面試官：爲什麼使用 Mapper 接口就能對數據庫進行訪問？

宏觀理解

拿到的 mapper 其實是一個代理對象，底層調用的是 SqlSession 的方法，SqlSession 的方法裏其實調用的 Executor 的方法。

源碼驗證

從 Mapper 到 SqlSession

跟 getMapper：
類 SqlSession

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

每一個 SqlSession 都包含一個 Configuration 的引用，所有的配置信息都被解讀在裏面了。

跟 getMapper：
類 Configuration

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

MapperRegistry

public class MapperRegistry {

  private final Configuration config;//config對象，mybatis全局唯一的
  //記錄了mapper接口與對應MapperProxyFactory之間的關係
  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);
  }

  //將mapper接口的工廠類添加到mapper註冊中心
  //XMLMapperBuilder裏的parse()裏的bindMapperForNamespace()裏的
  //configuration.addMapper(boundType)調用
  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 {
    	//實例化Mapper接口的代理工程類，並將信息添加至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.
        //解析接口上的註解信息，並添加至configuration對象
        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對象
  private final Class<T> mapperInterface;
  //key是mapper接口中的某個方法的method對象，value是對應的MapperMethod，MapperMethod對象不記錄任何狀態信息，所以它可以在多個代理對象之間共享
  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) {
	//創建實現了mapper接口的動態代理對象
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
	 //每次調用都會創建新的MapperProxy對象，MapperProxy繼承了InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

可以看出來最後是拿到的代理對象。

MapperProxy

學過 JDK 的動態代理就會知道：調用 mapper 接口的方法，其實就是調用這個對象裏的 invoke 方法

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;//記錄關聯的SqlSession對象
  private final Class<T> mapperInterface;//mapper接口對應的class對象；
  //key是mapper接口中的某個方法的method對象，value是對應的MapperMethod，MapperMethod對象不記錄任何狀態信息，所以它可以在多個代理對象之間共享
  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本身的方法不增強
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //從緩存中獲取mapperMethod對象，如果緩存中沒有，則創建一個，並添加到緩存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //調用execute方法執行sql，可以看出並沒有執行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

封裝了 mapper 接口中對應方法的信息，以及對應的 sql 語句的信息
它是 mapper 接口與映射配置文件中 sql 語句的橋樑
MapperMethod 對象不記錄任何狀態信息，所以它可以在多個代理對象之間共享

它有3個內部類：

• SqlCommand ： 從 configuration 中獲取方法的命名空間.方法名以及 sql 語句的類型
• MethodSignature：封裝 mapper 接口方法的相關信息（入參，返回類型）
• ParamNameResolver： 解析 mapper 接口方法中的入參，將多個參數轉成 Map

public class MapperMethod {
  //從configuration中獲取方法的命名空間.方法名以及SQL語句的類型（SELECT等）
  private final SqlCommand command;
  //封裝mapper接口方法的相關信息（入參，返回類型）
  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;
    //根據sql語句類型以及接口返回的參數選擇調用不同的
    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()) {//返回值爲void
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {//返回值爲集合或者數組
          //這個方法裏面其實就是調用的SqlSession的方法
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {//返回值爲map
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {//返回值爲遊標
          result = executeForCursor(sqlSession, args);
        } else {//處理返回爲單一對象的情況
          //通過參數解析器解析解析參數
          Object param = method.convertArgsToSqlCommandParam(args);
          //調用SqlSession的方法，重點看
          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
}

從 SqlSession 到 Executor

之前的代碼已經看到，當返回對象爲單一對象的情況，調用的是 selectOne 方法

但最終，會調用到 selectList 方法：

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //從configuration中獲取要執行的sql語句的配置信息
      MappedStatement ms = configuration.getMappedStatement(statement);
      //通過executor執行語句，並返回指定的結果集
      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：
類 Executor

會調哪個方法呢？

SqlSession sqlSession = sqlSessionFactory.openSession();

這是我們獲取 sqlSession 的代碼
跟 openSession：
類 DefaultSqlSessionFactory：

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

跟 openSessionFromDataSource：

  //從數據源獲取數據庫連接
  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
    	//獲取mybatis配置文件中的environment對象
      final Environment environment = configuration.getEnvironment();
      //從environment獲取transactionFactory對象
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //創建事務對象
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //根據配置創建executor
      final Executor executor = configuration.newExecutor(tx, execType);
      //創建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" />節點，
    //通過裝飾器，添加二級緩存的能力
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //通過interceptorChain遍歷所有的插件爲executor增強，添加插件的功能
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

看見沒有，裝飾器模式，會給你的 executor，套上 CachingExecutor。
而 cacheEnabled 在 Configuration 裏默認是 true 的。

而有了這些，並不是真的開啓了二級緩存。要開啓二級緩存，得在 Mapper.xml 文件里加上 <cache> 相關標籤，這個會在 Mybatis 初始化時，被 XMLMapperBuilder 解析到，交給 builderAssistant 創建 Cache 並存儲到 Configuration 的一個 Map裏，以 namespace 爲 key。

詳情請看 Mybatis 的初始化與建造者模式

CachingExecutor

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	//獲取sql語句信息，佔位符，參數等信息
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //拼裝緩存的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 {
	//從MappedStatement中獲取二級緩存
    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);//從二級緩存中獲取數據
        if (list == null) {
          //二級緩存爲空，纔會調用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);
  }

開啓了二級緩存，而沒有，或者沒開啓二級緩存，都會執行 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) {//檢查當前executor是否關閉
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {//非嵌套查詢，並且FlushCache配置爲true，則需要清空一級緩存
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;//查詢層次加一
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;//查詢以及緩存
      if (list != null) {
    	 //針對調用存儲過程的結果處理
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
    	 //緩存未命中，從數據庫加載數據
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }


    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {//延遲加載處理
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {//如果當前sql的一級緩存配置爲STATEMENT，查詢完既清空一集緩存
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

  //真正訪問數據庫獲取結果的方法
  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);//在緩存中添加佔位符
    try {
      //調用抽象方法doQuery，方法查詢數據庫並返回結果，可選的實現包括：simple、reuse、batch
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);//在緩存中刪除佔位符
    }
    localCache.putObject(key, list);//將真正的結果對象添加到一級緩存
    if (ms.getStatementType() == StatementType.CALLABLE) {//如果是調用存儲過程
      localOutputParameterCache.putObject(key, parameter);//緩存輸出類型結果參數
    }
    return list;
  }

跟 doQuery：

可以看出，這是一個模板方法，模板模式的體現。默認的是 SimpleExecutor 實現。

BaseExecutor 定義了一套骨架流程：

紅色的部分就是不同的 doQuery 實現。

SimpleExecutor

  @Override
  //查詢的實現
  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();//獲取configuration對象
      //創建StatementHandler對象，
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //StatementHandler對象創建stmt,並使用parameterHandler對佔位符進行處理
      stmt = prepareStatement(handler, ms.getStatementLog());
      //通過statementHandler對象調用ResultSetHandler將結果集轉化爲指定對象返回
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

---

doQuery 裏的 newStatementHandler

---

先跟 newStatementHandler：
類 Configuration

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
	//創建RoutingStatementHandler對象，實際由statmentType來指定真實的StatementHandler來實現
	StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

StatementHandler

創建了一個 StatementHandler 的引用，對象是 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據mappedStatment的配置，生成一個對應的StatementHandler對象並賦值給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());
    }

  }

可以看出來這是個靜態代理，根據配置選擇代理對象。默認是 PREPARED，也就是預編譯的。

---

doQuery 裏的 prepareStatement

---

回去跟 prepareStatement：

  //創建Statement
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    //獲取connection對象的動態代理，添加日誌能力；
    Connection connection = getConnection(statementLog);
    //通過不同的StatementHandler，利用connection創建（prepare）Statement
    stmt = handler.prepare(connection, transaction.getTimeout());
    //使用parameterHandler處理佔位符
    handler.parameterize(stmt);
    return stmt;
  }

跟 prepare：
類 StatementHandler

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

由前面可以知道，這個 delegate 是 PreparedStatementHandler。但由於這個方法它並沒有重寫，所以還是得看它的父類 BaseStatementHandler

  @Override
  //使用模板模式，定義了獲取Statement的步驟，其子類實現實例化Statement的具體的方式；
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      //通過不同的子類實例化不同的Statement，分爲三類：simple(statment)、prepare(prepareStatement)、callable(CallableStatementHandler)
      statement = instantiateStatement(connection);
      //設置超時時間
      setStatementTimeout(statement, transactionTimeout);
      //設置數據集大小
      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：

這就是一個需要子類重寫的骨架方法了

  @Override
  //使用底層的prepareStatement對象來完成對數據庫的操作
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    //根據mappedStatement.getKeyGenerator字段，創建prepareStatement
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {//對於insert語句
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
    	//返回數據庫生成的主鍵
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
    	//返回數據庫生成的主鍵填充至keyColumnNames中指定的列
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
     //設置結果集是否可以滾動以及其遊標是否可以上下移動，設置結果集是否可更新
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      //創建PrepareStatement對象，這是熟悉的JDBC代碼裏了
      return connection.prepareStatement(sql);
    }
  }

  //回去看前面最近一條分界線下面的方法，緊接着prepareStatement就是下面這個方法了
  @Override
  //使用parameterHandler對sql語句的佔位符進行處理
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

跟 setParameters：

ParameterHandler

對 sql 語句的佔位符進行處理

public interface ParameterHandler {

  Object getParameterObject();

  void setParameters(PreparedStatement ps)
      throws SQLException;

}

public class DefaultParameterHandler implements ParameterHandler {

//typeHandler註冊中心
  private final TypeHandlerRegistry typeHandlerRegistry;
  //對應的sql節點的信息
  private final MappedStatement mappedStatement;
  //用戶傳入的參數
  private final Object parameterObject;
  //SQL語句信息，其中還包括佔位符和參數名稱信息
  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());
    //從boundSql中獲取sql語句的佔位符對應的參數信息
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    //遍歷這個參數列表，把參數設置到PreparedStatement中
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {//對於存儲過程中的參數不處理
          Object value;//綁定的實參
          String propertyName = parameterMapping.getProperty();//參數的名字
          if (boundSql.hasAdditionalParameter(propertyName)) { // 獲取對應的實參值
            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();//從parameterMapping中獲取typeHandler對象
          JdbcType jdbcType = parameterMapping.getJdbcType();//獲取參數對應的jdbcType
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
        	 //爲statment中的佔位符綁定參數
            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 裏的 query

---

類 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

找到映射匹配規則、反射實例化目標對象、根據規則填充屬性值

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;

}

類 DefaultResultSetHandler

  @Override
  public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    //用於保存結果集對象
    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    //statment可能返回多個結果集對象，這裏先取出第一個結果集
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    //獲取結果集對應resultMap，本質就是獲取字段與java屬性的映射規則
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);//結果集和resultMap不能爲空，爲空拋出異常
    while (rsw != null && resultMapCount > resultSetCount) {
     //獲取當前結果集對應的resultMap
      ResultMap resultMap = resultMaps.get(resultSetCount);
      //根據映射規則（resultMap）對結果集進行轉化，轉換成目標對象以後放入multipleResults中
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);//獲取下一個結果集
      cleanUpAfterHandlingResultSet();//清空nestedResultObjects對象
      resultSetCount++;
    }
    //獲取多結果集。多結果集一般出現在存儲過程的執行，存儲過程返回多個resultset，
    //mappedStatement.resultSets屬性列出多個結果集的名稱，用逗號分割；
    //多結果集的處理不是重點
    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) {//處理多結果集的嵌套映射
        handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
      } else {
        if (resultHandler == null) {//如果resultHandler爲空，實例化一個人默認的resultHandler
          DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
          //對ResultSet進行映射，映射結果暫存在resultHandler中
          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
          //將暫存在resultHandler中的映射結果，填充到multipleResults
          multipleResults.add(defaultResultHandler.getResultList());
        } else {
          //使用指定的rusultHandler進行轉換
          handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
        }
      }
    } finally {
      // issue #228 (close resultsets)
      //調用resultset.close()關閉結果集
      closeResultSet(rsw.getResultSet());
    }
  }

跟 handleRowValues：

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

跟 handleRowValuesForSimpleResultMap：

  //簡單映射處理
  private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
	//創建結果上下文，所謂的上下文就是專門在循環中緩存結果對象的
    DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
    //1.根據分頁信息，定位到指定的記錄
    skipRows(rsw.getResultSet(), rowBounds);
    //2.shouldProcessMoreRows判斷是否需要映射後續的結果，實際還是翻頁處理，避免超過limit
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      //3.進一步完善resultMap信息，主要是處理鑑別器的信息
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      //4.讀取resultSet中的一行記錄並進行映射，轉化並返回目標對象
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      //5.保存映射結果對象
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }