Hibernate Java中的會話上的空指針異常

Question

我正在創建一個簡單的應用程序，由於某種原因，我不斷收到空指針異常。 我的猜測是，這與我的session.open是否與if語句有關？ 我得到的錯誤是在finally塊上，它說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();
            }

        }

    }
}

錯誤：

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

該行上的NullPointerException堆棧跟蹤中沒有其他內容僅表示session為null。 據我所知，可能發生的唯一方法是session = sessionFactory.openSession(); 分配從未執行-這意味着在sessionFactory.openSession()或HibernateUtil.getSessionFactory()中發生異常，該異常被finally塊中引發的更高版本的異常隱藏。

我希望，如果您在調試器中單步執行此代碼，您將看到執行流從這兩行之一跳到catch塊，然后在transaction.rollback()上命中NullPointerException （因為事務分配從未發生過）和跳轉到finally塊，在此撞上另一個NullPointerException並退出。

Answer 2

由於在try / catch塊之外將session和sessionFactory都初始化為null ，所以永遠不要假設在finally子句中它們將不為null並直接調用close() 。 將其更改為以下內容：

} catch (Exception ex) {
   if(transaction != null) {
      try {
         transaction.rollback();
      } catch (Exception e) { // Log error }
   }
   ex.printStackTrace();
   System.out.println("Transaction is rolled back.");
} finally {
   if(session != null) {
      try {
         session.close();
      } catch (Exception e) { // Log error }
   }

   if(sessionFactory != null) {
      try {
         sessionFactory.close();
      } catch (Exception e) { // Log error }
   }
}