Mittwoch, 1. Juli 2015

Introduction to Hibernate Search with any JPA implementor (in this example: EclipseLink)

Hibernate Search currently is only associated with Hibernate ORM/OGM. For many people this comes at no surprise as it already has “Hibernate” in its name. However during this year’s Google Summer of Code I am working on an integration of Hibernate Search that aims to be able to be used with any JPA implementor (so far Hibernate, EclipseLink and OpenJPA are supported).

In the following article I will show you how you can leverage the power of Hibernate Search even though you chose not to use a Hibernate based persistence provider in your application by using the new Hibernate Search GenericJPA project.


The example project



Imagine a Book store (ok, a really simple one). Information about their books are stored in their MySQL database and accessed via EclipseLink. A sample persistence.xml and Maven pom.xml for that would look like this:



With the Book entity looking like this:



Now, the manager of the store wants to be able to search for a specific book in the database to help the customers find what they want. This is where a normal database like MySQL gets into trouble. Yes, you can do queries like:


SELECT b FROM BOOK b WHERE b.name like ‘%Hobbit%’;


But these kind of queries don’t have all the power a fully fletched search-engine has. In such an engine you have the power to decide over how the name would be indexed. For example, if you want to have fuzzy queries that still return the right book even if you entered “Wobbit” a normal RDBMS is not sufficient. This is where the power of Hibernate Search comes in. Under the hood it uses Lucene, a powerful search-engine and improves it by adding features like clustering and support for mappings of Java Beans.


Adding Search to our Book store



Now, let’s take a look at how this is done for our Book store. First, we need to annotate our JPA entity with some extra information.


What did we do?

  1. Class-Level:
    1. added @Indexed annotation
      1. This is needed for Hibernate Search to recognize this entity-class as a Top-Level index class.
    2. added @InIndex annotation
      1. This is a special annotation needed by Hibernate Search GenericJPA on every entity that somehow is found in any index. Just put it there and you’ll be fine
  2. Field-Level:
    1. added @DocumentId/@Field on name
      1. Hibernate Search needs to know which field is used to identify this entity in the index. this produces a field with the name “id” in the index. We also want to store the name into a field called “name”.
    2. added @Field on author
      1. apart from searching for the name we also want to be able to search for the author of the book. this is stored into the field called “author” in the index.


Starting up the engine



As Hibernate Search GenericJPA is not integrated into Hibernate ORM we have to manually start everything up. But this is not hard, at all:


You may have noticed loading a properties file in this snippet. This is the configuration needed for Hibernate Search. Let’s take a look at it next.


Now we could start using hibernate search for queries.


But wait, why didn’t we receive any Books even though our query was right? Well, that’s because we forgot to add some additional classes to our persistence.xml. In order to keep the index up-to-date we have to specify special entities that can hold the information about changes in the database. These will be queried by our Hibernate Search GenericJPA engine and then the index is updated accordingly. For our example, we need this special entity:


It just has to be in our persistence.xml and the engine will automatically recognize it:


(Note that every table that is related to any entity in the indexed graph has to be mapped like this. But for the sake of keeping it simple, we didn’t include any mapping tables in this example.)


That’s it. Now we should be able to query our index properly and leverage Hibernate Search’s capabilities.


What’s next?


This example is quite simple as it doesn’t make use of Hibernate Search’s possibilities to index a complete hierarchy with many different entities in the graph. Examples on how to do that can be found in the Hibernate Search documentation (http://docs.jboss.org/hibernate/search/5.3/reference/en-US/html_single/). The only thing to keep in mind is that the *Updates entities will have to be created even for the mapping tables.

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