵分两路,一方面深入到SQL查询及结果集的设置子流程1中,然后再深入到SQL查询操作和结果集的封装子流程2,因为还会回到这里,所以就来一张调用栈的特写吧:
子流程1:SQL查询参数的设置
// SimplyExecutor类 private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; // 获取一个Connection Connection connection = getConnection(statementLog); stmt = handler.prepare(connection, transaction.getTimeout()); handler.parameterize(stmt); // 设置SQL查询中的参数值 return stmt; }
通过getConnection方法来获取一个Connection,调用prepare方法来获取一个Statement(这里的handler类型是RoutingStatementHandler,RoutingStatementHandler的prepare方法调用的是PrepareStatementHandler的prepare方法,因为PrepareStatementHandler并没有覆盖其父类的prepare方法,其实最后调用的是BaseStatementHandler中的prepare方法。是不是绕晕了,那就再看一遍吧 :) )。调用parameterize方法来设置SQL的参数值(这里最后调用的是PrepareStatementHandler中的parameterize方法,而PrepareStatementHandler.parameterize方法调用的是DefaultParameterHandler中的setParameters方法)。
// PrepareStatementHandler类 @Override public void parameterize(Statement statement) throws SQLException { parameterHandler.setParameters((PreparedStatement) statement); }
// DefaultParameterHandler类 @Override public void setParameters(PreparedStatement ps) { ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId()); List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); if (parameterMappings != null) { for (int i = 0; i < parameterMappings.size(); i++) { ParameterMapping parameterMapping = parameterMappings.get(i); if (parameterMapping.getMode() != ParameterMode.OUT) { Object value; String propertyName = parameterMapping.getProperty(); if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params value = boundSql.getAdditionalParameter(propertyName); } else if (parameterObject == null) { value = null; } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) { value = parameterObject; } else { MetaObject metaObject = configuration.newMetaObject(parameterObject); value = metaObject.getValue(propertyName); } TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if (value == null && jdbcType == null) { jdbcType = configuration.getJdbcTypeForNull(); } try { typeHandler.setParameter(ps, i + 1, value, jdbcType); } catch (TypeException e) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e); } catch (SQLException e) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e); } } } } }
到这里为止,已经给Statement设置了最初传递进去的参数(值为1)了,那么接着分析流程2:
流程2:SQL查询及结果集的设置
// RoutingStatementHandler类 @Override public <E> List<E> query(Statement statement) throws SQLException { return delegate.<E>query(statement); }
// RoutingStatementHandler类 @Override public <E> List<E> query(Statement statement) throws SQLException { // 这里就到了熟悉的PreparedStatement了 PreparedStatement ps = (PreparedStatement) statement; // 执行SQL查询操作 ps.execute(); // 结果交给ResultHandler来处理 return resultSetHandler.<E> handleResultSets(ps); }
// DefaultResultSetHandler类(封装返回值,将查询结果封装成Object对象) @Override 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(); 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.getResultSets(); 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); }
ResultSetWrapper是ResultSet的包装类,调用getFirstResultSet方法获取第一个ResultSet,同时获取数据库的MetaData数据,包括数据表列名、列的类型、类序号等,这些信息都存储在ResultSetWrapper类中了。然后调用handleResultSet方法来来进行结果集的封装。
// DefaultResultSetHandler类 private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException { try { if (parentMapping != null) { handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping); } else { if (resultHandler == null) { DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory); handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null); multipleResults.add(defaultResultHandler.getResultList()); } else { handleRowValues(rsw, resultMap, resultHandler, rowBounds, null); } } } finally { // issue #228 (close resultsets) closeResultSet(rsw.getResultSet()); } }
这里调用handleRowValues方法来进行值的设置:
// DefaultResultSetHandler类 public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { if (resultMap.hasNestedResultMaps()) { ensureNoRowBounds(); checkResultHandler(); handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); } else { // 封装数据 handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); } }
// DefaultResultSetHandler类 // 封装数据 private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>(); skipRows(rsw.getResultSet(), rowBounds); while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) { ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null); Object rowValue = getRowValue(rsw, discriminatedResultMap); storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet()); } }
// DefaultResultSetHandler类 private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException { final ResultLoaderMap lazyLoader = new ResultLoaderMap(); // createResultObject为新创建的对象,数据表对应的类 Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null); if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) { final MetaObject metaObject = configuration.newMetaObject(resultObject); boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty(); if (shouldApplyAutomaticMappings(resultMap, false)) { // 这里把数据填充进去,metaObject中包含了resultObject信息 foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues; } foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues; foundValues = lazyLoader.size() > 0 || foundValues; resultObject = foundValues ? resultObject : null; return resultObject; } return resultObject; }
// DefaultResultSetHandler类(把ResultSet中查询结果填充到JavaBean中) private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException { List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix); boolean foundValues = false; if (autoMapping.size() > 0) { // 这里进行for循环调用,因为user表中总共有7项,所以也就调用7次 for (UnMappedColumnAutoMapping mapping : autoMapping) { // 这里将esultSet中查询结果转换为对应的实际类型 final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column); if (value != null) { foundValues = true; } if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) { // gcode issue #377, call setter on nulls (value is not 'found') metaObject.setValue(mapping.property, value); } } } return foundValues; }
mapping.typeHandler.getResult会获取查询结果值的实际类型,比如我们user表中id字段为int类型,那么它就对应Java中的Integer类型,然后通过调用statement.getInt("id")来获取其int值,其类型为Integer。metaObject.setValue方法会把获取到的Integer值设置到Java类中的对应字段。
// MetaObject类 public void setValue(String name, Object value) { PropertyTokenizer prop = new PropertyTokenizer(name); if (prop.hasNext()) { MetaObject metaValue = metaObjectForProperty(prop.getIndexedName()); if (metaValue == SystemMetaObject.NULL_META_OBJECT) { if (value == null && prop.getChildren() != null) { // don't instantiate child path if value is null return; } else { metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory); } } metaValue.setValue(prop.getChildren(), value); } else { objectWrapper.set(prop, value); } }
metaValue.setValue方法最后会调用到Java类中对应数据域的set方法,这样也就完成了SQL查询结果集的Java类封装过程。最后贴一张调用栈到达Java类的set方法中的快照:
参考:
2、《深入分析Java Web技术内幕》的iBatis章节
3、《深入理解mybatis原理》 MyBatis的架构设计以及实例分析
4、