解释：
Dao接口，就是人们常说的Mapper接口，接口的全限名，就是映射文件中的namespace的值，接口的方法名，就是映射文件中MappedStatement的id值，接口方法内的参数，就是传递给sql的参数。Mapper接口是没有实现类的，当调用接口方法时，接口全限名+方法名拼接字符串作为key值，可唯一定位一个MappedStatement，举例：com.mybatis3.mappers.StudentDao.findStudentById，可以唯一找到namespace为com.mybatis3.mappers.StudentDao下面id = findStudentById的MappedStatement。在Mybatis中，每一个<select>、<insert>、<update>、<delete>标签，都会被解析为一个MappedStatement对象。
下面来看这个问题：
如何通过全限名+方法名来定位一个mapper的？
初始化
首先mybatis会把各种mapper映射进行初始化
对于mybatis与spring结合的项目，最开始是从SqlSessionFactoryBean开始
**SqlSessionFactoryBean类**
```java
protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
//其他省略...
if (xmlConfigBuilder != null) {
try {
//初始化mybatis config配置文件
xmlConfigBuilder.parse();
LOGGER.debug(() -> "Parsed configuration file: '" + this.configLocation + "'");
} catch (Exception ex) {
throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
} finally {
ErrorContext.instance().reset();
}
}
//其他省略...
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
//格式化mapper xml文件
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'");
//其他省略...
return this.sqlSessionFactoryBuilder.build(targetConfiguration);
}
```
**XMLMapperBuilder类**
```java
/**
* 解析mapper映射配置文件
*/
public void parse() {
//判断是否已经加载过该映射文件
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
//注册 Mapper 接 口
bindMapperForNamespace();
}
//处理 configurationElement ()方法中解析失败的<resultMap>节点
parsePendingResultMaps();
//处理 configurationElement ()方法中 解析失败的< cache-ref>节点
parsePendingCacheRefs();
//处理 configurationElement ()方法中 解析失败的 SQL 语句节点
parsePendingStatements();
}
```
**bindMapperForNamespace()方法**
```java
//绑定Mapper接口
private void bindMapperForNamespace() {
//获取当前namespace名称
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
//如果还没有加载
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}
```
你从configuration.addMapper(boundType)进入，到最后你会发现，会以类全限定名为key，mapper代理作为value放入knownMappers 中
**MapperRegistry类**
```java
public <T> void addMapper(Class<T> type) {
//....
try {
//放入knownMappers中
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);
}
}
}
}
```
另外还有一个地方也会初始化，在初始化mybatis config配置文件的时候，可以看**XMLConfigBuilder.java**中mapperElement方法
### 定位
测试用例
```java
@Test
public void shouldSelectBlogWithPostsUsingSubSelect() throws Exception {
SqlSession session = sqlSessionFactory.openSession();
try {
//getMapper返回一个MapperProxy对象
BoundBlogMapper mapper = session.getMapper(BoundBlogMapper.class);
Blog b = mapper.selectBlogWithPostsUsingSubSelect(1);
assertEquals(1, b.getId());
session.close();
assertNotNull(b.getAuthor());
assertEquals(101, b.getAuthor().getId());
assertEquals("jim", b.getAuthor().getUsername());
assertEquals("********", b.getAuthor().getPassword());
assertEquals(2, b.getPosts().size());
} finally {
session.close();
}
}
```
MapperProxy类实现了InvocationHandler接口，代理类调用的时候会执行invoke方法
**MapperProxy类**
```java
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//如果目标方法继承自Object，则直接调用目标方法
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
//对jdk7以上版本，动态语言的支持
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
//从缓存中获取 MapperMethod对象，如果缓存中没有，则创建新的 MapperMethod对象并添加到缓存中
final MapperMethod mapperMethod = cachedMapperMethod(method);
//调用 MapperMethod.execute ()方法执行 SQL 语 句
return mapperMethod.execute(sqlSession, args);
}
```
看 cachedMapperMethod(method)方法
```java
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
创建 MapperMethod 对象 ， 并添加到 methodCache 集合 中缓存
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
```
MapperMethod 中封装了 Mapper接口中对应方法的信息，以及对应 SQL 语句的信息
```java
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
//记录了 SQL语句的名称和类型
this.command = new SqlCommand(config, mapperInterface, method);
//Mapper 接 口中对应方法的相关信息
this.method = new MethodSignature(config, mapperInterface, method);
}
```
看SqlCommand -->resolveMappedStatement你会发现，sql语句的名称是由：Mapper 接口的名称与对应的方法名称组成的