具体代码如下：
//前三步都相同InputStream resourceAsStream = Resources.getResourceAsStream(“sqlMapConfig.xml”);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);SqlSession sqlSession = sqlSessionFactory.openSession();//这?不再调?SqlSession的api,?是获得了接?对象，调?接?中的?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?个问题，通常的Mapper接?我们都没有实现的?法却可以使?，是为什么呢？答案很简单：动态代理
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?下MyBatis初始化时对接?的处理：MapperRegistry是Configuration中的?个属性，它内部维护?个HashMap?于存放mapper接?的??类，每个接?对应?个??类。mappers中可以配置接?的包路径，或者某个具体的接?类。
<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?件时，会再将对应配置?件中的增删改查标签 封装成MappedStatement对象，存?mappedStatements中。(上?介绍了)当判断解析到接?时，会建此接?对应的MapperProxyFactory对象，存?HashMap中，key =接?的字节码对象，value =此接?对应MapperProxyFactory对象。
源码剖析-getmapper()
进? sqlSession.getMapper(UserMapper.class )中
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???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?了重载?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?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?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?mapper类中的?法了，但代理对象调??法，执?是在MapperProxy中的invoke?法中。
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?法，直接调? 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?了MapperMethod执?的?法 return mapperMethod.execute(sqlSession, args); }}
进?execute?法：
public class MapperMethod { public Object execute(SqlSession sqlSession, Object[] args) { Object result; //判断mapper中的?法类型，最终调?的还是SqlSession中的?法 switch(command.getType()) switch (command.getType()) { case INSERT: { //转换参数 Object param = method.convertArgsToSqlCommandParam(args); //执?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: //?返回，并且有ResultHandler?法参数，则将查询的结果，提交给 ResultHandler 进?处理 if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; //执?查询，返回列表 } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); //执?查询，返回Map } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); //执?查询，返回Cursor } else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args); //执?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; }}
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