面试官：为什么使用 Mapper 接口就能对数据库进行访问？

宏观理解

拿到的 mapper 其实是一个代理对象，底层调用的是 SqlSession 的方法，SqlSession 的方法里其实调用的 Executor 的方法。

源码验证

从 Mapper 到 SqlSession

跟 getMapper：
类 SqlSession

  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

每一个 SqlSession 都包含一个 Configuration 的引用，所有的配置信息都被解读在里面了。

跟 getMapper：
类 Configuration

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry

public class MapperRegistry {

  private final Configuration config;//config对象，mybatis全局唯一的
  //记录了mapper接口与对应MapperProxyFactory之间的关系
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @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);
    }
  }

  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }

  //将mapper接口的工厂类添加到mapper注册中心
  //XMLMapperBuilder里的parse()里的bindMapperForNamespace()里的
  //configuration.addMapper(boundType)调用
  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 {
    	//实例化Mapper接口的代理工程类，并将信息添加至knownMappers
        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.
        //解析接口上的注解信息，并添加至configuration对象
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }

}

MapperProxyFactory

public class MapperProxyFactory<T> {

  //mapper接口的class对象
  private final Class<T> mapperInterface;
  //key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，所以它可以在多个代理对象之间共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

  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) {
	//创建实现了mapper接口的动态代理对象
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
	 //每次调用都会创建新的MapperProxy对象，MapperProxy继承了InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

可以看出来最后是拿到的代理对象。

MapperProxy

学过 JDK 的动态代理就会知道：调用 mapper 接口的方法，其实就是调用这个对象里的 invoke 方法

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;//记录关联的SqlSession对象
  private final Class<T> mapperInterface;//mapper接口对应的class对象；
  //key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，所以它可以在多个代理对象之间共享
  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 {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {//如果是Object本身的方法不增强
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //从缓存中获取mapperMethod对象，如果缓存中没有，则创建一个，并添加到缓存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用execute方法执行sql，可以看出并没有执行method.invoke
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
  }

  @UsesJava7
  private Object invokeDefaultMethod(Object proxy, Method method, Object[] args)
      throws Throwable {
    final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
        .getDeclaredConstructor(Class.class, int.class);
    if (!constructor.isAccessible()) {
      constructor.setAccessible(true);
    }
    final Class<?> declaringClass = method.getDeclaringClass();
    return constructor
        .newInstance(declaringClass,
            MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
                | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
        .unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
  }

  /**
   * Backport of java.lang.reflect.Method#isDefault()
   */
  private boolean isDefaultMethod(Method method) {
    return (method.getModifiers()
        & (Modifier.ABSTRACT | Modifier.PUBLIC | Modifier.STATIC)) == Modifier.PUBLIC
        && method.getDeclaringClass().isInterface();
  }
}

MapperMethod

封装了 mapper 接口中对应方法的信息，以及对应的 sql 语句的信息
它是 mapper 接口与映射配置文件中 sql 语句的桥梁
MapperMethod 对象不记录任何状态信息，所以它可以在多个代理对象之间共享

它有3个内部类：

• SqlCommand ： 从 configuration 中获取方法的命名空间.方法名以及 sql 语句的类型
• MethodSignature：封装 mapper 接口方法的相关信息（入参，返回类型）
• ParamNameResolver： 解析 mapper 接口方法中的入参，将多个参数转成 Map

public class MapperMethod {
  //从configuration中获取方法的命名空间.方法名以及SQL语句的类型（SELECT等）
  private final SqlCommand command;
  //封装mapper接口方法的相关信息（入参，返回类型）
  private final MethodSignature method;

  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, mapperInterface, method);
  }

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根据sql语句类型以及接口返回的参数选择调用不同的
    switch (command.getType()) {
      case INSERT: {
    	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:
        if (method.returnsVoid() && method.hasResultHandler()) {//返回值为void
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {//返回值为集合或者数组
          //这个方法里面其实就是调用的SqlSession的方法
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {//返回值为map
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {//返回值为游标
          result = executeForCursor(sqlSession, args);
        } else {//处理返回为单一对象的情况
          //通过参数解析器解析解析参数
          Object param = method.convertArgsToSqlCommandParam(args);
          //调用SqlSession的方法，重点看
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = OptionalUtil.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 method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }
  //省略了大部分方法
}

从 SqlSession 到 Executor

之前的代码已经看到，当返回对象为单一对象的情况，调用的是 selectOne 方法

但最终，会调用到 selectList 方法：

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //从configuration中获取要执行的sql语句的配置信息
      MappedStatement ms = configuration.getMappedStatement(statement);
      //通过executor执行语句，并返回指定的结果集
      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();
    }
  }

跟 query：
类 Executor

会调哪个方法呢？

SqlSession sqlSession = sqlSessionFactory.openSession();

这是我们获取 sqlSession 的代码
跟 openSession：
类 DefaultSqlSessionFactory：

  @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

跟 openSessionFromDataSource：

  //从数据源获取数据库连接
  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
    	//获取mybatis配置文件中的environment对象
      final Environment environment = configuration.getEnvironment();
      //从environment获取transactionFactory对象
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //创建事务对象
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //根据配置创建executor
      final Executor executor = configuration.newExecutor(tx, execType);
      //创建DefaultSqlSession
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

跟 newExecutor：

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    //如果有<setting name="cacheEnabled" value="true" />节点，
    //通过装饰器，添加二级缓存的能力
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //通过interceptorChain遍历所有的插件为executor增强，添加插件的功能
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

看见没有，装饰器模式，会给你的 executor，套上 CachingExecutor。
而 cacheEnabled 在 Configuration 里默认是 true 的。

而有了这些，并不是真的开启了二级缓存。要开启二级缓存，得在 Mapper.xml 文件里加上 <cache> 相关标签，这个会在 Mybatis 初始化时，被 XMLMapperBuilder 解析到，交给 builderAssistant 创建 Cache 并存储到 Configuration 的一个 Map里，以 namespace 为 key。

详情请看 Mybatis 的初始化与建造者模式

CachingExecutor

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	//获取sql语句信息，占位符，参数等信息
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //拼装缓存的key值
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
  
  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
	//从MappedStatement中获取二级缓存
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);//从二级缓存中获取数据
        if (list == null) {
          //二级缓存为空，才会调用BaseExecutor.query
          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);
  }

开启了二级缓存，而没有，或者没开启二级缓存，都会执行 delegate.<E> query

BaseExecutor

  @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) {//检查当前executor是否关闭
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {//非嵌套查询，并且FlushCache配置为true，则需要清空一级缓存
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;//查询层次加一
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;//查询以及缓存
      if (list != null) {
    	 //针对调用存储过程的结果处理
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
    	 //缓存未命中，从数据库加载数据
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }


    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {//延迟加载处理
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {//如果当前sql的一级缓存配置为STATEMENT，查询完既清空一集缓存
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

  //真正访问数据库获取结果的方法
  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);//在缓存中添加占位符
    try {
      //调用抽象方法doQuery，方法查询数据库并返回结果，可选的实现包括：simple、reuse、batch
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);//在缓存中删除占位符
    }
    localCache.putObject(key, list);//将真正的结果对象添加到一级缓存
    if (ms.getStatementType() == StatementType.CALLABLE) {//如果是调用存储过程
      localOutputParameterCache.putObject(key, parameter);//缓存输出类型结果参数
    }
    return list;
  }

跟 doQuery：

可以看出，这是一个模板方法，模板模式的体现。默认的是 SimpleExecutor 实现。

BaseExecutor 定义了一套骨架流程：

红色的部分就是不同的 doQuery 实现。

SimpleExecutor

  @Override
  //查询的实现
  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();//获取configuration对象
      //创建StatementHandler对象，
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //StatementHandler对象创建stmt,并使用parameterHandler对占位符进行处理
      stmt = prepareStatement(handler, ms.getStatementLog());
      //通过statementHandler对象调用ResultSetHandler将结果集转化为指定对象返回
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

---

doQuery 里的 newStatementHandler

---

先跟 newStatementHandler：
类 Configuration

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
	//创建RoutingStatementHandler对象，实际由statmentType来指定真实的StatementHandler来实现
	StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

StatementHandler

创建了一个 StatementHandler 的引用，对象是 RoutingStatementHandler：

跟 RoutingStatementHandler：

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    //RoutingStatementHandler最主要的功能就是根据mappedStatment的配置，生成一个对应的StatementHandler对象并赋值给delegate
    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

可以看出来这是个静态代理，根据配置选择代理对象。默认是 PREPARED，也就是预编译的。

---

doQuery 里的 prepareStatement

---

回去跟 prepareStatement：

  //创建Statement
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    //获取connection对象的动态代理，添加日志能力；
    Connection connection = getConnection(statementLog);
    //通过不同的StatementHandler，利用connection创建（prepare）Statement
    stmt = handler.prepare(connection, transaction.getTimeout());
    //使用parameterHandler处理占位符
    handler.parameterize(stmt);
    return stmt;
  }

跟 prepare：
类 StatementHandler

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    return delegate.prepare(connection, transactionTimeout);
  }

由前面可以知道，这个 delegate 是 PreparedStatementHandler。但由于这个方法它并没有重写，所以还是得看它的父类 BaseStatementHandler

  @Override
  //使用模板模式，定义了获取Statement的步骤，其子类实现实例化Statement的具体的方式；
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      //通过不同的子类实例化不同的Statement，分为三类：simple(statment)、prepare(prepareStatement)、callable(CallableStatementHandler)
      statement = instantiateStatement(connection);
      //设置超时时间
      setStatementTimeout(statement, transactionTimeout);
      //设置数据集大小
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

跟 instantiateStatement：

这就是一个需要子类重写的骨架方法了

  @Override
  //使用底层的prepareStatement对象来完成对数据库的操作
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    //根据mappedStatement.getKeyGenerator字段，创建prepareStatement
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {//对于insert语句
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
    	//返回数据库生成的主键
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
    	//返回数据库生成的主键填充至keyColumnNames中指定的列
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
     //设置结果集是否可以滚动以及其游标是否可以上下移动，设置结果集是否可更新
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      //创建PrepareStatement对象，这是熟悉的JDBC代码里了
      return connection.prepareStatement(sql);
    }
  }

  //回去看前面最近一条分界线下面的方法，紧接着prepareStatement就是下面这个方法了
  @Override
  //使用parameterHandler对sql语句的占位符进行处理
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

跟 setParameters：

ParameterHandler

对 sql 语句的占位符进行处理

public interface ParameterHandler {

  Object getParameterObject();

  void setParameters(PreparedStatement ps)
      throws SQLException;

}

public class DefaultParameterHandler implements ParameterHandler {

//typeHandler注册中心
  private final TypeHandlerRegistry typeHandlerRegistry;
  //对应的sql节点的信息
  private final MappedStatement mappedStatement;
  //用户传入的参数
  private final Object parameterObject;
  //SQL语句信息，其中还包括占位符和参数名称信息
  private final BoundSql boundSql;
  private final Configuration configuration;

  public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    this.mappedStatement = mappedStatement;
    this.configuration = mappedStatement.getConfiguration();
    this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
    this.parameterObject = parameterObject;
    this.boundSql = boundSql;
  }

  @Override
  public Object getParameterObject() {
    return parameterObject;
  }

  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    //从boundSql中获取sql语句的占位符对应的参数信息
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    //遍历这个参数列表，把参数设置到PreparedStatement中
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {//对于存储过程中的参数不处理
          Object value;//绑定的实参
          String propertyName = parameterMapping.getProperty();//参数的名字
          if (boundSql.hasAdditionalParameter(propertyName)) { // 获取对应的实参值
            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();//从parameterMapping中获取typeHandler对象
          JdbcType jdbcType = parameterMapping.getJdbcType();//获取参数对应的jdbcType
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
        	 //为statment中的占位符绑定参数
            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);
          }
        }
      }
    }
  }
}

---

doQuery 里的 query

---

类 PreparedStatementHandler

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    //JDBC方法
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }

ResultSetHandler

找到映射匹配规则、反射实例化目标对象、根据规则填充属性值

public interface ResultSetHandler {

  <E> List<E> handleResultSets(Statement stmt) throws SQLException;

  <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;

  void handleOutputParameters(CallableStatement cs) throws SQLException;

}

类 DefaultResultSetHandler

  @Override
  public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    //用于保存结果集对象
    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    //statment可能返回多个结果集对象，这里先取出第一个结果集
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    //获取结果集对应resultMap，本质就是获取字段与java属性的映射规则
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);//结果集和resultMap不能为空，为空抛出异常
    while (rsw != null && resultMapCount > resultSetCount) {
     //获取当前结果集对应的resultMap
      ResultMap resultMap = resultMaps.get(resultSetCount);
      //根据映射规则（resultMap）对结果集进行转化，转换成目标对象以后放入multipleResults中
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);//获取下一个结果集
      cleanUpAfterHandlingResultSet();//清空nestedResultObjects对象
      resultSetCount++;
    }
    //获取多结果集。多结果集一般出现在存储过程的执行，存储过程返回多个resultset，
    //mappedStatement.resultSets属性列出多个结果集的名称，用逗号分割；
    //多结果集的处理不是重点
    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);
  }

跟 handleResultSet：

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
    try {
      if (parentMapping != null) {//处理多结果集的嵌套映射
        handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
      } else {
        if (resultHandler == null) {//如果resultHandler为空，实例化一个人默认的resultHandler
          DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
          //对ResultSet进行映射，映射结果暂存在resultHandler中
          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
          //将暂存在resultHandler中的映射结果，填充到multipleResults
          multipleResults.add(defaultResultHandler.getResultList());
        } else {
          //使用指定的rusultHandler进行转换
          handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
        }
      }
    } finally {
      // issue #228 (close resultsets)
      //调用resultset.close()关闭结果集
      closeResultSet(rsw.getResultSet());
    }
  }

跟 handleRowValues：

public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    if (resultMap.hasNestedResultMaps()) {//处理有嵌套resultmap的情况
      ensureNoRowBounds();
      checkResultHandler();
      handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    } else {//处理没有嵌套resultmap的情况
      handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    }
  }

跟 handleRowValuesForSimpleResultMap：

  //简单映射处理
  private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
	//创建结果上下文，所谓的上下文就是专门在循环中缓存结果对象的
    DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
    //1.根据分页信息，定位到指定的记录
    skipRows(rsw.getResultSet(), rowBounds);
    //2.shouldProcessMoreRows判断是否需要映射后续的结果，实际还是翻页处理，避免超过limit
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      //3.进一步完善resultMap信息，主要是处理鉴别器的信息
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      //4.读取resultSet中的一行记录并进行映射，转化并返回目标对象
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      //5.保存映射结果对象
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }