TypedQuery<T>.getSingleResult 返回 Class 类型的对象而不是 T 类型

Question

我已经花了两天时间解决这个问题，但我离解决它还差得远。我希望那里的一位专家可以提供帮助。

这就是问题所在。我正在使用 TomEE 1.6.0.2 (OpenEJB 4.6.0.2) 并尝试做一个返回单个结果的简单 TypedQyery，我得到了最奇怪的返回值。这不是我期望的 bean 对象——它是一个 java.lang.Class！

代码片段如下：

public static Account getAccount(String type, String key, EntityManager em) {

    try {
        TypedQuery<Account> query = em.createNamedQuery("Account.getByTypeAndKey", Account.class)
            .setParameter("type", type)
            .setParameter("key", key)
            .setMaxResults(1);

        Account account = query.getSingleResult();  // This statement throws an exception
    }
    catch (Exception e) {
        System.out.println(e.getMessage());         // "java.lang.Class cannot be cast to beans.Account"
    }

    try {
        TypedQuery<Account> query = em.createNamedQuery("Account.getByTypeAndKey", Account.class)
                .setParameter("type", type)
                .setParameter("key", key)
                .setMaxResults(1);

        Object object = query.getSingleResult();    // This works

        System.out.printf("object is '%s'\n", object.toString());
        // "object is 'class beans.Account'"

        Account account = new Account();
        System.out.printf("account is '%s'\n", account.toString());
        // "account is 'Account{id=0, type='null, etc... }'"

        return account;
    }
    catch (Exception e) {
        System.out.println(e.getMessage());         // Never gets here
    }
}

这是 Account 类的一个片段：

@Entity
@Table(name="Account")
@NamedQueries({
    @NamedQuery(name = "Account.getByTypeAndKey", query = "SELECT Account FROM Account rec WHERE rec.key = :key AND rec.type = :type"),
})
public class Account implements Serializable {

    private int id;
    private String type;

    // The usual stuff ...
}

这里是persistence.xml：

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="2.0">

    <persistence-unit name="unipagosPersistenceUnit" transaction-type="JTA">
        <provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider>

        <jta-data-source>PAY_AccountDSJta</jta-data-source>
        <non-jta-data-source>PAY_AccountDSNonJta</non-jta-data-source>

        <class>beans.Account</class>

        <properties>
            <property name="openjpa.DynamicEnhancementAgent" value="true"/>
            <property name="openjpa.RuntimeUnenhancedClasses" value="supported"/>
            <property name="openjpa.Log" value="SQL=TRACE"/>
            <property name="openjpa.ConnectionFactoryProperties" value="PrettyPrint=true, PrettyPrintLineLength=72, PrintParameters=true, MaxActive=10, MaxIdle=5, MinIdle=2, MaxWait=60000"/>
        </properties>
    </persistence-unit>

</persistence>

我确信这很容易，但我无法弄清楚这一点。怎么回事？！

TIA 为您提供帮助...

Answer 1

您的查询是错误的。应该是

SELECT rec FROM Account rec WHERE rec.key = :key AND rec.type = :type
        ^-- use the alias here, and not the class name