Mybatis中Mapper动态代理的实现原理

Posted 2022-12-03 想作会飞的鱼

一、概述

我们知道，Mybatis实现增删改查需要进行XML的配置，其基本的配置如下：

<?xml version="1.0" encoding="UTF-8" ?>  
<!DOCTYPE mapper  
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">  
<mapper namespace="org.mybatis.example.BlogMapper">  
  <select id="selectBlog" resultType="Blog">  
    select * from Blog where id = #id  
  </select>  
</mapper>  

上面的配置表示我们在命名空间org.mybatis.example.BlogMapper下定义一个ID为selectBlog的查询操作，其操作结果集为Blog，对应的语句为select * from Blog where id = #id

那么在实际使用时，我们可以采用如下形式：

SqlSession session = sqlSessionFactory.openSession();  
try   
  Blog blog = (Blog) session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);  
 finally   
  session.close();  
  

这种方式有很明显的缺点就是通过字符串去调用标签定义的SQL，第一容易出错，第二是当XML当中的id修改过以后你不知道在程序当中有多少个地方使用了这个ID,需要手工查找并一一修改。

在Mybatis的新版本中做了一些改进，支持这种方式调用：定义一个接口 方法名，参数需要与XML定义保持一致。

• 1、新建一个com.kang.mapper的包，定义map接口，接口名任意，这里是UserMapper。

package com.kang.mapper;  


import java.util.List;  
import com.kang.pojo.User;  
public interface UserMapper   
    //根据用户id查询用户信息  
    public User findUserById(int id) throws Exception;  
    //查询用户列表  
    public List<User> findUserByUsername(String username) throws Exception;  
    //添加用户信息  
    public void insertUser(User user)throws Exception;   
  

• 2、配置xml文件 UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>  
<!DOCTYPE mapper  
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">  
<mapper namespace="com.kang.mapper.UserMapper">  
<!-- 注意这里的 namespace必须对应着map接口的全类名-->  
    <select id="findUserById" parameterType="int" resultType="user">  
        select * from user where id = #id  
    </select>  

    <select id="findUserByUsername" parameterType="java.lang.String"  
        resultType="user">  
        select * from user where username like '%$value%'  
    </select>  

    <insert id="insertUser" parameterType="user">  
        <selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">  
            select LAST_INSERT_ID()  
        </selectKey>  
        insert into user(username,birthday,sex,address)  
        values(#username,#birthday,#sex,#address)  
    </insert>  
</mapper>  

注意到配置文件中的id属性值和map接口中的方法名是一一对应的。

• 3、在SqlMapConfig.xml中加入映射文件

<!-- 加载 映射文件 -->  
    <mappers>  
        <mapper resource="map/UserMapper.xml" />  
    </mappers>  

• 4、调用方法

        //获取session  
        SqlSession session = sqlSessionFactory.openSession();  
        //获取mapper接口的代理对象  
        UserMapper userMapper = session.getMapper(UserMapper.class);  
        //调用代理对象方法  
        User user = userMapper.findUserById(27);  
        System.out.println(user);  
        //关闭session  
        session.close();  
        System.out.println("---------执行完毕-----------");  

这样以来当我们修改了XML的ID以后，只需要修改接口中的方法就可以了，编译器会在其他使用该接口的地方报错，很容易进行修改。当然好处还不只这些，还可以通过与spring进行无缝集成，动态注入 等等。
关于Mybatis的Mapp接口使用方法请参考这篇博文Mybatis之Mapper动态代理
在上面的例子当中呢，UserMapper是一个接口 它并没有实现类，为什么接口可以直接使用呢? 那是因为MyBbatis使用了JDK动态代理机制动态生成了代理类，那么代理类又是如何多SqlSession进行封装的呢?带着这些疑问，让我们通过分析源代码的方式来解释这些问题。

二、源码解析

Mybatis关于包装Mapper的代码都在org.apache.ibatis.binding 这个包下面。其中有4个类：
MapperRegistry 类是注册Mapper接口与获取代理类实例的工具类。其源码如下：

package org.apache.ibatis.binding;
import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
import org.apache.ibatis.io.ResolverUtil;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
//这个类通过名字就可以看出 是用来注册Mapper接口与获取生成代理类实例的工具类 
public class MapperRegistry 
  //全局配置文件对象
  private Configuration config;
  //一个HashMap Key是mapper的类型对象, Value是MapperProxyFactory对象
  //这个MapperProxyFactory是创建Mapper代理对象的工厂 我们一会再分析
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
  public MapperRegistry(Configuration config) 
    this.config = config;
  
  //获取生成的代理对象
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) 
    //通过Mapper的接口类型 去Map当中查找 如果为空就抛异常
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null)
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    try 
      //否则创建一个当前接口的代理对象 并且传入sqlSession
      return mapperProxyFactory.newInstance(sqlSession);
     catch (Exception e) 
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    
  

  public <T> boolean hasMapper(Class<T> type) 
    return knownMappers.containsKey(type);
  
  //注册Mapper接口
  public <T> void addMapper(Class<T> type) 
    if (type.isInterface()) 
      if (hasMapper(type)) 
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      
      boolean loadCompleted = false;
      try 
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
       finally 
        if (!loadCompleted) 
          knownMappers.remove(type);
        
      
    
  
  public Collection<Class<?>> getMappers() 
    return Collections.unmodifiableCollection(knownMappers.keySet());
  
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) 
      addMapper(mapperClass);
    
  
  //通过包名扫描下面所有接口
  public void addMappers(String packageName) 
    addMappers(packageName, Object.class);
  

类的getMapper方法里面最后会去调用MapperProxyFactory类的newInstance方法。从上面的源码可以看出来，在调用getMapper方法前会初始化MapperProxyFactory，它是创建Mapper代理对象的工厂，其源码如下：

package org.apache.ibatis.binding;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import org.apache.ibatis.session.SqlSession;
//这个类负责创建具体Mapper接口代理对象的工厂类
public class MapperProxyFactory<T> 
  //具体Mapper接口的Class对象
  private final Class<T> mapperInterface;
  //该接口下面方法的缓存 key是方法对象 value是对接口中方法对象的封装
  private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
  //构造参数没啥好说的
  public MapperProxyFactory(Class<T> mapperInterface) 
    this.mapperInterface = mapperInterface;
  
  public Class<T> getMapperInterface() 
    return mapperInterface;
  
  public Map<Method, MapperMethod> getMethodCache() 
    return methodCache;
  
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) 
    //创建了一个代理类并返回
    //关于Proxy的API 可以查看java官方的API
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]  mapperInterface , mapperProxy);
  
  //在这里传入sqlSession 创建一个Mapper接口的代理类
  public T newInstance(SqlSession sqlSession) 
    //在这里创建了MapperProxy对象 这个类实现了JDK的动态代理接口 InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    //调用上面的方法 返回一个接口的代理类
    return newInstance(mapperProxy);
  

上述代码中的关键代码是

final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);

在这里创建了MapperProxy对象 这个类实现了JDK的动态代理接口 InvocationHandler,其源码如下：

package org.apache.ibatis.binding;
import java.io.Serializable;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map;
import org.apache.ibatis.session.SqlSession;
//实现了JDK动态代理的接口 InvocationHandler
//在invoke方法中实现了代理方法调用的细节
public class MapperProxy<T> implements InvocationHandler, Serializable 
  private static final long serialVersionUID = -6424540398559729838L;
  //SqlSession
  private final SqlSession sqlSession;
  //接口的类型对象
  private final Class<T> mapperInterface;
  //接口中方法的缓存 有MapperProxyFactory传递过来的。
  private final Map<Method, MapperMethod> methodCache;
  //构造参数
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) 
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  
  //接口代理对象所有的方法调用 都会调用该方法
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable 
    //判断是不是基础方法 比如toString() hashCode()等，这些方法直接调用不需要处理
    if (Object.class.equals(method.getDeclaringClass())) 
      return method.invoke(this, args);
    
    //这里进行缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用mapperMethod.execute 核心的地方就在这个方法里，这个方法对才是真正对SqlSession进行的包装调用
    return mapperMethod.execute(sqlSession, args);
  
  //缓存处理
  private MapperMethod cachedMapperMethod(Method method) 
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) 
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    
    return mapperMethod;
  

从源码的意思：从缓存中获得执行方法对应的MapperMethod类实例。如果MapperMethod类实例不存在的情况，创建加入缓存并返回相关的实例。最后调用MapperMethod类的execute方法。
到这里我们可以看到，getMapper方法就是用来获得相关的数据操作类接口。而事实数据操作类邦定了动态代理。所以操据操作类执行方法的时候，会触动每个方法相应的MapperProxy类的invoke方法。所以invoke方法返回的结果就是根据操作类执行方法的结果。这样子我们就知道最后的任务交给了MapperMethod类实例。
那么MapperMethod的详细源码如下：

package org.apache.ibatis.binding;
import org.apache.ibatis.annotations.MapKey;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlCommandType;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;
import java.lang.reflect.Array;
import java.lang.reflect.Method;
import java.util.*;
//这个类是整个代理机制的核心类，对Sqlsession当中的操作进行了封装
public class MapperMethod 
  //一个内部封 封装了SQL标签的类型 insert update delete select
  private final SqlCommand command;
  //一个内部类 封装了方法的参数信息 返回类型信息等
  private final MethodSignature method;
  //构造参数
  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) 
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
  
  //这个方法是对SqlSession的包装调用
  public Object execute(SqlSession sqlSession, Object[] args) 
    //定义返回结果
    Object result;
    //如果是INSERT操作
    if (SqlCommandType.INSERT == command.getType()) 
      //处理参数
      Object param = method.convertArgsToSqlCommandParam(args);
      //调用sqlSession的insert方法
      result = rowCountResult(sqlSession.insert(command.getName(), param));
      //如果是UPDATE操作 同上
     else if (SqlCommandType.UPDATE == command.getType()) 
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
      //如果是DELETE操作 同上
     else if (SqlCommandType.DELETE == command.getType()) 
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
      //如果是SELECT操作 那么情况会多一些 但是也都和sqlSession的查询方法一一对应
     else if (SqlCommandType.SELECT == command.getType()) 
      //如果返回void 并且参数有resultHandler
      //则调用 void select(String statement, Object parameter, ResultHandler handler);方法
      if (method.returnsVoid() && method.hasResultHandler()) 
        executeWithResultHandler(sqlSession, args);
        result = null;
      //如果返回多行结果这调用 <E> List<E> selectList(String statement, Object parameter);
      //executeForMany这个方法调用的
       else if (method.returnsMany()) 
        result = executeForMany(sqlSession, args);
      //如果返回类型是MAP 则调用executeForMap方法
       else if (method.returnsMap()) 
        result = executeForMap(sqlSession, args);
       else 
        //否则就是查询单个对象
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      
     else 
      //如果全都不匹配 说明mapper中定义的方法不对
      throw new BindingException("Unknown execution method for: " + command.getName());
    
    //如果返回值为空 并且方法返回值类型是基础类型 并且不是VOID 则抛出异常
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) 
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    
    return result;
  
  private Object rowCountResult(int rowCount) 
    final Object result;
    if (method.returnsVoid()) 
      result = null;
     else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) 
      result = rowCount;
     else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) 
      result = (long) rowCount;
     else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) 
      result = (rowCount > 0);
     else 
      throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
    
    return result;
  
  private void executeWithResultHandler(SqlSession sqlSession, Object[] args) 
    MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
    if (void.class.equals(ms.getResultMaps().get(0).getType())) 
      throw new BindingException("method " + command.getName() 
          + " needs either a @ResultMap annotation, a @ResultType annotation," 
          + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
    
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) 
      RowBounds rowBounds = method.extractRowBounds(args);
      sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
     else 
      sqlSession.select(command.getName(), param, method.extractResultHandler(args));
    
  
  //返回多行结果 调用sqlSession.selectList方法
  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) 
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    //如果参数含有rowBounds则调用分页的查询
    if (method.hasRowBounds()) 
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
     else 
      //没有分页则调用普通查询
      result = sqlSession.<E>selectList(command.getName(), param);
    
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) 
      if (method.getReturnType().isArray()) 
        return convertToArray(result);
       else 
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      
    
    return result;
  
  private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) 
    Object collection = config.getObjectFactory().create(method.getReturnType());
    MetaObject metaObject = config.newMetaObject(collection);
    metaObject.addAll(list);
    return collection;
  
  @SuppressWarnings("unchecked")
  private <E> E[] convertToArray(List<E> list) 
    E[] array = (E[]) Array.newInstance(method.getReturnType().getComponentType(), list.size());
    array = list.toArray(array);
    return array;
  
  private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) 
    Map<K, V> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) 
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds);
     else 
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey());
    
    return result;
  
  public static class ParamMap<V> extends HashMap<String, V> 
    private static final long serialVersionUID = -2212268410512043556L;
    @Override
    public V get(Object key) 
      if (!super.containsKey(key)) 
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet());
      
      return super.get(key);
    
  

MapperMethod类里面有俩个成员：SqlCommand类和MethodSignature类。从名字上我们大概的能想到一个可能跟SQL语句有关系，一个可能跟要执行的方法有关系。事实也是如此。
上面代码使用一个内部类SqlCommand来封装底层的增删改查操作，确切来讲这一部分的内容跟XxxMapper的XML配置文件里面的select节点、delete节点等有关。我们都会知道节点上有id属性值。那么MyBatis框架会把每一个节点(如：select节点、delete节点）生成一个MappedStatement类。要找到MappedStatement类就必须通过id来获得。有一个细节要注意：代码用到的id = 当前接口类 + XML文件的节点的ID属性。其源码如下：

//一个内部类 封装了具体执行的动作
  public static class SqlCommand 
    //xml标签的id
    private final String name;
    //insert update delete select的具体类型
    private final SqlCommandType type;
    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) throws BindingException 
      //拿到全名 比如 org.mybatis.example.BlogMapper.selectBlog
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      //获取MappedStatement对象 这个对象封装了XML当中一个标签的所有信息 比如下面
      //<select id="selectBlog" resultType="Blog">
      //select * from Blog where id = #id
      //</select>
      if (configuration.hasStatement(statementName)) 
        ms = configuration.getMappedStatement(statementName);
       else if (!mapperInterface.equals(method.getDeclaringClass().getName()))  // 这里是一个BUG
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) 
          ms = configuration.getMappedStatement(parentStatementName);
        
      
      //为空抛出异常
      if (ms == null) 
        throw new BindingException("Invalid bound statement (not found): " + statementName);
      
      name = ms.getId();
      type = ms.getSqlCommandType();
      //判断SQL标签类型 未知就抛异常
      if (type == SqlCommandType.UNKNOWN) 
        throw new BindingException("Unknown execution method for: " + name);
      
    
    public String getName() 
      return name;
    
    public SqlCommandType getType() 
      return type;
    
  
  //内部类 封装了接口当中方法的 参数类型 返回值类型 等信息
  public static class MethodSignature 
    //是否返回多调结果
    private final boolean returnsMany;
    //返回值是否是MAP
    private final boolean returnsMap;
    //返回值是否是VOID
    private final boolean returnsVoid;
    //返回值类型
    private final Class<?> returnType;
    //mapKey
    private final String mapKey;
    //resultHandler类型参数的位置
    private final Integer resultHandlerIndex;
    //rowBound类型参数的位置
    private final Integer rowBoundsIndex;
    //用来存放参数信息
    private final SortedMap<Integer, String> params;
    //是否存在命名参数
    private final boolean hasNamedParameters;
    //在这里对上面的属性进行初始化 就不一一详细说明了 具体实现细节可以看下面的代码。
    public MethodSignature(Configuration configuration, Method method) throws BindingException 
      this.returnType = method.getReturnType();
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      this.hasNamedParameters = hasNamedParams(method);
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
    
    public Object convertArgsToSqlCommandParam(Object[] args) 
      final int paramCount = params.size();
      if (args == null || paramCount == 0) 
        return null;
       else if (!hasNamedParameters && paramCount == 1) 
        return args[params.keySet().iterator().next()];
       else 
        final Map<String, Object> param = new ParamMap<Object>();
        int i = 0;
        for (Map.Entry<Integer, String> entry : params.entrySet()) 
          param.put(entry.getValue(), args[entry.getKey()]);
          // issue #71, add param names as param1, param2...but ensure backward compatibility
          final String genericParamName = "param" + String.valueOf(i + 1);
          if (!param.containsKey(genericParamName)) 
            param.put(genericParamName, args[entry.getKey()]);
          
          i++;
        
        return param;
      
    
    public boolean hasRowBounds() 
      return (rowBoundsIndex != null);
    
    public RowBounds extractRowBounds(Object[] args) 
      return (hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null);
    
    public boolean hasResultHandler() 
      return (resultHandlerIndex != null);
    
    public ResultHandler extractResultHandler(Object[] args) 
      return (hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null);
    
    public String getMapKey() 
      return mapKey;
    
    public Class<?> getReturnType() 
      return returnType;
    
    public boolean returnsMany() 
      return returnsMany;
    
    public boolean returnsMap() 
      return returnsMap;
    
    public boolean returnsVoid() 
      return returnsVoid;
    
    private Integer getUniqueParamIndex(Method method, Class<?> paramType) 
      Integer index = null;
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) 
        if (paramType.isAssignableFrom(argTypes[i])) 
          if (index == null) 
            index = i;
           else 
            throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
          
        
      
      return index;
    
    private String getMapKey(Method method) 
      String mapKey = null;
      if (Map.class.isAssignableFrom(method.getReturnType())) 
        final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class);
        if (mapKeyAnnotation != null) 
          mapKey = mapKeyAnnotation.value();
        
      
      return mapKey;
    
    private SortedMap<Integer, String> getParams(Method method, boolean hasNamedParameters) 
      final SortedMap<Integer, String> params = new TreeMap<Integer, String>();
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) 
        if (!RowBounds.class.isAssignableFrom(argTypes[i]) && !ResultHandler.class.isAssignableFrom(argTypes[i])) 
          String paramName = String.valueOf(params.size());
          if (hasNamedParameters) 
            paramName = getParamNameFromAnnotation(method, i, paramName);
          
          params.put(i, paramName);
        
      
      return params;
    
    private String getParamNameFromAnnotation(Method method, int i, String paramName) 
      final Object[] paramAnnos = method.getParameterAnnotations()[i];
      for (Object paramAnno : paramAnnos) 
        if (paramAnno instanceof Param) 
          paramName = ((Param) paramAnno).value();
        
      
      return paramName;
    
    private boolean hasNamedParams(Method method) 
      boolean hasNamedParams = false;
      final Object[][] paramAnnos = method.getParameterAnnotations();
      for (Object[] paramAnno : paramAnnos) 
        for (Object aParamAnno : paramAnno) 
          if (aParamAnno instanceof Param) 
            hasNamedParams = true;
            break;
          
        
      
      return hasNamedParams;
    
  

看到这里的时候，我们就可以回头去找一找在什么时候增加了MappedStatement类。上面之所以可以执行是建立在XML配置信息都被加载进来了。所以MappedStatement类也一定是在加载配置的时候就进行的。请读者们自行查看一下MapperBuilderAssistant类的addMappedStatement方法——加深理解。SqlCommand类的name成员和type成员我们还是关注一下。name成员就是节点的ID，type成员表示查寻还是更新或是删除。至于MethodSignature类呢。他用于说明方法的一些信息，主要有返回信息。