Accessing Lucene natively in Java

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Accessing Lucene natively
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the examples were generated outside the Hibernate Search framework, we were required to explicitly close them. In the next section, we ll show you how to obtain an instance of an IndexReader within the confines of Hibernate Search so that the framework will take care of closing the reader for you.
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11.2 Obtaining and using a Lucene IndexReader within the framework
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Looking at listing 11.6 you can see that before you obtain an instance of an IndexReader you must first get a reference to a ReaderProvider instance.
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Listing 11.6 Obtaining an instance of a Lucene IndexReader
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DirectoryProvider orderProvider = searchFactory.getDirectoryProviders(Order.class) [0]; DirectoryProvider clientProvider = searchFactory.getDirectoryProviders(Client.class) [0];
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Obtain DirectoryProviders
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ReaderProvider readerProvider = searchFactory.getReaderProvider(); Obtain a reader from IndexReader reader = a ReaderProvider readerProvider.openReader(orderProvider, clientProvider); try { //do read-only operations on the reader } finally { readerProvider.closeReader(reader); Close in a } finally block
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When ReaderProviders are created, they take into account the ReaderStrategy configuration property hibernate.search.reader.strategy, which can have any full classname of an implementation (see 9.2.2.1 for details), or use the shared and notshared keywords, the default value being shared. This option is present because one of the most time-consuming operations in Lucene is opening an IndexReader on a particular index. If your code has many users, and the reader strategy is set to notshared, this could result in performance problems depending on how often new index readers are needed. Using the shared IndexReader will make most queries much more efficient. To solve this problem and maximize performance, Hibernate Search caches all instances of IndexReader when the ReaderProvider is set to shared. This means that there are some simple additional good citizen rules that you ll have to follow:
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Never call indexReader.close() on a reader that was obtained via a ReaderProvider, but always call readerProvider.closeReader (reader); a finally block is the best place to do this, as shown in listing 11.6.
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Writing a DirectoryProvider your way
This IndexReader must not be used for modification operations (it s highly likely that in a future release we won t allow this to happen). If you want to use a read/write IndexReader, create an instance of one by calling the static IndexReader.open(Directory directory) method, like this:
DirectoryProvider[] provider = searchFactory.getDirectoryProviders(Order.class); org.apache.lucene.store.Directory directory = provider[0].getDirectory(); IndexReader reader = IndexReader.open(directory);
WARNING Failure to follow these simple rules will result in clients not being able
to access indexes. Worse, clients will fail at unpredictable times, making this a very difficult problem to track down. Also, and most important, if you create your own read/write IndexReader, you are responsible for closing it. Aside from these rules, you can use the IndexReader freely. On some occasions you may be required to furnish a custom DirectoryProvider. In the next section we will examine a couple of use cases from Emmanuel s customers who had specific storage requirements. This example shows what you must consider when writing a custom DirectoryProvider.
11.3 Writing a DirectoryProvider your way
It s impossible to cover even a small number of permutations with persistent storage configurations present in business today. First we ll discuss the FSSlaveDirectoryProvider classes methods and what you must take into consideration for each of them. Then we ll talk about two situations that exemplify what you must take into account when writing your own DirectoryProvider. We won t go into detail with code, but we will discuss the use case requirements. The FSSlaveDirectoryProvider is configured with these two directory settings:
hibernate.search.default.sourceBase = directory of master copy hibernate.search.default.indexBase = directory of local copy for querying
These settings specify where the master index is located and also where the copies of the master index are to be placed. Now let s investigate the FSSlaveDirectoryProvider class hierarchy. Examining figure 11.3 we see that there are four methods of the DirectoryProvider interface that it must implement:
void initialize(String directoryProviderName, Properties properties, SearchFactoryImplementor searchFactoryImplementor); This method is a
lightweight initialization that determines the location of the master index from the sourceBase setting and the slave index location(s) from the indexBase setting. It also determines the index name to allow the equals and hashCode to work as they must.
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