ibatis源码分析之---缓存

 public Object executeQueryForObject(RequestScope request, Transaction trans, Object parameterObject, Object resultObject)        throws SQLException    {        CacheKey cacheKey = getCacheKey(request, parameterObject);        cacheKey.update("executeQueryForObject");        Object object = cacheModel.getObject(cacheKey);        if(object == CacheModel.NULL_OBJECT)            object = null;        else        if(object == null)        {            object = statement.executeQueryForObject(request, trans, parameterObject, resultObject);            cacheModel.putObject(cacheKey, object);        }        return object;    }

?ibatis的配置文件里面有一个cache项：

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sqlMapConfig >       < settings        cacheModelsEnabled ="true"            enhancementEnabled ="true"            lazyLoadingEnabled ="true"            errorTracingEnabled ="true"            maxRequests ="32"                 maxSessions ="10"                 maxTransactions ="5"                useStatementNamespaces ="false"           />  

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???? cacheModelsEnabled 设置了ibatis在进行查询时是不是要进行缓存，如果设置了，那么在查询时先到缓存中查询，如果缓存中没有，那么就到数据库查询，并且把查询结果存储到缓存里面。

?? ibatis里面的jdbc操作最终是由SqlMapExecutorDelegate类执行的，比如查询

? ：

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public Object queryForObject(SessionScope session, String id, Object paramObject, Object resultObject)        throws SQLException    {        Object object;        MappedStatement ms;        Transaction trans;        boolean autoStart;        object = null;        ms = getMappedStatement(id);        trans = getTransaction(session);        autoStart = trans == null;        RequestScope request;        trans = autoStartTransaction(session, autoStart, trans);        request = popRequest(session, ms);        object = ms.executeQueryForObject(request, trans, paramObject, resultObject);        pushRequest(request);        break MISSING_BLOCK_LABEL_85;        Exception exception;        exception;        pushRequest(request);        throw exception;        autoCommitTransaction(session, autoStart);        autoEndTransaction(session, autoStart);        break MISSING_BLOCK_LABEL_114;        Exception exception1;        exception1;        autoEndTransaction(session, autoStart);        throw exception1;        return object;    }

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?? 如果在配置文嘉里面设置了缓存，那么这时候会使用一个CachingStatement：

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 public Object executeQueryForObject(RequestScope request, Transaction trans, Object parameterObject, Object resultObject)        throws SQLException    {        CacheKey cacheKey = getCacheKey(request, parameterObject);        cacheKey.update("executeQueryForObject");        Object object = cacheModel.getObject(cacheKey);        if(object == CacheModel.NULL_OBJECT)            object = null;        else        if(object == null)        {            object = statement.executeQueryForObject(request, trans, parameterObject, resultObject);            cacheModel.putObject(cacheKey, object);        }        return object;    }

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??? 其中的CashModel是一个线程安全的类。用来根据key取得缓存的对象。

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