Entity创建Primary Key的方法

对于EJB的实体Bean来说，最重要的一个方面便是Primary Key，因为，对于实体来说，每一条记录都是一个对象，必须要有一个唯一标识来获得这个对象，然后对该对象做Update 和Remove操作。本文主要讲述如何创建Primary Key。

 

对于表格无主键的创建Primary Key主要有两种方法

1.创建联合Primary Key

（1）通过创建主键类来标注，例子如下：

实体类：Customer.java

import java.io.Serializable; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.IdClass; import javax.persistence.NamedQueries; import javax.persistence.NamedQuery; @Entity @NamedQueries({ @NamedQuery(name = "Customer.findAll", query = "select o from Customer o") }) @IdClass(CustomerPK.class) public class Customer implements Serializable { @Column(length = 40) private String address; private Long area; @Column(length = 30) private String city; @Column(length = 2000) private String comments; private Double creditlimit; @Column(nullable = false) private Long custid; @Id @Column(nullable = false, length = 50) private String email; @Id @Column(nullable = false, length = 45) private String name; private Long optlock; @Column(length = 9) private String phone; @Column(nullable = false) private Long repid; @Column(length = 2) private String state; @Column(length = 9) private String zip; public Customer() { } public Customer(String address, Long area, String city, String comments, Double creditlimit, Long custid, String email, String name, Long optlock, String phone, Long repid, String state, String zip) { this.address = address; this.area = area; this.city = city; this.comments = comments; this.creditlimit = creditlimit; this.custid = custid; this.email = email; this.name = name; this.optlock = optlock; this.phone = phone; this.repid = repid; this.state = state; this.zip = zip; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public Long getArea() { return area; } public void setArea(Long area) { this.area = area; } public String getCity() { return city; } public void setCity(String city) { this.city = city; } public String getComments() { return comments; } public void setComments(String comments) { this.comments = comments; } public Double getCreditlimit() { return creditlimit; } public void setCreditlimit(Double creditlimit) { this.creditlimit = creditlimit; } public Long getCustid() { return custid; } public void setCustid(Long custid) { this.custid = custid; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Long getOptlock() { return optlock; } public void setOptlock(Long optlock) { this.optlock = optlock; } public String getPhone() { return phone; } public void setPhone(String phone) { this.phone = phone; } public Long getRepid() { return repid; } public void setRepid(Long repid) { this.repid = repid; } public String getState() { return state; } public void setState(String state) { this.state = state; } public String getZip() { return zip; } public void setZip(String zip) { this.zip = zip; } }

主键类 CusomerPK.java

import java.io.Serializable; public class CustomerPK implements Serializable { private String email; private String name; public CustomerPK() { } public CustomerPK(String email, String name) { this.email = email; this.name = name; } public boolean equals(Object other) { if (other instanceof CustomerPK) { final CustomerPK otherCustomerPK = (CustomerPK) other; final boolean areEqual = (otherCustomerPK.email.equals(email) && otherCustomerPK.name.equals(name)); return areEqual; } return false; } public int hashCode() { return super.hashCode(); } String getEmail() { return email; } void setEmail(String email) { this.email = email; } String getName() { return name; } void setName(String name) { this.name = name; } }

(2) 通过EmbededID来实现

具体方法如：

实体类：Student.java

 import java.io.Serializable; import javax.persistence.Column; import javax.persistence.EmbeddedId; import javax.persistence.Entity; import javax.persistence.IdClass; import javax.persistence.NamedQueries; import javax.persistence.NamedQuery; import javax.persistence.Id; @Entity @NamedQueries({ @NamedQuery(name = "Student.findAll", query = "select o from Student o") }) public class Student implements Serializable { @Column(length = 20) private String firstname; @EmbeddedId private StudentId id; public Student() { } public Student(String firstname, String lastname,Long ssn) { this.firstname = firstname; this.id = new StudentId(lastname,ssn); } public String getFirstname() { return firstname; } public void setFirstname(String firstname) { this.firstname = firstname; } public String getLastname(){ return this.id.getLastname(); } public Long getSsn(){ return this.id.getSsn(); } public void setId(StudentId id) { this.id = id; } public StudentId getId() { return id; } }

 

StudentID.java

import java.io.Serializable; import javax.persistence.Column; import javax.persistence.EmbeddedId; import javax.persistence.Entity; import javax.persistence.IdClass; import javax.persistence.NamedQueries; import javax.persistence.NamedQuery; import javax.persistence.Id; @Entity @NamedQueries({ @NamedQuery(name = "Student.findAll", query = "select o from Student o") }) public class Student implements Serializable { @Column(length = 20) private String firstname; @EmbeddedId private StudentId id; public Student() { } public Student(String firstname, String lastname,Long ssn) { this.firstname = firstname; this.id = new StudentId(lastname,ssn); } public String getFirstname() { return firstname; } public void setFirstname(String firstname) { this.firstname = firstname; } public String getLastname(){ return this.id.getLastname(); } public Long getSsn(){ return this.id.getSsn(); } public void setId(StudentId id) { this.id = id; } public StudentId getId() { return id; } }

2.通过应用ROWID来实现

在实体类中添加

@Id
private ROWID rowid;

这个方法应该适用于Oracle数据库，但据说在插入记录时，有些问题，不知新的TopLink有无解决该Bug