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First suggestion: move to 7.2, where sorting results is a built-in capability: https://portal.liferay.dev/docs/7-2/user/-/knowledge_base/u/sorting-search-results-with-the-sort-widget
On 7.1, this appears to be the general pattern fo building a sort and adding it to the search context:

SortFactory sortFactory = new SortFactoryImpl();

Sort sort = sortFactory.create(String fieldName, int type, boolean reverse);

searchContext.setSorts(sort));

If that doesn't push you toward a solution, you might have to be more specific with your question. I don't know if I can help you, but it makes it easier for a real expert to help if it's clear what you're trying to do, what you've tried, and how it failed.
Good luck!

Thanks for your help, but it is still not working.

Same as like 7.2 i want to implement into 7.1,  i tried many time but still not working.
I write one IndexerPostProcessor and below is a code snipest.    

@Override
    public void postProcessSearchQuery(BooleanQuery searchQuery, BooleanFilter booleanFilter,
            SearchContext searchContext) throws Exception {

        searchContext.setSorts(new Sort(Field.PUBLISH_DATE, Sort.SCORE_TYPE, false));
    }

Hy ,

Don't forget to add your sorted column on this property (portal-ext.properties) : 

index.sortable.text.fields=...