1. 简介

SQL查询执行流程图如下

image.png

通过这个流程图可以发现这里的JDK动态代理Mapper接口很有趣，那么接下来对这部分进行分析

2. JDK动态代理Mapper接口解析

image.png

1. 由上面的流程图发现Mapper接口的代理对象是从SqlSession的getMapper获取得到的，如下代码：

  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.getMapper(type, this);
  }

通过代码发现，getMapper方法最终调用的是对Configuration对象的引用的getMapper方法完成的,然后继续往下看

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry是存放所有Mapper接口代理对象的容器，属性knownMappers是一个Map<Class<?>, MapperProxyFactory<?>>对象的引用，用于存放所有mapper接口的代理工厂对象.

  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

在MapperProxyFactory中生成代理对象的代码如下

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

3. 调用查询方法具体解析

1. 上面我们知道代理对象是由MapperProxyFactory的代理工厂通过JDK动态代理模式生成代理对象，代理类为MapperProxy。代理类的invoke执行部分如下

  @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);
    }
  }

cachedInvoker方法会先从Map<Method, MapperMethodInvoker> methodCache根据Method取基于MapperMethodInvoker接口的实现类PlainMethodInvoker，如果取不到就创建PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()))对象，并把对象put到methodCache集合中。
PlainMethodInvoker的invoke方法执行体如下

    @Override
    public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
      return mapperMethod.execute(sqlSession, args);
    }

MapperMethod的execute的执行体如下

  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 {
          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;
  }

MapperMethod类有两个比较重要的对象，SqlCommand和MethodSignature。这两个类的作用单独用一篇文章来讲吧。