深入浅出Mybatis系列（五）---TypeHandler简介及配置（mybatis源码篇）

上篇文章《深入浅出Mybatis系列（四）---配置详解之typeAliases别名（mybatis源码篇）》为大家介绍了mybatis中别名的使用，以及其源码。本篇将为大家介绍TypeHandler， 并简单分析其源码。

Mybatis中的TypeHandler是什么？

　　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>      <!--           当配置package的时候，mybatis会去配置的package扫描TypeHandler          <package name="com.dy.demo"/>       -->            <!-- handler属性直接配置我们要指定的TypeHandler -->      <typeHandler handler=""/>            <!-- javaType 配置java类型，例如String, 如果配上javaType, 那么指定的typeHandler就只作用于指定的类型 -->      <typeHandler javaType="" handler=""/>            <!-- jdbcType 配置数据库基本数据类型，例如varchar, 如果配上jdbcType, 那么指定的typeHandler就只作用于指定的类型  -->      <typeHandler jdbcType="" handler=""/>            <!-- 也可两者都配置 -->      <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============================================================

ExampleTypeHandler:

@MappedJdbcTypes(JdbcType.VARCHAR)  //此处如果不用注解指定jdbcType, 那么，就可以在配置文件中通过"jdbcType"属性指定， 同理， javaType 也可通过 @MappedTypes指定public class ExampleTypeHandler extends BaseTypeHandler<String> {  @Override  public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType) throws SQLException {    ps.setString(i, parameter);  }  @Override  public String getNullableResult(ResultSet rs, String columnName) throws SQLException {    return rs.getString(columnName);  }  @Override  public String getNullableResult(ResultSet rs, int columnIndex) throws SQLException {    return rs.getString(columnIndex);  }  @Override  public String getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {    return cs.getString(columnIndex);  }}

然后，就该配置我们的自定义TypeHandler了：

<configuration>  <typeHandlers>      <!-- 由于自定义的TypeHandler在定义时已经通过注解指定了jdbcType, 所以此处不用再配置jdbcType -->      <typeHandler handler="ExampleTypeHandler"/>  </typeHandlers>    ......  </configuration>

也就是说，我们在自定义TypeHandler的时候，可以在TypeHandler通过@MappedJdbcTypes指定jdbcType, 通过 @MappedTypes 指定javaType, 如果没有使用注解指定，那么我们就需要在配置文件中配置。

好啦，本篇文章到此结束。