为什么我在尝试开始交易时收到 NullPointerException？

Question

在我在这一行中得到 NullPointerException 之前

EntityManager manager = emf.createEntityManager();

但我通过在上面添加一行来修复它

emf =  Persistence.createEntityManagerFactory("main");

现在我在这一行得到 NullPointerException

transaction.begin();

我不明白为什么。我试图从 try catch 块中取出它，但得到了相同的结果。同样在最后我尝试添加 emf.close();但错误没有得到修复。另外，我包含 @Named 注释是因为有人建议我这样做。

import java.io.Serializable;
import javax.inject.Named; 
import javax.inject.Inject; 
import java.sql.*;
import javax.annotation.Resource;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.PersistenceUnit;
import javax.persistence.TypedQuery ; 
import javax.transaction.UserTransaction;
import javax.transaction.RollbackException; 
import javax.persistence.criteria.*;
import java.util.List;
import java.util.*;

@Named
public class UserManager implements Serializable{

   @PersistenceUnit(unitName="main")
   private EntityManagerFactory emf;

   @Resource
   private UserTransaction transaction;

   public UserManager() {}

   public UserTransaction getTransaction() { return transaction; }
   public void setTransaction(UserTransaction transaction) { this.transaction = transaction; }

   public void addUser(User v) throws Exception {
    emf =  Persistence.createEntityManagerFactory("main");
        EntityManager manager = emf.createEntityManager();
        boolean commit;
        try {   
    transaction.begin();
        manager.joinTransaction();
    commit = false;
        try {
        manager.persist(v);
        transaction.commit();
        commit = true;
        }finally {
                if(commit==false){
            transaction.rollback();
                }
         }
        }finally {     
         manager.close();
             //emf.close();
    }
   }



@Named("subBean")
@RequestScoped
public class UserBean implements Serializable{

   private List<User> users;
   private UserManager vm;
   private User v;
   private int total;

   public UserBean() {
      this.total= 0;
      users = new ArrayList<User>();
      vm = new UserManager();
      v = new User();
   }

   public String addUserAction() throws Exception {
      vm.addUser(v);
      users = getUsers();
      return "submit1";

   }

   //getters and setters 
}

Answer 1

正如我在评论中指出的，问题在于您手动创建了UserManager bean 的实例，而 CDI 无法注入变量。此处注明：

@Named("subBean")
@RequestScoped
public class UserBean implements Serializable{
   //variable is just declared...
   private UserManager vm;

   public UserBean() {
      //and here you're initializing it manually...
      vm = new UserManager();
   }

   //rest of your code...
}

让 CDI 通过注入变量来完成它的工作：

@Named("subBean")
@RequestScoped
public class UserBean implements Serializable{
   //variable is just declared
   //and notify CDI it should inject an instance when creating an instance of the client class
   @Inject
   private UserManager vm;

   public UserBean() {
      //no need to initialize it ever
      //vm = new UserManager();
   }

   //rest of your code...
}

通过这样做，您也不需要创建任何应该注入到其他 bean 中的字段，这意味着您不需要手动初始化 emf 字段：

//this line should be commented as well...
//emf =  Persistence.createEntityManagerFactory("main");