State-fields that are mapped in serialized form or as lobs may not be portably used in conditional expressions. Note The implementation is not expected to perform such query operations involving such fields in memory rather than in the database. String literals in queries, like Java String literals, use unicode character encoding. The use of Java escape notation is not supported in query string literals Exact numeric literals support the use of Java integer literal syntax as well as SQL exact numeric literal syntax. Approximate literals support the use Java floating point literal syntax as well as SQL approximate numeric literal syntax.
|Published (Last):||22 March 2015|
|PDF File Size:||18.80 Mb|
|ePub File Size:||19.62 Mb|
|Price:||Free* [*Free Regsitration Required]|
Sample Code Sample Application The code snippets discussed in the article are taken from the Java source files used in the sample application accompanying the article.
Looking through the sample archive, you may notice that this is a simple Web application based on the Java Servlet and Java Persistence API technologies. Figure 1 illustrates the sample entities structure. As you can see, it contains a set of entities related to each other with relationships of different types.
Figure 1 Relationships among the entities utilized within the sample application For a detailed instruction on how to set up and then launch the sample app, you can refer to the readme. Both are used to access and manipulate database data, in the long run.
And both use nonprocedural statements—commands recognized by a special interpreter. In particular, these are find, merge, and remove methods. The use of those methods, however, is typically limited to a single entity instance, unless cascading takes effect, of course.
In contrast, JPQL statements do not have such a limitation—you can define bulk update and delete operations over sets of entities, and define queries returning sets of entity instances. Obtain an instance of EntityManager, using injection or explicitly through an EntityManagerFactory instance. In the case of an update or delete operation, though, you must use the executeUpdate method, which returns the number of entity instances updated or deleted.
Now that you have a rough idea of how you can create and then issue a JPQL query, you might want to see some practical examples. Of special interest here are the createQuery method of the EntityManager instance and the getResultList method of the Query instance.
If you need to retrieve a single result, the Query API interface offers the getSingleResult method, as shown in the following example. Note, however, that using getSingleResult will cause an exception if you get multiple results back. With setParameter, you can bind both named and positional parameters.
Here, though, you bind a named parameter. In some situations, you may need to retrieve only some information from the target entity instance or instances, defining a JPQL query against a certain entity field or fields. In JPQL, you also can use a comprised select list, selecting the data only from the entity fields of interest. In that case, however, you need to create the class to which you will cast the query result. In the following section, you will see an example of a JPQL join query whose select list is comprised of the fields derived from more than one entity.
In contrast, the select list of a JPQL join query typically includes a single entity or even a single entity field. The reason for this lies in the nature of the JPA technology. Once you obtain an entity instance, you can then navigate to its related instances using corresponding getter methods.
This approach makes it unnecessary for you to define a query that will return all related entity instances of interest at once. For example, to obtain information about orders along with their line items in SQL, you would need to define a join query on both the purchaseOrders and orderLineItems tables, specifying the fields from the both tables in the select list of the query.
The following snippet shows an example of JPQL join query in action. To better understand how the entities involved are related to each other, you can turn back to Figure 1 shown in the Sample Application section earlier in the article. In the above example, you use the MAX aggregate function in the SELECT clause of the join query in order to determine the highest price product, of those that have been supplied by Tortuga Trading and have been ordered at least once.
A more common situation, however, is when you need to calculate, say, the total price of the ordered products, which have been supplied by a certain supplier. This is where the SUM aggregate function may come in handy.
However, there are ways to work around this issue. In the following example, you define class LineItemSum whose constructor is then used in the select list of the query, taking p. What the LineItemSum constructor does is multiply p. The following snippet shows how all this can be implemented in code: Copy package jpqlexample. LineItemSum p. In most cases, however, you will have to deal with queries that receive an instance or a list of instances of a certain entity.
Retrieved Entity Instances and the Current Persistence Context The query results in the article examples so far have been simply printed out. In real-world applications, though, you may need to perform some further operations on the query results. For example, you may need to update the retrieved instances and then persist them back to the database. This raises the question: are the instances being retrieved by a JPQL query ready to be further processed by the application, or some additional steps are required to make them ready for that?
In particular, it would be interesting to learn in what state, concerning the current persistence context, retrieved entity instances are. If you have some experience with Java Persistence, you should know what a persistence context is. To recap, a persistence context is a set of entity instances managed by the EntityManager instance associated with that context.
Actually, the EntityManager API includes more than twenty methods to manage the lifecycle of entity instances, control transactions, and create instances of Query whose methods are then used to execute the query specified and retrieve the query result. With respect to a persistence context, an entity instance can be in one of the following four states: new, managed, detached, or removed.
To be precise, the entity instances in the removed state are also synchronized, meaning the database records corresponding to those instances are removed from the database. This is because that new PurchaseOrder instance has not been attached to the persistence context.
Here is what the code might look like: Copy This means entity instances retrieved by a JPQL query become automatically managed. The fact is that the first time you access an associated instance it becomes managed automatically.
Here is a simple example showing how all this works in practice: Copy In spite of this fact, the changes made to the first line item in the order will be persisted to the database upon committing the transaction. This happens because both the retrieved entity instances and their associations are automatically attached to the current persistence context.
As mentioned earlier, the former become managed when they are retrieved, and the latter are attached to the context as you access them. In some situations, you may want associations to be attached to the context upon the query execution, rather than upon the first access.
Say, you want to obtain all the orders belonging to a certain customer, upon retrieving that customer instance. Being not part of the explicit query result, the PurchaseOrder entity instances associated with the Customer instance retrieved here are also retrieved and attached to the current persistence context upon the query execution.
The most important thing to understand about native SQL queries created with EntityManager methods is that they, like JPQL queries, return entity instances, rather than database table records. Here is a simple example of a dynamic native SQL query: Copy Among other things, the above example illustrates that you can bind arguments to native query parameters.
In particular, you can bind arguments to positional parameters in the same way as if you were dealing with a JPQL query. The main disadvantage of native queries is complexity of result binding. In the example, the query produces a single result of a simple type, thus avoiding this problem. In practice, however, you often have to deal with a result set of a complex type. In this case, you will have to declare an entity to which you can map your native query, or define a complex result set mapped to multiple entities or to a blend of entities and scalar results.
Using Stored Procedures Another disadvantage of native queries is that your Java code becomes directly dependent on the underlying database structure. To work around this problem, while still using native queries, you might take advantage of stored procedures, moving complex SQL queries into programs stored and executed inside the database, and then calling those stored programs instead of making direct calls to the underlying tables.
What this means in practice is that stored procedures may save you the trouble of dealing with the underlying tables directly from the queries that are hard-coded in your Java code. Instead, only the stored procedures will need fixing. Conclusion As you learned in this article, JPQL is a powerful tool when it comes to accessing relational data from within Java applications utilizing Java Persistence. Yuli Vasiliev is a software developer, freelance author, and consultant currently specializing in open source development, Java technologies, databases, and SOA.
JPA JPQL Introduction
Greater or equal then: author. Is null: author. In: author. Operators for collection expressions: Is empty: author. Size: size author. Member of: :myBook member of author.
Querying JPA Entities with JPQL and Native SQL
The field values are extracted from or projected out of entity objects to form the query results. The results of the above query are received as a list of String values: TypedQueryjavax. See JavaDoc Reference Page Nested path expressions are also supported. Such queries are useful mainly for displaying information efficiently. They are less productive with operations that update or delete entity objects, in which managed entity objects are needed.