# MyBatis 的执行流程

本文基于 MyBatis3.5.5 版本源码

1、Mapper 接口和映射文件是如何进行绑定的
2、MyBatis 中 SQL 语句的执行流程
3、自定义 MyBatis 中的参数设置处理器 typeHandler
4、自定义 MyBatis 中结果集处理器 typeHandler

# 概要

在 MyBatis 中，利用编程式进行数据查询，主要就是下面几行代码：

SqlSession session = sqlSessionFactory.openSession();
UserMapper userMapper = session.getMapper(UserMapper.class);
List<LwUser> userList = userMapper.listUserByUserName("钵钵鸡");

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# 获取 Mapper 接口(getMapper)

1、在调用 getMapper 之后，会去 Configuration 对象中获取 Mapper 对象，因为在项目启动的时候就会把 Mapper 接口加载并解析存储到 Configuration 对象

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

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2、通过 Configuration 对象中的 MapperRegistry 对象属性，继续调用 getMapper 方法

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

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3、根据 type 类型，从 MapperRegistry 对象中的 knownMappers 获取到当前类型对应的代理工厂类，然后通过代理工厂类生成对应 Mapper 的代理类

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);
    }
}

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4、最终获取到我们接口对应的代理类 MapperProxy 对象

# Mapper 接口和映射文件是何时关联的

1、首先我们会手动调用 SqlSessionFactoryBuilder 方法中的 build()方法：

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

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2、然后会构造一个 XMLConfigBuilder 对象，并调用其 parse 方法

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

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3、然后会继续调用自己的 parseConfiguration 来解析配置文件，这里面就会分别去解析全局配置文件的顶级节点

  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
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

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4、继续调用自己的 mapperElement 来解析 mappers 文件

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) { // 1、解析package配置方式
          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) { // 2、解析通过reaource配置全限定路径xml映射方式
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) { // 3、解析通过url配置本地或者远程xml映射方式
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) { // 4、解析通过配置Mapper接口路径方式（class）
            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.");
          }
        }
      }
    }
  }

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5、我们先看第2种和第3中（直接配置 xml 映射文件的解析方式），会构建一个 XMLMapperBuilder 对象并调用其 parse 方法。

  public void parse() {
      // 这里地resource就是xml文件地全路径，com/xxx/xxx/xxx.xml
    if (!configuration.isResourceLoaded(resource)) { // 判断是否加载过
      configurationElement(parser.evalNode("/mapper")); // 解析mapper节点下地9个顶层节点
      configuration.addLoadedResource(resource); // 将加载之后的资源全路径放入Set<String> loadedResources
      bindMapperForNamespace(); // 注册mapper接口
    }

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

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/**
* configurationElement在解析ResultMaps时，因为ResultMaps有extend属性，如果解析到extend属性时，
* 发现父类还没解析ResultMaps，就会抛出IncompleteElementException，并且被加入到Configuration的incompleteResultMaps集合中
* 此处就是解析incompleteResultMaps内还没有解析完成的ResultMap
*/
  private void parsePendingResultMaps() {
    Collection<ResultMapResolver> incompleteResultMaps = configuration.getIncompleteResultMaps();
    synchronized (incompleteResultMaps) {
      Iterator<ResultMapResolver> iter = incompleteResultMaps.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().resolve();
          iter.remove();
        } catch (IncompleteElementException e) {
          // ResultMap is still missing a resource...
        }
      }
    }
  }

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/**
* 和上面类似，因为cache-ref是引用自其他映射文件缓存，所以可能存在它所引用的映射文件还没完成解析时
* 会被加入Configuration到中的incompleteCacheRefs
* 解析未完成的缓存
*/
  private void parsePendingCacheRefs() {
    Collection<CacheRefResolver> incompleteCacheRefs = configuration.getIncompleteCacheRefs();
    synchronized (incompleteCacheRefs) {
      Iterator<CacheRefResolver> iter = incompleteCacheRefs.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().resolveCacheRef();
          iter.remove();
        } catch (IncompleteElementException e) {
          // Cache ref is still missing a resource...
        }
      }
    }
  }

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/**
* Select标签上可以使用useCache,需要确保当前的映射文件缓存所属的映射文件已经被加载，如果没有被加载，则
* 当前的标签会被加入到Configuration中的incompleteStatements
* 此处就是解析未完成的Statements(Sql语句)
*/
  private void parsePendingStatements() {
    Collection<XMLStatementBuilder> incompleteStatements = configuration.getIncompleteStatements();
    synchronized (incompleteStatements) {
      Iterator<XMLStatementBuilder> iter = incompleteStatements.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().parseStatementNode();
          iter.remove();
        } catch (IncompleteElementException e) {
          // Statement is still missing a resource...
        }
      }
    }
  }

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6、解析完映射文件之后，调用自身方法 bindMapperForNamespace，开始绑定 Mapper 接口和映射文件

  private void bindMapperForNamespace() {
    String namespace = builderAssistant.getCurrentNamespace(); // 根据命名空间来获取对应的Mapper接口
    if (namespace != null) {
      Class<?> boundType = null;
      try {
        boundType = Resources.classForName(namespace);
      } catch (ClassNotFoundException e) {
        //ignore, bound type is not required
      }
      if (boundType != null) {
        if (!configuration.hasMapper(boundType)) {
          // Spring may not know the real resource name so we set a flag
          // to prevent loading again this resource from the mapper interface
          // look at MapperAnnotationBuilder#loadXmlResource
          configuration.addLoadedResource("namespace:" + namespace);
          configuration.addMapper(boundType);
        }
      }
    }
  }

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7、调用 Configuration 对象的 addMapper

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

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// MapperRegistry.addMapper
  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(); // 9、注意上面红框里面的代码，又调用了一次parse方法，这个parse方法主要是解析注解，比如下面的语句：
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

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@Select("select * from lw_user")
List<LwUser> listAllUser();

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8、调用 Configuration 对象的属性 MapperRegistry 内的 addMapper 方法，这个方法就是正式将 Mapper 接口添加到 knownMappers，所以上面 getMapper 可以直接获取：
解析完成之后，还会对 Mapper 接口中的方法进行解析，并将每个方法的全限定类名作为 key 存入存入 Configuration 中的 mappedStatements 属性

需要指出的是，这里存储的时候，同一个 value 会存储 2 次，一个全限定名作为 key，另一个就是只用方法名(sql 语句的 id)来作为 key ：事实上如果我们通过接口的方式来编程的话，最后来 getStatement 的时候，都是根据全限定名来取的，所以即使有重名对我们也没有影响，而之所以要这么做的原因其实还是为了兼容早期版本的用法，那就是不通过接口，而是直接通过方法名的方式来进行查询：

# sql 执行流程分析

# 寻找 sql

1、了解代理模式的应该都知道，调用被代理对象的方法之后实际上执行的就是代理对象的 invoke 方法

// org.apache.ibatis.binding.MapperProxy
  @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);
    }
  }

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2、因为我们这里并没有调用 Object 类中的方法，所以肯定走的 else。
- else 中会继续调用 MapperProxy 内部类MapperMethodInvoker中的方法cachedInvoker，这里面会有一个判断，判断一下我们是不是 default 方法，因为 Jdk1.8 中接口中可以新增 default 方法，而 default 方法是并不是一个抽象方法，所以也需要特殊处理

private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
    try {
      return methodCache.computeIfAbsent(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;
    }
  }

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public PlainMethodInvoker(MapperMethod mapperMethod) {
      super();
      this.mapperMethod = mapperMethod;
    }

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public class MapperMethod {

  private final SqlCommand command;
  private final MethodSignature method;
}

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这里面就会把要执行的 sql 语句，请求参数，方法返回值全部解析封装成 MapperMethod 对象，然后后面就可以开始准备执行 sql 语句了

# 执行 sql 语句

1、我们继续上面的流程进入 execute 方法

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;
  }

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2、这里面会根据语句类型以及返回值类型来决定如何执行，本人这里返回的是一个集合，故而我们进入 executeForMany 方法

  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> 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); // 正常情况下
    }
    // 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;
  }

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3、这里面首先会将前面存好的参数进行一次转换，然后绕了这么一圈，回到了起点SqlSession对象，继续调用selectList方法

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      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();
    }
  }

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4、接下来又讲流程委派给了 Execute 去执行 query 方法，最终又会去调用 queryFromDatabase 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);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

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5、到这里之后，终于要进入正题了，一般带了这种do开头的方法就是真正做事的，Spring中很多地方也是采用的这种命名方式：

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
  }

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6、继续进入PreparedStatementHandler对象的query方法，可以看到，这一步就是调用了jdbc操作对象PreparedStatement中的execute方法，最后一步就是转换结果集然后返回。

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

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# 参数映射

// org.apache.ibatis.type.BaseTypeHandler
  @Override
  public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
    if (parameter == null) {
      if (jdbcType == null) {
        throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
      }
      try {
        ps.setNull(i, jdbcType.TYPE_CODE);
      } catch (SQLException e) {
        throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . "
              + "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. "
              + "Cause: " + e, e);
      }
    } else {
      try {
        setNonNullParameter(ps, i, parameter, jdbcType);
      } catch (Exception e) {
        throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . "
              + "Try setting a different JdbcType for this parameter or a different configuration property. "
              + "Cause: " + e, e);
      }
    }
  }

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类型映射

public TypeHandlerRegistry(Configuration configuration) {
    this.unknownTypeHandler = new UnknownTypeHandler(configuration);

    register(Boolean.class, new BooleanTypeHandler());
    register(boolean.class, new BooleanTypeHandler());
    register(JdbcType.BOOLEAN, new BooleanTypeHandler());
    register(JdbcType.BIT, new BooleanTypeHandler());

    register(Byte.class, new ByteTypeHandler());
    register(byte.class, new ByteTypeHandler());
    register(JdbcType.TINYINT, new ByteTypeHandler());

    register(Short.class, new ShortTypeHandler());
    register(short.class, new ShortTypeHandler());
    register(JdbcType.SMALLINT, new ShortTypeHandler());

    register(Integer.class, new IntegerTypeHandler());
    register(int.class, new IntegerTypeHandler());
    register(JdbcType.INTEGER, new IntegerTypeHandler());

    register(Long.class, new LongTypeHandler());
    register(long.class, new LongTypeHandler());

    register(Float.class, new FloatTypeHandler());
    register(float.class, new FloatTypeHandler());
    register(JdbcType.FLOAT, new FloatTypeHandler());

    register(Double.class, new DoubleTypeHandler());
    register(double.class, new DoubleTypeHandler());
    register(JdbcType.DOUBLE, new DoubleTypeHandler());

    register(Reader.class, new ClobReaderTypeHandler());
    register(String.class, new StringTypeHandler());
    register(String.class, JdbcType.CHAR, new StringTypeHandler());
    register(String.class, JdbcType.CLOB, new ClobTypeHandler());
    register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.LONGVARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
    register(JdbcType.CHAR, new StringTypeHandler());
    register(JdbcType.VARCHAR, new StringTypeHandler());
    register(JdbcType.CLOB, new ClobTypeHandler());
    register(JdbcType.LONGVARCHAR, new StringTypeHandler());
    register(JdbcType.NVARCHAR, new NStringTypeHandler());
    register(JdbcType.NCHAR, new NStringTypeHandler());
    register(JdbcType.NCLOB, new NClobTypeHandler());

    register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
    register(JdbcType.ARRAY, new ArrayTypeHandler());

    register(BigInteger.class, new BigIntegerTypeHandler());
    register(JdbcType.BIGINT, new LongTypeHandler());

    register(BigDecimal.class, new BigDecimalTypeHandler());
    register(JdbcType.REAL, new BigDecimalTypeHandler());
    register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
    register(JdbcType.NUMERIC, new BigDecimalTypeHandler());

    register(InputStream.class, new BlobInputStreamTypeHandler());
    register(Byte[].class, new ByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
    register(byte[].class, new ByteArrayTypeHandler());
    register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
    register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.BLOB, new BlobTypeHandler());

    register(Object.class, unknownTypeHandler);
    register(Object.class, JdbcType.OTHER, unknownTypeHandler);
    register(JdbcType.OTHER, unknownTypeHandler);

    register(Date.class, new DateTypeHandler());
    register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
    register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
    register(JdbcType.TIMESTAMP, new DateTypeHandler());
    register(JdbcType.DATE, new DateOnlyTypeHandler());
    register(JdbcType.TIME, new TimeOnlyTypeHandler());

    register(java.sql.Date.class, new SqlDateTypeHandler());
    register(java.sql.Time.class, new SqlTimeTypeHandler());
    register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());

    register(String.class, JdbcType.SQLXML, new SqlxmlTypeHandler());

    register(Instant.class, new InstantTypeHandler());
    register(LocalDateTime.class, new LocalDateTimeTypeHandler());
    register(LocalDate.class, new LocalDateTypeHandler());
    register(LocalTime.class, new LocalTimeTypeHandler());
    register(OffsetDateTime.class, new OffsetDateTimeTypeHandler());
    register(OffsetTime.class, new OffsetTimeTypeHandler());
    register(ZonedDateTime.class, new ZonedDateTimeTypeHandler());
    register(Month.class, new MonthTypeHandler());
    register(Year.class, new YearTypeHandler());
    register(YearMonth.class, new YearMonthTypeHandler());
    register(JapaneseDate.class, new JapaneseDateTypeHandler());

    // issue #273
    register(Character.class, new CharacterTypeHandler());
    register(char.class, new CharacterTypeHandler());
  }

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正是因为MyBatis中默认提供了常用数据类型的映射，所以我们写Sql的时候才可以省略参数映射关系，可以直接采用下面的方式，系统可以根据我们参数的类型，自动选择合适的typeHander进行映射：

select user_id,user_name from lw_user where user_name=#{userName}

select user_id,user_name from lw_user where user_name=#{userName,jdbcType=VARCHAR}

或者说我们可以直接指定typeHandler：

select user_id,user_name from lw_user where user_name=#{userName,jdbcType=VARCHAR,typeHandler=org.apache.ibatis.type.IntegerTypeHandler}

typeHandler

这里因为我们配置了typeHandler，所以会优先以配置的typeHandler为主不会再去读取默认的映射，如果类型不匹配就会直接报错了：

如果我们自己自定义一个typeHandler，然后就可以配置成我们自己的自定义类。所以接下来就让我们看看如何自定义一个typeHandler

# 自定义typeHandler

自定义typeHandler需要实现BaseTypeHandler接口，BaseTypeHandler有4个方法，包括结果集映射，为了节省篇幅，代码没有写上来：

package com.lonelyWolf.mybatis.typeHandler;

import org.apache.ibatis.type.BaseTypeHandler;
import org.apache.ibatis.type.JdbcType;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MyTypeHandler extends BaseTypeHandler<String> {

    @Override
    public void setNonNullParameter(PreparedStatement preparedStatement, int index, String param, JdbcType jdbcType) throws SQLException {
        System.out.println("自定义typeHandler生效了");
        preparedStatement.setString(index,param);
    }
}

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select user_id,user_name from lw_user where user_name=#{userName,jdbcType=VARCHAR,typeHandler=com.lonelyWolf.mybatis.typeHandler.MyTypeHandler}

# 结果集映射

# 自定义typeHandler结果集

package com.lonelyWolf.mybatis.typeHandler;

import org.apache.ibatis.type.BaseTypeHandler;
import org.apache.ibatis.type.JdbcType;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MyTypeHandler extends BaseTypeHandler<String> {

    /**
     * 设置参数
     */
    @Override
    public void setNonNullParameter(PreparedStatement preparedStatement, int index, String param, JdbcType jdbcType) throws SQLException {
        System.out.println("设置参数->自定义typeHandler生效了");
        preparedStatement.setString(index,param);
    }
    /**
     * 根据列名获取结果
     */
    @Override
    public String getNullableResult(ResultSet resultSet, String columnName) throws SQLException {
        System.out.println("根据columnName获取结果->自定义typeHandler生效了");
        return resultSet.getString(columnName);
    }

    /**
     * 根据列的下标来获取结果
     */
    @Override
    public String getNullableResult(ResultSet resultSet, int columnIndex) throws SQLException {
        System.out.println("根据columnIndex获取结果->自定义typeHandler生效了");
        return resultSet.getString(columnIndex);
    }

    /**
     * 处理存储过程的结果集
     */
    @Override
    public String getNullableResult(CallableStatement callableStatement, int columnIndex) throws SQLException {
        return callableStatement.getString(columnIndex);
    }
}

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改写Mapper映射文件配置：

 <resultMap id="MyUserResultMap" type="lwUser">
        <result column="user_id" property="userId" jdbcType="VARCHAR" typeHandler="com.lonelyWolf.mybatis.typeHandler.MyTypeHandler" />
        <result column="user_name" property="userName" jdbcType="VARCHAR" />
    </resultMap>

<select id="listUserByUserName" parameterType="String" resultMap="MyUserResultMap">
        select user_id,user_name from lw_user where user_name=#{userName,jdbcType=VARCHAR,typeHandler=com.lonelyWolf.mybatis.typeHandler.MyTypeHandler}
    </select>

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