Mybatis 系列5-结合源码解析TypeHandler

上篇系列4中为大家介绍了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，先看看怎么配置吧？

配置TypeHandler:

 1 <configuration>
 2     <typeHandlers>
 3       <!-- 
 4           当配置package的时候，mybatis会去配置的package扫描TypeHandler
 5           <package name="com.dy.demo"/>
 6        -->
 7       
 8       <!-- handler属性直接配置我们要指定的TypeHandler -->
 9       <typeHandler handler=""/>
10       
11       <!-- 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 -->
12       <typeHandler javaType="" handler=""/>
13       
14       <!-- 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  -->
15       <typeHandler jdbcType="" handler=""/>
16       
17       <!-- 也可两者都配置 -->
18       <typeHandler javaType="" jdbcType="" handler=""/>
19       
20   </typeHandlers>
21   
22   ......
23   
24 </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：

 1 /**
 2  * 解析typeHandlers节点
 3  */
 4 private void typeHandlerElement(XNode parent) throws Exception {
 5     if (parent != null) {
 6       for (XNode child : parent.getChildren()) {
 7         //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
 8         if ("package".equals(child.getName())) {
 9           String typeHandlerPackage = child.getStringAttribute("name");
10           typeHandlerRegistry.register(typeHandlerPackage);
11         } else {
12           //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
13           //javaType 是指定java类型
14           //jdbcType 是指定jdbc类型（数据库类型： 如varchar）
15           String javaTypeName = child.getStringAttribute("javaType");
16           String jdbcTypeName = child.getStringAttribute("jdbcType");
17           //handler就是我们配置的typeHandler
18           String handlerTypeName = child.getStringAttribute("handler");
19           //resolveClass方法就是我们上篇文章所讲的TypeAliasRegistry里面处理别名的方法
20           Class<?> javaTypeClass = resolveClass(javaTypeName);
21           //JdbcType是一个枚举类型，resolveJdbcType方法是在获取枚举类型的值
22           JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
23           Class<?> typeHandlerClass = resolveClass(handlerTypeName);
24           //注册typeHandler, typeHandler通过TypeHandlerRegistry这个类管理
25           if (javaTypeClass != null) {
26             if (jdbcType == null) {
27               typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
28             } else {
29               typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
30             }
31           } else {
32             typeHandlerRegistry.register(typeHandlerClass);
33           }
34         }
35       }
36     }
37 }

接下来看看TypeHandler的管理注册类：

TypeHandlerRegistry：

  1 /**
  2  * typeHandler注册管理类
  3  */
  4 public final class TypeHandlerRegistry {
  5 
  6   //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
  7 
  8   //基本数据类型与其包装类
  9   private static final Map<Class<?>, Class<?>> reversePrimitiveMap = new HashMap<Class<?>, Class<?>>() {
 10     private static final long serialVersionUID = 1L;
 11     {
 12       put(Byte.class, byte.class);
 13       put(Short.class, short.class);
 14       put(Integer.class, int.class);
 15       put(Long.class, long.class);
 16       put(Float.class, float.class);
 17       put(Double.class, double.class);
 18       put(Boolean.class, boolean.class);
 19       put(Character.class, char.class);
 20     }
 21   };
 22 
 23   //这几个MAP不用说就知道存的是什么东西吧，命名的好处
 24   private final Map<JdbcType, TypeHandler<?>> JDBC_TYPE_HANDLER_MAP = new EnumMap<JdbcType, TypeHandler<?>>(JdbcType.class);
 25   private final Map<Type, Map<JdbcType, TypeHandler<?>>> TYPE_HANDLER_MAP = new HashMap<Type, Map<JdbcType, TypeHandler<?>>>();
 26   private final TypeHandler<Object> UNKNOWN_TYPE_HANDLER = new UnknownTypeHandler(this);
 27   private final Map<Class<?>, TypeHandler<?>> ALL_TYPE_HANDLERS_MAP = new HashMap<Class<?>, TypeHandler<?>>();
 28 
 29   //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
 30   //具体如下
 31   public TypeHandlerRegistry() {
 32     register(Boolean.class, new BooleanTypeHandler());
 33     register(boolean.class, new BooleanTypeHandler());
 34     register(JdbcType.BOOLEAN, new BooleanTypeHandler());
 35     register(JdbcType.BIT, new BooleanTypeHandler());
 36 
 37     register(Byte.class, new ByteTypeHandler());
 38     register(byte.class, new ByteTypeHandler());
 39     register(JdbcType.TINYINT, new ByteTypeHandler());
 40 
 41     register(Short.class, new ShortTypeHandler());
 42     register(short.class, new ShortTypeHandler());
 43     register(JdbcType.SMALLINT, new ShortTypeHandler());
 44 
 45     register(Integer.class, new IntegerTypeHandler());
 46     register(int.class, new IntegerTypeHandler());
 47     register(JdbcType.INTEGER, new IntegerTypeHandler());
 48 
 49     register(Long.class, new LongTypeHandler());
 50     register(long.class, new LongTypeHandler());
 51 
 52     register(Float.class, new FloatTypeHandler());
 53     register(float.class, new FloatTypeHandler());
 54     register(JdbcType.FLOAT, new FloatTypeHandler());
 55 
 56     register(Double.class, new DoubleTypeHandler());
 57     register(double.class, new DoubleTypeHandler());
 58     register(JdbcType.DOUBLE, new DoubleTypeHandler());
 59 
 60     register(String.class, new StringTypeHandler());
 61     register(String.class, JdbcType.CHAR, new StringTypeHandler());
 62     register(String.class, JdbcType.CLOB, new ClobTypeHandler());
 63     register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
 64     register(String.class, JdbcType.LONGVARCHAR, new ClobTypeHandler());
 65     register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
 66     register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
 67     register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
 68     register(JdbcType.CHAR, new StringTypeHandler());
 69     register(JdbcType.VARCHAR, new StringTypeHandler());
 70     register(JdbcType.CLOB, new ClobTypeHandler());
 71     register(JdbcType.LONGVARCHAR, new ClobTypeHandler());
 72     register(JdbcType.NVARCHAR, new NStringTypeHandler());
 73     register(JdbcType.NCHAR, new NStringTypeHandler());
 74     register(JdbcType.NCLOB, new NClobTypeHandler());
 75 
 76     register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
 77     register(JdbcType.ARRAY, new ArrayTypeHandler());
 78 
 79     register(BigInteger.class, new BigIntegerTypeHandler());
 80     register(JdbcType.BIGINT, new LongTypeHandler());
 81 
 82     register(BigDecimal.class, new BigDecimalTypeHandler());
 83     register(JdbcType.REAL, new BigDecimalTypeHandler());
 84     register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
 85     register(JdbcType.NUMERIC, new BigDecimalTypeHandler());
 86 
 87     register(Byte[].class, new ByteObjectArrayTypeHandler());
 88     register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
 89     register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
 90     register(byte[].class, new ByteArrayTypeHandler());
 91     register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
 92     register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
 93     register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
 94     register(JdbcType.BLOB, new BlobTypeHandler());
 95 
 96     register(Object.class, UNKNOWN_TYPE_HANDLER);
 97     register(Object.class, JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);
 98     register(JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);
 99 
100     register(Date.class, new DateTypeHandler());
101     register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
102     register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
103     register(JdbcType.TIMESTAMP, new DateTypeHandler());
104     register(JdbcType.DATE, new DateOnlyTypeHandler());
105     register(JdbcType.TIME, new TimeOnlyTypeHandler());
106 
107     register(java.sql.Date.class, new SqlDateTypeHandler());
108     register(java.sql.Time.class, new SqlTimeTypeHandler());
109     register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());
110 
111     // issue #273
112     register(Character.class, new CharacterTypeHandler());
113     register(char.class, new CharacterTypeHandler());
114   }
115 
116   public boolean hasTypeHandler(Class<?> javaType) {
117     return hasTypeHandler(javaType, null);
118   }
119 
120   public boolean hasTypeHandler(TypeReference<?> javaTypeReference) {
121     return hasTypeHandler(javaTypeReference, null);
122   }
123 
124   public boolean hasTypeHandler(Class<?> javaType, JdbcType jdbcType) {
125     return javaType != null && getTypeHandler((Type) javaType, jdbcType) != null;
126   }
127 
128   public boolean hasTypeHandler(TypeReference<?> javaTypeReference, JdbcType jdbcType) {
129     return javaTypeReference != null && getTypeHandler(javaTypeReference, jdbcType) != null;
130   }
131 
132   public TypeHandler<?> getMappingTypeHandler(Class<? extends TypeHandler<?>> handlerType) {
133     return ALL_TYPE_HANDLERS_MAP.get(handlerType);
134   }
135 
136   public <T> TypeHandler<T> getTypeHandler(Class<T> type) {
137     return getTypeHandler((Type) type, null);
138   }
139 
140   public <T> TypeHandler<T> getTypeHandler(TypeReference<T> javaTypeReference) {
141     return getTypeHandler(javaTypeReference, null);
142   }
143 
144   public TypeHandler<?> getTypeHandler(JdbcType jdbcType) {
145     return JDBC_TYPE_HANDLER_MAP.get(jdbcType);
146   }
147 
148   public <T> TypeHandler<T> getTypeHandler(Class<T> type, JdbcType jdbcType) {
149     return getTypeHandler((Type) type, jdbcType);
150   }
151 
152   public <T> TypeHandler<T> getTypeHandler(TypeReference<T> javaTypeReference, JdbcType jdbcType) {
153     return getTypeHandler(javaTypeReference.getRawType(), jdbcType);
154   }
155 
156   private <T> TypeHandler<T> getTypeHandler(Type type, JdbcType jdbcType) {
157     Map<JdbcType, TypeHandler<?>> jdbcHandlerMap = TYPE_HANDLER_MAP.get(type);
158     TypeHandler<?> handler = null;
159     if (jdbcHandlerMap != null) {
160       handler = jdbcHandlerMap.get(jdbcType);
161       if (handler == null) {
162         handler = jdbcHandlerMap.get(null);
163       }
164     }
165     if (handler == null && type != null && type instanceof Class && Enum.class.isAssignableFrom((Class<?>) type)) {
166       handler = new EnumTypeHandler((Class<?>) type);
167     }
168     @SuppressWarnings("unchecked")
169     // type drives generics here
170     TypeHandler<T> returned = (TypeHandler<T>) handler;
171     return returned;
172   }
173 
174   public TypeHandler<Object> getUnknownTypeHandler() {
175     return UNKNOWN_TYPE_HANDLER;
176   }
177 
178   public void register(JdbcType jdbcType, TypeHandler<?> handler) {
179     JDBC_TYPE_HANDLER_MAP.put(jdbcType, handler);
180   }
181 
182   //
183   // REGISTER INSTANCE
184   //
185 
186   /**
187    * 只配置了typeHandler, 没有配置jdbcType 或者javaType
188    */
189   @SuppressWarnings("unchecked")
190   public <T> void register(TypeHandler<T> typeHandler) {
191     boolean mappedTypeFound = false;
192     //在自定义typeHandler的时候，可以加上注解MappedTypes 去指定关联的javaType
193     //因此，此处需要扫描MappedTypes注解
194     MappedTypes mappedTypes = typeHandler.getClass().getAnnotation(MappedTypes.class);
195     if (mappedTypes != null) {
196       for (Class<?> handledType : mappedTypes.value()) {
197         register(handledType, typeHandler);
198         mappedTypeFound = true;
199       }
200     }
201     // @since 3.1.0 - try to auto-discover the mapped type
202     if (!mappedTypeFound && typeHandler instanceof TypeReference) {
203       try {
204         TypeReference<T> typeReference = (TypeReference<T>) typeHandler;
205         register(typeReference.getRawType(), typeHandler);
206         mappedTypeFound = true;
207       } catch (Throwable t) {
208         // maybe users define the TypeReference with a different type and are not assignable, so just ignore it
209       }
210     }
211     if (!mappedTypeFound) {
212       register((Class<T>) null, typeHandler);
213     }
214   }
215 
216   /**
217    * 配置了typeHandlerhe和javaType
218    */
219   public <T> void register(Class<T> javaType, TypeHandler<? extends T> typeHandler) {
220     register((Type) javaType, typeHandler);
221   }
222 
223   private <T> void register(Type javaType, TypeHandler<? extends T> typeHandler) {
224     //扫描注解MappedJdbcTypes
225     MappedJdbcTypes mappedJdbcTypes = typeHandler.getClass().getAnnotation(MappedJdbcTypes.class);
226     if (mappedJdbcTypes != null) {
227       for (JdbcType handledJdbcType : mappedJdbcTypes.value()) {
228         register(javaType, handledJdbcType, typeHandler);
229       }
230       if (mappedJdbcTypes.includeNullJdbcType()) {
231         register(javaType, null, typeHandler);
232       }
233     } else {
234       register(javaType, null, typeHandler);
235     }
236   }
237 
238   public <T> void register(TypeReference<T> javaTypeReference, TypeHandler<? extends T> handler) {
239     register(javaTypeReference.getRawType(), handler);
240   }
241 
242   /**
243    * typeHandlerhe、javaType、jdbcType都配置了
244    */
245   public <T> void register(Class<T> type, JdbcType jdbcType, TypeHandler<? extends T> handler) {
246     register((Type) type, jdbcType, handler);
247   }
248 
249   /**
250    * 注册typeHandler的核心方法
251    * 就是向Map新增数据而已
252    */
253   private void register(Type javaType, JdbcType jdbcType, TypeHandler<?> handler) {
254     if (javaType != null) {
255       Map<JdbcType, TypeHandler<?>> map = TYPE_HANDLER_MAP.get(javaType);
256       if (map == null) {
257         map = new HashMap<JdbcType, TypeHandler<?>>();
258         TYPE_HANDLER_MAP.put(javaType, map);
259       }
260       map.put(jdbcType, handler);
261       if (reversePrimitiveMap.containsKey(javaType)) {
262         register(reversePrimitiveMap.get(javaType), jdbcType, handler);
263       }
264     }
265     ALL_TYPE_HANDLERS_MAP.put(handler.getClass(), handler);
266   }
267 
268   //
269   // REGISTER CLASS
270   //
271 
272   // Only handler type
273 
274   public void register(Class<?> typeHandlerClass) {
275     boolean mappedTypeFound = false;
276     MappedTypes mappedTypes = typeHandlerClass.getAnnotation(MappedTypes.class);
277     if (mappedTypes != null) {
278       for (Class<?> javaTypeClass : mappedTypes.value()) {
279         register(javaTypeClass, typeHandlerClass);
280         mappedTypeFound = true;
281       }
282     }
283     if (!mappedTypeFound) {
284       register(getInstance(null, typeHandlerClass));
285     }
286   }
287 
288   // java type + handler type
289 
290   public void register(Class<?> javaTypeClass, Class<?> typeHandlerClass) {
291     register(javaTypeClass, getInstance(javaTypeClass, typeHandlerClass));
292   }
293 
294   // java type + jdbc type + handler type
295 
296   public void register(Class<?> javaTypeClass, JdbcType jdbcType, Class<?> typeHandlerClass) {
297     register(javaTypeClass, jdbcType, getInstance(javaTypeClass, typeHandlerClass));
298   }
299 
300   // Construct a handler (used also from Builders)
301 
302   @SuppressWarnings("unchecked")
303   public <T> TypeHandler<T> getInstance(Class<?> javaTypeClass, Class<?> typeHandlerClass) {
304     if (javaTypeClass != null) {
305       try {
306         Constructor<?> c = typeHandlerClass.getConstructor(Class.class);
307         return (TypeHandler<T>) c.newInstance(javaTypeClass);
308       } catch (NoSuchMethodException ignored) {
309         // ignored
310       } catch (Exception e) {
311         throw new TypeException("Failed invoking constructor for handler " + typeHandlerClass, e);
312       }
313     }
314     try {
315       Constructor<?> c = typeHandlerClass.getConstructor();
316       return (TypeHandler<T>) c.newInstance();
317     } catch (Exception e) {
318       throw new TypeException("Unable to find a usable constructor for " + typeHandlerClass, e);
319     }
320   }
321 
322  
323   /**
324    * 根据指定的pacakge去扫描自定义的typeHander，然后注册
325    */
326   public void register(String packageName) {
327     ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
328     resolverUtil.find(new ResolverUtil.IsA(TypeHandler.class), packageName);
329     Set<Class<? extends Class<?>>> handlerSet = resolverUtil.getClasses();
330     for (Class<?> type : handlerSet) {
331       //Ignore inner classes and interfaces (including package-info.java) and abstract classes
332       if (!type.isAnonymousClass() && !type.isInterface() && !Modifier.isAbstract(type.getModifiers())) {
333         register(type);
334       }
335     }
336   }
337   
338   // get information
339   
340   /**
341    * 通过configuration对象可以获取已注册的所有typeHandler
342    */
343   public Collection<TypeHandler<?>> getTypeHandlers() {
344     return Collections.unmodifiableCollection(ALL_TYPE_HANDLERS_MAP.values());
345   }
346   
347 }

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:

 1 @MappedJdbcTypes(JdbcType.VARCHAR)  
 2 //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
 3 public class ExampleTypeHandler extends BaseTypeHandler<String> {
 4 
 5   @Override
 6   public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType) throws SQLException {
 7     ps.setString(i, parameter);
 8   }
 9 
10   @Override
11   public String getNullableResult(ResultSet rs, String columnName) throws SQLException {
12     return rs.getString(columnName);
13   }
14 
15   @Override
16   public String getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
17     return rs.getString(columnIndex);
18   }
19 
20   @Override
21   public String getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
22     return cs.getString(columnIndex);
23   }
24 }

然后，就该配置我们的自定义TypeHandler了：

1 <configuration>
2   <typeHandlers>
3       <!-- 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 -->
4       <typeHandler handler="ExampleTypeHandler"/>
5   </typeHandlers>
6   
7   ......
8   
9 </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。

预知后事如何，请听下回分解。

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原文地址：https://www.cnblogs.com/yizhiamumu/p/8996904.html