MyBatis的SQL语句执行过程是什么

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1 SQL语句的执行过程介绍

MyBatis核心执行组件：

2 SQL执行的入口分析

2.1 为Mapper接口创建代理对象

// 方式1：User user = session.selectOne("com.oldlu.dao.UserMapper.findUserById", 101);// 方式2：UserMapper mapper = session.getMapper(UserMapper.class);List userList = mapper.findAll();

2.2 执行代理逻辑

方式1入口分析：
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  List selectList(String statement, Object parameter, RowBoundsrowBounds) {  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(); }}

方式2入口分析：
获取代理对象：

//DefaultSqlSession类 ====================>@Overridepublic  T getMapper(Class type) {  return configuration.getMapper(type, this);  }// Configuration类 ====================>public  T getMapper(Class type, SqlSession sqlSession) {  return mapperRegistry.getMapper(type, sqlSession);}//MapperRegistry ----> apperProxyFactory.newInstance ====================>public  T getMapper(Class type, SqlSession sqlSession) {  //从缓存中获取该Mapper接口的代理工厂对象  final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory)knownMappers.get(type);  //如果该Mapper接口没有注册过，则抛异常  if (mapperProxyFactory == null) {    throw new BindingException("Type " + type + " is not known to theMapperRegistry."); }  try {    //【使用代理工厂创建Mapper接口的代理对象】    return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) {    throw new BindingException("Error getting mapper instance. Cause: " + e,e); }}//MapperProxyFactory  --->此时生成代理对象 ====================>protected T newInstance(MapperProxy mapperProxy) {  //Mybatis底层是调用JDK的Proxy类来创建代理实例  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), newClass[] { mapperInterface }, mapperProxy);}public T newInstance(SqlSession sqlSession) {  final MapperProxy mapperProxy = new MapperProxy<>(sqlSession,mapperInterface, methodCache);  return newInstance(mapperProxy);}

代理对象执行逻辑：

//MapperProxy   ====================>/**代理对象执行的方法，代理以后，所有Mapper的方法调用时，都会调用这个invoke方法*/public Object invoke(Object proxy, Method method, Object[] args) throwsThrowable { try {  if (Object.class.equals(method.getDeclaringClass())) {   //如果是Object方法，则调用方法本身   return method.invoke(this, args); } else {   //调用接口方法：根据被调用接口的Method对象，从缓存中获取MapperMethodInvoker对象   //apper接口中的每一个方法都对应一个MapperMethodInvoker对象，而MapperMethodInvoker对象里面的MapperMethod保存着对应的SQL信息和返回类型以完成SQL调用 ...   return cachedInvoker(method).invoke(proxy, method, args, sqlSession); }} catch (Throwable t) {  throw ExceptionUtil.unwrapThrowable(t);}}/**获取缓存中MapperMethodInvoker，如果没有则创建一个，而MapperMethodInvoker内部封装这一个MethodHandler*/private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {  try {    return methodCache.computeIfAbsent(method, m -> {      if (m.isDefault()) {        //如果调用接口的是默认方法（default方法）        try {          if (privateLookupInMethod == null) {            return newDefaultMethodInvoker(getMethodHandleJava8(method));         } else {            return newDefaultMethodInvoker(getMethodHandleJava9(method));         }       } catch (IllegalAccessException | InstantiationException |InvocationTargetException            | NoSuchMethodException e) {          throw new RuntimeException(e);       }     } else {        //如果调用的普通方法（非default方法），则创建一个PlainMethodInvoker并放入缓存，其中MapperMethod保存对应接口方法的SQL以及入参和出参的数据类型等信息        return new PlainMethodInvoker(new MapperMethod(mapperInterface,method, sqlSession.getConfiguration()));     }   }); } catch (RuntimeException re) {    Throwable cause = re.getCause();throw cause == null ? re : cause; }}// MapperProxy内部类： PainMethodInvoker  ====================>// 当cacheInvoker返回了PalinMethodInvoker实例之后，紧接着调用了这个实例的PlainMethodInvoker:invoke方法@Overridepublic Object invoke(Object proxy, Method method, Object[] args, SqlSessionsqlSession) throws Throwable { //Mybatis实现接口方法的核心: MapperMethod::execute方法： return mapperMethod.execute(sqlSession, args);}// MapperMethod  ====================>public Object execute(SqlSession sqlSession, Object[] args) {  Object result;  switch (command.getType()) {    case INSERT: {      // 将args进行解析，如果是多个参数则，则根据@Param注解指定名称将参数转换为Map，如果是封装实体则不转换      Object param = method.convertArgsToSqlCommandParam(args);      result = rowCountResult(sqlSession.insert(command.getName(),param));      break;   }    case UPDATE: {      Object param = method.convertArgsToSqlCommandParam(args);      result = rowCountResult(sqlSession.update(command.getName(),param));      break;   }    case DELETE: {      Object param = method.convertArgsToSqlCommandParam(args);      result = rowCountResult(sqlSession.delete(command.getName(),param));      break;   }    case SELECT:      //查询操作       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 if (method.returnsCursor()) {        result = executeForCursor(sqlSession, args);     } else {        //解析参数，因为SqlSession::selectOne方法参数只能传入一个，但是我们Mapper中可能传入多个参数，        //有可能是通过@Param注解指定参数名，所以这里需要将Mapper接口方法中的多个参数转化为一个ParamMap,        //也就是说如果是传入的单个封装实体，那么直接返回出来；如果传入的是多个参数，实际上都转换成了Map        Object param = method.convertArgsToSqlCommandParam(args);        //可以看到动态代理最后还是使用SqlSession操作数据库的        result = sqlSession.selectOne(command.getName(), param);        if (method.returnsOptional()          && (result == null ||!method.getReturnType().equals(result.getClass()))) {          result = Optional.ofNullable(result);       }     }      break;    case FLUSH:      result = sqlSession.flushStatements();      break;    default:      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 methodwith a primitive return type (" + method.getReturnType() + ")."); }  return result;}// 此时我们发现： 回到了sqlsession中private  Object executeForMany(SqlSession sqlSession, Object[] args) {  List result;  Object param = method.convertArgsToSqlCommandParam(args);  if (method.hasRowBounds()) {   RowBounds rowBounds = method.extractRowBounds(args);   result = sqlSession.selectList(command.getName(), param, rowBounds); } else {   result = sqlSession.selectList(command.getName(), param); } // ...  return result;}

3 查询语句的执行过程分析

3.1 selectOne方法分析

// DefaultSqlSession类  ===============>// selectOne@Overridepublic  T selectOne(String statement, Object parameter) {  // //selectOne()会调用selectList()。  List list = this.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 bereturned by selectOne(), but found: " + list.size()); } else {    return null; }}// selectListpublic  List selectList(String statement, Object parameter, RowBoundsrowBounds) {  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(); }}

3.2 sql获取

// CachingExecutor ===============>public  List query(MappedStatement ms, Object parameterObject, RowBoundsrowBounds, ResultHandler resultHandler) throws SQLException {  // 获取绑定的sql命令，比如"SELECT * FROM xxx"  BoundSql boundSql = ms.getBoundSql(parameterObject);   CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}@Overridepublic  List query(MappedStatement ms, Object parameterObject, RowBoundsrowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)  throws SQLException {  Cache cache = ms.getCache();  if (cache != null) {    flushCacheIfRequired(ms);    if (ms.isUseCache() && resultHandler == null) {      ensureNoOutParams(ms, boundSql);      @SuppressWarnings("unchecked")      List list = (List) tcm.getObject(cache, key);      if (list == null) {        list = delegate.query(ms, parameterObject, rowBounds,resultHandler, key, boundSql);        tcm.putObject(cache, key, list); // issue #578 and #116    }      return list;   } }  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key,boundSql);}//真正执行query操作的是SimplyExecutor代理来完成的，SimplyExecutor的父类BaseExecutor的query方法中:// BaseExecutor类:SimplyExecutor的父类 =================>@Overridepublic  List query(MappedStatement ms, Object parameter, RowBoundsrowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throwsSQLException {  ErrorContext.instance().resource(ms.getResource()).activity("executing aquery").object(ms.getId());  if (closed) {    throw new ExecutorException("Executor was closed."); }  if (queryStack == 0 && ms.isFlushCacheRequired()) {    clearLocalCache(); }  List 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) 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();   }    deferredLoads.clear();    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {      clearLocalCache();   }}  return list;}//第一次，没有缓存，所以会调用queryFromDatabase方法来执行查询。private  List queryFromDatabase(...) throws SQLException {  List list;  localCache.putObject(key, EXECUTION_PLACEHOLDER);  try {    // 查询    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;}// SimpleExecutor类 ============================>public  List doQuery(...) throws SQLException {  Statement stmt = null;  try {    Configuration configuration = ms.getConfiguration();    StatementHandler handler = configuration.newStatementHandler(....);    // 1：SQL查询参数的设置    stmt = prepareStatement(handler, ms.getStatementLog());    // StatementHandler封装了Statement    // 2：SQL查询操作和结果集的封装    return handler.query(stmt); } finally {    closeStatement(stmt); }}

3.3 参数设置

// SimplyExecutor类 ============================>// 【1】 参数设置： prepareStatementprivate Statement prepareStatement(StatementHandler handler, Log statementLog)throws SQLException {  Statement stmt;  // 通过getConnection方法来获取一个Connection，  Connection connection = getConnection(statementLog);  // 调用prepare方法来获取一个Statement  stmt = handler.prepare(connection, transaction.getTimeout());   // 设置SQL查询中的参数值 ***  handler.parameterize(stmt);  return stmt;}// RoutingStatementHandler ============================>// PreparedStatementHandler ============================>@Overridepublic void parameterize(Statement statement) throws SQLException {  parameterHandler.setParameters((PreparedStatement) statement);}// DefaultParameterHandler ============================> 此时参数设置成功@Overridepublic void setParameters(PreparedStatement ps) {  ErrorContext.instance().activity("settingparameters").object(mappedStatement.getParameterMap().getId());  List 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)) {          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 | SQLException e) {          throw new TypeException("Could not set parameters formapping.....");       }     }   } }}

3.4 SQL执行和结果集的封装

// RoutingStatementHandler ============================>@Overridepublic  List query(Statement statement) throws SQLException {  return delegate.query(statement);}// PreparedStatementHandler ============================>@Overridepublic  List query(Statement statement, ResultHandler resultHandler)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. handleResultSets(ps);}// DefaultResultSetHandler类（封装返回值，将查询结果封装成Object对象）@Overridepublic List