An in-depth explanation of the Mybatis series (4)---detailed configuration of typeAliases aliases (mybatis source code)

The previous article "In-depth introduction to Mybatis series (3)---Detailed configuration of properties and environments (mybatis source code)" introduced properties and environments. This article continues to talk about one of the remaining configuration nodes: typeAliases. The typeAliases node is mainly used to set aliases. In fact, this is a very useful function. By configuring aliases, we no longer need to specify the complete package name, and can also take aliases.

For example: When we use com.demo.entity.UserEntity, we can directly configure an alias user, so that when we use com.demo.entity.UserEntity in the configuration file in the future, we can use it directly Just User.

Taking the above example as an example, let’s implement it and see how typeAliases is configured:

<configuration>
    <typeAliases>
      <!--
      通过package, 可以直接指定package的名字， mybatis会自动扫描你指定包下面的javabean,
      并且默认设置一个别名，默认的名字为： javabean 的首字母小写的非限定类名来作为它的别名。
      也可在javabean 加上注解@Alias 来自定义别名， 例如： @Alias(user) 
      <package name="com.dy.entity"/>       -->
      <typeAlias alias="UserEntity" type="com.dy.entity.User"/>
  </typeAliases>
  
  ......  
</configuration>

Write another test code to see if it takes effect: (I only write a pseudocode )

Configuration con = sqlSessionFactory.getConfiguration();
Map<String, Class<?>> typeMap = con.getTypeAliasRegistry().getTypeAliases();
for(Entry<String, Class<?>> entry: typeMap.entrySet()) {
    System.out.println(entry.getKey() + " ================> " + entry.getValue().getSimpleName());
}

========================================== I am the source code Dividing line==============================================

The above gives you a brief introduction to the usage of typeAliases. Next, let’s take a look at the source code in Mybatis:

Old rules, let’s start with the parsing of xml:

 1 /** 
 2  * 解析typeAliases节点 
 3  */ 
 4 private void typeAliasesElement(XNode parent) {
 5     if (parent != null) { 
 6       for (XNode child : parent.getChildren()) { 
 7         //如果子节点是package, 那么就获取package节点的name属性， mybatis会扫描指定的package 
 8         if ("package".equals(child.getName())) { 
 9           String typeAliasPackage = child.getStringAttribute("name");
 10           //TypeAliasRegistry 负责管理别名， 这儿就是通过TypeAliasRegistry 进行别名注册， 下面就会看看TypeAliasRegistry源码
 11           configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
 12         } else {
 13           //如果子节点是typeAlias节点，那么就获取alias属性和type的属性值
 14           String alias = child.getStringAttribute("alias");
 15           String type = child.getStringAttribute("type");
 16           try {
 17             Class<?> clazz = Resources.classForName(type);
 18             if (alias == null) {
 19               typeAliasRegistry.registerAlias(clazz);
 20             } else {
 21               typeAliasRegistry.registerAlias(alias, clazz);
 22             }
 23           } catch (ClassNotFoundException e) {
 24             throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
 25           }
 26         }
 27       }
 28     }
 29   }

The important source code is here:

TypeAliasRegistry:

public class TypeAliasRegistry {  
  //这就是核心所在啊， 原来别名就仅仅通过一个HashMap来实现， key为别名， value就是别名对应的类型（class对象）
  private final Map<String, Class<?>> TYPE_ALIASES = new HashMap<String, Class<?>>();  /**
   * 以下就是mybatis默认为我们注册的别名   */
  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);
  }  
  /**
   * 处理别名， 直接从保存有别名的hashMap中取出即可   */
  @SuppressWarnings("unchecked")  public <T> Class<T> resolveAlias(String string) {    
  try {      
  if (string == null) return null;
      String key = string.toLowerCase(Locale.ENGLISH); // issue #748
      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 &#39;" + string + "&#39;.  Cause: " + e, e);
    }
  }  
  /**
   * 配置文件中配置为package的时候， 会调用此方法，根据配置的报名去扫描javabean ，然后自动注册别名
   * 默认会使用 Bean 的首字母小写的非限定类名来作为它的别名
   * 也可在javabean 加上注解@Alias 来自定义别名， 例如： @Alias(user)   */
  public void registerAliases(String packageName){
    registerAliases(packageName, Object.class);
  }  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);
      }
    }
  }  public void registerAlias(Class<?> type) {
    String alias = type.getSimpleName();
    Alias aliasAnnotation = type.getAnnotation(Alias.class);    
    if (aliasAnnotation != null) {
      alias = aliasAnnotation.value();
    } 
    registerAlias(alias, type);
  }  //这就是注册别名的本质方法， 其实就是向保存别名的hashMap新增值而已， 呵呵， 别名的实现太简单了，对吧
  public void registerAlias(String alias, Class<?> value) {    
  if (alias == null) throw new TypeException("The parameter alias cannot be null");
    String key = alias.toLowerCase(Locale.ENGLISH); // issue #748
    if (TYPE_ALIASES.containsKey(key) && TYPE_ALIASES.get(key) != null && !TYPE_ALIASES.get(key).equals(value)) {      
    throw new TypeException("The alias &#39;" + alias + "&#39; is already mapped to the value &#39;" + TYPE_ALIASES.get(key).getName() + "&#39;.");
    }
    TYPE_ALIASES.put(key, value);
  }  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);
    }
  }  
  /**
   * 获取保存别名的HashMap, Configuration对象持有对TypeAliasRegistry的引用，因此，如果需要，我们可以通过Configuration对象获取   */
  public Map<String, Class<?>> getTypeAliases() {    
  return Collections.unmodifiableMap(TYPE_ALIASES);
  }
}

It can be seen from the source code that the principle of setting alias is so simple. Mybatis sets it for us by default. There are many aliases, which can be seen in the above code.

Okay, the content of this article is that simple, that’s it. The next article will continue to explain the configuration nodes that have not been finished yet.

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