Leveraging Hibernate’s Criteria API

Liferay allows you to use custom SQL queries to retrieve data from the database. Sometimes, however, it’s more convenient to build queries dynamically at runtime than it is to invoke predefined SQL queries. Liferay allows you to build queries dynamically using its DynamicQuery API, which wraps Hibernate’s Criteria API. Using Liferay’s DynamicQuery API allows you to build queries without writing a single line of SQL. The DynamicQuery API helps you think in terms of objects and member variables instead of in terms of tables and columns. Complex queries constructed via Hibernate’s Criteria API can be significantly easier to understand and maintain than the equivalent custom SQL (or HQL) queries. While you technically don’t need to know SQL to construct queries via Hibernate’s Criteria API, you still need to take care to construct efficient queries. For information on Hibernate’s Criteria API, please see Hibernate’s manual. In this tutorial, you’ll learn how to create custom finders for Liferay plugins using Service Builder and Liferay’s Dynamic Query API.

To use Liferay’s Dynamic Query API, you need to create a finder implementation for your model entity. You can define model entities in service.xml and run Service Builder to generate model, persistence, and service layers for your application. See the Writing a Data-Driven Application learning path for more information on using Service Builder. Once you’ve used Service Builder to generate model, persistence, and service layers for your application, you can create custom finders using Liferay’s Dynamic Query API by following these steps:

  1. Create a custom -FinderImpl class and define a findBy- finder method in this class. Run Service Builder to generate the required interfaces and utility classes.

  2. Implement your finder method using Liferay’s Dynamic Query API.

  3. Add a method to your -LocalServiceImpl class that accesses your finder method. Run Service Builder to add the required method to the service interface.

Once you’ve taken these steps, you can access your custom finder as a service method. Note: You can create multiple or overloaded findBy- finder methods in your -FinderImpl class. Next, let’s examine these steps in more detail.

Step 1: Defining a Custom Finder Method

To define any custom query, either by specifying custom SQL or by defining a dynamic query, you need a finder class. Create a [Entity]FinderImpl class in the generated [Plugin Package Path].service.persistence package of your plugin’s docroot/WEB-INF/src folder. Then define a findBy- finder method in this class. Make sure to add any required arguments to your finder method’s method signature.

For example, consider the Guestbook application that we discussed in the Writing a Data-Driven Application. learning path. In that application, there are two entities: guestbooks and entries. Each entry belongs to a guestbook so the entry entity has a guestbookId field as a foreign key. Suppose you need to create a custom finder to search for guestbook entries by the entry name and the guestbook name. In this case, you’d add a custom finder method to GuestbookFinderImpl and name it something like findByEntryNameGuestbookName. The full method signature would appear as findByEntryNameGuestbookName(String entryName, String guestbookName).

Once you’ve created a finder method with the appropriate method signature in your finder class, run Service Builder to generate the appropriate interface and utility class in the [Plugin Package Path].service.persistence package in your plugin’s docroot/WEB-INF/service folder.

For example, after adding findByEntryNameGuestbookName(String entryName, String guestbookName) to GuestbookFinderImpl and running Service Builder, the interface com.liferay.docs.guestbook.service.persistence.GuestbookFinder and the utility class com.liferay.docs.guestbook.service.persistence.GuestbookFinderUtil are generated.

Once the finder interface has been generated, make sure that the finder class implements the interface. For example, the class declaration should look like this:

public class GuestbookFinderImpl extends BasePersistenceImpl<Guestbook> implements GuestbookFinder

Your next step is to actually define your query in your custom finder method using the Dynamic Query API.

Step 2: Implementing Your Custom Finder Method Using Dynamic Query

Your first step in implementing your custom finder method in your -FinderImpl class is to open a new Hibernate session. Since your -FinderImpl class extends BasePersistenceImpl<Entity> and BasePersistenceImpl<Entity> contains a session factory object and an openSession method, you can simply invoke the openSession method of your -FinderImpl’s parent class to open a new Hibernate session. The basic structure of your finder method should look like this:

public List<Entity> findBy-(...) {

    Session session = null;
    try {
            /*
            Try to open a new Hibernate session and create a dynamic
            query to retrieve and return the desired list of entity
            objects
            */
    } catch (Exception e) {
            // Exception handling
    } finally {
            closeSession(session);
    }
    return null;/*
                Return null only if there was an error returning the
                desired list of entity objects in the try block
                */
}

For example, in the case of the Guestbook application, you could write the following finder method to retrieve a list of Guestbook entries that have a specific name and that also belong to a Guestbook of a specific name:

public List<Event> findByEntryNameGuestbookName(String entryName, String guestbookName) {

    Session session = null;
    try {
        session = openSession();

        DynamicQuery guestbookQuery = DynamicQueryFactoryUtil.forClass(Guestbook.class)
            .add(RestrictionsFactoryUtil.eq("name", guestbookName))
            .setProjection(ProjectionFactoryUtil.property("guestbookId"));

        Order order = OrderFactoryUtil.desc("modifiedDate");

        DynamicQuery entryQuery = DynamicQueryFactoryUtil.forClass(Entry.class)
            .add(RestrictionsFactoryUtil.eq("name", entryName))
            .add(PropertyFactoryUtil.forName("guestbookId").in(guestbookQuery))
            .addOrder(order);

        List<Event> entries = EventLocalServiceUtil.dynamicQuery(entryQuery);

        return entries;

    } catch (Exception e) {

        try {

            throw new SystemException(e);

        } catch (SystemException se) {

            se.printStackTrace();

        }

    } finally {

        closeSession(session);

    }

Notice that in Liferay, you don’t create criteria objects directly from the Hibernate session. Instead, you create dynamic query objects using Liferay’s DynamicQueryFactoryUtil service. Thus, instead of

Criteria entryCriteria = session.createCriteria(Entry.class);

you use

DynamicQuery entryQuery = DynamicQueryFactoryUtil.forClass(Entry.class);

Most features of Hibernate’s Criteria API, including restrictions, projections, and orders, can be used on Liferay’s dynamic query objects. Restrictions in Hibernate’s Criteria API roughly correspond to the where clause of an SQL query: they offer a variety of ways to limit the results returned by the query. You can use restrictions, for example, to cause a query to return only results where a certain field has a particular value, or a value in a certain range, or a non-null value, etc.

Projections in Hibernate’s Criteria API allow you to modify the kind of results returned by a query. For example, if you don’t want your query to return a list of entity objects (the default), you can set a projection on a query so that only a list of the values of a certain entity field, or fields, is returned. You can also use projections on a query to return the maximum or minimum value of an entity field, or the sum of all the values of a field, or the average, etc. For more information on restrictions and projections, please refer to Hibernate’s documentation.

Orders, another feature of Hibernate’s Criteria API, allow you to control the order of the elements in the list returned by a query. You can choose which property or properties to apply an order to and you can choose for the properties to appear in ascending or descending order in the list.

Like Hibernate criteria, Liferay’s dynamic queries are chainable. This means that you can add criteria to, set projections on, and add orders to Liferay’s dynamic query objects just by appending the appropriate method calls to the query object. For example, the following snippet demonstrates chaining the addition of a restriction criterion and a projection to a dynamic query object declaration:

DynamicQuery guestbookQuery = DynamicQueryFactoryUtil.forClass(Guestbook.class)
    .add(RestrictionsFactoryUtil.eq("name", guestbookName))
    .setProjection(ProjectionFactoryUtil.property("guestbookId"));

When you need to add restrictions to a dynamic query in Liferay, don’t call Hibernate’s Restrictions class directly. Instead, use the methods of Liferay’s RestrictionsFactoryUtil service. You’ll find the same methods in Liferay’s RestrictionsFactoryUtil service class that you’re used to from Hibernate’s Restrictions class: in, between, like, eq, ne, gt, ge, lt, le, etc.

Thus, instead of

entryCriteria.add(Restrictions.eq("name", guestbookName));

to specify that a guestbook must have a certain name, you use

entryQuery.add(RestrictionsFactoryUtil.eq("name", guestbookName));

Similarly, to set projections, you create properties via Liferay’s PropertyFactoryUtil service instead of through Hibernate’s Property class. Thus, instead of

entryCriteria.setProjection(Property.forName("guestbookId"));

you use

entryQuery.setProjection(PropertyFactoryUtil.forName("guestbookId"));

Notice that in the custom findByGuestbookNameEntryName finder method, there are two distinct dynamic queries. The first query retrieves a list of guestbook IDs corresponding to guestbook names that match the guestbookName parameter of the finder method. The second query retrieves a list of guestbook entries with entry names that match the entryName parameter and have guestbookId foreign keys belonging to the list returned by the first query.

Here’s the first query:

DynamicQuery guestbookQuery = DynamicQueryFactoryUtil.forClass(Guestbook.class)
    .add(RestrictionsFactoryUtil.eq("name", guestbookName))
    .setProjection(ProjectionFactoryUtil.property("guestbookId"));

By default, DynamicQueryFactoryUtil.forClass(Guestbook.class) returns a query that retrieves a list of all guestbook entities. Adding the .add(RestrictionsFactoryUtil.eq("name", guestbookName)) restriction limits the results to only those guestbooks whose guestbook names match the guestbookName parameter. The .setProjection(ProjectionFactoryUtil.property("guestbookId")) projection changes the result set from a list of guestbook entries to a list of guestbook IDs. This is useful since guestbook IDs are much less expensive to retrieve than full guestbook entities and the guestbook IDs are all that the guestbook entry query requires.

Next is an order which applies to the list of entries returned by the findByEntryNameGuestbookName finder method:

Order order = OrderFactoryUtil.desc("modifiedDate");

When this order is applied to a query, the list of results returned by the query are arranged in descending order of the query entity’s modifiedDate attribute. Thus the most recently modified entities (guestbook entries, in our example) appear first and the least recently modified entities appear last.

Here’s the second query:

DynamicQuery eventQuery = DynamicQueryFactoryUtil.forClass(Entry.class)
    .add(RestrictionsFactoryUtil.eq("name", entryName))
    .add(PropertyFactoryUtil.forName("guestbookId").in(guestbookQuery))
    .addOrder(order);

List<Event> entries = EventLocalServiceUtil.dynamicQuery(entryQuery);

By default, DynamicQueryFactoryUtil.forClass(Entry.class) returns of list of all guestbook entry entities. The .add(RestrictionsFactoryUtil.eq("name", entryName)) restriction limits the results to only those guestbook entries whose names match the entryName parameter of the finder method. PropertyFactoryUtil is a Liferay utility class with a single method, forName(String propertyName), that returns the specified property. This property can be passed to another Liferay dynamic query. This is exactly what happens in the following line of our example:

.add(PropertyFactoryUtil.forName("guestbookId").in(guestbookQuery))

Here, the code makes sure that the guestbook IDs (foreign keys) of the entry entities in the entityQuery must belong to the list of guestbook IDs returned by the guestbookQuery. Declaring that an entity property in one query must belong to the result list of another query is a way to use Liferay’s dynamic query API to create complex queries, similar to SQL joins.

Lastly, you apply the order defined earlier to the entries returned by the findByEntryNameGuestbookName finder method:

.addOrder(order);

This orders the list of guestbook entities by the modifiedDate attribute, from most recent to least recent.

Note: Service Builder not only generates a public List dynamicQuery(DynamicQuery dynamicQuery) class in -LocalServiceBaseImpl but it also generates public List dynamicQuery(DynamicQuery dynamicQuery, int start, int end) and public List dynamicQuery(DynamicQuery dynamicQuery, int start, int end, OrderByComparator orderByComparator) methods. You can go back to step 1 and either modify your custom finder method or create overloaded versions of your custom finder method to take advantage of these extra methods and their parameters. The int start and int end parameters are useful when paginating a result list. start is the lower bound of the range of model entity instances and end is the upper bound. The OrderByComparator orderByComparator is the comparator by which to order the results.

To use the overloaded dynamicQuery methods of your -LocalServiceBaseImpl class in the (optionally overloaded) custom finders of your -FinderImpl class, just choose the appropriate methods for running the dynamic queries: EventLocalServiceUtil.dynamicQuery(eventQuery), or EventLocalServiceUtil.dynamicQuery(eventQuery, start, end) or EventLocalServiceUtil.dynamicQuery(eventQuery, start, end, orderByComparator).

Great! You’ve now created a custom finder method using Liferay’s Dynamic Query API. Your last step is to add a service method that calls your finder.

Step 3: Accessing Your Custom Finder Method from the Service Layer

So far, you’ve created a -FinderImpl class and generated a -FinderUtil utility class. Your portlet class, however, should not use the finder utility class directly: only a local or remote service implementation (i.e., -LocalServiceImpl or -ServiceImpl) in your plugin project should invoke the -FinderUtil class. This encourages a proper separation of concerns: the portlet classes invoke the business logic of the services. The services, in turn, access the data model using the persistence layer’s finder classes. So you’ll add a method in the -LocalServiceImpl class that invokes the finder method implementation via the -FinderUtil class. Then you’ll provide the portlet and JSPs access to this service method by rebuilding the service.

For example, for the Guestbook application, you’d add the following method to the EntryLocalServiceImpl class:

public List<Entry> findByEntryNameGuestbookName(String entryName,
    String guestbookName)
throws SystemException {

    return EntryFinderUtil.findByEntryNameGuestbookName(String entryName,
        String guestbookName);
}   

After you’ve added your findBy- method to your -LocalServiceImpl class, run Service Builder to generate the interface and make the finder method available in the EntryLocalServiceUtil class.

Now you can indirectly call the finder method from your portlet class or from a JSP by calling EntryLocalServiceUtil.findByEntryNameGuestbookName(...)!

Congratulations on following the three step process of developing a dynamic query in a custom finder and exposing it as a service for your portlet!

Related Topics

Running Service Builder and Understanding the Generated Code

Using Service Builder to Generate a Persistence Framework

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