How set where in templates using jpa in CriteriaQuery

I am beginner in java EE and I need know, how set a clausule where in this code of criteria

public <T> List<T> findEntity(Class<T> entityClass) {
    CriteriaQuery<T> criteria = builder.createQuery(entityClass);
    Root<T> entityRoot = criteria.from(entityClass);;
    return em.createQuery(criteria).getResultList();

They will notice that I am using templates in java (<T>) to make this code work with various entities from my database.

Then I pass the sql (in postgresql) code and the entity class.


  id integer NOT NULL,
  name text NOT NULL,
  _modified timestamp without time zone,
  _user integer,
  _enable boolean,

And class entity

public class activity implements Serializable {
    private static final long serialVersionUID = 1L;

    @GeneratedValue( strategy = GenerationType.SEQUENCE, generator = "activity_id_activity_seq" )
    @SequenceGenerator( name = "actividad_id_actividad_seq", sequenceName = "actividad_id_actividad_seq", allocationSize = 1, initialValue = 110 )
    @Column(name = "id")
    private Integer id;
    @Column(name = "name")
    private String name;
    @Column(name = "_modified")
    private Date modified;
    @Column(name = "_enable")
    private Boolean enable;
    @Column(name = "_user")
    private Integer user;

I need to know is how to add a where clause in the code of the function findEntity using methods template.

The where clause of criteria should be matching the column _Enable, this column this column mentioned is repeated in 4 tables in my database, so you note that it is better to reuse code in that function.



Here is an example how to add where() clause:

public static <T> List<T> findEntity(Class<T> entityClass, boolean isEnabled) {
    CriteriaQuery<T> criteria = builder.createQuery(entityClass);
    Root<T> entityRoot = criteria.from(entityClass);;

            entityRoot.get("enable"), //path expression
            builder.parameter(Boolean.class, "isEnabled")) //parameter expression

    return em.createQuery(criteria)
            .setParameter("isEnabled", isEnabled)

The entityRoot.get("enable") statement defines Path<String> expression. This is equivalent of a.enable which uses a colon to denote a parameter in JPQL, i.e.

SELECT a FROM Activity a WHERE a.enable = :isEnabled

To build conditional expression one must also create a ParameterExpression<Boolean> which corresponds to the parameter of typed query that executes our criteria.

Using this approach you can easily extend your generic method with new parameters.

I hope it helps.

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