java - Hibernate Java 中 session 的空指针异常

Question

我正在创建一个简单的应用程序，由于某种原因，我不断收到空指针异常。我的猜测是，这与我的 session.open 位于 if 语句内有关？我收到的错误出现在finally block 上，其中显示session.close();

import java.util.*;
import javax.persistence.*;
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class OrderTester {
    private static SessionFactory sessionFactory;
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        Session session = null;
        Transaction transaction = null;
        System.out.println("Press 1 to log in or press 2 to sign up");
        int n = scan.nextInt();

        if (n == 1) {
            Customer c = new Customer();
            // c.logIn();
        } else if (n == 2) {
            try {
                sessionFactory = HibernateUtil.getSessionFactory();
                session = sessionFactory.openSession();
                transaction = session.beginTransaction();
                Customer c = new Customer();
                Address a = new Address();
                session.save(c);
                session.save(a);

                Scanner read = new Scanner(System.in);
                System.out.println("Enter a username");
                String userName = read.next();
                c.setUsername(userName);

                System.out.println("Enter a password");
                String password = read.next();
                c.setPassword(password);

                System.out.println("Enter the street name");
                String streetName = read.next();
                a.setStreetName(streetName);

                System.out.println("Enter city");
                String city = read.next();
                a.setCity(city);

                System.out.println("Enter state");
                String state = read.next();
                a.setState(state);

                System.out.println("Enter zipcode");
                String zipcode = read.next();
                a.setZipCode(zipcode);
                read.close();

                transaction.commit();
            } catch (Exception ex) {
                transaction.rollback();
                System.out.println("Transaction is rolled back.");
            } finally {
                session.close();
                sessionFactory.close();
            }

        }

    }
}

错误:

Press 1 to log in or press 2 to sign up
2
Mar 05, 2016 4:51:44 PM org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {4.0.5.Final}
Mar 05, 2016 4:51:44 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate Core {4.3.11.Final}
Mar 05, 2016 4:51:44 PM org.hibernate.cfg.Environment <clinit>
INFO: HHH000206: hibernate.properties not found
Mar 05, 2016 4:51:44 PM org.hibernate.cfg.Environment buildBytecodeProvider
INFO: HHH000021: Bytecode provider name : javassist
Mar 05, 2016 4:51:44 PM org.hibernate.cfg.Configuration configure
INFO: HHH000043: Configuring from resource: /hibernate.cfg.xml
Mar 05, 2016 4:51:44 PM org.hibernate.cfg.Configuration getConfigurationInputStream
INFO: HHH000040: Configuration resource: /hibernate.cfg.xml
Mar 05, 2016 4:51:45 PM org.hibernate.internal.util.xml.DTDEntityResolver resolveEntity
WARN: HHH000223: Recognized obsolete hibernate namespace http://hibernate.sourceforge.net/. Use namespace http://www.hibernate.org/dtd/ instead. Refer to Hibernate 3.6 Migration Guide!
Exception in thread "main" java.lang.NullPointerException
    at Pizza.OrderTester.main(OrderTester.java:65)

Answer 1

该行上的 NullPointerException 在堆栈跟踪中没有其他内容只能意味着 session 为空。据我所知，唯一可能发生的情况是，如果 session = sessionFactory.openSession(); 赋值从未执行 - 这意味着在 中发生异常sessionFactory.openSession() 或 HibernateUtil.getSessionFactory() 中，该异常被 finally block 中引发的后续异常隐藏。

我希望，如果您在调试器中单步执行此代码，您将看到执行流程从这两行之一跳转到 catch block ，然后遇到 NullPointerException on transaction.rollback() (因为事务分配也从未发生过)并跳转到 finally block ，在那里它遇到另一个 NullPointerException 并且摆脱困境。