Programming Design Notes

以 PostgreSQL 為例。

首先先在 PostgreSQL 內新增一個 Sequence 和 Table。

User ID Sequence:

CREATE SEQUENCE user_id_seq
  INCREMENT 1
  MINVALUE 1
  MAXVALUE 9223372036854775807
  START 1
  CACHE 1;

User Table:

CREATE TABLE "user"
(
  uid integer NOT NULL DEFAULT nextval('user_id_seq'::regclass),
  username character varying(255) NOT NULL,
  "password" character varying(255) NOT NULL,
  CONSTRAINT user_pk PRIMARY KEY (uid)
)
WITH (
  OIDS=FALSE
);

然後新增一個 Entity class。

User entity:

package com.blogspot.lawpronotes.entity;

import java.io.Serializable;

import javax.persistence.Basic;
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", schema = "public")
public class User implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "USER_UID_GENERATOR")
    @SequenceGenerator(name = "USER_UID_GENERATOR", sequenceName = "user_id_seq", allocationSize = 1)
    @Column(name = "uid")
    private Integer uid;

    @Basic
    @Column(name = "username", nullable = false, unique = true)
    private String username;

    @Basic
    @Column(name = "password", nullable = false, unique = false)
    private String password;

    public User() {
    }

    public Integer getUid() {
        return this.uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    public String getName() {
        return this.username;
    }

    public void setName(String username) {
        this.username = username;
    }

    public String getPassword() {
        return this.password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

}

其中的關鍵:

@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "USER_UID_GENERATOR")
@SequenceGenerator(name = "USER_UID_GENERATOR", sequenceName = "user_id_seq", allocationSize = 1)

GeneratedValue 的 strategy 是 GenerationType.SEQUENCE，即是不斷地加同一個數值，達到一致。而 generator 即是數值產生器的名稱，數值產生器即是 SequenceGenerator，SequenceGenerator 的 sequenceName 要跟資料庫內的 sequence 名稱一樣，因為是靠資料庫內的 sequence 產生數值的， allocationSize 即是每次將數值加多少，因為資料庫內的 sequence 設定了每次加 1 ，allocationSize 也要是 1 才行，系統預設的 allocationSize 是 50。現在應該可以自動產生 ID 了。

相關書籍: