MyBatis 学习记录2 Mapper对象是如何生成的

主题

　　以前我一直有一个问题不懂.并且觉得很神奇.就是Mybatis我们开发的时候只需要定义接口,并没有写实现类,为什么我们运行的时候就可以直接使用? 现在我想分享下这部分大致是怎么实现的.

在启动的时候

根据之前的分享,在初始化阶段Build SqlSessionFactory的时候需要用到XMLConfigBuilder去parse XML文件生成Configuration对象,在 parse的步骤中其中有一步就是parse mappers节点

在parse mapper过程中会用到XMLMapperBuilder去parse.一步一步进入断点.

会发现后面会调用configuration的addMapper方法.它会调用MapperRegistry的addMapper方法

MapperRegistry相当用knownMappers这个hashmap于为每个Mapper注册一次,其中key是你自定义的Mapper接口的class,Value是MapperProxyFactory类的对象.

Factory一个就是一个工厂类,它肯定需要生产对应的对象,从名字上也能发现它生产的就是MapperProxy

 1 /**
 2  * Copyright 2009-2015 the original author or authors.
 3  * <p>
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  * <p>
 8  * http://www.apache.org/licenses/LICENSE-2.0
 9  * <p>
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package org.apache.ibatis.binding;
17 
18 import org.apache.ibatis.session.SqlSession;
19 
20 import java.lang.reflect.Method;
21 import java.lang.reflect.Proxy;
22 import java.util.Map;
23 import java.util.concurrent.ConcurrentHashMap;
24 
25 /**
26  * @author Lasse Voss
27  */
28 public class MapperProxyFactory<T> {
29 
30     private final Class<T> mapperInterface;
31     private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
32 
33     public MapperProxyFactory(Class<T> mapperInterface) {
34         this.mapperInterface = mapperInterface;
35     }
36 
37     public Class<T> getMapperInterface() {
38         return mapperInterface;
39     }
40 
41     public Map<Method, MapperMethod> getMethodCache() {
42         return methodCache;
43     }
44 
45     @SuppressWarnings("unchecked")
46     protected T newInstance(MapperProxy<T> mapperProxy) {
47         return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
48     }
49 
50     public T newInstance(SqlSession sqlSession) {
51         final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
52         return newInstance(mapperProxy);
53     }
54 
55 }

View Code

 1 /**
 2  * Copyright 2009-2015 the original author or authors.
 3  * <p>
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  * <p>
 8  * http://www.apache.org/licenses/LICENSE-2.0
 9  * <p>
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package org.apache.ibatis.binding;
17 
18 import org.apache.ibatis.reflection.ExceptionUtil;
19 import org.apache.ibatis.session.SqlSession;
20 
21 import java.io.Serializable;
22 import java.lang.reflect.InvocationHandler;
23 import java.lang.reflect.Method;
24 import java.util.Map;
25 
26 /**
27  * @author Clinton Begin
28  * @author Eduardo Macarron
29  */
30 public class MapperProxy<T> implements InvocationHandler, Serializable {
31 
32     private static final long serialVersionUID = -6424540398559729838L;
33     private final SqlSession sqlSession;
34     private final Class<T> mapperInterface;
35     private final Map<Method, MapperMethod> methodCache;
36 
37     public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
38         this.sqlSession = sqlSession;
39         this.mapperInterface = mapperInterface;
40         this.methodCache = methodCache;
41     }
42 
43     @Override
44     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
45         if (Object.class.equals(method.getDeclaringClass())) {
46             try {
47                 return method.invoke(this, args);
48             } catch (Throwable t) {
49                 throw ExceptionUtil.unwrapThrowable(t);
50             }
51         }
52         final MapperMethod mapperMethod = cachedMapperMethod(method);
53         return mapperMethod.execute(sqlSession, args);
54     }
55 
56     private MapperMethod cachedMapperMethod(Method method) {
57         MapperMethod mapperMethod = methodCache.get(method);
58         if (mapperMethod == null) {
59             mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
60             methodCache.put(method, mapperMethod);
61         }
62         return mapperMethod;
63     }
64 
65 }

View Code

从代码中我们可以得知,这里是用了JDK动态代理,MapperProxy类implements了InvocationHandler,

如果是调用自定义Mapper的Object类中的方法,比如toString,那就直接调用,否则的话调用mapperMethod.execute去执行对应的方法(比如selectById).那么mapperMethod是什么呢?

这个类的对象其实就是对应你写的Mapper里的方法,你的每个方法对应1个MapperMethod,相当于是Java的Method的包装.

另外还包含了你在XML里定义的SQL字符串, 是select还是insert还是update,delete操作等信息.相当于融合了你定义的Mapper里的Method和你为每个Method在XML里的写的信息.

这样你调用mapperMethod.execute的时候就能找到对应的SQL去执行了.

通过SqlSession获取Mapper

初始化完成后就如同之前的介绍,会在confuguration的mapperRegistry里注册好了各种MapperFactory.那么通过SqlSession去获取Mapper的时候也是类似的.会调用configuration去获取Mapper,内部会调用mapperRegistry去获取Mapper