MyBatis-Select 流程

mybatis 版本：3.5.1

测试代码：

public interface MyUserMapperAnnotation {
    @Select("select * from myuser where id = #{id}")
    MyUser selectMyUser(Integer id);
}

public static void main(String[] args) throws IOException {
    SqlSession session = null;
    try {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        session = sqlSessionFactory.openSession();
        MyUserMapperAnnotation mapper = session.getMapper(MyUserMapperAnnotation.class);

        System.out.println(mapper.selectMyUser(4));
    } finally {
        if (session != null) {
            session.close();
        }
    }
}

System.out.println(mapper.selectMyUser(4))

org.apache.ibatis.binding.MapperProxy

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    // 代理以后，所有 Mapper 的方法调用时，都会调用这个 invoke 方法
    try {
        // 并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是 Object 中通用的方法（toString、hashCode等）则无需执行
        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);
    }
    // 去缓存中找 MapperMethod
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // 执行
    return mapperMethod.execute(sqlSession, args);
}

return mapperMethod.execute(this.sqlSession, args)

org.apache.ibatis.binding.MapperMethod

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    // 执行时就是4种情况，insert|update|delete|select，分别调用 SqlSession 的 4 大类方法
    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()) {
                // 如果有结果集处理器
                executeWithResultHandler(sqlSession, args);
                result = null;
            } else if (method.returnsMany()) {
                // 如果结果有多条记录
                result = executeForMany(sqlSession, args);
            } else if (method.returnsMap()) {
                // 如果结果是 Map
                result = executeForMap(sqlSession, args);
            } else if (method.returnsCursor()) {
                // 如果结果是 Cursor
                result = executeForCursor(sqlSession, args);
            } else {
                // 封装查询的入参
                Object param = method.convertArgsToSqlCommandParam(args);
                // 结果是一条记录
                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;
}

result = sqlSession.selectOne(this.command.getName(), param)

org.apache.ibatis.session.defaults.DefaultSqlSession

@Override
public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    // 转而去调用selectList，如果得到 0 条则返回null，得到1条则返回1条，得到多条报 TooManyResultsException 异常
    // 特别需要主要的是当没有查询到结果的时候就会返回null。因此一般建议在 mapper中编写 resultType 的时候使用包装类型
    List<T> list = this.selectList(statement, parameter);
    if (list.size() == 1) {
        return list.get(0);
    } else if (list.size() > 1) {
        throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
        return null;
    }
}
@Override
public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        // 根据 statement id 找到对应的 MappedStatement
        MappedStatement ms = configuration.getMappedStatement(statement);
        // 用执行器来查询结果，这里传入的 ResultHandler 是 null
        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();
    }
}
private Object wrapCollection(final Object object) {
    // 把参数包装成 Collection
    if (object instanceof Collection) {
        StrictMap<Object> map = new StrictMap<>();
        // 参数若是 Collection 类型，做 collection 标记
        map.put("collection", object);
        if (object instanceof List) {
            // 参数若是 List 类型，做 list 标记
            map.put("list", object);
        }
        return map;
    } else if (object != null && object.getClass().isArray()) {
        StrictMap<Object> map = new StrictMap<>();
        // 参数若是数组类型，做 array 标记
        map.put("array", object);
        return map;
    }
    // 参数若不是集合类型，直接返回原来值
    return object;
}

return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER)

org.apache.ibatis.executor.CachingExecutor

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    // 获取执行的 SQL
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    // 生成缓存的key，query 时传入
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    Cache cache = ms.getCache();
    // 默认情况下是没有开启缓存的(二级缓存).要开启二级缓存,你需要在你的 SQL 映射文件中添加一行: <cache/>
    // 简单的说，就是先查 CacheKey，查不到再委托给实际的执行器去查
    if (cache != null) {
        flushCacheIfRequired(ms);
        if (ms.isUseCache() && resultHandler == null) {
            ensureNoOutParams(ms, boundSql);
            @SuppressWarnings("unchecked")
            List<E> list = (List<E>) tcm.getObject(cache, key);
            if (list == null) {
                list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                tcm.putObject(cache, key, list); // issue #578 and #116
            }
            return list;
        }
    }
    // 二级缓存没有开启，或未在二级缓存中找到，再继续查询
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql)

org.apache.ibatis.executor.BaseExecutor

@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    // 如果已经关闭，报错
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
        throw new ExecutorException("Executor was closed.");
    }
    // 先清局部缓存，再查询。但仅查询堆栈为0，才清。为了处理递归调用
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
        clearLocalCache();
    }
    List<E> list;
    try {
        // 加一，这样递归调用到上面的时候就不会再清局部缓存了
        queryStack++;
        // 先根据 cachekey 从 localCache（一级缓存）去查
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            // 若查到 localCache 缓存，处理 localOutputParameterCache
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            // 从数据库查
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        // 清空堆栈
        queryStack--;
    }
    if (queryStack == 0) {
        // 延迟加载队列中所有元素
        for (DeferredLoad deferredLoad : deferredLoads) {
            deferredLoad.load();
        }
        // issue #601
        // 清空延迟加载队列
        deferredLoads.clear();
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            // issue #482
            // 如果是 STATEMENT，清本地缓存
            clearLocalCache();
        }
    }
    return list;
}
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    // 先向缓存中放入占位符？？？
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
        // 查询数据
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        // 最后删除占位符
        localCache.removeObject(key);
    }
    // 加入缓存
    localCache.putObject(key, list);
    // 如果是存储过程，OUT 参数也加入缓存
    if (ms.getStatementType() == StatementType.CALLABLE) {
        localOutputParameterCache.putObject(key, parameter);
    }
    return list;
}

list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql)

org.apache.ibatis.executor.SimpleExecutor

@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
        Configuration configuration = ms.getConfiguration();
        // 更具 StatementType 创建 StatementHandler，这里传入了 resultHandler 和 boundSql，这一步会还会创建 ParameterHandler 和 ResultSetHandler
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 准备语句
        stmt = prepareStatement(handler, ms.getStatementLog());
        // StatementHandler.query
        return handler.query(stmt, resultHandler);
    } finally {
        // 关闭资源
        closeStatement(stmt);
    }
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    // 进行参数预编译，这一步会应用 StatementHandler 插件
    stmt = handler.prepare(connection, transaction.getTimeout());
    // 根据 TypeHandler 设置 SQL 参数,这一步会应用 ParameterHandler 插件
    handler.parameterize(stmt);
    return stmt;
}

return handler.query(stmt, resultHandler)

org.apache.ibatis.executor.statement.PreparedStatementHandler

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    // 查询
    ps.execute();
    // 根据 TypeHandlee 封装结果集（getPropertyMappingValue()），这一步会应用 ResultSetHandler 插件
    return resultSetHandler.handleResultSets(ps);
}

时序图

查询流程

MyBatis 框架分层架构

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https://github.com/tuguangquan/mybatis/tree/master/src/main/java/org/apache/ibatis

http://www.gulixueyuan.com/course/43