转载自   mybatis源码阅读(四)：mapper(dao)实例化

在开始分析之前，先来了解一下这个模块中的核心组件之间的关系，如图：

1.MapperRegistry&MapperProxyFactory

MapperRegistry是Mapper接口及其对应的代理对象工程的注册中心，Configuration是Mybatis全局性的配置对象，在初始化的过程中，所有配置信息会被解析成相应的对象并记录到Configuration对象中，这在之前也详细介绍了。Configuration.mapperRegistry字段记录当前使用的MapperRegistry对象，

public class MapperRegistry {// 全局唯一的配置对象，其中包含了所有的配置信息private final Configuration config;// 记录Mapper接口与对应MapperProxyFactory之间的关系private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
}private void bindMapperForNamespace() {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) {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#loadXmlResourceconfiguration.addLoadedResource("namespace:" + namespace);configuration.addMapper(boundType);}}}
}
public <T> void addMapper(Class<T> type) {mapperRegistry.addMapper(type);
}
public <T> void addMapper(Class<T> type) {if (type.isInterface()) {//是否为接口if (hasMapper(type)) {//是否已经加载过throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {knownMappers.put(type, new MapperProxyFactory<T>(type));// 注解处理MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}
}

在需要执行SQL语句时，会先获取mapper借口的代理对象，例如：

@Test
public void findUserById() {SqlSession sqlSession = getSessionFactory().openSession();UserDao userMapper = sqlSession.getMapper(UserDao.class);User user = userMapper.findUserById(1);Assert.assertNotNull("没找到数据", user);
}

DefaultSqlSession类中方法如下，实际上是通过JDK动态代理生成的代理对象

public <T> T getMapper(Class<T> type) {return this.configuration.getMapper(type, this);
}

Configuration类方法如下：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {return mapperRegistry.getMapper(type, sqlSession);
}

MapperRegistry类中方法如下：

@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {//查找指定type对象的MapperProxyFactory对象final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {//如果为空抛出异常throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {// 创建实现了type接口的代理对象return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}
}

MapperProxyFactory主要负责创建代理对象

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);
}

2.MapperProxy

MapperProxy实现了InvocationHandler接口，对动态代理的可以先去了解这篇文章https://my.oschina.net/u/3737136/blog/1786175

public class MapperProxy<T> implements InvocationHandler, Serializable {private static final long serialVersionUID = -6424540398559729838L;// 记录关联的SQLSession对象private final SqlSession sqlSession;// mapper接口对应的class对象private final Class<T> mapperInterface;private final Map<Method, MapperMethod> methodCache;public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {this.sqlSession = sqlSession;this.mapperInterface = mapperInterface;this.methodCache = methodCache;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {// 如果目标方法是Object类继承来的，直接调用目标方法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);}// 从缓存中获取MapperMethod 对象,如果没有就创建新的并添加final MapperMethod mapperMethod = cachedMapperMethod(method);// 执行sql 语句return mapperMethod.execute(sqlSession, args);}}

3.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，

public class MapperMethod {// 记录SQL语句的名称和类型private final SqlCommand command;// mapper接口中对应方法的相关信息private final MethodSignature method;public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {this.command = new SqlCommand(config, mapperInterface, method);this.method = new MethodSignature(config, mapperInterface, method);}public Object execute(SqlSession sqlSession, Object[] args) {Object result;switch (command.getType()) {case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}case SELECT:if (method.returnsVoid() && method.hasResultHandler()) {// 处理返回值为void ,ResultSet 通过ResultHand处理的方法executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {// 处理返回值为集合或者数组的方法result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {// 处理返回值为map的方法result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {// 处理返回值为cursor的方法result = executeForCursor(sqlSession, args);} else {// 处理返回值为单一对象的方法Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}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() + ").");}return result;}
}