MyBatis 學習記錄2 Mapper對象是如何生成的

時間 2019-12-08

標籤 mybatis 學習記錄 mapper 對象如何生成欄目 MyBatis 简体版

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主題

　　之前我一直有一個問題不懂.而且以爲很神奇.就是Mybatis咱們開發的時候只須要定義接口,並無寫實現類,爲何咱們運行的時候就能夠直接使用? 如今我想分享下這部分大體是怎麼實現的.java

在啓動的時候

根據以前的分享,在初始化階段Build SqlSessionFactory的時候須要用到XMLConfigBuilder去parse XML文件生成Configuration對象,在 parse的步驟中其中有一步就是parse mappers節點sql

在parse mapper過程當中會用到XMLMapperBuilder去parse.一步一步進入斷點.express

會發現後面會調用configuration的addMapper方法.它會調用MapperRegistry的addMapper方法apache

MapperRegistry至關用knownMappers這個hashmap於爲每一個Mapper註冊一次,其中key是你自定義的Mapper接口的class,Value是MapperProxyFactory類的對象.session

Factory一個就是一個工廠類,它確定須要生產對應的對象,從名字上也能發現它生產的就是MapperProxyapp

 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

從代碼中咱們能夠得知,這裏是用了JDK動態代理,MapperProxy類implements了InvocationHandler,less

若是是調用自定義Mapper的Object類中的方法,好比toString,那就直接調用,不然的話調用mapperMethod.execute去執行對應的方法(好比selectById).那麼mapperMethod是什麼呢?ide

這個類的對象其實就是對應你寫的Mapper裏的方法,你的每一個方法對應1個MapperMethod,至關因而Java的Method的包裝.ui

另外還包含了你在XML裏定義的SQL字符串, 是select仍是insert仍是update,delete操做等信息.至關於融合了你定義的Mapper裏的Method和你爲每一個Method在XML裏的寫的信息.this

這樣你調用mapperMethod.execute的時候就能找到對應的SQL去執行了.

經過SqlSession獲取Mapper

初始化完成後就如同以前的介紹,會在confuguration的mapperRegistry裏註冊好了各類MapperFactory.那麼經過SqlSession去獲取Mapper的時候也是相似的.會調用configuration去獲取Mapper,內部會調用mapperRegistry去獲取Mapper

而後經過MapperProxyFactory去建立一個MapperProxy並返還