Mybatis中Dao接口的工作原理

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