MyBatis源码剖析之Mapper代理方式细节

news2024/11/24 12:39:27

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 {
  @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 { 
  @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;
  }
}