Mybatis可以把Mapper.xml檔案直接對映到對應的介面，呼叫介面方法會自動去Mapper.xml檔案中找到對應的標籤，這個功能就是利用java的動態代理在binding包中實現的。

一、註冊Mapper

在初始化時會把獲取到的Mapper介面註冊到MapperRegistry，註冊的時候建立一個Mapper代理工廠，這個工廠通過JDK的代理建立一個執行物件，建立代理需要的InvocationHandler為MapperProxy

public class MapperRegistry {
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
                
 
//取出MapperProxyFactory
        final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
        if (mapperProxyFactory == null)
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        try {
                        
 
//建立代理
            return mapperProxyFactory.newInstance(sqlSession);
        } catch (Exception e) {
            throw new BindingException("Error getting mapper instance. Cause: " + e, e);
        }
    }
}
 
//建立代理的工廠
public class MapperProxyFactory<T> {
    /**
     * 需要建立代理的介面
     */
    private
 
 final Class<T> mapperInterface;
    /**
     * 執行方法的快取,不需要每次都建立MapperMethod
     */
    private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
 
    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) {
        //建立代理, InvocationHanderl是MapperProxy
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface },
                mapperProxy);
    }
    /**
     * 傳人sqlSession建立代理
     * @param sqlSession
     * @return
     */
    public T newInstance(SqlSession sqlSession) {
        //把代理執行需要用到的物件傳入
        final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
    }
}

二、獲取介面物件

從knownMappers中根據介面型別取出對應的代理建立工廠，用該工廠建立代理。

1. public class MapperProxy<T> implements InvocationHandler, Serializable {
    
       private static final long serialVersionUID = -6424540398559729838L;
       private final SqlSession sqlSession;
       private final Class<T> mapperInterface;
       private final Map<Method, MapperMethod> methodCache;
    
       public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
           this.sqlSession = sqlSession;
           this.mapperInterface = mapperInterface;
           this.methodCache = methodCache;
       }
    
       public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
           //如果方法是Object裡面的則直接呼叫方法
           if (Object.class.equals(method.getDeclaringClass())) {
               try {
                   return method.invoke(this, args);
               } catch (Throwable t) {
                   throw ExceptionUtil.unwrapThrowable(t);
               }
           }
           //獲取執行方法的封裝物件
           final MapperMethod mapperMethod = cachedMapperMethod(method);
           //裡面就是找到對應的sql 執行sql語句
           return mapperMethod.execute(sqlSession, args);
       }
       //快取, 不需要每次都建立
       private MapperMethod cachedMapperMethod(Method method) {
           MapperMethod mapperMethod = methodCache.get(method);
           if (mapperMethod == null) {
               //傳人配置引數
               mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
               methodCache.put(method, mapperMethod);
           }
           return mapperMethod;
       }
   }

三、呼叫介面方法

呼叫代理方法會進入到MapperProxy的public Object invoke(Object proxy, Method method, Object[] args)方法

1. public class MapperProxy<T> implements InvocationHandler, Serializable {
    
       private static final long serialVersionUID = -6424540398559729838L;
       private final SqlSession sqlSession;
       private final Class<T> mapperInterface;
       private final Map<Method, MapperMethod> methodCache;
    
       public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
           this.sqlSession = sqlSession;
           this.mapperInterface = mapperInterface;
           this.methodCache = methodCache;
       }
    
       public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
           //如果方法是Object裡面的則直接呼叫方法
           if (Object.class.equals(method.getDeclaringClass())) {
               try {
                   return method.invoke(this, args);
               } catch (Throwable t) {
                   throw ExceptionUtil.unwrapThrowable(t);
               }
           }
           //獲取執行方法的封裝物件
           final MapperMethod mapperMethod = cachedMapperMethod(method);
           //裡面就是找到對應的sql 執行sql語句
           return mapperMethod.execute(sqlSession, args);
       }
       //快取, 不需要每次都建立
       private MapperMethod cachedMapperMethod(Method method) {
           MapperMethod mapperMethod = methodCache.get(method);
           if (mapperMethod == null) {
               //傳人配置引數
               mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
               methodCache.put(method, mapperMethod);
           }
           return mapperMethod;
       }
   }

最終執行sql會進入到MapperMethod中execute方法：

1. public class MapperMethod {
    
       private final SqlCommand command;
       private final MethodSignature method;
    
       public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
           //SqlCommand封裝該介面方法需要執行sql的相關屬性，如：id(name), 型別
           this.command = new SqlCommand(config, mapperInterface, method);
           //執行方法特性進行封裝，用於構造sql引數，判斷執行sql邏輯走哪條分支
           this.method = new MethodSignature(config, method);
       }
    
       public Object execute(SqlSession sqlSession, Object[] args) {
           Object result;
           //先找到對應的執行sql型別, sqlSession會呼叫不同方法
           if (SqlCommandType.INSERT == command.getType()) {
               Object param = method.convertArgsToSqlCommandParam(args);
               result = rowCountResult(sqlSession.insert(command.getName(), param));
           } else if (SqlCommandType.UPDATE == command.getType()) {
               Object param = method.convertArgsToSqlCommandParam(args);
               result = rowCountResult(sqlSession.update(command.getName(), param));
           } else if (SqlCommandType.DELETE == command.getType()) {
               Object param = method.convertArgsToSqlCommandParam(args);
               result = rowCountResult(sqlSession.delete(command.getName(), param));
           } else if (SqlCommandType.SELECT == command.getType()) {//如果是查詢, 需要對返回做判斷處理
               //根據方法的特性判斷進入哪個執行分支
               if (method.returnsVoid() && method.hasResultHandler()) {
                   executeWithResultHandler(sqlSession, args);
                   result = null;
               } else if (method.returnsMany()) {
                   result = executeForMany(sqlSession, args);
               } else if (method.returnsMap()) {
                   result = executeForMap(sqlSession, args);
               } else {
                   //只查一條資料
                   Object param = method.convertArgsToSqlCommandParam(args);
                   result = sqlSession.selectOne(command.getName(), param);
               }
           } else {
               throw new BindingException("Unknown execution method for: " + command.getName());
           }
           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;
       }
   }

上面就是根據介面、方法、配置引數找到對應的執行sql，並構造引數，解析執行結果，具體sql執行在sqlSession流程裡面，後面再看。