[已解決] org.hibernate.AnnotationException: 未為實體類指定識別符

最近我在一個Hibernate專案上工作，我添加了一些實體Bean，當執行時我得到了以下異常堆棧跟踪。

Initial SessionFactory creation failed.org.hibernate.AnnotationException: No identifier specified for entity: com.journaldev.hibernate.model.Address
org.hibernate.AnnotationException: No identifier specified for entity: com.journaldev.hibernate.model.Address
	at org.hibernate.cfg.InheritanceState.determineDefaultAccessType(InheritanceState.java:277)
	at org.hibernate.cfg.InheritanceState.getElementsToProcess(InheritanceState.java:224)
	at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:775)
	at org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:3788)
	at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3742)
	at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1410)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)
	at com.journaldev.hibernate.util.HibernateUtil.buildSessionFactory(HibernateUtil.java:22)
	at com.journaldev.hibernate.util.HibernateUtil.getSessionFactory(HibernateUtil.java:34)
	at com.journaldev.hibernate.main.HibernateExample.main(HibernateExample.java:15)
Exception in thread "main" java.lang.ExceptionInInitializerError
	at com.journaldev.hibernate.util.HibernateUtil.buildSessionFactory(HibernateUtil.java:29)
	at com.journaldev.hibernate.util.HibernateUtil.getSessionFactory(HibernateUtil.java:34)
	at com.journaldev.hibernate.main.HibernateExample.main(HibernateExample.java:15)

問題是由於我忘記在我的實體Bean中指定主鍵，我的Bean定義如下：Address.java

package com.journaldev.hibernate.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;

@Entity
@Table(name = "ADDRESS")
@Access(value=AccessType.FIELD)
public class Address {

	@Column(name = "emp_id", unique = true, nullable = false)
	@GeneratedValue(generator = "gen")
	@GenericGenerator(name = "gen", strategy = "foreign", parameters = { @Parameter(name = "property", value = "employee") })
	private long id;

	@Column(name = "address_line1")
	private String addressLine1;

	@Column(name = "zipcode")
	private String zipcode;

	@Column(name = "city")
	private String city;

	@OneToOne
	@PrimaryKeyJoinColumn
	private Employee employee;

	public long getId() {
		return id;
	}

	public void setId(long id) {
		this.id = id;
	}

	public String getAddressLine1() {
		return addressLine1;
	}

	public void setAddressLine1(String addressLine1) {
		this.addressLine1 = addressLine1;
	}

	public String getZipcode() {
		return zipcode;
	}

	public void setZipcode(String zipcode) {
		this.zipcode = zipcode;
	}

	public String getCity() {
		return city;
	}

	public void setCity(String city) {
		this.city = city;
	}

	public Employee getEmployee() {
		return employee;
	}

	public void setEmployee(Employee employee) {
		this.employee = employee;
	}

	@Override
	public String toString() {
		return "AddressLine1= " + addressLine1 + ", City=" + city
				+ ", Zipcode=" + zipcode;
	}
}

解決問題的唯一需要就是用@Id註釋標記主鍵字段。我將我的id字段聲明更改如下，問題解決了。

@Id
@Column(name = "emp_id", unique = true, nullable = false)
@GeneratedValue(generator = "gen")
@GenericGenerator(name = "gen", strategy = "foreign", parameters = { @Parameter(name = "property", value = "employee") })
private long id;

這是一個簡單的修復，但有一種情況會變得更加混亂。通常Hibernate會根據變量或getter-setter上使用的註釋自動找出訪問Bean屬性的方法。但是，我們可以明確定義實體Bean的訪問類型。有兩種訪問類型：

1. Field：在這種情況下，Hibernate將查找變量上的註釋，就像我們上面為Address類定義的那樣@Access(value=AccessType.FIELD)。
2. Property：在這種情況下，Hibernate將查找getter-setter方法上的註釋，語法是@Access(value=AccessType.PROPERTY)

如果註釋未按照訪問類型定義，那麼我們仍然會得到這個異常。例如，如果訪問類型為Property，我們將所有註釋都添加到Bean變量上，我們將得到此異常。下面是一個將拋出此異常的示例類。Employee.java

package com.journaldev.hibernate.model;

import javax.persistence.Access;
import javax.persistence.AccessType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;

@Entity
@Table(name = "EMPLOYEE")
@Access(value=AccessType.FIELD)
public class Employee {

	private long id;

	private String name;

	private double salary;

	private Address address;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "emp_id")
	public long getId() {
		return id;
	}

	public void setId(long id) {
		this.id = id;
	}

	@OneToOne(mappedBy = "employee")
	@Cascade(value = org.hibernate.annotations.CascadeType.ALL)
	public Address getAddress() {
		return address;
	}

	public void setAddress(Address address) {
		this.address = address;
	}

	@Column(name = "emp_name")
	public String getName() {
		System.out.println("Employee getName called");
		return name;
	}

	public void setName(String name) {
		System.out.println("Employee setName called");
		this.name = name;
	}

	@Column(name = "emp_salary")
	public double getSalary() {
		return salary;
	}

	public void setSalary(double salary) {
		this.salary = salary;
	}

	@Override
	public String toString() {
		return "Id= " + id + ", Name= " + name + ", Salary= " + salary
				+ ", {Address= " + address + "}";
	}

}

請注意，所有的JPA註釋都是與getter方法一起使用的，而訪問類型被定義為Field，只需將訪問類型更改為property即可解決此問題。