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
以上这篇解决spring结合mybatis时一级缓存失效的问题就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持。
