MyBatis从入门到精通—源码剖析之Mapper代理方式细节

MyBatis从入门到精通—源码剖析之Mapper代理方式细节

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 ids = new ArrayList(); ids.add(1); ids.add(2); List 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

public class Configuration { protected final MapperRegistry mapperRegistry = new MapperRegistry(this); } public class MapperRegistry { private final Map, MapperProxyFactory> knownMappers = new HashMap, 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。

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

public interface SqlSession extends Closeable { //调用configuration中的getMapper T getMapper(Class type); } public class DefaultSqlSession implements SqlSession { @Override public T getMapper(Class type) { //调用configuration 中的getMapper return configuration.getMapper(type, this); } } public class Configuration { public T getMapper(Class type, SqlSession sqlSession) { //调用MapperRegistry 中的getMapper return mapperRegistry.getMapper(type, sqlSession); } } public class MapperRegistry { @SuppressWarnings("unchecked") public T getMapper(Class type, SqlSession sqlSession) { //从 MapperRegistry 中的 HashMap 中拿 MapperProxyFactory final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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 { public T newInstance(SqlSession sqlSession) { //创建了 JDK动态代理的Handler类 final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache); //调⽤了重载⽅法 return newInstance(mapperProxy); } @SuppressWarnings("unchecked") protected T newInstance(MapperProxy mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); } } //MapperProxy 类，实现了 InvocationHandler 接⼝ public class MapperProxy implements InvocationHandler, Serializable { //省略部分源码 private final SqlSession sqlSession; private final Class mapperInterface; private final Map 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 mapperInterface, Map 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。

public class MapperProxy 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：

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; } }

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