truncate/delete from given the entity class

问题

I have my entity class available via a method. I'm trying to figure out, how via the JPA JPQL or Criteria API's I could issue a truncate or delete from. I think that the criteria api is more natural for working with classes, and truncate is a faster operation so these are prefered. This is what I put together so far, but not sure what to add/change about it.

    CriteriaBuilder cb = this._em().getCriteriaBuilder();
    cb.createQuery( _entityClass() ).from( _entityClass() );

note: _entityClass returns MyEntity.class, I have no other references to MyEntity this is a more generalized implementation.

回答1:

Assuming that MyEntity refers to the table you want to drop you can proceed as follows:

// Criteria API (JPA 2.1 and above)
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaDelete<MyEntity> query = builder.createCriteriaDelete(MyEntity.class);
query.from(MyEntity.class);
em.createQuery(query).executeUpdate();

or with a generalized approach:

public <T> int deleteAllEntities(Class<T> entityType) {
    CriteriaBuilder builder = em.getCriteriaBuilder();
    CriteriaDelete<T> query = builder.createCriteriaDelete(entityType);
    query.from(entityType);
    return em.createQuery(query).executeUpdate();
}

Similarly for JPQL/SQL queries:

// JPQL
em.createQuery("DELETE FROM MyEntity e").executeUpdate();

// SQL
em.createNativeQuery("TRUNCATE TABLE MyEntity").executeUpdate();

or with a generalized approach:

public static <T> int deleteAllEntities(Class<T> entityType) {
    String query = new StringBuilder("DELETE FROM ")
                            .append(entityType.getSimpleName())
                            .append(" e")
                            .toString();
    return em.createQuery(query).executeUpdate();
}

public static <T> int truncateTable(Class<T> entityType) {
    String query = new StringBuilder("TRUNCATE TABLE ")
                            .append(entityType.getSimpleName())
                            .toString();        
    return em.createNativeQuery(query).executeUpdate();
}

With Criteria API you can only use SELECT, UPDATE, DELETE statements therefore TRUNCATE is not possible.

来源：https://stackoverflow.com/questions/23269885/truncate-delete-from-given-the-entity-class