IndexReader is an abstract class, providing an interface for accessing an index. Search of an index is done entirely through this abstract interface, so that any subclass which implements it is searchable.
Concrete subclasses of IndexReader are usually constructed with a call to one of the static open() methods, e.g. {@link #open(Directory,boolean)}.
For efficiency, in this API documents are often referred to via document numbers, non-negative integers which each name a unique document in the index. These document numbers are ephemeral--they may change as documents are added to and deleted from an index. Clients should thus not rely on a given document having the same number between sessions.
An IndexReader can be opened on a directory for which an IndexWriter is opened already, but it cannot be used to delete documents from the index then.
NOTE: for backwards API compatibility, several methods are not listed as abstract, but have no useful implementations in this base class and instead always throw UnsupportedOperationException. Subclasses are strongly encouraged to override these methods, but in many cases may not need to.
NOTE: as of 2.4, it's possible to open a read-only IndexReader using the static open methods that accept the boolean readOnly parameter. Such a reader has better concurrency as it's not necessary to synchronize on the isDeleted method. You must specify false if you want to make changes with the resulting IndexReader.
NOTE: {@link IndexReader} instances are completely threadsafe, meaning multiple threads can call any of its methods, concurrently. If your application requires external synchronization, you should not synchronize on the IndexReader instance; use your own (non-Lucene) objects instead.