Question

我有NetBeans生成的EJB外观，并且JPA提取的实体中的排序集合不起作用。这是代码：

@Stateless
public class CompetitionFacade extends AbstractFacade<Competition> implements CompetitionFacadeLocal {

@PersistenceContext(unitName = "mot_persistence_unit")
private EntityManager em;

@Override
protected EntityManager getEntityManager() {
    return em;
}

public CompetitionFacade() {
    super(Competition.class);
}

@Override
public Competition findAndInitializeGD(Integer idCompetition) {
    Competition entity = em.find(Competition.class, idCompetition);

    entity.getGroupDetailsList().forEach(p -> System.out.println("First lambda " + p.getIdGroupName()));
    new ArrayList<>(entity.getGroupDetailsList()).forEach(p -> System.out.println("Second lambda " + p.getIdGroupName()));

    Collections.sort(entity.getGroupDetailsList(), new Comparator<GroupDetails>() {

        @Override
        public int compare(GroupDetails o1, GroupDetails o2) {
            throw new UnsupportedOperationException("First sorting");
        }

    });

    Collections.sort(new ArrayList<>(entity.getGroupDetailsList()), new Comparator<GroupDetails>() {

        @Override
        public int compare(GroupDetails o1, GroupDetails o2) {
            throw new UnsupportedOperationException("Second sorting");
        }

    });

    return entity;
}

}

AbstractFacade：

public abstract class AbstractFacade<T> {
private Class<T> entityClass;

public AbstractFacade(Class<T> entityClass) {
    this.entityClass = entityClass;
}

protected abstract EntityManager getEntityManager();

// auto-generated methods like find/edit etc.
}

调用此方法会产生以下异常：

Caused by: java.lang.UnsupportedOperationException: Second sorting

为什么排序不会抛出第一个异常？看起来lambda表达式也不起作用。添加这两行后

entity.getGroupDetailsList().forEach(p -> System.out.println("First lambda " + p.getIdGroupName()));
new ArrayList<>(entity.getGroupDetailsList()).forEach(p -> System.out.println("Second lambda " + p.getIdGroupName()));

输出

Info:   Second lambda entities.GroupName[ idGroupName=709 ]
Info:   Second lambda entities.GroupName[ idGroupName=706 ]
Info:   Second lambda entities.GroupName[ idGroupName=707 ]
Info:   Second lambda entities.GroupName[ idGroupName=708 ]

编辑：

这是来自实体的字段

@OneToMany(cascade = CascadeType.ALL, mappedBy = "competition")
private List<GroupDetails> groupDetailsList = new ArrayList<>();

正常的吸气剂

public List<GroupDetails> getGroupDetailsList() {
    return groupDetailsList;
}

EDIT2：我在接受的答案之后找到了令人满意的答案https://stackoverflow.com/a/27980037/3803447，这正确地暗示了我。

Answer 1

如果启用了延迟加载（这是默认设置），则getGroupDetailsList返回的集合实际上是一个特殊的包装类，由JPA提供程序提供，用于在访问后为中间人提取数据集它。尝试调试并查看实际返回的类。

Collections.sort在java 8中没有调用

1 个答案: