Sharing a mapper across multiple threads?

[Romain Vernoux]

Hi,

I have been using MyBatis in my Spring project (with mybatis-spring) for quite some time now, and I after reading a lot of exchanges about the issue of multihreading and thread-safety in mybatis(-spring) I still cannot understand what I should really do in this case. 

The following code resides in a Spring service, in which DataVisualizationDao is a mapper configured with an XML file and @Autowired in the service.

For performance reasons, I want that one call to my service runs multiple queries through the same Dao in parallel. I first came up with the following code:

 

@Autowired 

private DataVisualizationDao dataVisualizationDao;

 

public myFunction(List<...> filters) {

// ... More code 

final ExecutorService executor = Executors.newFixedThreadPool(filters.size());

for (int i = 0; i < filters.size(); i++) {

int index = i;

executor.execute(() -> {

aSynchronizedMap.put(index, dataVisualizationDao.aQuery(index));

});

}

// ... More code 

} 

 

Very randomly though, I get the following error:

Exception in thread "pool-2-thread-1" org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.binding.BindingException: Parameter 'null' not found. Available parameters are [filters, aggregationConfig, param3, lessorId, param1, param2]

at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:75)

at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:371)

at com.sun.proxy.$Proxy54.selectList(Unknown Source)

at org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:198)

at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:114)

at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:58)

at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:43)

The fact that it occurs very randomly points me towards concurrency issues. I've also had random ArrayIndexOutOfBoundsExceptions and NPEs there before but I did not save the Stacktrace. I believe they are the symptoms of the same core issue.

I understand that since DataVisualizationDao is a Spring Bean, it is a singleton in the calling thread (i.e., the thread handling the http request and calling my service). But it looks like the dao implementation itself is not thread-safe. What should I do then?

I've read about something like this, using the fact that the SqlSessionFactory is thread-safe:

@Autowired

private SqlSessionFactory sqlSessionFactory;

public myFunction(List<...> filters) {

// ... More code

final ExecutorService executor = Executors.newFixedThreadPool(filters.size());

for (int i = 0; i < filters.size(); i++) {

int index = i;

executor.execute(() -> { 

SqlSession session = sqlSessionFactory.openSession();

try {

DataVisualizationDao dataVisualizationDao = session.getMapper(DataVisualizationDao.class);

aSynchronizedHashmap.put(index, dataVisualizationDao.aQuery(index));

} finally {

session.close();

}

});

} 

// ... More code

}

But after debugging this I still see that many objects inside the different dao instances are shared. Since the error is super-hard to reproduce to begin with, I can't know for sure if this solution fixes my issue or not.