解决spring结合mybatis时一级缓存失效的问题

之前了解到mybatis的一级缓存是默认开启的，作用域是sqlSession,是基 HashMap的本地缓存。不同的SqlSession之间的缓存数据区域互不影响。

当进行select、update、delete操作后并且commit事物到数据库之后，sqlSession中的Cache自动被清空

<setting name="localCacheScope" value="SESSION"/>

结论

spring结合mybatis后，一级缓存作用：

在未开启事物的情况之下，每次查询，spring都会关闭旧的sqlSession而创建新的sqlSession,因此此时的一级缓存是没有启作用的

在开启事物的情况之下，spring使用threadLocal获取当前资源绑定同一个sqlSession，因此此时一级缓存是有效的

案例

情景一:未开启事物

@Service("countryService")
public class CountryService {

 @Autowired
 private CountryDao countryDao;

 // @Transactional 未开启事物
 public void noTranSactionMethod() throws JsonProcessingException {
  CountryDo countryDo = countryDao.getById(1L);
  CountryDo countryDo1 = countryDao.getById(1L);
  ObjectMapper objectMapper = new ObjectMapper();
  String json = objectMapper.writeValueAsString(countryDo);
  String json1 = objectMapper.writeValueAsString(countryDo1);
  System.out.println(json);
  System.out.println(json1);
 }
}

测试案例:

@Test
public void transactionTest() throws JsonProcessingException {
 countryService.noTranSactionMethod();
}

结果:

[DEBUG] SqlSessionUtils Creating a new SqlSession
[DEBUG] SpringManagedTransaction JDBC Connection [com.mysql.jdbc.JDBC4Connection@14a54ef6] will not be managed by Spring
[DEBUG] getById ==> Preparing: SELECT * FROM country WHERE country_id = ?
[DEBUG] getById ==> Parameters: 1(Long)
[DEBUG] getById <==  Total: 1
[DEBUG] SqlSessionUtils Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@3359c978]
[DEBUG] SqlSessionUtils Creating a new SqlSession
[DEBUG] SqlSessionUtils SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@2aa27288] was not registered for synchronization because synchronization is not active
[DEBUG] SpringManagedTransaction JDBC Connection [com.mysql.jdbc.JDBC4Connection@14a54ef6] will not be managed by Spring
[DEBUG] getById ==> Preparing: SELECT * FROM country WHERE country_id = ?
[DEBUG] getById ==> Parameters: 1(Long)
[DEBUG] getById <==  Total: 1
[DEBUG] SqlSessionUtils Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@2aa27288]
{"countryId":1,"country":"Afghanistan","lastUpdate":"2006-02-15 04:44:00.0"}
{"countryId":1,"country":"Afghanistan","lastUpdate":"2006-02-15 04:44:00.0"}

可以看到，两次查询，都创建了新的sqlSession,并向数据库查询，此时缓存并没有起效果

情景二: 开启事物

打开@Transactional注解:

@Service("countryService")
public class CountryService {

 @Autowired
 private CountryDao countryDao;

 @Transactional
 public void noTranSactionMethod() throws JsonProcessingException {
  CountryDo countryDo = countryDao.getById(1L);
  CountryDo countryDo1 = countryDao.getById(1L);
  ObjectMapper objectMapper = new ObjectMapper();
  String json = objectMapper.writeValueAsString(countryDo);
  String json1 = objectMapper.writeValueAsString(countryDo1);
  System.out.println(json);
  System.out.println(json1);
 }
}

使用原来的测试案例，输出结果:

[DEBUG] SqlSessionUtils Creating a new SqlSession
[DEBUG] SqlSessionUtils Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@109f5dd8]
[DEBUG] SpringManagedTransaction JDBC Connection [com.mysql.jdbc.JDBC4Connection@55caeb35] will be managed by Spring
[DEBUG] getById ==> Preparing: SELECT * FROM country WHERE country_id = ?
[DEBUG] getById ==> Parameters: 1(Long)
[DEBUG] getById <==  Total: 1
[DEBUG] SqlSessionUtils Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@109f5dd8]
// 从当前事物中获取sqlSession
[DEBUG] SqlSessionUtils Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@109f5dd8] from current transaction
[DEBUG] SqlSessionUtils Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@109f5dd8]
{"countryId":1,"country":"Afghanistan","lastUpdate":"2006-02-15 04:44:00.0"}
{"countryId":1,"country":"Afghanistan","lastUpdate":"2006-02-15 04:44:00.0"}

可以看到，两次查询，只创建了一次sqlSession,说明一级缓存起作用了

跟踪源码

从SqlSessionDaoSupport作为路口，这个类在mybatis-spring包下，sping为sqlSession做了代理

public abstract class SqlSessionDaoSupport extends DaoSupport {

 private SqlSession sqlSession;

 private boolean externalSqlSession;

 public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
 if (!this.externalSqlSession) {
  this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
 }
 }
 //....omit
}

创建了SqlSessionTemplate后,在SqlSessionTemplate中:

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
 PersistenceExceptionTranslator exceptionTranslator) {

 notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
 notNull(executorType, "Property 'executorType' is required");

 this.sqlSessionFactory = sqlSessionFactory;
 this.executorType = executorType;
 this.exceptionTranslator = exceptionTranslator;
 //代理了SqlSession
 this.sqlSessionProxy = (SqlSession) newProxyInstance(
  SqlSessionFactory.class.getClassLoader(),
  new Class[] { SqlSession.class },
  new SqlSessionInterceptor());
}

再看SqlSessionInterceptor,SqlSessionInterceptor是SqlSessionTemplate的内部类:

public class SqlSessionTemplate implements SqlSession, DisposableBean {
 // ...omit..
 private class SqlSessionInterceptor implements InvocationHandler {
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  SqlSession sqlSession = getSqlSession(
   SqlSessionTemplate.this.sqlSessionFactory,
   SqlSessionTemplate.this.executorType,
   SqlSessionTemplate.this.exceptionTranslator);
  try {
   Object result = method.invoke(sqlSession, args);
   //如果尚未开启事物(事物不是由spring来管理)，则sqlSession直接提交
   if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
   // force commit even on non-dirty sessions because some databases require
   // a commit/rollback before calling close()
   // 手动commit
   sqlSession.commit(true);
   }
   return result;
  } catch (Throwable t) {
   Throwable unwrapped = unwrapThrowable(t);
   if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
   // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
   closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
   sqlSession = null;
   Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
   if (translated != null) {
    unwrapped = translated;
   }
   }
   throw unwrapped;
  } finally {
   //一般情况下，默认都是关闭sqlSession
   if (sqlSession != null) {
   closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
   }
  }
  }
 }
}

再看getSqlSession方法，这个方法是在SqlSessionUtils.java中的：

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {

 notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
 notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);
 //获取holder
 SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
 //从sessionHolder中获取SqlSession
 SqlSession session = sessionHolder(executorType, holder);
 if (session != null) {
 return session;
 }

 if (LOGGER.isDebugEnabled()) {
 LOGGER.debug("Creating a new SqlSession");
 }

 //如果sqlSession不存在，则创建一个新的
 session = sessionFactory.openSession(executorType);
 //将sqlSession注册在sessionHolder中
 registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

 return session;
}

private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
  PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
 SqlSessionHolder holder;
 //在开启事物的情况下
 if (TransactionSynchronizationManager.isSynchronizationActive()) {
  Environment environment = sessionFactory.getConfiguration().getEnvironment();

  //由spring来管理事物的情况下
  if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
  if (LOGGER.isDebugEnabled()) {
   LOGGER.debug("Registering transaction synchronization for SqlSession [" + session + "]");
  }

  holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
  //将sessionFactory绑定在sessionHolde相互绑定
  TransactionSynchronizationManager.bindResource(sessionFactory, holder);
  TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
  holder.setSynchronizedWithTransaction(true);
  holder.requested();
  } else {
  if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
   if (LOGGER.isDebugEnabled()) {
   LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
   }
  } else {
   throw new TransientDataAccessResourceException(
    "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
  }
  }
 } else {
  if (LOGGER.isDebugEnabled()) {
  LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
  }
 }

再看TransactionSynchronizationManager.bindResource的方法:

public abstract class TransactionSynchronizationManager {

 //omit...
 private static final ThreadLocal<Map<Object, Object>> resources =
   new NamedThreadLocal<Map<Object, Object>>("Transactional resources");

  // key:sessionFactory, value:SqlSessionHolder(Connection)
  public static void bindResource(Object key, Object value) throws IllegalStateException {
  Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
  Assert.notNull(value, "Value must not be null");
  //从threadLocal类型的resources中获取与当前线程绑定的资源，如sessionFactory，Connection等等
  Map<Object, Object> map = resources.get();
  // set ThreadLocal Map if none found
  if (map == null) {
   map = new HashMap<Object, Object>();
   resources.set(map);
  }
  Object oldValue = map.put(actualKey, value);
  // Transparently suppress a ResourceHolder that was marked as void...
  if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
   oldValue = null;
  }
  if (oldValue != null) {
   throw new IllegalStateException("Already value [" + oldValue + "] for key [" +
    actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
  }
  if (logger.isTraceEnabled()) {
   logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" +
    Thread.currentThread().getName() + "]");
  }
  }
}

这里可以看到，spring是如何做到获取到的是同一个SqlSession,前面的长篇大论，就是为使用ThreadLocal将当前线程绑定创建SqlSession相关的资源，从而获取同一个sqlSession

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