mybatis的解析和运行原理(四)

左手的ㄟ右手 2022-10-02 14:56 180阅读 0赞

# 前言: #

之前的SqlSession的运行过程还未讲完,还差一个SqlSession下的最重要的四大对象,这一篇文章将会讲解四大对象之间的关系.

# SqlSession下的四大对象 #

之前讲解了映射器其实就是一个动态代理的对象,进入到MapperMethod的execute方法,那么进入到execute的方法又是如何执行的呢?肯定是通过我们所写的Sql语句,但是我们关心的不是这些细节,而是框架的设计这些,现在我们来分析这些对象.  
1.Executor 执行器,用于调度其他对象  
2.StatementHandler 调用statement执行操作  
3.ParameterHandler 处理Sql参数  
4.ResultHandler 处理最后的结果集封装并返回

## 一.Executor ##

用于与数据库进行交互,为我们提供了查询,更新,事务等方法,这和其他框架并无不同,那么先让我们看看Executor如何创建的把

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        executorType = executorType == null ? defaultExecutorType : executorType;
        executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
        Executor executor;
        if (ExecutorType.BATCH == executorType) {
            executor = new BatchExecutor(this, transaction);
        } else if (ExecutorType.REUSE == executorType) {
            executor = new ReuseExecutor(this, transaction);
        } else {
            executor = new SimpleExecutor(this, transaction);
        }
        if (cacheEnabled) {
            executor = new CachingExecutor(executor);
        }
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
    }

他会根据你选择的三种执行器的哪一种去创建一个对象,创建对象后回去执行最后这句(Executor) interceptorChain.pluginAll(executor),其实这句就是mybatis的插件,它为我们构建了动态代理对象,然后在Executor执行之前先配置mybatis插件.

然后我们再从他的一个BaseExecutor中的updata再来进行分析

@Override
    public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
        Statement stmt = prepareStatement(handler, ms.getStatementLog());
        return handler.update(stmt);
    }
    private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Statement stmt;
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        if (hasStatementFor(sql)) {
            stmt = getStatement(sql);
            applyTransactionTimeout(stmt);
        } else {
            Connection connection = getConnection(statementLog);
            stmt = handler.prepare(connection, transaction.getTimeout());
            putStatement(sql, stmt);
        }
        handler.parameterize(stmt);
        return stmt;
    }

上面内容很显然Mybatis根据Configuration来构建StatementHandler,然后使用prepareStatement对SQL语句进行编译,它先调用了StatementHandler的prepare()方法进行预编译,然后通过StatementHandler的parameterize()方法来设置参数,resultHandler组装结果并返回结果,看到上面这段代码,我们的注意力就要从Executor中出来了,那么再让我们去分析一下StatementHandler这个对象吧.

## 二.StatementHandler ##

StatementHandler专门用来处理数据库之间的会话类,同样的我们来先看看Mybatis如何创建这个类的把.

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
      }

这么几句话很显然创建的真实对象其实是RoutingStatementHandler只是实现了StatementHandler这个接口,那么它和Executor一样,都只是用代理对象进行了一步步的封装,他和Executor一样为我们提供了三种执行器分别时SimpleStatementHandler,PreparedStatementHandler,  
CallableStatementHandler,他会根据上下文决定创建那个执行器

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        switch (ms.getStatementType()) {
          case STATEMENT:
            delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          case PREPARED:
            delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          case CALLABLE:
            delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          default:
            throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
        }
    
      }

这是RouingStatementHandler类下的构造方法,它提供了一个是配置delegate,他的作用是给实现类对象的使用提供一个统一,简易的适配器.再来看看他的主要三个方法之一的prepare,之后再按照顺序执行parameterize,query

public Statement prepare(Connection connection) throws SQLException {
        ErrorContext.instance().sql(boundSql.getSql());
        Statement statement = null;
        try {
          statement = instantiateStatement(connection);
          setStatementTimeout(statement);
          setFetchSize(statement);
          return statement;
        } catch (SQLException e) {
          closeStatement(statement);
          throw e;
        } catch (Exception e) {
          closeStatement(statement);
          throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
        }
      }

以上代码中的instantiateStatement方法是对SQL进行了预编译和基础配置然后调用parameterize方法去设置参数

public void parameterize(Statement statement) throws SQLException {
        delegate.parameterize(statement);
      }

继续查看query方法

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {  
        PreparedStatement ps = (PreparedStatement) statement; 
        ps.execute();
        return resultSetHandler.<E> handleResultSets(ps);  
      }

现在就非常明确了,在得到DefaultSqlSession对象时初始化了Executor执行器。然后在调用Executor的doQuery方法查询的时候，会初始化StatementHandler对象，然后在初始化StatementHandler的过程中，会初始化ParameterHandler和ResultSetHandler对象，生成的StatementHandler对象会保存这两个对象的引用。直到这时，Executor的职责就差不多完成了，通过其他的三大组件来完成接下来的数据库查询工作并返回结果。

## 三.ParameterHandler ##

public interface ParameterHandler {
      Object getParameterObject();
      void setParameters(PreparedStatement ps)
          throws SQLException;
    }

getParameterObject方法作用是返回参数对象,setParameter设置预编译SQL语句的参数.  
那么我们来分析一下setParameter的实现吧.

@Override
      public void setParameters(PreparedStatement ps) {
        ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
        if (parameterMappings != null) {
          for (int i = 0; i < parameterMappings.size(); i++) {
            ParameterMapping parameterMapping = parameterMappings.get(i);
            if (parameterMapping.getMode() != ParameterMode.OUT) {
              Object value;
              String propertyName = parameterMapping.getProperty();
              if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
                value = boundSql.getAdditionalParameter(propertyName);
              } else if (parameterObject == null) {
                value = null;
              } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                value = parameterObject;
              } else {
                MetaObject metaObject = configuration.newMetaObject(parameterObject);
                value = metaObject.getValue(propertyName);
              }
              TypeHandler typeHandler = parameterMapping.getTypeHandler();
              JdbcType jdbcType = parameterMapping.getJdbcType();
              if (value == null && jdbcType == null) {
                jdbcType = configuration.getJdbcTypeForNull();
              }
              try {
                typeHandler.setParameter(ps, i + 1, value, jdbcType);
              } catch (TypeException e) {
                throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
              } catch (SQLException e) {
                throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
              }
            }
          }
        }
      }
    }

从parameterObject中取参数,根据typeHandler进行参数处理.typeHandler在Mybatis初始化的时候,就已经注册在了Configuration中.

## 四.ResultHandler ##

接口代码我就不贴出来了,根据前面的描述它只是对最终的结果进行了封装处理,我就不在这里多废话了.

# 总结: #

SqlSession是通过Executor创建StatementHandler来运行,二StatementHandler要进过下面三步:  
1.prepared 预编译  
2.parameterize 设置参数  
3. query 执行SQL.  
以上就是SqlSession的执行过程,明白了四大对象是如何协作的了.