第4行对索引进行清理，清理过程中将把所有标记为删除的文档物理删除。
The fourth line cleans up the index. IndexWriter physically deletes from the disk the documents that have been marked as deleted.
第4行对索引进行清理，清理过程中将把所有标记为删除的文档物理删除。
The fourth line cleans up the index. IndexWriter physically deletes from the disk the documents that have been marked as deleted.
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