mybatis代理sql执行实现流程和类图分析

本文讲解Mybatis在使用上通过Mapper接口即可实现实际数据库sql操作的流程，中间涉及到SqlSession、MybatisMapperProxy等类，会对类之间的关系做分析。

目录

Mapper接口到sql执行的类图

1、从SqlSession到MybatisMapperProxyFactory

2、产生MybatisMapperProxy

3、MybatisMapperProxy调用MybatisMapperMethod

4、类图

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sql执行实现流程

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Mapper接口到sql执行的类图

首先，Mapper接口具体实现的过程是:

T mapperInterface
SqlSession sqlSession = sqlSessionFactory.openSession();
T mapper = sqlSession.getMapper(mapperInterface.getClass());

1、从SqlSession到MybatisMapperProxyFactory

sqlSession.getMapper里是通过configuration.getMapper(Class<T> type, SqlSession sqlSession) 方法实现的，configuration里有一个mapperRegistry属性，它维护了Map<Class<?>, MybatisMapperProxyFactory<?>> knownMappers，即mapperInterface的类与mapperProxy工厂的映射关系。

SqlSessionMapper.java:

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

Configuration.java:

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

MybatisMapperRegistry.java:

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        // TODO 这里换成 MybatisMapperProxyFactory 而不是 MapperProxyFactory
        final MybatisMapperProxyFactory<T> mapperProxyFactory = (MybatisMapperProxyFactory<T>) knownMappers.get(type);
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MybatisPlusMapperRegistry.");
        }
        try {
            return mapperProxyFactory.newInstance(sqlSession);
        } catch (Exception e) {
            throw new BindingException("Error getting mapper instance. Cause: " + e, e);
        }
    }

2、产生MybatisMapperProxy

MybatisMapperProxyFactory对象是与mapperInterface的类型绑定的，它可以产生一个MybatisMapperProxy对象，这个对象实现了InvocationHandler接口，也就是java动态代理的接口。

    protected T newInstance(MybatisMapperProxy<T> mapperProxy) {
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
    }

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

3、MybatisMapperProxy调用MybatisMapperMethod

MybatisMapperProxy在invoke方法中会调用MybatisMapperMethod的execute(sqlSession, args)方法，到这里上层的类调用关系就结束了，剩下的就是实际的sql操作级别的封装代码。

   public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            if (Object.class.equals(method.getDeclaringClass())) {
                return method.invoke(this, args);
            } else if (method.isDefault()) {
                return invokeDefaultMethod(proxy, method, args);
            }
        } catch (Throwable t) {
            throw ExceptionUtil.unwrapThrowable(t);
        }
        final MybatisMapperMethod mapperMethod = cachedMapperMethod(method);
        return mapperMethod.execute(sqlSession, args);
    }

4、类图

 

sql执行实现流程

MybatisMapperMethod中的Invoke方法，根据方法的类型(insert、update还是select)来进入不同的实现方法：

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);
                    // TODO 这里下面改了
                    if (IPage.class.isAssignableFrom(method.getReturnType()) && args != null
                        && IPage.class.isAssignableFrom(args[0].getClass())) {
                        result = ((IPage<?>) args[0]).setRecords(executeForIPage(sqlSession, args));
                        // TODO 这里上面改了
                    } else {
                        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;
    }

以查询方法为例，executeForMany最终使用的是sqlSession的selectList方法：

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
        List<E> result;
        Object param = method.convertArgsToSqlCommandParam(args);
        if (method.hasRowBounds()) {
            RowBounds rowBounds = method.extractRowBounds(args);
            result = sqlSession.selectList(command.getName(), param, rowBounds);
        } else {
            result = sqlSession.selectList(command.getName(), param);
        }
        // issue #510 Collections & arrays support
        if (!method.getReturnType().isAssignableFrom(result.getClass())) {
            if (method.getReturnType().isArray()) {
                return convertToArray(result);
            } else {
                return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
            }
        }
        return result;
    }

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  sql执行是通过Exceutor执行的，

<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

可以看到用到了Statement, parameter(参数)，resultHandler(行处理器，用于从查询结果到java对象的转换）。