具体代码如下：
代码语言：javascript复制//前三步都相同
InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
//这⾥不再调⽤SqlSession的api,⽽是获得了接⼝对象，调⽤接⼝中的⽅法。
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
//模拟ids的数据
List<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
List<User> userList = mapper.findByIds(ids);思考⼀个问题，通常的Mapper接⼝我们都没有实现的⽅法却可以使⽤，是为什么呢？ 答案很简单：动态代理
代码语言：javascript复制public class Configuration {
protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
}
public class MapperRegistry {
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
}开始之前介绍⼀下MyBatis初始化时对接⼝的处理：MapperRegistry是Configuration中的⼀个属性，它内部维护⼀个HashMap⽤于存放mapper接⼝的⼯⼚类，每个接⼝对应⼀个⼯⼚类。mappers中可以配置接⼝的包路径，或者某个具体的接⼝类。
代码语言：javascript复制<mappers>
<mapper class="com.zjq.mapper.UserMapper"/>
<package name="com.zjq.mapper"/>
</mappers>当解析mappers标签时，它会判断解析到的是mapper配置⽂件时，会再将对应配置⽂件中的增删改查标签 封装成MappedStatement对象，存⼊mappedStatements中。(上⽂介绍了)当判断解析到接⼝时，会建此接⼝对应的MapperProxyFactory对象，存⼊HashMap中，key =接⼝的字节码对象，value =此接⼝对应MapperProxyFactory对象。
源码剖析-getmapper()
进⼊ sqlSession.getMapper(UserMapper.class )中
代码语言：javascript复制public interface SqlSession extends Closeable {
//调用configuration中的getMapper
<T> T getMapper(Class<T> type);
}
public class DefaultSqlSession implements SqlSession {
@Override
public <T> T getMapper(Class<T> type) {
//调用configuration 中的getMapper
return configuration.<T>getMapper(type, this);
}
}
public class Configuration {
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//调用MapperRegistry 中的getMapper
return mapperRegistry.getMapper(type, sqlSession);
}
}
public class MapperRegistry {
@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//从 MapperRegistry 中的 HashMap 中拿 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 {
//调用MapperProxyFactory的newInstance通过动态代理⼯⼚⽣成实例
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " e, e);
}
}
}
public class MapperProxyFactory<T> {
public T newInstance(SqlSession sqlSession) {
//创建了 JDK动态代理的Handler类
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
//调⽤了重载⽅法
return newInstance(mapperProxy);
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
}
//MapperProxy 类，实现了 InvocationHandler 接⼝
public class MapperProxy<T> implements InvocationHandler, Serializable {
//省略部分源码
private final SqlSession sqlSession;
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache;
//构造，传⼊了 SqlSession，说明每个session中的代理对象的不同的！
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
//省略部分源码
}源码剖析-invoke()
在动态代理返回了示例后，我们就可以直接调⽤mapper类中的⽅法了，但代理对象调⽤⽅法，执⾏是在MapperProxy中的invoke⽅法中。
代码语言：javascript复制public class MapperProxy<T> implements InvocationHandler, Serializable {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//如果是Object定义的⽅法，直接调⽤
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
// 获得 MapperMethod 对象
final MapperMethod mapperMethod = cachedMapperMethod(method);
//重点在这：最终调⽤了MapperMethod执⾏的⽅法
return mapperMethod.execute(sqlSession, args);
}
}进⼊execute⽅法：
代码语言：javascript复制public class MapperMethod {
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//判断mapper中的⽅法类型，最终调⽤的还是SqlSession中的⽅法 switch(command.getType())
switch (command.getType()) {
case INSERT: {
//转换参数
Object param = method.convertArgsToSqlCommandParam(args);
//执⾏INSERT操作
//转换rowCount
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
//转换参数
Object param = method.convertArgsToSqlCommandParam(args);
// 转换 rowCount
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
//转换参数
Object param = method.convertArgsToSqlCommandParam(args);
// 转换 rowCount
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
//⽆返回，并且有ResultHandler⽅法参数，则将查询的结果，提交给 ResultHandler 进⾏处理
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
//执⾏查询，返回列表
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
//执⾏查询，返回Map
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
//执⾏查询，返回Cursor
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
//执⾏查询，返回单个对象
} else {
//转换参数
Object param = method.convertArgsToSqlCommandParam(args);
//查询单条
result = sqlSession.selectOne(command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " command.getName());
}
//返回结果为null，并且返回类型为基本类型，则抛出BindingException异常
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() ").");
}
//返回结果
return result;
}
}
