Mybatis源码分析别名实现机制。

在Mybatis中我们经常会用到将某个bean作为参数类型parameterType或者结果返回值类型ResultType，所以很多时候我们需要把完成的Bean的包名在mapper文件中

<select id="selectUser" parameterType="com.test.entity.User" resultType="com.test.entity.User">  
    select * from user where c_id=#{id}  
</select>  

Mybatis给我们提供了一种叫别名的机制，意思就是对某个具体的类设置别名，在mybatis的配置文件中配置如下：

<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.test.entity"/>
       -->
      <typeAlias alias="user" type="com.test.entity.User"/>
  </typeAliases>

  ......

</configuration>

这样之后mapper文件中的select可以写成如下格式：

<select id="selectUser" parameterType="user" resultType="user">  
    select * from user where c_id=#{id}  
</select>  

Mybatis相关的解析源码：

//类别名解析

private void typeAliasesElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
		//如果子节点是package,那么就获取package节点的name属性
        if ("package".equals(child.getName())) {
          String typeAliasPackage = child.getStringAttribute("name");
          configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
        } else {
		//如果子节点是typeAlias节点，那么就获取alias属性和type的属性
          String alias = child.getStringAttribute("alias");
          String type = child.getStringAttribute("type");
          try {
			//通过type的值来加载获得类
            Class<?> clazz = Resources.classForName(type);
            if (alias == null) {
			//typeAliasRegistry会进行别名注册
              typeAliasRegistry.registerAlias(clazz);
            } else {
              typeAliasRegistry.registerAlias(alias, clazz);
            }
          } catch (ClassNotFoundException e) {
            throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
          }
        }
      }
    }

解析alias来获得别名，解析type元素来获得类名，通过Resources.classForName(type)获得类信息，然后通过typeAliasRegistry.registerAlias(alias, clazz)将类别名注册到typeAliasRegistry中，这样就完成了mybatis中配置文件的解析。


TypeAliasRegistry类源码分析
TypeAliasRegistry：是用来记录别名alias和类clazz之间的对应关系的，它可以看做是一个Map，
alias作为key，类名作为value，详看源码如下：

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
  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);
    }
  }



  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);
  }

  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);
  }

  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);
  }

}

解析mybatis的配置文件，将alias元素的值作为Map的key，通过反射机制获得的type元素对应的类名的类作为Map的value值，在真正使用时通过alias别名来获取真正的类。

但是如果配置package情况要复杂的多，下面看下配置package的处理过程。

 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);
      }
    }
  }

我们继续分析ResolverUtil.java的方法，这里IsA是个闭包，里面主要实现一个类视为为parent的子类。 find函数会遍历这个package下面所有的class，如果是Object的子类并切能被classloader加载
缓存起来， resolverUtil.getClasses()就是返回我们缓存的class。

public static class IsA implements Test {
    private Class<?> parent;

    /** Constructs an IsA test using the supplied Class as the parent class/interface. */
    public IsA(Class<?> parentType) {
      this.parent = parentType;
    }

    @Override
    public boolean matches(Class<?> type) {
      return type != null && parent.isAssignableFrom(type);
    }

    @Override
    public String toString() {
      return "is assignable to " + parent.getSimpleName();
    }
  }


   public ResolverUtil<T> find(Test test, String packageName) {
    String path = getPackagePath(packageName);

    try {
      List<String> children = VFS.getInstance().list(path);
      for (String child : children) {
        if (child.endsWith(".class")) {
          addIfMatching(test, child);
        }
      }
    } catch (IOException ioe) {
      log.error("Could not read package: " + packageName, ioe);
    }

    return this;
  }

  protected String getPackagePath(String packageName) {
    return packageName == null ? null : packageName.replace('.', '/');
  }

   public List<String> list(String path) throws IOException {
    List<String> names = new ArrayList<String>();
    for (URL url : getResources(path)) {
      names.addAll(list(url, path));
    }
    return names;
  }

       protected static List<URL> getResources(String path) throws IOException {
    		return Collections.list(Thread.currentThread().getContextClassLoader().getResources(path));
 	 }



protected void addIfMatching(Test test, String fqn) {
    try {
      String externalName = fqn.substring(0, fqn.indexOf('.')).replace('/', '.');
      ClassLoader loader = getClassLoader();
      if (log.isDebugEnabled()) {
        log.debug("Checking to see if class " + externalName + " matches criteria [" + test + "]");
      }

      Class<?> type = loader.loadClass(externalName);
      if (test.matches(type)) {
        matches.add((Class<T>) type);
      }
    } catch (Throwable t) {
      log.warn("Could not examine class '" + fqn + "'" + " due to a " +
          t.getClass().getName() + " with message: " + t.getMessage());
    }
  }