Mybatis 3源码跟踪

CREATE TABLE `user` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4;

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/UserMapper.xml"/>
    </mappers>
</configuration>

   @Test
    public void sqlSession() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml");
        org.apache.ibatis.session.SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(resourceAsStream);
        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
            UserMapper mapper = sqlSession.getMapper(UserMapper.class);
            List<User> users = mapper.listAll();
            System.out.println("users = " + users);
        }

        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
            //为什么listAll会可以; 和后面的StrictMap有关系,
            List listAll = (List) sqlSession.selectList("listAll", null);
            System.out.println("listAll = " + listAll);
        }
    }

  public SqlSessionFactory build(InputStream inputStream) {
    return build(inputStream, null, null);
  }
  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

public class XMLConfigBuilder extends BaseBuilder {

    //--- 下面三个是BaseBuilder的变量
  protected final Configuration configuration;
  protected final TypeAliasRegistry typeAliasRegistry;
  protected final TypeHandlerRegistry typeHandlerRegistry;
    //---

  private boolean parsed;
  private final XPathParser parser;
  private String environment;
  private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();


  public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

  private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      // 这里xml中可以配置多个environment,但是元素的id和environments一致的第一个会被真正的使用
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));

      //这里是类型转化; 可以自定义,比如枚举就适合写一个handler
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

// 看源码有一部分原因是对xml有些好奇; 这里是通过XMLMapperBuiler进行xml解析的
  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            try(InputStream inputStream = Resources.getResourceAsStream(resource)) {
            //我们一般创建xml与mapper文件的时候会在这里解析
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
              mapperParser.parse();
            }
            // 这里亮瞎了我的眼,mybatis竟然支持remote资源;点进去后发现远端的必须的是xml格式的;
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            try(InputStream inputStream = Resources.getUrlAsStream(url)){
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
              mapperParser.parse();
            }
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

}

public class Configuration {

  protected Environment environment;

  protected boolean safeRowBoundsEnabled;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase;
  protected boolean aggressiveLazyLoading;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys;
  protected boolean useColumnLabel = true;
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls;
  protected boolean useActualParamName = true;
  protected boolean returnInstanceForEmptyRow;
  protected boolean shrinkWhitespacesInSql;

  protected String logPrefix;
  protected Class<? extends Log> logImpl;
  protected Class<? extends VFS> vfsImpl;
  protected Class<?> defaultSqlProviderType;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<>(Arrays.asList("equals", "clone", "hashCode", "toString"));
  protected Integer defaultStatementTimeout;
  protected Integer defaultFetchSize;
  protected ResultSetType defaultResultSetType;
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
  protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;

  protected Properties variables = new Properties();
  protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();

  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://github.com/mybatis/old-google-code-issues/issues/300'>Issue 300 (google code)</a>
   */
  protected Class<?> configurationFactory;

  protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
  protected final InterceptorChain interceptorChain = new InterceptorChain();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry(this);
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
      .conflictMessageProducer((savedValue, targetValue) ->
          ". please check " + savedValue.getResource() + " and " + targetValue.getResource());
  protected final Map<String, Cache> caches = new StrictMap<>("Caches collection");
  protected final Map<String, ResultMap> resultMaps = new StrictMap<>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<>("XML fragments parsed from previous mappers");

  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<>();

  /*
   * A map holds cache-ref relationship. The key is the namespace that
   * references a cache bound to another namespace and the value is the
   * namespace which the actual cache is bound to.
   */
  protected final Map<String, String> cacheRefMap = new HashMap<>();


  public void addMappers(String packageName, Class<?> superType) {
    mapperRegistry.addMappers(packageName, superType);
  }

  public void addMappers(String packageName) {
    mapperRegistry.addMappers(packageName);
  }

  public <T> void addMapper(Class<T> type) {
    mapperRegistry.addMapper(type);
  }

  // 这里是将有哪些mapper的映射一一留存下来;比如我们需要使用UserMapper, 那么就从knownMappers中获取这个类的动态代理类;代理类后面分析
  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 {
        knownMappers.put(type, new MapperProxyFactory<>(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.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
}

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;
}

  public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      //解析xml文件,mapper是root节点
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
      bindMapperForNamespace();
    }

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

  private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.isEmpty()) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
      //针对具体的每一个语句进行解析
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

  //语句解析
   private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
        //后面分析下这个类
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

public class XMLStatementBuilder extends BaseBuilder {

    //注意这个类也是继承了BaseBuilder类; 因此下面三个变量也会存在的
  protected final Configuration configuration;
  protected final TypeAliasRegistry typeAliasRegistry;
  protected final TypeHandlerRegistry typeHandlerRegistry;


  private final MapperBuilderAssistant builderAssistant;
  private final XNode context;
  private final String requiredDatabaseId;


  public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);

    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
      resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }
}

    @Override
    @SuppressWarnings("unchecked")
    public V put(String key, V value) {
      if (containsKey(key)) {
        throw new IllegalArgumentException(name + " already contains value for " + key
            + (conflictMessageProducer == null ? "" : conflictMessageProducer.apply(super.get(key), value)));
      }
      if (key.contains(".")) {
        final String shortKey = getShortName(key);
        if (super.get(shortKey) == null) {
          super.put(shortKey, value);
        } else {
          super.put(shortKey, (V) new Ambiguity(shortKey));
        }
      }
      return super.put(key, value);
    }

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


  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      //这里我们因为是mysql环境; 所以对应的是JdbcTransactionFactory
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //tx = JdbcTransaction
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);
      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();
    }
  }

public class JdbcTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(JdbcTransaction.class);

  protected Connection connection;
  protected DataSource dataSource;
  protected TransactionIsolationLevel level;
  protected boolean autoCommit;

    @Override
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

 protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommit);
  }

  //后面分析close的时候sqlSession的close触发executor的close,executor的close触发了事务的close, 事务的colse触发了这个jdbc的close
  @Override
  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      connection.close();
    }
  }
}

public class DefaultSqlSession implements SqlSession {

  //executeForMany的执行
  private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public void close() {
    try {
      executor.close(isCommitOrRollbackRequired(false));
      closeCursors();
      dirty = false;
    } finally {
      ErrorContext.instance().reset();
    }
  }

}
//这里偷个懒; 直接将这个类粘贴到这里
public class CachingExecutor implements Executor {
  @Override
  public void close(boolean forceRollback) {
    try {
      // issues #499, #524 and #573
      if (forceRollback) {
        tcm.rollback();
      } else {
        //对于autoCommit=false的; close的时候进行了提交,完成闭环
        tcm.commit();
      }
    } finally {
        //这里-> BaseExecutor.close,内部会关闭事务,而事务的关闭会关闭连接
      delegate.close(forceRollback);
    }
  }
}

public class MapperRegistry {

  private final Configuration config;
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }
    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 {
        knownMappers.put(type, new MapperProxyFactory<>(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.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
}

  public void parse() {
    String resource = type.toString();
    if (!configuration.isResourceLoaded(resource)) {
      loadXmlResource();
      configuration.addLoadedResource(resource);
      assistant.setCurrentNamespace(type.getName());
      parseCache();
      parseCacheRef();
      for (Method method : type.getMethods()) {
        if (!canHaveStatement(method)) {
          continue;
        }
        if (getAnnotationWrapper(method, false, Select.class, SelectProvider.class).isPresent()
            && method.getAnnotation(ResultMap.class) == null) {
          parseResultMap(method);
        }
        try {
          //如果有Select,Update,SelectProvider等注解形式的方法注解进行解析并存放到configuration的mappedStatements中
          parseStatement(method);
        } catch (IncompleteElementException e) {
          configuration.addIncompleteMethod(new MethodResolver(this, method));
        }
      }
    }
    parsePendingMethods();
  }

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -4724728412955527868L;
  private static final int ALLOWED_MODES = MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
      | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC;
  private static final Constructor<Lookup> lookupConstructor;
  private static final Method privateLookupInMethod;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethodInvoker> methodCache;

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else {
        return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
  }

  private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
    try {
      return MapUtil.computeIfAbsent(methodCache, method, m -> {
        if (m.isDefault()) {
          try {
            if (privateLookupInMethod == null) {
              return new DefaultMethodInvoker(getMethodHandleJava8(method));
            } else {
              return new DefaultMethodInvoker(getMethodHandleJava9(method));
            }
          } catch (IllegalAccessException | InstantiationException | InvocationTargetException
              | NoSuchMethodException e) {
            throw new RuntimeException(e);
          }
        } else {
          return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
        }
      });
    } catch (RuntimeException re) {
      Throwable cause = re.getCause();
      throw cause == null ? re : cause;
    }
  }
}

public class MapperMethod {

    //类创建时会通过Configuration来获取MappedStatement,进一步可以获取到command
  private final SqlCommand command;
  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;
    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()) {
          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 {
          Object param = method.convertArgsToSqlCommandParam(args);
          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 method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

  // 这里sqlSession中存在configuration,因此执行是可以通过configuration获取到MappedStatement来执行sql查询
  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      //注意 sqlSession里面有configuration ,所以什么都可以拿到
      result = sqlSession.selectList(command.getName(), param, rowBounds);
    } else {
      result = sqlSession.selectList(command.getName(), param);
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }