Question

Mybatis里面的TypeHandler是做什么的

Answer 1

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
//