Mybatis中的TypeHandler源码解析(四)

无论是 MyBatis 在预处理语句（PreparedStatement）中设置一个参数时，还是从结果集中取出一个值时，都会用类型处理器将获取的值以合适的方式转换成 Java 类型。Mybatis默认为我们实现了许多TypeHandler, 当我们没有配置指定TypeHandler时，Mybatis会根据参数或者返回结果的不同，默认为我们选择合适的TypeHandler处理。

那么，Mybatis为我们实现了哪些TypeHandler呢? 我们怎么自定义实现一个TypeHandler ? 这些都会在接下来的mybatis的源码中看到。

在看源码之前，还是像之前一样，先看看怎么配置吧？

配置TypeHandler:

<configuration>
    <typeHandlers>
      
      
      
      <typeHandler handler=""/>
      
      
      <typeHandler javaType="" handler=""/>
      
      
      <typeHandler jdbcType="" handler=""/>
      
      
      <typeHandler javaType="" jdbcType="" handler=""/>
      
  typeHandlers>
  
  ......
  
configuration>

上面简单介绍了一下TypeHandler, 下面就看看mybatis中TypeHandler的源码了。

=我是源码分割线==

老规矩，先从对xml的解析讲起：

/**
 * 解析typeHandlers节点
 */
private void typeHandlerElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        //子节点为package时，获取其name属性的值，然后自动扫描package下的自定义typeHandler
        if ("package".equals(child.getName())) {
          String typeHandlerPackage = child.getStringAttribute("name");
          typeHandlerRegistry.register(typeHandlerPackage);
        } else {
          //子节点为typeHandler时， 可以指定javaType属性， 也可以指定jdbcType, 也可两者都指定
          //javaType 是指定java类型
          //jdbcType 是指定jdbc类型（数据库类型： 如varchar）
          String javaTypeName = child.getStringAttribute("javaType");
          String jdbcTypeName = child.getStringAttribute("jdbcType");
          //handler就是我们配置的typeHandler
          String handlerTypeName = child.getStringAttribute("handler");
          //resolveClass方法就是我们上篇文章所讲的TypeAliasRegistry里面处理别名的方法
          Class<?> javaTypeClass = resolveClass(javaTypeName);
          //JdbcType是一个枚举类型，resolveJdbcType方法是在获取枚举类型的值
          JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
          Class<?> typeHandlerClass = resolveClass(handlerTypeName);
          //注册typeHandler, typeHandler通过TypeHandlerRegistry这个类管理
          if (javaTypeClass != null) {
            if (jdbcType == null) {
              typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
            } else {
              typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
            }
          } else {
            typeHandlerRegistry.register(typeHandlerClass);
          }
        }
      }
    }
}

接下来看看TypeHandler的管理注册类：

TypeHandlerRegistry：

/**
 * typeHandler注册管理类
 */
public final class TypeHandlerRegistry {

  //源码一上来，二话不说，几个大大的HashMap就出现，这不又跟上次讲的typeAliases的注册类似么

  //基本数据类型与其包装类
  private static final Map<Class<?>, Class<?>> reversePrimitiveMap = new HashMap<Class<?>, Class<?>>() {
    private static final long serialVersionUID = 1L;
    {
      put(Byte.class, byte.class);
      put(Short.class, short.class);
      put(Integer.class, int.class);
      put(Long.class, long.class);
      put(Float.class, float.class);
      put(Double.class, double.class);
      put(Boolean.class, boolean.class);
      put(Character.class, char.class);
    }
  };

  //这几个MAP不用说就知道存的是什么东西吧，命名的好处
  private final Map<JdbcType, TypeHandler<?>> JDBC_TYPE_HANDLER_MAP = new EnumMap<JdbcType, TypeHandler<?>>(JdbcType.class);
  private final Map<Type, Map<JdbcType, TypeHandler<?>>> TYPE_HANDLER_MAP = new HashMap<Type, Map<JdbcType, TypeHandler<?>>>();
  private final TypeHandler<Object> UNKNOWN_TYPE_HANDLER = new UnknownTypeHandler(this);
  private final Map<Class<?>, TypeHandler<?>> ALL_TYPE_HANDLERS_MAP = new HashMap<Class<?>, TypeHandler<?>>();

  //就像上篇文章讲的typeAliases一样，mybatis也默认给我们注册了不少的typeHandler
  //具体如下
  public TypeHandlerRegistry() {
    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(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 ClobTypeHandler());
    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 ClobTypeHandler());
    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(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, UNKNOWN_TYPE_HANDLER);
    register(Object.class, JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);
    register(JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);

    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());

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

  public boolean hasTypeHandler(Class<?> javaType) {
    return hasTypeHandler(javaType, null);
  }

  public boolean hasTypeHandler(TypeReference<?> javaTypeReference) {
    return hasTypeHandler(javaTypeReference, null);
  }

  public boolean hasTypeHandler(Class<?> javaType, JdbcType jdbcType) {
    return javaType != null && getTypeHandler((Type) javaType, jdbcType) != null;
  }

  public boolean hasTypeHandler(TypeReference<?> javaTypeReference, JdbcType jdbcType) {
    return javaTypeReference != null && getTypeHandler(javaTypeReference, jdbcType) != null;
  }

  public TypeHandler<?> getMappingTypeHandler(Class<? extends TypeHandler<?>> handlerType) {
    return ALL_TYPE_HANDLERS_MAP.get(handlerType);
  }

  public <T> TypeHandler<T> getTypeHandler(Class<T> type) {
    return getTypeHandler((Type) type, null);
  }

  public <T> TypeHandler<T> getTypeHandler(TypeReference<T> javaTypeReference) {
    return getTypeHandler(javaTypeReference, null);
  }

  public TypeHandler<?> getTypeHandler(JdbcType jdbcType) {
    return JDBC_TYPE_HANDLER_MAP.get(jdbcType);
  }

  public <T> TypeHandler<T> getTypeHandler(Class<T> type, JdbcType jdbcType) {
    return getTypeHandler((Type) type, jdbcType);
  }

  public <T> TypeHandler<T> getTypeHandler(TypeReference<T> javaTypeReference, JdbcType jdbcType) {
    return getTypeHandler(javaTypeReference.getRawType(), jdbcType);
  }

  private <T> TypeHandler<T> getTypeHandler(Type type, JdbcType jdbcType) {
    Map<JdbcType, TypeHandler<?>> jdbcHandlerMap = TYPE_HANDLER_MAP.get(type);
    TypeHandler<?> handler = null;
    if (jdbcHandlerMap != null) {
      handler = jdbcHandlerMap.get(jdbcType);
      if (handler == null) {
        handler = jdbcHandlerMap.get(null);
      }
    }
    if (handler == null && type != null && type instanceof Class && Enum.class.isAssignableFrom((Class<?>) type)) {
      handler = new EnumTypeHandler((Class<?>) type);
    }
    @SuppressWarnings("unchecked")
    // type drives generics here
    TypeHandler<T> returned = (TypeHandler<T>) handler;
    return returned;
  }

  public TypeHandler<Object> getUnknownTypeHandler() {
    return UNKNOWN_TYPE_HANDLER;
  }

  public void register(JdbcType jdbcType, TypeHandler<?> handler) {
    JDBC_TYPE_HANDLER_MAP.put(jdbcType, handler);
  }

  //
  // REGISTER INSTANCE
  //

  /**
   * 只配置了typeHandler, 没有配置jdbcType 或者javaType
   */
  @SuppressWarnings("unchecked")
  public <T> void register(TypeHandler<T> typeHandler) {
    boolean mappedTypeFound = false;
    //在自定义typeHandler的时候，可以加上注解MappedTypes 去指定关联的javaType
    //因此，此处需要扫描MappedTypes注解
    MappedTypes mappedTypes = typeHandler.getClass().getAnnotation(MappedTypes.class);
    if (mappedTypes != null) {
      for (Class<?> handledType : mappedTypes.value()) {
        register(handledType, typeHandler);
        mappedTypeFound = true;
      }
    }
    // @since 3.1.0 - try to auto-discover the mapped type
    if (!mappedTypeFound && typeHandler instanceof TypeReference) {
      try {
        TypeReference<T> typeReference = (TypeReference<T>) typeHandler;
        register(typeReference.getRawType(), typeHandler);
        mappedTypeFound = true;
      } catch (Throwable t) {
        // maybe users define the TypeReference with a different type and are not assignable, so just ignore it
      }
    }
    if (!mappedTypeFound) {
      register((Class<T>) null, typeHandler);
    }
  }

  /**
   * 配置了typeHandlerhe和javaType
   */
  public <T> void register(Class<T> javaType, TypeHandler<? extends T> typeHandler) {
    register((Type) javaType, typeHandler);
  }

  private <T> void register(Type javaType, TypeHandler<? extends T> typeHandler) {
    //扫描注解MappedJdbcTypes
    MappedJdbcTypes mappedJdbcTypes = typeHandler.getClass().getAnnotation(MappedJdbcTypes.class);
    if (mappedJdbcTypes != null) {
      for (JdbcType handledJdbcType : mappedJdbcTypes.value()) {
        register(javaType, handledJdbcType, typeHandler);
      }
      if (mappedJdbcTypes.includeNullJdbcType()) {
        register(javaType, null, typeHandler);
      }
    } else {
      register(javaType, null, typeHandler);
    }
  }

  public <T> void register(TypeReference<T> javaTypeReference, TypeHandler<? extends T> handler) {
    register(javaTypeReference.getRawType(), handler);
  }

  /**
   * typeHandlerhe、javaType、jdbcType都配置了
   */
  public <T> void register(Class<T> type, JdbcType jdbcType, TypeHandler<? extends T> handler) {
    register((Type) type, jdbcType, handler);
  }

  /**
   * 注册typeHandler的核心方法
   * 就是向Map新增数据而已
   */
  private void register(Type javaType, JdbcType jdbcType, TypeHandler<?> handler) {
    if (javaType != null) {
      Map<JdbcType, TypeHandler<?>> map = TYPE_HANDLER_MAP.get(javaType);
      if (map == null) {
        map = new HashMap<JdbcType, TypeHandler<?>>();
        TYPE_HANDLER_MAP.put(javaType, map);
      }
      map.put(jdbcType, handler);
      if (reversePrimitiveMap.containsKey(javaType)) {
        register(reversePrimitiveMap.get(javaType), jdbcType, handler);
      }
    }
    ALL_TYPE_HANDLERS_MAP.put(handler.getClass(), handler);
  }

  //
  // REGISTER CLASS
  //

  // Only handler type

  public void register(Class<?> typeHandlerClass) {
    boolean mappedTypeFound = false;
    MappedTypes mappedTypes = typeHandlerClass.getAnnotation(MappedTypes.class);
    if (mappedTypes != null) {
      for (Class<?> javaTypeClass : mappedTypes.value()) {
        register(javaTypeClass, typeHandlerClass);
        mappedTypeFound = true;
      }
    }
    if (!mappedTypeFound) {
      register(getInstance(null, typeHandlerClass));
    }
  }

  // java type + handler type

  public void register(Class<?> javaTypeClass, Class<?> typeHandlerClass) {
    register(javaTypeClass, getInstance(javaTypeClass, typeHandlerClass));
  }

  // java type + jdbc type + handler type

  public void register(Class<?> javaTypeClass, JdbcType jdbcType, Class<?> typeHandlerClass) {
    register(javaTypeClass, jdbcType, getInstance(javaTypeClass, typeHandlerClass));
  }

  // Construct a handler (used also from Builders)

  @SuppressWarnings("unchecked")
  public <T> TypeHandler<T> getInstance(Class<?> javaTypeClass, Class<?> typeHandlerClass) {
    if (javaTypeClass != null) {
      try {
        Constructor<?> c = typeHandlerClass.getConstructor(Class.class);
        return (TypeHandler<T>) c.newInstance(javaTypeClass);
      } catch (NoSuchMethodException ignored) {
        // ignored
      } catch (Exception e) {
        throw new TypeException("Failed invoking constructor for handler " + typeHandlerClass, e);
      }
    }
    try {
      Constructor<?> c = typeHandlerClass.getConstructor();
      return (TypeHandler<T>) c.newInstance();
    } catch (Exception e) {
      throw new TypeException("Unable to find a usable constructor for " + typeHandlerClass, e);
    }
  }
  /**
   * 根据指定的pacakge去扫描自定义的typeHander，然后注册
   */
  public void register(String packageName) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(TypeHandler.class), packageName);
    Set<Class<? extends Class<?>>> handlerSet = resolverUtil.getClasses();
    for (Class<?> type : handlerSet) {
      //Ignore inner classes and interfaces (including package-info.java) and abstract classes
      if (!type.isAnonymousClass() && !type.isInterface() && !Modifier.isAbstract(type.getModifiers())) {
        register(type);
      }
    }
  }
  
  // get information
  
  /**
   * 通过configuration对象可以获取已注册的所有typeHandler
   */
  public Collection<TypeHandler<?>> getTypeHandlers() {
    return Collections.unmodifiableCollection(ALL_TYPE_HANDLERS_MAP.values());
  }
  
}

由源码可以看到， mybatis为我们实现了那么多TypeHandler, 随便打开一个TypeHandler，看其源码，都可以看到，它继承自一个抽象类：BaseTypeHandler， 那么我们是不是也能通过继承BaseTypeHandler，从而实现自定义的TypeHandler ? 答案是肯定的， 那么现在下面就为大家演示一下自定义TypeHandler:

=自定义TypeHandler分割线========

自定义 TypeHandler

自定义类型处理器是通过实现 org.apache.ibatis.type.TypeHandler 接口实现的。这个接口定义了类型处理器的基本功能，接口定义如下所示。

public interface TypeHandler<T> {
    void setParameter(PreparedStatement var1, int var2, T var3, JdbcType var4) throws SQLException;

    T getResult(ResultSet var1, String var2) throws SQLException;

    T getResult(ResultSet var1, int var2) throws SQLException;

    T getResult(CallableStatement var1, int var2) throws SQLException;
}

其中 setParameter 方法用于把 java 对象设置到 PreparedStatement 的参数中，getResult 方法用于从 ResultSet（根据列名或者索引位置获取） 或 CallableStatement（根据存储过程获取） 中取出数据转换为 java 对象。

实际开发中，我们可以继承 org.apache.ibatis.type.BaseTypeHandler 类型来实现自定义类型处理器。这个类型是抽象类型，实现了 TypeHandler 的方法进行通用流程的封装，做了异常处理，并定义了几个类似的抽象方法，如下所示。继承 BaseTypeHandler 类型可以极大地降低开发难度。

类型转换器还可以通过注解配置 java 类型和 jdbc 类型：

@MappedTypes：注解配置 java 类型
@MappedJdbcTypes：注解配置 jdbc 类型

自定义枚举类型处理器示例：

自定义一个枚举基类

/**
 * @description:
 * @author: zhangcb
 * @create: 2020-04-18 15:42
 **/
public interface EnumBase<T> extends Serializable {

    public T getCode();

    public String getDesc();

}

定义一个枚举类UserStatus

public enum UserStatus implements EnumBase<Integer> {

    DISABLED(0, "禁用"),
    ENABLED(1, "启用")
    ;

    private int code;
    private String desc;

    UserStatus(int code, String desc) {
        this.code = code;
        this.desc = desc;
    }

    @Override
    public Integer getCode() {
        return code;
    }

    @Override
    public String getDesc() {
        return desc;
    }
}

自定义枚举类型处理器

/**
 * @description: 自定义枚举类型处理器
 * @author: zhangcb
 * @create: 2020-04-18 15:49
 **/

/***
 * 类型转换器还可以通过注解配置 java 类型和 jdbc 类型,也可以在mybaits配置文件里面配置
 */
//@MappedJdbcTypes(JdbcType.TINYINT)
//@MappedTypes(UserStatus.class)
public class EnumTypeHandler extends BaseTypeHandler<EnumBase> {

    private Class<EnumBase> type;

    private final EnumBase[] enums;

    public EnumTypeHandler(Class<EnumBase> type) {
        if (type == null)
            throw new IllegalArgumentException("Type argument cannot be null");
        this.type = type;
        this.enums = type.getEnumConstants();
        if (this.enums == null)
            throw new IllegalArgumentException(type.getSimpleName()
                    + " does not represent an enum type.");
    }

    @Override
    public EnumBase getNullableResult(ResultSet rs, String columnName) throws SQLException {
        int i = rs.getInt(columnName);

        if (rs.wasNull()) {
            return null;
        } else {
            return convert(i);
        }
    }

    @Override
    public EnumBase getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        int i = rs.getInt(columnIndex);
        if (rs.wasNull()) {
            return null;
        } else {
            return convert(i);
        }
    }

    @Override
    public EnumBase getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        int i = cs.getInt(columnIndex);
        if (cs.wasNull()) {
            return null;
        } else {
            return convert(i);
        }
    }

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, EnumBase parameter, JdbcType jdbcType)
            throws SQLException {
        ps.setInt(i, Integer.parseInt(parameter.getCode() + ""));

    }

    private EnumBase convert(int code) {
        for (EnumBase status : enums) {
            if (Integer.parseInt(status.getCode().toString()) == code) {
                return status;
            }
        }
        throw new IllegalArgumentException("未知的枚举类型!");
    }
}

Javabean，Dao层，mapper层处理忽略，将自定义的类型处理器配置到程序中即可。

public class User implements Serializable {
    private Long id;

    private String name;

    private Integer age;

    private Integer sex;

    private UserStatus status;
    ...
}

把TypeHandler配置到程序中有三种方法：

1.在Mapper.xml中声明

<resultMap id="BaseResultMap" type="com.github.chengbin.mybatisdemo.entity.User">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="age" jdbcType="INTEGER" property="age" />
    <result column="sex" jdbcType="TINYINT" property="sex" />
    <result column="status" jdbcType="TINYINT" property="status" javaType="UserStatus" typeHandler="EnumTypeHandler" />
  resultMap>

2.在mybatis配置文件中设置

<typeAliases>
		<typeAlias alias="UserStatus" type="com.github.chengbin.mybatisdemo.enums.UserStatus" />
		<typeAlias alias="EnumTypeHandler" type="com.github.chengbin.mybatisdemo.enums.EnumTypeHandler" />
	
	typeAliases>

	<typeHandlers>
		<typeHandler handler="com.github.chengbin.mybatisdemo.enums.EnumTypeHandler" javaType="com.github.chengbin.mybatisdemo.enums.UserStatus"/>
	typeHandlers>

3.在springboot的yml配置文件中设置类型处理器所在的包名

mybatis:
  type-handlers-package: com.xxx.handler

源码分析

MyBatis 启动后首先把 typeHandler 注册进去。首先尝试读取 MappedTypes 注解，如果有这个注解定义了 java 类型，则把这个类型处理器注册到相应的 java 类型的处理器中。如果没有使用注解，但是继承了 TypeReference 类型，比如前面提到的 BaseTypeHandler，则通过 TypeReference 的接口获取原始类型注册到相应的 java 类型的处理器中。如果实在是获取不到 java 类型，则按照无类型处理。

public <T> void register(TypeHandler<T> typeHandler) {
    boolean mappedTypeFound = false;
    MappedTypes mappedTypes = typeHandler.getClass()
        .getAnnotation(MappedTypes.class);
    if (mappedTypes != null) {
      for (Class<?> handledType : mappedTypes.value()) {
        register(handledType, typeHandler);
        mappedTypeFound = true;
      }
    }
    // @since 3.1.0 - try to auto-discover the mapped type
    if (!mappedTypeFound && typeHandler instanceof TypeReference) {
      try {
        TypeReference<T> typeReference = (TypeReference<T>) typeHandler;
        register(typeReference.getRawType(), typeHandler);
        mappedTypeFound = true;
      } catch (Throwable t) {
        // maybe users define the TypeReference with a different type and 
are not assignable, so just ignore it
      }
    }
    if (!mappedTypeFound) {
      register((Class<T>) null, typeHandler);
    }
  }

MyBatis 在预处理语句设置参数时调用 TypeHandler 进行 java 对象到 jdbc 的 PreparedStatement 参数值的转换。以下为其中一个调用片段。

TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
    jdbcType = configuration.getJdbcTypeForNull();
}
try {
    typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException 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);
}

MyBatis 查询数据库完成后，调用 TypeHandler 的方法读取数据转换成 java 对象。以下为其中一个调用片段。

private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, 
  ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
  throws SQLException {
    if (propertyMapping.getNestedQueryId() != null) {
        return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, 
            lazyLoader, columnPrefix);
    } else if (propertyMapping.getResultSet() != null) {
        addPendingChildRelation(rs, metaResultObject, propertyMapping);   
        // TODO is that OK?
        return DEFERED;
    } else {
        final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
        final String column = prependPrefix(propertyMapping.getColumn(), 
            columnPrefix);
        return typeHandler.getResult(rs, column);
    }
}