Defining an Object-Relational Map with Service Builder

In this tutorial, you’ll learn how to define an object relational map for your application so that it can persist data. As an example, you’ll examine the existing Liferay Bookmarks application that uses Service Builder.

The Bookmarks application is an example portlet project that an organization can use to bookmark assets in Liferay. The application defines two entities, or model types, to represent an organization’s bookmarks and their folders. These entities are called bookmark entries and bookmark folders. Since a bookmark must have a folder (even if it’s a root folder), the entry entity references a folder entity as one of its attributes.

You can design your application’s modules anyway you like, but for the Bookmarks application, its Java sources reside in the bookmarks-api, bookmarks-service, and bookmarks-web modules. Notice the and files in the com.liferay.bookmarks.web.portlet package in the bookmarks-web module. These portlet classes extend Liferay’s MVCPortlet class. They act as the controllers in the MVC pattern. These classes contain the business logic that invokes the Service Builder generated bookmarks services that you’ll learn how to create in this section. The application’s view layer is implemented in the JSPs in the bookmarks-web/src/main/resources/META-INF/resources folder.

You can learn how to generate a generic modular application from scratch that includes the *api, *service, and *web modules by default in the Modularizing an Existing Portlet tutorial. This tutorial assumes you’ve assembled your application’s modules similarly to the linked tutorial above. Be sure to also visit the Fundamentals tutorial for additional info on the *api, *service, and *web modules.

The first step in using Service Builder is to define your model classes and their attributes in a service.xml file. This file’s location typically resides in the root folder of the *-service module, although you can configure your build tool to recognize it from other directories. In Service Builder terminology, your model classes are called entities. For example, the Bookmarks application has two entities: BookmarksEntry and BookmarksFolder. The requirements for each of these entities are defined in the bookmarks-service module’s service.xml listed in the <column /> elements.

Once Service Builder reads the service.xml file, you can define your entities. Liferay Developer Studio makes it very easy to define entities in your application’s service.xml file. To define a custom entity, follow these steps:

  1. Create the service.xml file in your project’s *-service module. It resides in the root folder of that module, if one does not already exist there.

  2. Define global information for the service.

  3. Define service entities.

  4. Define the columns (attributes) for each service entity.

  5. Define relationships between entities.

  6. Define a default order for the entity instances to be retrieved from the database.

  7. Define finder methods that retrieve objects from the database based on specified parameters.

You’ll examine these steps in detail next, starting with creating a service.xml file.

Step 1: Creating the service.xml File

To define a service for your portlet project, you must create a service.xml file. The DTD (Document Type Declaration) file specifies the format and requirements of the XML to use. You can create your service.xml file manually, following the DTD, or you can use Liferay Developer Studio. Developer Studio helps you build the service.xml file piece-by-piece, taking the guesswork out of creating XML that adheres to the DTD.

If a default service.xml file already exists in your *-service module folder, check to see if it has an <entity /> element named Foo. If it has the Foo entity, remove the entire <entity name="Foo" ...> ... </entity> element. The Liferay Developer Studio project wizard creates the Foo entity as an example. It has no practical use for you.

If you don’t already have a service.xml file, create one in your *-service module. Once it’s created, open it. Liferay Developer Studio provides a Diagram mode and a Source mode to give you different perspectives of the service information in your service.xml file. Diagram mode is helpful for creating and visualizing relationships between service entities. Source mode brings up the service.xml file’s raw XML content in the editor. You can switch between these modes as you wish.

Next, you can start filling out the global information for your service.

Step 2: Defining Global Service Information

A service’s global information applies to all of its entities, so it’s a good place to start. In Liferay Developer Studio, select the Service Builder node in the upper left corner of the Overview mode of your service.xml file. The main section of the view now shows the Service Builder form in which you can enter your service’s global information. The fields include the service’s package path, author, and namespace options.

Figure 1: This is the Service Builder form from a fictitious Event Listing applications service.xml.

Figure 1: This is the Service Builder form from a fictitious Event Listing application's `service.xml`.

The package path specifies the package in which the service and persistence classes are generated. The package path defined above ensures that the service classes are generated in the package in the *-api module. The persistence classes are generated in a package of the same name in the *-service module. For examples, you can look in the Bookmarks application’s bookmarks-api and bookmarks-service modules to see an example of how these are automatically generated for you. Refer to the Running Service Builder and Understanding the Generated Code tutorial for a description of the contents of these packages.

Service Builder uses the service namespace in naming the database tables it generates for the service. For example, Event could serve as the namespace for an Event Listing portlet service.


Service Builder uses the namespace in the following SQL scripts it generates in your src/main/resources/META-INF/sql folder:

  • indexes.sql
  • sequences.sql
  • tables.sql

Liferay DXP uses these scripts to create database tables for all the entities defined in the service.xml file. Service Builder prepends the namespace to the database table names. Since the namespace value above is Event, the names of the database tables created for the entities start with Event_ as their prefix. The namespace for each Service Builder project must be unique. Separate plugins should use separate namespaces and should not use a namespace already used by Liferay (such as Users or Groups). Check the table names in Liferay’s database if you’re wondering which namespaces are already in use.

Warning: Use caution when assigning namespace values. Some databases have strong restrictions on database table and column name lengths. The Service Builder Gradle and Maven plugin parameter databaseNameMaxLength sets the maximum length you can use for your table and column names. Here are paraphrased examples of setting databaseNameMaxLength in build files:

Gradle build.gradle

buildService {
    databaseNameMaxLength = 64

Maven pom.xml


As the last piece of global information, enter your name as the service’s author in your service.xml file. Service Builder adds @author annotations with the specified name to all of the generated Java classes and interfaces. Save your service.xml file to preserve the information you added. Next, you’ll add entities for your service’s events and locations.

Step 3: Defining Service Entities

Entities are the heart and soul of a service. Entities represent the map between the model objects in Java and the fields and tables in your database. Once your entities are defined, Service Builder handles the mapping automatically, giving you a facility for taking Java objects and persisting them. For the Bookmarks application, two entities are created according to its service.xml –one for bookmark entries and one for bookmark folders.

Here’s a summary of the information used for the BookmarksEntry entity:

  • Name: BookmarksEntry
  • Local service: yes
  • Remote service: yes

And here’s what was used for the BookmarksFolder entity:

  • Name: BookmarksFolder
  • Local service: yes
  • Remote service: yes

To create your entities using Liferay Developer Studio, select the Entities node under the Service Builder node in the outline on the left side of the service.xml editor in Overview mode. In the main part of the view, notice that the Entities table is empty. Create an entity by clicking on the Add Entity icon (Add) to the right of the table. Enter your entity’s name and if you’d like to generate local and remote services for that entity. Add as many entities as you need.

Figure 2: Adding service entities is easy with Liferay Developer Studios Overview mode of your service.xml file.

Figure 2: Adding service entities is easy with Liferay Developer Studio's *Overview* mode of your `service.xml` file.

An entity’s name is used to name the database table for persisting instances of the entity. The actual name of the database table is prefixed with the namespace; the Bookmarks example creates one database table named Bookmarks_BookmarksEntry and another named Bookmarks_BookmarksFolder.

Setting the local service attribute to true instructs Service Builder to generate local interfaces for the entity’s services. The default value for local service is false. Local services can only be invoked from the Liferay server on which they’re deployed. Therefore, if your application will be deployed to Liferay, the service will be local from your Liferay server’s point of view.

Setting the remote service attribute to true instructs Service Builder to generate remote interfaces for the service. The default value for remote service is true. You could build a fully-functional application without generating remote services. In that case, you could set local service to true and remote service to false for your entities. If, however, you want to enable remote access to your application’s services, you should set both local service and remote service to true.

Now that you’ve seen how to create your application’s entities, you’ll learn how to describe their attributes using entity columns.

Step 4: Defining the Columns (Attributes) for Each Service Entity

Each entity is described by its columns, which represent an entity’s attributes. These attributes map on the one side to fields in a table and on the other side to attributes of a Java object. To add attributes for your entity, you need to drill down to its columns in the Overview mode outline of the service.xml file. From the outline, expand the Entities node and expand an entity node. Then select the Columns node. Liferay Developer Studio displays a table of the entity’s columns.

Service Builder creates a database field for each column you add to the service.xml file. It maps a database field type appropriate to the Java type specified for each column, and it does this across all the databases Liferay supports. Once Service Builder runs, it generates a Hibernate configuration that handles the object-relational mapping. Service Builder automatically generates getter/setter methods in the model class for these attributes. The column’s Name specifies the name used in the getters and setters that are created for the entity’s Java field. The column’s Type indicates the Java type of this field for the entity. If a column’s Primary (i.e., primary key) attribute value is set to true, then the column becomes part of the primary key for the entity. An entity’s primary key is a unique identifier for the entity. If only one column has Primary set to true, then that column represents the entire primary key for the entity. This is the case in the Event Listing example. However, it’s possible to use multiple columns as the primary key for an entity. In this case, the combination of columns makes up a compound primary key for the entity.

Similar to the way you used the form table for adding entities, add attribute columns for each of your entities. Create each attribute by clicking on the Add icon (Add). Then fill in the name of the attribute, select its type, and specify whether it is a primary key for the entity. While your cursor is in a column’s Type field, an option icon appears. Click this icon to select the appropriate type for the column. Create a column for each attribute of your entity or entities.

In addition to columns for your entity’s primary key and attributes, it’s recommended to add columns for portal instance ID and site ID. They allow your portlet to support the multi-tenancy features of Liferay, so that each portal instance and each site in a portal instance can have independent sets of portlet data. To hold the site’s ID, add a column called groupId of type long. To hold the portal instance’s ID, add a column called companyId of type long. If you’d like to add these columns to your entities, follow the table below.

Portal and site scope columns


You’ll also want to know who owns each entity instance. To keep track of that, add a column called userId of type long.

User column


Lastly, you can add columns to help audit your entities. For example, you could create a column named createDate of type Date to note the date an entity instance was created. And add a column named modifiedDate of type Date to track the last time an entity instance was modified.

Audit columns


Great! Your entities are set with the columns that not only represent their attributes, but also support multi-tenancy and entity auditing. Next, you’ll learn how to specify the relationship service entities.

Step 5: Defining Relationships Between Service Entities

Often you’ll want to reference one type of entity in the context of another entity. That is, you’ll want to relate the entities. Liferay’s Bookmarks application defines a relationship between an entry and its folder.

As mentioned earlier, each bookmark must have a folder. Therefore, each BookmarksEntry entity must relate to a BookmarksFolder entity. Liferay Developer Studio’s Diagram mode for service.xml makes relating entities easy. First, select Diagram mode for the service.xml file. Then select the Relationship option under Connections in the palette on the right side of the view. This relationship tool helps you draw relationships between entities in the diagram. Click your first entity and move your cursor over to the entity you’d like to relate it with. Liferay Developer Studio draws a dashed line from your selected entity to the cursor. Click the second entity to complete drawing the relationship. Liferay IDE turns the dashed line into a solid line, with an arrow pointing to the second entity. Save the service.xml file.

Congratulations! You’ve related two entities. Their relationship should show in Diagram mode and look similar to that of the figure below.

Figure 3: Relating entities is a snap in Liferay Developer Studios Diagram mode for service.xml.

Figure 3: Relating entities is a snap in Liferay Developer Studio's *Diagram* mode for `service.xml`.

Switch to Source mode in the editor for your service.xml file and note that Liferay Developer Studio created a column element in the first selected entity to hold the ID of the corresponding entity instance reference. For example:

<column name="folderId" type="long" />

Now that your entity columns are in place, you can specify the default order in which the entity instances are retrieved from the database.

Step 6: Defining Ordering of Service Entity Instances

Often, you want to retrieve multiple instances of a given entity and list them in a particular order. Liferay lets you specify the default order of the entities in your service.xml file.

Suppose you want to return BookmarksEntry entities alphabetically by name. It’s easy to specify these default orderings using Liferay Developer Studio. Switch back to Overview mode in the editor for your service.xml file. Then select the Order node under the entity node in the outline on the left side of the view. The IDE displays a form for ordering the chosen entity. Check the Specify ordering checkbox to show the form for specifying the ordering. Create an order column by clicking the Add icon (Add) to the right of the table. Enter the column name (e.g., name, date, etc.) to use in ordering the entity. Click the Browse icon (Browse) to the right of the By field and choose the asc or desc option. This orders the entity in ascending or descending order.

Now that you know how to order your service entities, the last thing to do is to define the finder methods for retrieving entity instances from the database.

Step 7: Defining Service Entity Finder Methods

Finder methods retrieve entity objects from the database based on specified parameters. You’ll probably want to create at least one finder method for each entity you create in your services. Service Builder generates several methods based on each finder you create for an entity. It creates methods to fetch, find, remove, and count entity instances based on the finder’s parameters.

For many applications, it’s important to be able to find its entities per site. You can specify these finders using Liferay Developer Studio’s Overview mode of service.xml. Select the Finders node under the entity node in the Outline on the left side of the screen. The IDE displays an empty Finders table in the main part of the view. Create a new finder by clicking the Add icon (Add) to the right of the table. Give your finder a name and return type. Use the Java camel-case naming convention when naming finders since the finder’s name is used to name the methods that Service Builder creates. The IDE creates a new finder sub-node under the Finders node in the outline. Next, you’ll learn how to specify the finder column for this node.

Under the new finder node, the IDE created a Finder Columns node. Select the Finder Columns node to specify the columns for your finder’s parameters. Create a new finder column by clicking the Add icon and specifying the column’s name. Keep in mind that you can specify multiple parameters (columns) for a finder.

Figure 4: Creating Finder entities is easy with Liferay Developer Studio.

Figure 4: Creating Finder entities is easy with Liferay Developer Studio.

If you’re creating site-scoped entities (entities whose data should be unique to each site), you should follow the steps described above to create finders by groupId for retrieving your entities. Remember to save your service.xml file after editing it to preserve the finders you define.

When you run Service Builder, it generates finder-related methods (e.g., fetchByGroupId, findByGroupId, removeByGroupId, countByGroupId) for the your entities in the *Persistence and *PersistenceImpl classes. The first of these classes is the interface; the second is its implementation. For example, Liferay’s Bookmarks application generates its entity finder methods in the -Persistence classes found in the /bookmarks-api/src/main/java/com/liferay/bookmarks/service/persistence folder and the -PersistenceImpl classes in the /bookmarks-service/src/main/java/com/liferay/bookmarks/service/persistence/impl folder.

Now you know to configure Service Builder to create finder methods for your entity. Terrific!

Now that you’ve specified the service for your project, you’re ready to build the service by running Service Builder. To learn how to run Service Builder and to learn about the code that Service Builder generates, please refer to the Running Service Builder and Understanding the Generated Code tutorial.

What is Service Builder?

Running Service Builder and Understanding the Generated Code

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