Mybatis Mapper Bean 生成源码分析二
问题
在java程序中，我们往往一个@Autowired注解就可以获取到一个Mapper接口实例，一个未经我们实现的接口实例，那么这个Bean实例是怎么在Spring初始化的过程中注入的呢？ 让我们来找找答案。
探寻内容偏长，没有DEBUG过且不耐烦可直接跳过去看总结~
MapperFactoryBean创建工厂
public T getObject() throws Exception { // getSqlSession() 即 SqlSessionTemplate; return this.getSqlSession().getMapper(this.mapperInterface); }
关键类org.mybatis.spring.SqlSessionTemplate
在org.mybatis.spring.SqlSessionTemplate中有一个getMapper方法，如下：
public <T> T getMapper(Class<T> type) { return this.getConfiguration().getMapper(type, this); }
这个SqlSessionTemplate会被注入到容器中，而且能够在我们代码中使用，获取到一个Mapper。使用如下：
@Autowired public SqlSessionTemplate sqlSession; public void demo() { sqlSession.getMapper(UserMapper.class); }
显然这个SqlSessionTemplate类可以做到根据接口取到实例，那么Spring启动时其实也是根据它来获取的。
spring启动过程注入一个动态代理对象
对于如下一个需要Mapper的服务，启动过程中Mapper必然会被注入进来：
@Component @RestController public class UserController { @Autowired UserMapper userMapper; }
那么其会走spring的如下方法来获取Bean:
// org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean // 方法很长，将会省略很多 protected <T> T doGetBean( String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException { // beanName即userMapper String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. // 获取的sharedInstance即为工厂 Object sharedInstance = getSingleton(beanName); // 启动时，第一次实际产生工厂代码为下方代码，生成了MapperFactoryBean sharedInstance = getSingleton(beanName, () -> { try { // 作为懒加载的方法，最终创建工厂对象，值得一看 return createBean(beanName, mbd, args); } catch (BeansException ex) { destroySingleton(beanName); throw ex; } }); // 通过工厂来生成Bean实例 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); return (T) bean; }
// org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException { Object object; try { if (System.getSecurityManager() != null) { // 省略 } else { // 重点，通过工厂进行获取Bean object = factory.getObject(); } } return object; }
其堆栈最终如下图：
所以可以看到在下方代码中，返回了一个JDK动态代理的对象，就是我们服务内会注入的名为“userMapper”的Bean了：
// com.baomidou.mybatisplus.core.MybatisMapperRegistry#getMapper @SuppressWarnings("unchecked") @Override public <T> T getMapper(Class<T> type, SqlSession sqlSession) { // 代理工厂 final MybatisMapperProxyFactory<T> mapperProxyFactory = (MybatisMapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MybatisPlusMapperRegistry."); } try { // 通过代理工厂，来生成对应的代理实例 return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } }
代理工厂生成一个怎样的实例
来看看MybatisMapperProxyFactory，究竟是怎么样一个工厂
// com.baomidou.mybatisplus.core.override.MybatisMapperProxyFactory public class MybatisMapperProxyFactory<T> { // 接口名称 @Getter private final Class<T> mapperInterface; @Getter private final Map<Method, MybatisMapperProxy.MapperMethodInvoker> methodCache = new ConcurrentHashMap<>(); public MybatisMapperProxyFactory(Class<T> mapperInterface) { this.mapperInterface = mapperInterface; } @SuppressWarnings("unchecked") protected T newInstance(MybatisMapperProxy<T> mapperProxy) { // 最终包装成JDK动态代理的对象 return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy); } // debug可查看实际走这一个方法来产生 public T newInstance(SqlSession sqlSession) { final MybatisMapperProxy<T> mapperProxy = new MybatisMapperProxy<>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); } }
// com.baomidou.mybatisplus.core.override.MybatisMapperProxypublic class MybatisMapperProxy<T> implements InvocationHandler, Serializable { // bean最后实际会调用到这个代理的invoke方法，由此方法做路由到具体方法 @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } else { return cachedInvoker(method).invoke(proxy, method, args, sqlSession); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } private MapperMethodInvoker cachedInvoker(Method method) throws Throwable { try { return CollectionUtils.computeIfAbsent(methodCache, method, m -> { if (m.isDefault()) { try { if (privateLookupInMethod == null) { return new DefaultMethodInvoker(getMethodHandleJava8(method)); } else { return new DefaultMethodInvoker(getMethodHandleJava9(method)); } } catch (IllegalAccessException | InstantiationException | InvocationTargetException | NoSuchMethodException e) { throw new RuntimeException(e); } } else { // 缓存一个Mapper方法 return new PlainMethodInvoker(new MybatisMapperMethod(mapperInterface, method, sqlSession.getConfiguration())); } }); } catch (RuntimeException re) { Throwable cause = re.getCause(); throw cause == null ? re : cause; } } }
- 最终使用的方法：通过这个方法最终决定走查询、插入、更新等等命令。
// com.baomidou.mybatisplus.core.override.MybatisMapperMethod#execute 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()) { executeWithResultHandler(sqlSession, args); result = null; } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); } else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args); } else { // TODO 这里下面改了 if (IPage.class.isAssignableFrom(method.getReturnType())) { result = executeForIPage(sqlSession, args); // TODO 这里上面改了 } else { Object param = method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(command.getName(), param); if (method.returnsOptional() && (result == null || !method.getReturnType().equals(result.getClass()))) { result = Optional.ofNullable(result); } } } 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; }
总结
有如下几步，来进行Mapper接口Bean的注入
- spring doGetBean 方法获取到bean工厂
- bean工厂通过SqlSessionTemplate来进行获取（创建）Mapper实例
- MybatisMapperProxyFactory工厂通过newInstance构建了一个通过JDK代理过的MybatisMapperProxy对象。
- 该代理对象的invoke方法完成了Mapper接口方法的路由，到具体SQL命令中去。
