MyBatis源码剖析之Mapper代理方式细节-编程知识

MyBatis是一个流行的Java持久层框架，它提供了多种方式来执行数据库操作，其中之一就是通过Mapper代理方式。通过Mapper代理方式，开发者可以编写接口，然后MyBatis会动态地生成接口的实现类，从而避免了繁琐的SQL映射配置。

具体代码如下：
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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<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。
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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。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> T getMapper(Class<T> type);
}public class DefaultSqlSession implements SqlSession {@Overridepublic <T> T getMapper(Class<T> type) {//调用configuration 中的getMapperreturn configuration.<T>getMapper(type, this);}
}public class Configuration {public <T> T getMapper(Class<T> type, SqlSession sqlSession) {//调用MapperRegistry 中的getMapperreturn 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 MapperProxyFactoryfinal 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。

public class MapperProxy<T> implements InvocationHandler, Serializable { @Overridepublic 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rowCountresult = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {//转换参数Object param = method.convertArgsToSqlCommandParam(args);// 转换 rowCountresult = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {//转换参数Object param = method.convertArgsToSqlCommandParam(args);// 转换 rowCountresult = 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;}
}