java - getFirstResult() 返回 0

Question

我最近在做 Java 教程，但遇到了查询问题。起初我将人员添加到数据库。

图片 1:

图片 2:

现在，当我查询此人是否在场时，get firstResult() 返回零 :/。

Person 类示例，带有查询:

@Entity
@NamedQuery(name="findQuery", query="select p from Person p where p.id=:id")
public class Person {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    //@Column(name="personId")
    private long id;
    //@Column(name="personName")
    private String name;
    //@Column(name="personAge")
    private int age;
    //@Column(name="personNumber")
    private int number;

现在检查查询的类:

@Override
public String read(long id) {


    TypedQuery<Person> createNamedQuery = em.createNamedQuery("findQuery", Person.class);
    createNamedQuery.setParameter("id", id);

    int firstResult = createNamedQuery.getFirstResult();
    System.out.println("test: "+firstResult);

    if (createNamedQuery.getFirstResult()>0) {
        Person singleResult = createNamedQuery.getSingleResult();
        creationMessage = "User: " + singleResult.getName() + " added to database with success";
    } else {
        creationMessage = "User was not added to database";
    }
    return creationMessage;
}

当我尝试查找 ID 为 1 的人时，getFirstResult 返回零:/。有人可以帮助我吗，我做错了什么？

我不知道告诉它是否重要，但数据源是 JTA。 :)

Answer 1

来自 Query#getFirstResult() 的 Javadoc :

The position of the first result the query object was set to retrieve. Returns 0 if setFirstResult was not applied to the query object

您从未调用过 setFirstResult，因此 getFirstResult 应该返回零。我怀疑您将此方法混淆为可以检测是否存在匹配记录的方法。相反，为此使用以下代码:

TypedQuery<Person> createNamedQuery = em.createNamedQuery("findQuery", Person.class);
createNamedQuery.setParameter("id", id);
try {
    Person singleResult = createNamedQuery.getSingleResult();
    creationMessage = "User: " + singleResult.getName()+" added to database with success";
}
catch (Exception e) {
    System.out.println("Either less than one row, more than one row, or some other error");
}

getSingleResult() 如果没有找到用户记录，或者如果找到多个记录，将抛出异常。通过捕获异常，您可以确定查询是否成功。迭代结果集通常以这种方式工作；您一次阅读一份记录，并在阅读过程中弄清楚事情。