NamedQuery is another useful feature provided by hibernate. Sometimes, we require a query or a bunch of queries multiple times in the life of an application; at such a time, this feature helps.
Let's create a scenario to understand this feature.
Let's consider that we want to search a category by name. The following code shows how NamedQuery would help us in this case.
For this, we will use the @NamedQuery and @NamedQueries annotations in a class:
@NamedQuery: This annotation is used to define a single named query@NamedQueries: This annotation is used to define multiple queries
Update the following code in their respective files:
Source file: Category.java
@NamedQuery(name="getCategoryNameByName", query="FROM Category c WHERE c.name=:name")
@Entity
@Table(name = "category")
public class Category {
// fields and getters/setters
}Here, we defined NamedQuery using the @NamedQuery annotation, and we used two attributes.
The name attribute accepts string, which helps to identify NamedQuery. For example: name="getCategoryNameByName".
The query attribute accepts string. This defines a query, which can be either SQL or HQL. For example: query="FROM Category c WHERE c.name=:name".
Take a look at the following code:
/* Line 1 */ Query query = session.getNamedQuery("getCategoryNameByName");
/* Line 2 */ query.setString("name", "Stationary");
List list = query.list();
System.out.println("Category size: " + list.size());Hibernate: select category0_.id as id1_, category0_.created_on as created2_1_, category0_.name as name1_ from category category0_ where category0_.name=? Category size: 1
We can determine from the output that hibernate will get a query using the name parameter shown in Line 1. We set a named parameter, "name", using the setString(…) method shown in Line 2.
Note
The value in the name parameter should be unique to the application. If hibernate finds a value of a parameter multiple times, it will throw an error. This is similar to "Duplicate query mapping getCategoryNameByName", if the name getCategoryNameByName were defined multiple times in an application.
Now, let's take a look at how to use @NamedQueries to define multiple NamedQueries.
In the previous section, we created NamedQuery for "getCategoryNameByName". Now, we need one more query, such as "getCategoryNameById". Let's take a look at how to define it:
Source file: Category.java
@NamedQueries(
{
@NamedQuery(
name="getCategoryNameByName",
query="FROM Category c WHERE c.name=:name"
),
@NamedQuery(
name="getCategoryNameById",
query="FROM Category c WHERE c.id=:id"
),
}
)
@Entity
@Table(name = "category")
public class Category {
// fields and getters/setters
}This works in the same way as @NamedQuery. This is only used to define multiple NamedQueries.