EJB3.0实体主键自增

SARA520 2008-11-09 11:43:00

用O/R Mapping
建立一个实体
每增加一个实体要让实体的主键自动加1
怎么做？
用注释？

高手指导下啊
谢谢

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火龙果被占用了 2008-11-09

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方式一：采用数据库自身的自增长列，数据库需要支持自动增长。对于 Oracle 就不能采用这种方式。

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Table;



@Entity

@Table(name = "USER")

public class User {



    private long id;

    private String username;

    private String password;

    

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    public long getId() {

        return id;

    }

    public void setId(long id) {

        this.id = id;

    }

    

    @Column

    public String getPassword() {

        return password;

    }

    public void setPassword(String password) {

        this.password = password;

    }

    

    @Column(unique = true)

    public String getUsername() {

        return username;

    }

    

    public void setUsername(String username) {

        this.username = username;

    }

}

方式二：对于没有自增列的数据库，如：Oracle，采用 Sequence 方式。

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.SequenceGenerator;

import javax.persistence.Table;



@Entity

@Table(name = "USER")



// name 指定该 Sequence 在 @Id 中引用的名字

// sequenceName 表示数据库中的 Sequence 名称

// allocationSize 访问 Sequence 的递增值，默认为 50

@SequenceGenerator(name = "USER_SEQUENCE", sequenceName = "S_USER", allocationSize = 1)

public class User {



    private long id;

    private String username;

    private String password;

    

    @Id

    

    // generator 指的就是 @SequenceGenerator 中的 name 值

    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "USER_SEQUENCE")

    public long getId() {

        return id;

    }

    public void setId(long id) {

        this.id = id;

    }

    

    @Column

    public String getPassword() {

        return password;

    }

    public void setPassword(String password) {

        this.password = password;

    }

    

    @Column(unique = true)

    public String getUsername() {

        return username;

    }

    

    public void setUsername(String username) {

        this.username = username;

    }

}

方式三：需要在数据库中建一张主键表

按下面的代码为例：
主键表表名为：GENERATOR_TABLE
表结构：
PK_COLUMN VARCHAR NOT NULL,
PK_VALUE LONG NOT NULL

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Table;

import javax.persistence.TableGenerator;



@Entity

@Table(name = "USER")

public class User {



    private long id;

    private String username;

    private String password;

    

    @Id

    @TableGenerator(name = "USER_PK_GENERATOR",

                    table = "GENERATOR_TABLE",

                    pkColumnName = "PK_COLUMN",

                    valueColumnName = "PK_VALUE",

                    pkColumnValue = "USER_PK",

                    allocationSize = 1)

    @GeneratedValue(strategy = GenerationType.TABLE, generator = "USER_PK_GENERATOR")

    public long getId() {

        return id;

    }

    public void setId(long id) {

        this.id = id;

    }

    

    @Column

    public String getPassword() {

        return password;

    }

    public void setPassword(String password) {

        this.password = password;

    }

    

    @Column(unique = true)

    public String getUsername() {

        return username;

    }

    

    public void setUsername(String username) {

        this.username = username;

    }

}