Bootstrap

Mybatis的SQL执行流程

Mybatis的SQL执行流程

1、指定statementId和参数

public static void main(String[] args) throws Exception {
        // 指定全局配置文件
        String resource = "mybatis-config.xml";
        // 读取配置文件
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        sqlSession.selectOne("com.hyy.mybatis.demo.mapper.ActivityPoolMapper.selectByActivityType", 2);
    }

SqlSession的实现类DefaultSqlSession中的selectOne(statement, parameter)方法：

public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>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;
    }
  }

从代码可以看出，selectOne()实际上调用的是selectList()方法，其中如果查询出的记录数大于1，则会抛出Expected one result (or null) to be returned by selectOne(), but found: n异常，相信大家都遇到过这个异常。

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

Mybatis的Mapper.xml文件中的每一个select|insert|delete|update在Mybatis初始化时都被封装成一个MappedStatement对象存放在Configiration中的Map<String, MappedStatement> mappedStatements中，其中Key为*Mapper.xml中的select|insert|delete|update的id属性值和namespace + select|insert|delete|update的id属性值，Value为MappedStatement。
MappedStatements的数据结构
在这里插入图片描述
其中namespace + statementId可以唯一确定MappedStatement，那么如果存在多个相同的statementId会怎样呢？新建一份Mapper.xml，里面同样有一个statementId为selectAll的<select标签>。mappedStatements中Value的值变化如下：
在这里插入图片描述
从前两张图中可以发现，MappedStatements中Key为selectAll的Value值变成了Configuration类中的内部类Ambiguity。我们来看一下Configuration中，往HashMap中put的代码：

public V put(String key, V value) {
	  // 判断全限定名（namespace + statementId）是否存在
      if (containsKey(key))
        throw new IllegalArgumentException(name + " already contains value for " + key);
      if (key.contains(".")) {
        // 获取statementId
        final String shortKey = getShortName(key);
        // HashMap中不存在则直接将MappedStatement存入
        if (super.get(shortKey) == null) {
          super.put(shortKey, value);
        } else {
          // 如果存在，则新建一个Ambiguity对象后存入
          super.put(shortKey, (V) new Ambiguity(shortKey));
        }
      }
      return super.put(key, value);
    }

然后控制台会报如下错误：
在这里插入图片描述因此在不同的namespace中，可以存在id相同的select|insert|delete|update，但是只能使用全限定名来访问。
在获取到MappedStatement后，就会调用Excutor的query方法。该方法的主要作用是动态创建SQL语句、为查询创建缓存，以提高性能，执行SQL语句、以及对返回的数据进行封装。
CachingExecutor类中的query方法：

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	// 根据传入的参数动态构建SQL语句
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

BaseExecutor中的query方法：

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) throw new ExecutorException("Executor was closed.");
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
      	// 从数据库中查询数据
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      deferredLoads.clear(); // issue #601
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        clearLocalCache(); // issue #482
      }
    }
    return list;
  }
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

SimpleExecutor中的doQuery方法：

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();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      // 组装参数
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

SimpleStatementHandler中的query方法：

public <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException {
    String sql = boundSql.getSql();
    statement.execute(sql);
    // 将查询结果组装成List
    return resultSetHandler.<E>handleResultSets(statement);
  }

是不是看到这么多代码就烦？我也是！我们还是画个时序图来整理一下流程吧。
在这里插入图片描述

2、通过Mapper执行

public class MybatisTest {
    public static void main(String[] args) throws Exception {
        // 指定全局配置文件
        String resource = "mybatis-config.xml";
        // 读取配置文件
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        ActivityPoolMapper mapper = sqlSession.getMapper(ActivityPoolMapper.class);
        /*List<ActivityPool> activityPools = mapper.selectByActivityTypes(Arrays.asList(2, 21));*/
        /*ActivityPool activityPool = mapper.selectByTypeAndSubType(11, 1);*/
        ActivityPool activityPool = mapper.selectByActivityType(2);
    }
}

首先来看一下Mapper实例的获取流程：
DefaultSqlSession类：

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

从代码中可以看出，Mapper类都是存储在Configuration中。
Configuration类：

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

在Configruation中使用了MapperRegistry类来注册所有Mapper实例。
MapperRegistry类：

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

可以看到，getMapper()方法返回的是MapperProxy，即Mapper的代理对象。因此，我们在调用Mapper类中的方法时，实际上就是调用的MapperProxy类的invoke()方法。
在这里插入图片描述

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

在invoke()方法中，新建了一个MapperMethod实例，并放入methodCache中。
接下来就是调用mapperMethod.execute()方法：

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    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.convertArgsToSqlCommandParam(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 {
      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;
  }

这里if-else if-else就不多介绍了，讲一下MapperMethod.convertArgsToSqlCommandParam()方法：

public Object convertArgsToSqlCommandParam(Object[] args) {
      final int paramCount = params.size();
      if (args == null || paramCount == 0) {
        return null;
      } else if (!hasNamedParameters && paramCount == 1) {
        return args[params.keySet().iterator().next()];
      } else {
        final Map<String, Object> param = new ParamMap<Object>();
        int i = 0;
        for (Map.Entry<Integer, String> entry : params.entrySet()) {
          param.put(entry.getValue(), args[entry.getKey()]);
          // issue #71, add param names as param1, param2...but ensure backward compatibility
          final String genericParamName = "param" + String.valueOf(i + 1);
          if (!param.containsKey(genericParamName)) {
            param.put(genericParamName, args[entry.getKey()]);
          }
          i++;
        }
        return param;
      }
    }

代码中的params的类型为SortedMap<Integer, String>，其中Key为参数的index，Value为index或者@Param注解中的值。

List<ActivityPool> selectByActivityTypes(List<Integer> activityTypes);

在这里插入图片描述

ActivityPool selectByTypeAndSubType(@Param("activityType") int activityType, @Param("activitySubType") int activitySubType);

在这里插入图片描述
hasNamedParameters的作用是判断是否有@Param注解：

private boolean hasNamedParams(Method method) {
      boolean hasNamedParams = false;
      final Object[][] paramAnnos = method.getParameterAnnotations();
      for (Object[] paramAnno : paramAnnos) {
        for (Object aParamAnno : paramAnno) {
          if (aParamAnno instanceof Param) {
            hasNamedParams = true;
            break;
          }
        }
      }
      return hasNamedParams;
    }

随后的过程跟1中的一样

悦读

道可道，非常道；名可名，非常名。 无名，天地之始，有名，万物之母。 故常无欲，以观其妙，常有欲，以观其徼。 此两者，同出而异名，同谓之玄，玄之又玄，众妙之门。

最新收录
无声胜有声家具缓冲器，让家居对话成温柔
Linux(Ubuntu)/Windows-C++云备份实现
SpringBoot 采用@Transactional注解失效 事务问题
前端实现流文件下载的完整指南
【可控图像生成系列论文（四）】IP-Adapter 具体是如何训练的？1公式篇
在el-form中自定义校验规则（关联字段校验，需要在校验方法中用到this）
幂等性浅谈（防止重复支付）
Linux 开发环境工具 下载网址大全 --转
通过python简单实现了处理场内基金数据的一些小功能
Text函数的使用
;