iBatis一级缓存的探讨

前几天和朋友讨论数据库Cache的时候，谈到了iBatis框架不支持一级缓存，后来尝试作了一些扩展来支持一级缓存，放出来大家探讨一下。

 

首先让我们简单回顾一下缓存的概念。直白的说，缓存就是把从数据库里查出来的数据放到内存中，供后续操作使用。例如，某个应用需要让业务人员查询前日的数据报表，可能同时有很多人在查询该数据，并且数据没有实时的要求，则可以在第一次查询的时候把结果放到缓存中，以提高后续操作的效率。缓存分为一级缓存和二级缓存，所谓一级缓存通常是指在一次数据库事物范围内的缓存，当数据库事物提交，连接被释放后，该缓存就被销毁了；所谓二级缓存通常指存在时间比较长，例如1天等。一级缓存通常在事物开启的时候建立，在事物提交或回滚的时候销毁。Hibernate对一级缓存和二级缓存都支持的很好，而ibatis则只对应用二级缓存提供了内置的支持。因此本文重点讨论如何为ibatis提供一级缓存功能。

 

先介绍一下我的环境：Spring1.2.6 + iBatis2.1.3 + MySql5.0。Spring已经成为事实上的标准，对iBatis提供了非常好的支持。因此我们在此讨论如何在Spring环境下提供对iBatis的一级缓存支持。Spring对iBatis的封装主要是通过SqlMapClientTemplate来实现的，而事物方面这是通过TransactionTemplate来提供支持，因此要提供一级缓存的功能，就要在这两个类上做一些文章。 当前事物的缓存可以放在当前的线程（ThreadLocal）中，看下面的代码：

public class CacheModel {       private static final Log                              logger     = LogFactory                                                                          .getLog(CacheModel.class);       private static final ThreadLocal<Map<Object, Object>> cacheModel = new ThreadLocal<Map<Object, Object>>();       synchronized static boolean exist() {         return cacheModel.get() != null;     }       public synchronized static void add(Object key, Object value) {         if (exist()) {             cacheModel.get().put(key, value);         }     }       public synchronized static Object get(Object key) {         if (exist()) {             return cacheModel.get().get(key);         }         return null;     }       public synchronized static void initial() {         if (!exist()) {             cacheModel.set(new HashMap<Object, Object>());             logger.info("Initial current transaction's Cache.");         } else {             throw new RuntimeException("Current transaction's CacheModel allready initial!");         }     }       public synchronized static void destroy() {         cacheModel.set(null);         logger.info("Destroy current transaction's Cache.");     }   }

接下来扩展TransactionTemplate，实现在开启和结束事物的时候创建和销毁CacheModel：

public class CacheTransactionTemplate extends TransactionTemplate {     private static final long serialVersionUID = 489621858441822688L;       private boolean           cacheable        = true;       private boolean           isInitialized    = false;       /* (non-Javadoc)      * @see org.springframework.transaction.support.TransactionTemplate#afterPropertiesSet()      */     @Override     public void afterPropertiesSet() {         super.afterPropertiesSet();           isInitialized = true;     }       /* (non-Javadoc)      * @see org.springframework.transaction.support.TransactionTemplate#execute(org.springframework.transaction.support.TransactionCallback)      */     @Override     public Object execute(TransactionCallback action) throws TransactionException {         TransactionStatus status = getTransactionManager().getTransaction(this);           initialCacheModel(status);           Object result = null;         try {             result = action.doInTransaction(status);             logger.debug(action);         } catch (RuntimeException ex) {             // transactional code threw application exception -> rollback             rollbackOnException(status, ex);             throw ex;         } catch (Error err) {             // transactional code threw error -> rollback             rollbackOnException(status, err);             throw err;         }           try {             getTransactionManager().commit(status);         } finally {             destoryCacheModel(status);         }         return result;     }       /**      * Perform a rollback, handling rollback exceptions properly.      *      * @param status object representing the transaction      *      * @param ex the thrown application exception or error      *      * @throws TransactionException in case of a rollback error      */     private void rollbackOnException(TransactionStatus status, Throwable ex)                                                                             throws TransactionException {         logger.debug("Initiating transaction rollback on application exception", ex);         try {             getTransactionManager().rollback(status);         } catch (RuntimeException ex2) {             logger.error("Application exception overridden by rollback exception", ex);             throw ex2;         } catch (Error err) {             logger.error("Application exception overridden by rollback error", ex);             throw err;         } finally {             destoryCacheModel(status);         }     }       /**      * Initail current transaction's CacheModel.      *      * <p>      * Must be a new Transaction.      *      * Current transaction's CacheModel must have not been initialized.      * </p>      *      * @param status      */     private void initialCacheModel(TransactionStatus status) {         if (cacheable && status != null && status.isNewTransaction()) {             if (!CacheModel.exist()) {                 CacheModel.initial();             }         }     }       /**      * Destroy current transaction's CacheModel.      *      * <p>      * Must be a new Transaction.      *      * Current transaction's CacheModel must have been initialized.      * </p>      *      * @param status      */     private void destoryCacheModel(TransactionStatus status) {         if (cacheable && status != null && status.isNewTransaction()) {             if (CacheModel.exist()) {                 CacheModel.destroy();             }         }     }       /**      *      * @param cacheable      */     public void setCacheable(boolean cacheable) {         if (!isInitialized) {             this.cacheable = cacheable;         }     }   }

上面的代码重写了TransactionTemplate的 execute方法，加入了对CacheModel的支持。接下来再扩展SqlMapClientTemplate，并重写query方法：

public class CacheSqlMapClientTemplate extends SqlMapClientTemplate {       private static final Log    logger           = LogFactory                                                      .getLog(CacheSqlMapClientTemplate.class);       private static final String DEFAULT_SPARATOR = "_";       /* (non-Javadoc)      * @see org.springframework.orm.ibatis.SqlMapClientTemplate#queryForList(java.lang.String, java.lang.Object, int, int)      */     @Override     public List queryForList(String statementName, Object parameterObject, int skipResults,                              int maxResults) throws DataAccessException {           String key = statementName + DEFAULT_SPARATOR + generateKeyPart(parameterObject)                      + skipResults + maxResults;           List result = queryDataFromCacheModel(key, List.class);           if (result != null) {               return result;         }           result = super.queryForList(statementName, parameterObject, skipResults, maxResults);           addDataToCacheModel(key, result);           return result;     }       /* (non-Javadoc)      * @see org.springframework.orm.ibatis.SqlMapClientTemplate#queryForList(java.lang.String, java.lang.Object)      */     @Override     public List queryForList(String statementName, Object parameterObject)                                                                           throws DataAccessException {           String key = statementName + DEFAULT_SPARATOR + generateKeyPart(parameterObject);           List result = queryDataFromCacheModel(key, List.class);           if (result != null) {               return result;         }           result = super.queryForList(statementName, parameterObject);           addDataToCacheModel(key, result);           return result;     }       /* (non-Javadoc)      * @see org.springframework.orm.ibatis.SqlMapClientTemplate#queryForObject(java.lang.String, java.lang.Object, java.lang.Object)      */     @Override     public Object queryForObject(String statementName, Object parameterObject, Object resultObject)                                                                                                    throws DataAccessException {           String key = statementName + DEFAULT_SPARATOR + generateKeyPart(parameterObject)                      + DEFAULT_SPARATOR + generateKeyPart(resultObject);           Object result = queryDataFromCacheModel(key, Object.class);           if (result != null) {               return result;         }           result = super.queryForObject(statementName, parameterObject, resultObject);           addDataToCacheModel(key, result);           return result;     }       /* (non-Javadoc)      * @see org.springframework.orm.ibatis.SqlMapClientTemplate#queryForObject(java.lang.String, java.lang.Object)      */     @Override     public Object queryForObject(String statementName, Object parameterObject)                                                                               throws DataAccessException {           String key = statementName + DEFAULT_SPARATOR + generateKeyPart(parameterObject);         Object result = queryDataFromCacheModel(key, Object.class);         if (result != null) {             return result;         }           result = super.queryForObject(statementName, parameterObject);           addDataToCacheModel(key, result);           return result;     }       private <T> T queryDataFromCacheModel(Object key, Class<T> type) {         if (CacheModel.exist()) {               logger.info("Look up data from current transaction's CacheModel , KEY={" + key + "}");               Object value = CacheModel.get(key);               if (value != null) {                 return type.cast(value);             } else {                 logger                     .info("Current transaction's CacheModel dosen't contain any data relate with {"                           + key + "}");             }           }         return null;     }       private <T> void addDataToCacheModel(Object key, T value) {         if (null == value) {             return;         }         if (CacheModel.exist()) {             CacheModel.add(key, value);             logger.info("Add data to current transaction's CacheModel  , KEY={" + key + "}");         }     }       private String generateKeyPart(Object source) {         if (source == null) {             return "";         } else if (source instanceof String) {             return (String) source;         } else if (source instanceof Map) {             return generate((Map) source);         } else {             try {                 return generate(BeanUtils.describe(source));             } catch (Exception e) {                 return source.toString();             }         }     }       private String generate(Map props) {         try {             Iterator iProps = props.keySet().iterator();             StringBuffer retBuffer = new StringBuffer();             while (iProps.hasNext()) {                 String key = (String) iProps.next();                 if ("class".equals(key)) {                     continue;                 }                 retBuffer.append(key).append("=[").append(props.get(key)).append("]");                 if (iProps.hasNext()) {                     retBuffer.append(", ");                 }             }             return retBuffer.toString();         } catch (Exception e) {             return "";         }     } }

类扩展写好了，下面我们看一下如何配置：

<beans default-autowire="byName">     <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"        destroy-method="close">        <property name="driverClassName" value="com.mysql.jdbc.Driver"/>        <property name="url" value="jdbc:mysql://localhost:3306/batchtest"/>        <property name="username" value="root"/>        <property name="password" value="foxdesign"/>     </bean>       <bean id="transactionManager"         class="org.springframework.jdbc.datasource.DataSourceTransactionManager"/>       <bean id="transactionTemplate"        class="com.company.CacheTransactionTemplate">        <property name="cacheable">            <value>true</value>        </property>       </bean>          <bean id="customSqlMapClient"        class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">        <property name="configLocation" value="SqlMapConfig.xml"/>     </bean>         <bean id="sqlMapClientTemplate"        class="com.company.CacheSqlMapClientTemplate">        <property name="sqlMapClient" ref="customSqlMapClient"/>     </bean>            <bean id="userCacheDAO" class="com.company.imp.UserCacheDAOImpl">     </bean> </beans>

 

经过上面的扩展，执行queryForObject和queryForList方法的时候，查询出来的数据将在当前事物内缓存起来，可以看一下下面的测试代码：

public void testWithTrans() {           transactionTemplate.execute(new TransactionCallback() {               public Object doInTransaction(TransactionStatus arg0) {                   userCacheDAO.getUserByName("Tony");                   userCacheDAO.getUserByName("Tony");                   userCacheDAO.getUserByName("Aroma");                   return null;             }         });     }

 

该方案只是抛砖引玉的提供了一些想法，大家有什么更好的方法可以讨论一下。