[Mybatis] Mapper接口原理分析

上篇文章中，是直接用SqlSession.selectList()去执行一条SQL的，但是作为 一个ORM框架，我们往往是调用接口去执行，如下所示：

    InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
    SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
    SqlSessionFactory factory = builder.build(inputStream);

    SqlSession sqlSession = factory.openSession();
    UserMapper userMapper =sqlSession.getMapper(UserMapper.class);
    userMapper.selectAllUser();//直接调用接口执行

    sqlSession.close();

但是UserMapper只是一个接口，并没有相应的实现类。我们都知道接口要有实现类才能实例化进而调用，但是那么Myabtis是如何实现调用没有实现类的方法，还能完成相应的SQL处理逻辑。

其实原理很简单：这个是用了JDK的动态代理技术，首先为UserMapper生成一个代理对象，由代理对象去调用SqlSession方法。也就是说在调用链SqlSession作为参数一直传递下去，并交给代理对象，真正的执行操作还是由SqlSession去执行的。

下面我们来分析下这其中的过程，先看下sqlSession.getMapper()方法：

 @Override
  public <T> T getMapper(Class<T> type) {
    //从配置中获取，注意这两个参数，一个是接口UserMapper.class,另一个是SqlSession自身，特别是SqlSession这个参数，后文一系列的调用链都有这个参数的
    return configuration.<T>getMapper(type, this);
  }

再看下configuration.getMapper()

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //其实是调用了MapperRegistry的方法
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry类：

public class MapperRegistry {

  private final Configuration config;
  //Map类型，<Class,MapperProxyFactory>这两个的映射。后者就是Mapper代理工厂
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //获取Mapper代理工厂
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //从代理工厂创建实例，注意还有个SqlSession入参
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

}

继续往下看MapperProxyFactory，从名字中可以直接知道这个是：Mapper代理工厂。

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  //方法缓存，提高性能
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    //很明显，这就是一个动态代理，用于生产一个代理对象
    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);//创建一个Mapper代理
    return newInstance(mapperProxy);//实例化一个代理对象，也就是UserMapper的代理对象
  }

}

到了此步，创建代理对象就完成了，后面的内容注释使用代理对象执行SQL操作了。

来看代理类MapperProxy:

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  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;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //注意这个判断，获取该方法的对应的类，如果是Object，也就是说该方法为toString()，equals()等根类Object的方法。则调用代理类MapperProxy自身。
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //在代理对象中执行SQL
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

最后看下MapperMethod类，也就是真正执行SQL的类

public class MapperMethod {

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //判断这条SQL是什么类型的CRUD，最终有SqlSession去执行！也就是说整个调用还是交了SqlSession处理了！
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsT oSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      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 {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else if (SqlCommandType.FLUSH == command.getType()) {
        result = sqlSession.flushStatements();
    } else {
      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;
  }

}

框架流程总结如下：

1. MapperRegistry获取到MapperProxyFactory工厂
2. MapperProxyFactory工厂创建MapperProxy
3. MapperProxy调用SqlSession执行SQL操作