JPA Query selecting only specific columns without using Criteria Query?

问题

Is it possible to select, say, only properties A and B from an object using a JPA query without using criteria queries?

To select all properties I'd just do something like:

SELECT i FROM ObjectName i WHERE i.id = 10

But I have an object with many properties on a legacy system, and want to select just a few even though I'm aware selecting several properties is usually quick.

Is this possible without using criteria queries?

Thank you!

回答1:

Yes, like in plain sql you could specify what kind of properties you want to select:

SELECT i.firstProperty, i.secondProperty FROM ObjectName i WHERE i.id=10

Executing this query will return a list of Object[], where each array contains the selected properties of one object.

Another way is to wrap the selected properties in a custom object and execute it in a TypedQuery:

String query = "SELECT NEW CustomObject(i.firstProperty, i.secondProperty) FROM ObjectName i WHERE i.id=10";
TypedQuery<CustomObject> typedQuery = em.createQuery(query , CustomObject.class);
List<CustomObject> results = typedQuery.getResultList();

Examples can be found in this article.

UPDATE 29.03.2018:

@Krish:

@PatrickLeitermann for me its giving "Caused by: org.hibernate.hql.internal.ast.QuerySyntaxException: Unable to locate class ***" exception . how to solve this ?

I guess you’re using JPA in the context of a Spring application, don't you? Some other people had exactly the same problem and their solution was adding the fully qualified name (e. g. com.example.CustomObject) after the SELECT NEW keywords.

Maybe the internal implementation of the Spring data framework only recognizes classes annotated with @Entity or registered in a specific orm file by their simple name, which causes using this workaround.

回答2:

You can use something like this:

List<Object[]> list = em.createQuery("SELECT p.field1, p.field2 FROM Entity p").getResultList();

then you can iterate over it:

for (Object[] obj : list){
    System.out.println(obj[0]);
    System.out.println(obj[1]);
}

BUT if you have only one field in query, you get a list of the type not from Object[]

回答3:

Excellent answer! I do have a small addition. Regarding this solution:

TypedQuery<CustomObject> typedQuery = em.createQuery(query , String query = "SELECT NEW CustomObject(i.firstProperty, i.secondProperty) FROM ObjectName i WHERE i.id=100";
TypedQuery<CustomObject> typedQuery = em.createQuery(query , CustomObject.class);
List<CustomObject> results = typedQuery.getResultList();CustomObject.class);

To prevent a class not found error simply insert the full package name. Assuming org.company.directory is the package name of CustomObject:

String query = "SELECT NEW org.company.directory.CustomObject(i.firstProperty, i.secondProperty) FROM ObjectName i WHERE i.id=10";
TypedQuery<CustomObject> typedQuery = em.createQuery(query , CustomObject.class);
List<CustomObject> results = typedQuery.getResultList();

回答4:

Projections can be used to select only specific properties(columns) of an entity object.

From the docs

Spring Data Repositories usually return the domain model when using query methods. However, sometimes, you may need to alter the view of that model for various reasons. In this section, you will learn how to define projections to serve up simplified and reduced views of resources.

Define an interface with only the getters you want.

interface CustomObject {  
    String getA(); // Actual property name is A
    String getB(); // Actual property name is B 
}

Now return CustomObject from your repository like so :

public interface YOU_REPOSITORY_NAME extends JpaRepository<YOUR_ENTITY, Long> {
    CustomObject findByObjectName(String name);
}

回答5:

I suppose you could look at this link if I understood your question correctly http://www.javacodegeeks.com/2012/07/ultimate-jpa-queries-and-tips-list-part_09.html

For example they created a query like:

 select id, name, age, a.id as ADDRESS_ID, houseNumber, streetName ' +
 20' from person p join address a on a.id = p.address_id where p.id = 1'

回答6:

Yes, it is possible. All you have to do is change your query to something like SELECT i.foo, i.bar FROM ObjectName i WHERE i.id = 10. The result of the query will be a List of array of Object. The first element in each array is the value of i.foo and the second element is the value i.bar. See the relevant section of JPQL reference.

来源：https://stackoverflow.com/questions/24710626/jpa-query-selecting-only-specific-columns-without-using-criteria-query