Mybatis 源码解析(二) DefaultSqlSession 为什么不是线程安全的

小咪咪 2022-02-12 12:34 1234阅读 0赞

DefaultSqlSession 真正的执行代码是由 BaseExecutor 进行执行的，而BaseExecutor中的代码如下：

定义一个localCache字段，其实底层就是一个HashMap。

protected PerpetualCache localCache;

public class PerpetualCache implements Cache { 
    
      private final String id;
    
      private Map<Object, Object> cache = new HashMap<Object, Object>();
    
      public PerpetualCache(String id) { 
        this.id = id;
      }
      
      @Override
      public void putObject(Object key, Object value) { 
        cache.put(key, value);
      }
    
      @Override
      public Object getObject(Object key) { 
        return cache.get(key);
      }
    
      @Override
      public Object removeObject(Object key) { 
        return cache.remove(key);
      }
      // .....

BaseExecutor 用这个HashMap缓存了 SqlSession的 select/selectList的查询结果。

所以这个可以理解成一个共享竞争资源

我们看一下BaseExecutor的查询数据库代码

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);
        }
        // ①
        localCache.putObject(key, list);
        if (ms.getStatementType() == StatementType.CALLABLE) { 
          localOutputParameterCache.putObject(key, parameter);
        }
        return list;
      }

当不同的线程使用sqlsession进行查询的时候，会出现以下的情况

1.  线程1 使用select \* from user 进行查询，查询出5条数据，
2.  线程2同样使用语句，并且这个时候是线程1刚好执行到①处，则还没有放入缓存，这个时候直接查库，但是线程2查询的时候，数据库刚好新插入了1条，所以查询到的是6条，
3.  线程2把查询到的6条放入缓存
4.  然后线程1又把第5条放入了缓存，
5.  这样就导致了后续的查询都使用的是线程1的缓存，导致查询结果不正确。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTMwNzYwNDQ_size_16_color_FFFFFF_t_70]

**附一个线程安全的SqlSession**

实现思路就是创建一个包装类，每一次真正执行的时候，使用的是当前绑定线程的SqlSession

package com.wuhulala.springboot.mybatis.threadsafe;
    
    import org.apache.ibatis.cursor.Cursor;
    import org.apache.ibatis.executor.BatchResult;
    import org.apache.ibatis.session.Configuration;
    import org.apache.ibatis.session.ResultHandler;
    import org.apache.ibatis.session.RowBounds;
    import org.apache.ibatis.session.SqlSession;
    import org.apache.ibatis.session.SqlSessionFactory;
    import org.apache.ibatis.session.defaults.DefaultSqlSessionFactory;
    
    import java.sql.Connection;
    import java.util.List;
    import java.util.Map;
    
    
    public class ThreadSafeSqlSession implements SqlSession { 
    
        private static ThreadLocal<SqlSession> threadLocal = new ThreadLocal<>();
    
    
        private SqlSessionFactory factory = null;
    
        public ThreadSafeSqlSession(DefaultSqlSessionFactory sqlSessionFactory) { 
            this.factory = sqlSessionFactory;
        }
    
        @Override
        public <T> T selectOne(String statement) { 
            return getSqlSession().selectOne(statement);
        }
    
        @Override
        public <T> T selectOne(String statement, Object parameter) { 
            return getSqlSession().selectOne(statement, parameter);
        }
    
        @Override
        public <E> List<E> selectList(String statement) { 
            return getSqlSession().selectList(statement);
        }
    
        @Override
        public <E> List<E> selectList(String statement, Object parameter) { 
            return getSqlSession().selectList(statement, parameter);
        }
    
        @Override
        public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { 
            return getSqlSession().selectList(statement, parameter, rowBounds);
        }
    
        @Override
        public <K, V> Map<K, V> selectMap(String statement, String mapKey) { 
            return getSqlSession().selectMap(statement, mapKey);
        }
    
        @Override
        public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey) { 
            return getSqlSession().selectMap(statement, parameter, mapKey);
    
        }
    
        @Override
        public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) { 
            return getSqlSession().selectMap(statement, parameter, mapKey, rowBounds);
        }
    
    
        @Override
        public <T> Cursor<T> selectCursor(String statement) { 
            return getSqlSession().selectCursor(statement);
        }
    
        @Override
        public <T> Cursor<T> selectCursor(String statement, Object parameter) { 
            return getSqlSession().selectCursor(statement, parameter);
    
        }
    
        @Override
        public <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds) { 
            return getSqlSession().selectCursor(statement, parameter, rowBounds);
        }
    
        @Override
        public void select(String statement, Object parameter, ResultHandler handler) { 
            getSqlSession().select(statement, parameter, handler);
        }
    
        @Override
        public void select(String statement, ResultHandler handler) { 
            getSqlSession().select(statement, handler);
    
        }
    
        @Override
        public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) { 
            getSqlSession().select(statement, parameter, rowBounds, handler);
    
        }
    
        @Override
        public int insert(String statement) { 
            return getSqlSession().insert(statement);
        }
    
        @Override
        public int insert(String statement, Object parameter) { 
            return getSqlSession().insert(statement, parameter);
        }
    
        @Override
        public int update(String statement) { 
            return getSqlSession().update(statement);
        }
    
        @Override
        public int update(String statement, Object parameter) { 
            return getSqlSession().update(statement, parameter);
        }
    
        @Override
        public int delete(String statement) { 
            return getSqlSession().delete(statement);
        }
    
        @Override
        public int delete(String statement, Object parameter) { 
            return getSqlSession().delete(statement, parameter);
        }
    
        @Override
        public void commit() { 
            getSqlSession().commit();
        }
    
        @Override
        public void commit(boolean force) { 
            getSqlSession().commit(force);
    
        }
    
        @Override
        public void rollback() { 
            getSqlSession().rollback();
        }
    
        @Override
        public void rollback(boolean force) { 
            getSqlSession().rollback(force);
        }
    
        @Override
        public List<BatchResult> flushStatements() { 
            return getSqlSession().flushStatements();
        }
    
        @Override
        public void clearCache() { 
            getSqlSession().clearCache();
        }
    
        @Override
        public <T> T getMapper(Class<T> type) { 
            return getSqlSession().getMapper(type);
        }
    
        @Override
        public Connection getConnection() { 
            return getSqlSession().getConnection();
        }
    
        @Override
        public Configuration getConfiguration() { 
            return factory.getConfiguration();
        }
    
        /** * 关闭SqlSession */
        @Override
        public void close() { 
            SqlSession sqlSession = threadLocal.get();
            if (sqlSession != null) { 
                sqlSession.close();
                threadLocal.remove();
            }
        }
    
        /** * 获取SqlSession 代理方法 * * <p> 应该可以使用代理模式实现 </p> * @return SqlSession */
        private SqlSession getSqlSession() { 
            SqlSession sqlSession = threadLocal.get();  // 从当前线程获取
            if (sqlSession == null) { 
                sqlSession = factory.openSession();
                threadLocal.set(sqlSession);  // 将sqlSession与当前线程绑定
            }
            return sqlSession;
        }
    
    
    
    
    }

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