Criteria.uniqueResult 返回 null，但没有抛出错误

Question

我刚刚学习休眠，我正在尝试查询表以查找具有特定 ID 的人。当我调用criteria.unique 结果时，没有抛出错误，但它返回一个空值，谁能帮我弄清楚发生了什么？

@Entity
@Table(name="employees")
public class Employee {
private long emp_no; 
private Date birth_date; 
private String first_name;
private String last_name;
private char gender;
private Date hire_date; 

private Set<DepartmentEmp> department_emp = new HashSet<DepartmentEmp>(); 
private Set<DepartmentManager> department_manager = new HashSet<DepartmentManager>();
private Set<Projects> projects; 
private ContactInfo contactInfo; 


public Employee(){};
public Employee(long empt_no, Date birth_date, String first_name, String last_name, 
        char gender, Date hire_date){
        this.emp_no = emp_no;
        this.birth_date = birth_date; 
        this.first_name = first_name;
        this.last_name = last_name;
        this.gender = gender; 
        this.hire_date = hire_date; 
}

 @Id
 @Column(name="emp_no")
    public long getEmp_no() { return emp_no; }
    public void setEmp_no(long emp_no) {this.emp_no=emp_no;}


 @Column(name="birth_date")
    public Date getBirth_date() { return birth_date; }
    public void setBirth_date(Date birth_date) {this.birth_date = birth_date;}


 @Column(name="first_name")
    public String getFirst_name() { return first_name; } 
    public void setFirst_name(String name) { this.first_name = name; }



 @Column(name="last_name")
    public String getLast_name() { return last_name; } 
    public void setLast_name(String name) { this.last_name = name; }


 @Column(name="gender")
    public char getGender() { return gender; } 
    public void setGender(char gender) {this.gender = gender;}


  @Column(name="hire_date")
    public Date getHire_date() { return hire_date; } 
    public void setHire_date(Date hire_date) {this.hire_date = hire_date;}

    @OneToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
    @JoinColumn(name="emp_no")
    public ContactInfo getContactInfo() { return contactInfo; }
    public void setContactInfo(ContactInfo info) { this.contactInfo = info; }

  @OneToMany(mappedBy="employee", targetEntity=DepartmentEmp.class,
    cascade=CascadeType.ALL)
  @Fetch(value = FetchMode.SELECT) 
    public Set<DepartmentEmp> getDepartment_emp() { return department_emp; }
    public void setDepartment_emp(Set<DepartmentEmp> department_emp) { this.department_emp = department_emp; }



    @OneToMany(mappedBy="employee", targetEntity=DepartmentManager.class,
    cascade=CascadeType.ALL)
    @Fetch(value = FetchMode.SELECT) 

    public Set<DepartmentManager> getDepartment_manager() { return department_manager; }
    public void setDepartment_manager(Set<DepartmentManager> department_manager) { this.department_manager = department_manager; }

    @ManyToMany
      @JoinTable(name="employee_projects", 
            joinColumns={@JoinColumn(name="employee_emp_no")},
            inverseJoinColumns={@JoinColumn(name="projects_proj_num")})
    @Fetch(value = FetchMode.SELECT) 
        public Set<Projects> getProjects() { return projects; }
        public void setProjects(Set<Projects> projects) { this.projects = projects; }

    public void print() {
        System.out.printf("%d: %s %s %s %c %s\n", emp_no, birth_date.toString(), first_name, last_name, gender, hire_date.toString()); 
    } 

    public static void list()
    {
        Session session = HibernateContext.getSession();
        Criteria criteria = session.createCriteria(Employee.class);
        criteria.addOrder(Order.asc("emp_no"));

        List <Employee> employees = criteria.list();
        System.out.println("All Employees:");
        for (Employee employee : employees) {
            employee.print();
        }
        session.close();
    }

    //TODO finish 4 methods

    public static Employee find(long emp_no){
        Employee prototype = new Employee();
        prototype.setEmp_no(emp_no);
        Example ex = Example.create(prototype);

        Session sess = HibernateContext.getSession();
        Criteria criteria = sess.createCriteria(Employee.class);
        criteria.add(ex);

        Employee e = (Employee) criteria.uniqueResult();
        return e;
    }
    public static Employee find(String first, String last){
        Employee prototype = new Employee();
        prototype.setFirst_name(first);
        prototype.setLast_name(last);
        Example ex = Example.create(prototype);

        Session sess = HibernateContext.getSession();
        Criteria criteria = sess.createCriteria(Employee.class);
        criteria.add(ex);

        Employee e = (Employee) criteria.uniqueResult();

        sess.close();
        return e;
    }
    public static void main(String [] args){
           Class klasses [] = {Employee.class, DepartmentEmp.class, DepartmentManager.class, ContactInfo.class,
                                Salaries.class, Titles.class, Department.class, Projects.class};
            HibernateContext.addClasses(klasses);
            Employee.find((long)10001).print();
    }
  }

休眠上下文：

public class HibernateContext {

public static AnnotationConfiguration config = null;
public static SessionFactory factory = null;

/**
 * sets configuration if null
 */
private static void setConfiguration(){
    if(config == null){
        config = new AnnotationConfiguration();
        config.configure();
    }
}

/**
 * sets factory if null
 */
private static void setFactory(){
    if(factory == null){
        setConfiguration();
        factory = config.buildSessionFactory();
    }
}

/**
 * Opens session from the factory
 */
public static Session getSession(){
    setFactory();
    return factory.openSession();
}

/**
 * Creates a schema from the configuration
 */
public static void createSchema(){
    setConfiguration();
    (new SchemaExport(config)).create(true, true);

}

/**
 * Adds a new class object to the database
 */
public static void addClass(Class c){
    setConfiguration();
    config.addAnnotatedClass(c);
}

/**
 * Adds a list of class objects to the database
 */
public static void addClasses(Class classes[]){
    for(Class c : classes){
        addClass(c);
    }
}

}

我省略了其他类，因为它们不参与此查询，但是当我在 Employee 中运行主函数时，堆栈跟踪只是在 find.print(); 处显示空指针异常；线。我知道数据库已填充并且正在运行 Employee.list() 工作只是找到，所以我正在努力找出究竟是什么问题。感谢您的帮助！

Answer 1

没有堆栈跟踪，很难找到问题所在，这就是我的猜测：

如果我们看到你的打印方法；

public void print() {
        System.out.printf("%d: %s %s %s %c %s\n", emp_no, birth_date.toString(),
 first_name, last_name, gender, 
hire_date.toString()); 
    } 

即使 Employee 对象正确返回 .. 在 print 你得到的值 hire_date 如果您没有将hire_date 提供给employee object 并尝试调用hire_date.toString()，它将提供null pointer exception。

确定返回的Employee对象是否有hire_date