Mybatis源碼之（TypeAliasRegistry）TypeAlias別名實現機制

原文鏈接：http://blog.csdn.net/qq924862077/article/details/52612589

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在Mybatis編程中我們經常會用到將某個bean作為參數類型parameterType或者結果返回值類型ResultType，所以很多時候我們需要把完成的Bean的包名在mapper文件中寫上，如下：

[html]?view plaincopy  
 print?
 <select?id="selectUser"?parameterType="com.dy.entity.User"?resultType="com.dy.entity.User">????
 ????select?*?from?user?where?c_id=#{id}????
 </select>????

Mybatis給我們提供了一種叫別名的機制，意思就是對某個具體的類設置別名，在mybatis的配置文件中配置如下：

[html]?view plaincopy  
 print?
 <configuration>??
 ????<typeAliases>??
 ??????<!--??
 ??????通過package,?可以直接指定package的名字，?mybatis會自動掃描你指定包下面的javabean,??
 ??????并且默認設置一個別名，默認的名字為：?javabean?的首字母小寫的非限定類名來作為它的別名。??
 ??????也可在javabean?加上注解@Alias?來自定義別名，?例如：?@Alias(user)???
 ??????<package?name="com.dy.entity"/>??
 ???????-->?
 
 
 ??????<typeAlias?alias="user"?type="com.dy.entity.User"/>??
 
 
 ??</typeAliases>??
 ????
 ??......??
 ????
 </configuration>??

這樣之后mapper文件中的select可以寫成如下格式：

[html]?view plaincopy  
 print?
 <select?id="selectUser"?parameterType="user"?resultType="user">????
 ????select?*?from?user?where?c_id=#{id}????
 </select>????

這樣就可以在使用某個bean時使用別名就可以了，不需要寫完成的包名+類名

接下來我們介紹TypeAlias別名的實現機制

（1）我們在mybatis的配置文件中配置了typeAliases，我們首先分析XMLConfigBuilder類中對于typeAliases的解析，源碼如下：

[java]?view plaincopy  
 print?
 //類別名解析??
 ??private?void?typeAliasesElement(XNode?parent)?{??
 ????if?(parent?!=?null)?{??
 ??????for?(XNode?child?:?parent.getChildren())?{??
 ????????//如果子節點是package,那么就獲取package節點的name屬性??
 ????????if?("package".equals(child.getName()))?{??
 ??????????String?typeAliasPackage?=?child.getStringAttribute("name");??
 ??????????configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);??
 ????????}?else?{??
 ????????//如果子節點是typeAlias節點，那么就獲取alias屬性和type的屬性??
 ??????????String?alias?=?child.getStringAttribute("alias");??
 ??????????String?type?=?child.getStringAttribute("type");??
 ??????????try?{??
 ????????????//通過type的值來加載獲得類??
 ????????????Class<?>?clazz?=?Resources.classForName(type);??
 ????????????if?(alias?==?null)?{??
 ????????????//typeAliasRegistry會進行別名注冊??
 ??????????????typeAliasRegistry.registerAlias(clazz);??
 ????????????}?else?{??
 ??????????????typeAliasRegistry.registerAlias(alias,?clazz);??
 ????????????}??
 ??????????}?catch?(ClassNotFoundException?e)?{??
 ????????????throw?new?BuilderException("Error?registering?typeAlias?for?'"?+?alias?+?"'.?Cause:?"?+?e,?e);??
 ??????????}??
 ????????}??
 ??????}??
 ????}??

通過分析源碼我們可以得知，解析alias來獲得別名，解析type元素來獲得類名，通過Resources.classForName(type)獲得類信息，然后通過typeAliasRegistry.registerAlias(alias, clazz)將類別名注冊到typeAliasRegistry中，這樣就完成了mybatis中配置文件的解析。

（3）TypeAliasRegistry：是用來記錄別名alias和類clazz之間的對應關系的，它可以看做是一個Map，alias作為key，類名作為value，詳看源碼如下：

[java]?view plaincopy  
 print?
 //其實就是一個map結構，用來對象key別名和value具體的類??
 public?class?TypeAliasRegistry?{??
 ??
 ??private?final?Map<String,?Class<?>>?TYPE_ALIASES?=?new?HashMap<String,?Class<?>>();??
 ??
 ??public?TypeAliasRegistry()?{??
 ????registerAlias("string",?String.class);??
 ??
 ????registerAlias("byte",?Byte.class);??
 ????registerAlias("long",?Long.class);??
 ????registerAlias("short",?Short.class);??
 ????registerAlias("int",?Integer.class);??
 ????registerAlias("integer",?Integer.class);??
 ????registerAlias("double",?Double.class);??
 ????registerAlias("float",?Float.class);??
 ????registerAlias("boolean",?Boolean.class);??
 ??
 ????registerAlias("byte[]",?Byte[].class);??
 ????registerAlias("long[]",?Long[].class);??
 ????registerAlias("short[]",?Short[].class);??
 ????registerAlias("int[]",?Integer[].class);??
 ????registerAlias("integer[]",?Integer[].class);??
 ????registerAlias("double[]",?Double[].class);??
 ????registerAlias("float[]",?Float[].class);??
 ????registerAlias("boolean[]",?Boolean[].class);??
 ??
 ????registerAlias("_byte",?byte.class);??
 ????registerAlias("_long",?long.class);??
 ????registerAlias("_short",?short.class);??
 ????registerAlias("_int",?int.class);??
 ????registerAlias("_integer",?int.class);??
 ????registerAlias("_double",?double.class);??
 ????registerAlias("_float",?float.class);??
 ????registerAlias("_boolean",?boolean.class);??
 ??
 ????registerAlias("_byte[]",?byte[].class);??
 ????registerAlias("_long[]",?long[].class);??
 ????registerAlias("_short[]",?short[].class);??
 ????registerAlias("_int[]",?int[].class);??
 ????registerAlias("_integer[]",?int[].class);??
 ????registerAlias("_double[]",?double[].class);??
 ????registerAlias("_float[]",?float[].class);??
 ????registerAlias("_boolean[]",?boolean[].class);??
 ??
 ????registerAlias("date",?Date.class);??
 ????registerAlias("decimal",?BigDecimal.class);??
 ????registerAlias("bigdecimal",?BigDecimal.class);??
 ????registerAlias("biginteger",?BigInteger.class);??
 ????registerAlias("object",?Object.class);??
 ??
 ????registerAlias("date[]",?Date[].class);??
 ????registerAlias("decimal[]",?BigDecimal[].class);??
 ????registerAlias("bigdecimal[]",?BigDecimal[].class);??
 ????registerAlias("biginteger[]",?BigInteger[].class);??
 ????registerAlias("object[]",?Object[].class);??
 ??
 ????registerAlias("map",?Map.class);??
 ????registerAlias("hashmap",?HashMap.class);??
 ????registerAlias("list",?List.class);??
 ????registerAlias("arraylist",?ArrayList.class);??
 ????registerAlias("collection",?Collection.class);??
 ????registerAlias("iterator",?Iterator.class);??
 ??
 ????registerAlias("ResultSet",?ResultSet.class);??
 ??}??
 ??
 ??@SuppressWarnings("unchecked")??
 ??//?throws?class?cast?exception?as?well?if?types?cannot?be?assigned??
 ??/*?通過別名來找到具體的類**/??
 ??public?<T>?Class<T>?resolveAlias(String?string)?{??
 ????try?{??
 ??????if?(string?==?null)?{??
 ????????return?null;??
 ??????}??
 ??????//?issue?#748??
 ??????String?key?=?string.toLowerCase(Locale.ENGLISH);??
 ??????Class<T>?value;??
 ??????if?(TYPE_ALIASES.containsKey(key))?{??
 ????????value?=?(Class<T>)?TYPE_ALIASES.get(key);??
 ??????}?else?{??
 ????????value?=?(Class<T>)?Resources.classForName(string);??
 ??????}??
 ??????return?value;??
 ????}?catch?(ClassNotFoundException?e)?{??
 ??????throw?new?TypeException("Could?not?resolve?type?alias?'"?+?string?+?"'.??Cause:?"?+?e,?e);??
 ????}??
 ??}??
 ??/*?通過包名注冊類**/??
 ??public?void?registerAliases(String?packageName){??
 ????registerAliases(packageName,?Object.class);??
 ??}??
 ??/*?獲得包內的類，除去內部類和接口**/??
 ??public?void?registerAliases(String?packageName,?Class<?>?superType){??
 ????ResolverUtil<Class<?>>?resolverUtil?=?new?ResolverUtil<Class<?>>();??
 ????resolverUtil.find(new?ResolverUtil.IsA(superType),?packageName);??
 ????Set<Class<??extends?Class<?>>>?typeSet?=?resolverUtil.getClasses();??
 ????for(Class<?>?type?:?typeSet){??
 ??????//?Ignore?inner?classes?and?interfaces?(including?package-info.java)??
 ??????//?Skip?also?inner?classes.?See?issue?#6??
 ??????if?(!type.isAnonymousClass()?&&?!type.isInterface()?&&?!type.isMemberClass())?{??
 ????????registerAlias(type);??
 ??????}??
 ????}??
 ??}??
 ??/*?注冊類**/??
 ??public?void?registerAlias(Class<?>?type)?{??
 ????String?alias?=?type.getSimpleName();??
 ????Alias?aliasAnnotation?=?type.getAnnotation(Alias.class);??
 ????if?(aliasAnnotation?!=?null)?{??
 ??????alias?=?aliasAnnotation.value();??
 ????}???
 ????registerAlias(alias,?type);??
 ??}??
 ??/*?注冊類包括別名和類**/??
 ??public?void?registerAlias(String?alias,?Class<?>?value)?{??
 ????if?(alias?==?null)?{??
 ??????throw?new?TypeException("The?parameter?alias?cannot?be?null");??
 ????}??
 ????//?issue?#748??
 ????String?key?=?alias.toLowerCase(Locale.ENGLISH);??
 ????if?(TYPE_ALIASES.containsKey(key)?&&?TYPE_ALIASES.get(key)?!=?null?&&?!TYPE_ALIASES.get(key).equals(value))?{??
 ??????throw?new?TypeException("The?alias?'"?+?alias?+?"'?is?already?mapped?to?the?value?'"?+?TYPE_ALIASES.get(key).getName()?+?"'.");??
 ????}??
 ????TYPE_ALIASES.put(key,?value);??
 ??}??
 ??/*?注冊類包括別名和類名**/??
 ??public?void?registerAlias(String?alias,?String?value)?{??
 ????try?{??
 ??????registerAlias(alias,?Resources.classForName(value));??
 ????}?catch?(ClassNotFoundException?e)?{??
 ??????throw?new?TypeException("Error?registering?type?alias?"+alias+"?for?"+value+".?Cause:?"?+?e,?e);??
 ????}??
 ??}??
 ????
 ??/**?
 ???*?@since?3.2.2?
 ???*/??
 ??public?Map<String,?Class<?>>?getTypeAliases()?{??
 ????return?Collections.unmodifiableMap(TYPE_ALIASES);??
 ??}??
 ??
 }??

通過上面的源碼我們可以看到，它默認注冊了一些基本的類型基本類和包裝類，然后我們可以調用registerAliases來注冊其他類的別名。

（3）剛才我們看到了TypeAliasRegistry.registerAliases（）函數會登記別名及類名，我們也可以看到TypeAliasRegistry通過了resolveAlias函數來讓我們通過別名alias來獲取實際的類，源碼如下：

[java]?view plaincopy  
 print?
 /*?通過別名來找到具體的類**/??
 ??public?<T>?Class<T>?resolveAlias(String?string)?{??
 ????try?{??
 ??????if?(string?==?null)?{??
 ????????return?null;??
 ??????}??
 ??????//?issue?#748??
 ??????String?key?=?string.toLowerCase(Locale.ENGLISH);??
 ??????Class<T>?value;??
 ??????if?(TYPE_ALIASES.containsKey(key))?{??
 ????????value?=?(Class<T>)?TYPE_ALIASES.get(key);??
 ??????}?else?{??
 ????????value?=?(Class<T>)?Resources.classForName(string);??
 ??????}??
 ??????return?value;??
 ????}?catch?(ClassNotFoundException?e)?{??
 ??????throw?new?TypeException("Could?not?resolve?type?alias?'"?+?string?+?"'.??Cause:?"?+?e,?e);??
 ????}??
 ??}??

總結：這樣我們就對Mybatis的typaAlias的實現機制就有了一個簡單的了解，其實簡單說就是創建了一個Map<string,Class<?>>，解析mybatis的配置文件，將alias元素的值作為Map的key，通過反射機制獲得的type元素對應的類名的類作為Map的value值，在真正使用時通過alias別名來獲取真正的類。

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