之前瞭解到mybatis的一級快取是預設開啟的，作用域是sqlSession,是基 HashMap的本地快取。不同的SqlSession之間的快取資料區域互不影響。

當進行select、update、delete操作後並且commit事物到資料庫之後，sqlSession中的Cache自動被清空

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

結論

spring結合mybatis後，一級快取作用：

在未開啟事物的情況之下，每次查詢，spring都會關閉舊的sqlSession而建立新的sqlSession,因此此時的一級快取是沒有啟作用的

在開啟事物的情況之下，spring使用threadLocal獲取當前資源繫結同一個sqlSession，因此此時一級快取是有效的

案例

情景一:未開啟事物

@Service("countryService")
public class CountryService {

 @Autowired
 private CountryDao countryDao;

 // @Transactional 未開啟事物
 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);
 }
}

測試案例:

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

結果:

[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,"lastUpdate":"2006-02-15 04:44:00.0"}

可以看到，兩次查詢，都建立了新的sqlSession,並向資料庫查詢，此時快取並沒有起效果

情景二: 開啟事物

開啟@Transactional註解:

@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);
 }
}

使用原來的測試案例，輸出結果:

[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]
// 從當前事物中獲取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,"lastUpdate":"2006-02-15 04:44:00.0"}

可以看到，兩次查詢，只建立了一次sqlSession,說明一級快取起作用了

跟蹤原始碼

從SqlSessionDaoSupport作為路口，這個類在mybatis-spring包下，sping為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
}

建立了SqlSessionTemplate後,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的內部類:

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);
   //如果尚未開啟事物(事物不是由spring來管理)，則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()
   // 手動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 {
   //一般情況下，預設都是關閉sqlSession
   if (sqlSession != null) {
   closeSqlSession(sqlSession,SqlSessionTemplate.this.sqlSessionFactory);
   }
  }
  }
 }
}

再看getSqlSession方法，這個方法是在SqlSessionUtils.java中的：

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

 notNull(sessionFactory,NO_SQL_SESSION_FACTORY_SPECIFIED);
 notNull(executorType,NO_EXECUTOR_TYPE_SPECIFIED);
 //獲取holder
 SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
 //從sessionHolder中獲取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，則建立一個新的
 session = sessionFactory.openSession(executorType);
 //將sqlSession註冊在sessionHolder中
 registerSessionHolder(sessionFactory,executorType,exceptionTranslator,session);

 return session;
}

private static void registerSessionHolder(SqlSessionFactory sessionFactory,PersistenceExceptionTranslator exceptionTranslator,SqlSession session) {
 SqlSessionHolder holder;
 //在開啟事物的情況下
 if (TransactionSynchronizationManager.isSynchronizationActive()) {
  Environment environment = sessionFactory.getConfiguration().getEnvironment();

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

  holder = new SqlSessionHolder(session,exceptionTranslator);
  //將sessionFactory繫結在sessionHolde相互繫結
  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");
  //從threadLocal型別的resources中獲取與當前執行緒繫結的資源，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() + "]");
  }
  }
}

這裡可以看到，spring是如何做到獲取到的是同一個SqlSession,前面的長篇大論，就是為使用ThreadLocal將當前執行緒繫結建立SqlSession相關的資源，從而獲取同一個sqlSession

以上這篇解決spring結合mybatis時一級快取失效的問題就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支援我們。