Hibernate 無主鍵表(複合主鍵)映射

1. 爲何要有複合主鍵映射

在現實中咱們可能會遇到許多表多是沒有主鍵的，那麼咱們對其作映射後使用會是什麼樣的結果？能正常獲得咱們想要的嗎？結果應該是得不到想要的結果，而獲得的可能會是以下的報錯：

Caused by:org.hibernate.AnnotationException: No identifier specified for entity: xxx.xxx.xxx

這個結果告訴咱們：Hibernate映射表是須要主鍵的。

因此複合主鍵映射就應運而生了。

2. 注意點

複合主鍵使用一個可嵌入的類做爲主鍵表示,所以你須要使用@Id和@Embeddable兩個註解.還有一種方式是使用@EmbeddedId註解.固然還有一種方法那就是——使用@IdClass註解。具體請查看【hibernate-annotations-3.4.0.GA 2.2.6.映射覆合主鍵與外鍵】。

固然，只須要選擇其中的一種就能夠了。^_^

注意所依賴的類必須實現 serializable以及實現equals()/hashCode()方法.

舉一個具體事例：

2.1. User.java

<span style="font-size: 12px;">package com.sourcefour.bean;
// default package

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.Column;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Table;


/**
 * User entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table (name="t_user_composite_pk"
)

public class User implements java.io.Serializable {


 // Fields

 private UserId id;


 // Constructors

 /** default constructor */
 public User() {
 }


 /** full constructor */
 public User(UserId id) {
 this.id = id;
 }


 // Property accessors
 @EmbeddedId

 @AttributeOverrides( {
 @AttributeOverride(name="intId", column=@Column (name="intId", nullable=false) ),
 @AttributeOverride(name="varcName", column=@Column (name="varcName", length=50) ),
 @AttributeOverride(name="varcAddress", column=@Column (name="varcAddress", length=50) ),
 @AttributeOverride(name="intAge", column=@Column (name="intAge", nullable=false) ) } )

 public UserId getId() {
 return this.id;
 }

 public void setId(UserId id) {
 this.id = id;
 }
}
</span>

package com.sourcefour.bean; // default package import javax.persistence.AttributeOverride; import javax.persistence.AttributeOverrides; import javax.persistence.Column; import javax.persistence.EmbeddedId; import javax.persistence.Entity; import javax.persistence.Table; /** * User entity. @author MyEclipse Persistence Tools */ @Entity @Table(name="t_user_composite_pk" ) public class User implements java.io.Serializable { // Fields private UserId id; // Constructors /** default constructor */ public User() { } /** full constructor */ public User(UserId id) { this.id = id; } // Property accessors @EmbeddedId @AttributeOverrides( { @AttributeOverride(name="intId", column=@Column(name="intId", nullable=false) ), @AttributeOverride(name="varcName", column=@Column(name="varcName", length=50) ), @AttributeOverride(name="varcAddress", column=@Column(name="varcAddress", length=50) ), @AttributeOverride(name="intAge", column=@Column(name="intAge", nullable=false) ) } ) public UserId getId() { return this.id; } public void setId(UserId id) { this.id = id; } }

2.2. UserId.java

<span style="font-size: 12px;">package com.sourcefour.bean;

// default package

import javax.persistence.Column;
import javax.persistence.Embeddable;

/**
 * UserId entity. @author MyEclipse Persistence Tools
 */
@Embeddable
public class UserId implements java.io.Serializable {

 // Fields

 private int intId;
 private String varcName;
 private String varcAddress;
 private int intAge;

 // Constructors

 /** default constructor */
 public UserId() {
 }

 /** minimal constructor */
 public UserId(int intId, int intAge) {
 this.intId = intId;
 this.intAge = intAge;
 }

 /** full constructor */
 public UserId(int intId, String varcName, String varcAddress, int intAge) {
 this.intId = intId;
 this.varcName = varcName;
 this.varcAddress = varcAddress;
 this.intAge = intAge;
 }

 // Property accessors

 @Column (name = "intId", nullable = false)
 public int getIntId() {
 return this.intId;
 }

 public void setIntId(int intId) {
 this.intId = intId;
 }

 @Column (name = "varcName", length = 50)
 public String getVarcName() {
 return this.varcName;
 }

 public void setVarcName(String varcName) {
 this.varcName = varcName;
 }

 @Column (name = "varcAddress", length = 50)
 public String getVarcAddress() {
 return this.varcAddress;
 }

 public void setVarcAddress(String varcAddress) {
 this.varcAddress = varcAddress;
 }

 @Column (name = "intAge", nullable = false)
 public int getIntAge() {
 return this.intAge;
 }

 public void setIntAge(int intAge) {
 this.intAge = intAge;
 }

 public boolean equals(Object other) {
 if ((this == other))
 return true;
 if ((other == null))
 return false;
 if (!(other instanceof UserId))
 return false;
 UserId castOther = (UserId) other;

 return (this.getIntId() == castOther.getIntId())
 && ((this.getVarcName() == castOther.getVarcName()) || (this.getVarcName() != null
 && castOther.getVarcName() != null && this.getVarcName().equals(castOther.getVarcName())))
 && ((this.getVarcAddress() == castOther.getVarcAddress()) || (this.getVarcAddress() != null
 && castOther.getVarcAddress() != null && this.getVarcAddress().equals(
 castOther.getVarcAddress()))) && (this.getIntAge() == castOther.getIntAge());
 }

 public int hashCode() {
 int result = 17;

 result = 37 * result + this.getIntId();
 result = 37 * result + (getVarcName() == null ? 0 : this.getVarcName().hashCode());
 result = 37 * result + (getVarcAddress() == null ? 0 : this.getVarcAddress().hashCode());
 result = 37 * result + this.getIntAge();
 return result;
 }

}</span>

package com.sourcefour.bean; // default package import javax.persistence.Column; import javax.persistence.Embeddable; /** * UserId entity. @author MyEclipse Persistence Tools */ @Embeddable public class UserId implements java.io.Serializable { // Fields private int intId; private String varcName; private String varcAddress; private int intAge; // Constructors /** default constructor */ public UserId() { } /** minimal constructor */ public UserId(int intId, int intAge) { this.intId = intId; this.intAge = intAge; } /** full constructor */ public UserId(int intId, String varcName, String varcAddress, int intAge) { this.intId = intId; this.varcName = varcName; this.varcAddress = varcAddress; this.intAge = intAge; } // Property accessors @Column(name = "intId", nullable = false) public int getIntId() { return this.intId; } public void setIntId(int intId) { this.intId = intId; } @Column(name = "varcName", length = 50) public String getVarcName() { return this.varcName; } public void setVarcName(String varcName) { this.varcName = varcName; } @Column(name = "varcAddress", length = 50) public String getVarcAddress() { return this.varcAddress; } public void setVarcAddress(String varcAddress) { this.varcAddress = varcAddress; } @Column(name = "intAge", nullable = false) public int getIntAge() { return this.intAge; } public void setIntAge(int intAge) { this.intAge = intAge; } public boolean equals(Object other) { if ((this == other)) return true; if ((other == null)) return false; if (!(other instanceof UserId)) return false; UserId castOther = (UserId) other; return (this.getIntId() == castOther.getIntId()) && ((this.getVarcName() == castOther.getVarcName()) || (this.getVarcName() != null && castOther.getVarcName() != null && this.getVarcName().equals(castOther.getVarcName()))) && ((this.getVarcAddress() == castOther.getVarcAddress()) || (this.getVarcAddress() != null && castOther.getVarcAddress() != null && this.getVarcAddress().equals( castOther.getVarcAddress()))) && (this.getIntAge() == castOther.getIntAge()); } public int hashCode() { int result = 17; result = 37 * result + this.getIntId(); result = 37 * result + (getVarcName() == null ? 0 : this.getVarcName().hashCode()); result = 37 * result + (getVarcAddress() == null ? 0 : this.getVarcAddress().hashCode()); result = 37 * result + this.getIntAge(); return result; } }

3. 可能還會出現的問題

具體來講問題就是查詢出來的結果列表爲‘null’（這一點我此次在我機器上測試時沒有出現）。
若是出現了該問題，那麼看到這一點就應該能解決問題啦，若是不出現那就更好，呵呵！
可是我的以爲這一點應該仍是得說的。^_^
有時候查詢出來的結果列表爲‘null’，這使人非常費解，能夠想下這是什麼緣由？
直接上緣由，嘎嘎……。
緣由：做爲聯合主鍵的字段理論上不該該包含可能爲空的字段。
緣由分析：根據緣由，說明實體Bean中的某個（些）對應的表字段有空值。
解決方案：只須要將可能爲空的字段不做爲聯合主鍵的一部分就能夠。
說的估計暈頭了吧，直接來個事例吧（我的一直以爲，例子是解釋問題的最好說明）。

假設表中的varcName和varcAddress是可能爲空的，其它都不可能爲空，那麼映射應該是這樣的

User.java

……
private UserId id;
private String varcName;
private String varcAddress;
……
/*
這裏加入varcName和varcAddress映射內容，嗯仍是貼出來吧，反正電子檔的又不怕木有地方，嘎嘎……
*/
 @Column (name="varcName", length=50)
 public String getVarcName() {
 return this.varcName;
 }

 public void setVarcName(String varcName) {
 this.varcName = varcName;
 }

 @Column (name="varcAddress", length=50)
 public String getVarcAddress() {
 return this.varcAddress;
 }

 public void setVarcAddress(String varcAddress) {
 this.varcAddress = varcAddress;
 }

……
private UserId id;
private String varcName;
private String varcAddress;
……
/*
這裏加入varcName和varcAddress映射內容，嗯仍是貼出來吧，反正電子檔的又不怕木有地方，嘎嘎……
*/
	@Column(name="varcName", length=50)
    public String getVarcName() {
        return this.varcName;
    }
    
    public void setVarcName(String varcName) {
        this.varcName = varcName;
    }

    @Column(name="varcAddress", length=50)
    public String getVarcAddress() {
        return this.varcAddress;
    }
    
    public void setVarcAddress(String varcAddress) {
        this.varcAddress = varcAddress;
    }

UserId.java

……
private int intId;
private int intAge;
……