Java GAE数据存储使用JPA生成字符串密钥

因为不可能只使用长ID，所以我尝试使用生成的字符串键。我有三个类

User

，

Topic

，

Comments

，其中

User

-1:n-

Topic

-1:n-

Comments

课堂评论：

@Entity
public class Comment implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;
    @ManyToOne
    private User author;
    @ManyToOne
    private Topic topic;

@Entity
public class User implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;

    @Unique
    private String username;

@Entity
public class Topic implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
    private String key;
    @ManyToOne(cascade = CascadeType.ALL)
    private User author;
    @OneToMany(cascade = CascadeType.ALL)
    private List<Comment> comments;

类用户：

@Entity
public class Comment implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;
    @ManyToOne
    private User author;
    @ManyToOne
    private Topic topic;

@Entity
public class User implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;

    @Unique
    private String username;

@Entity
public class Topic implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
    private String key;
    @ManyToOne(cascade = CascadeType.ALL)
    private User author;
    @OneToMany(cascade = CascadeType.ALL)
    private List<Comment> comments;

课程主题：

@Entity
public class Comment implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;
    @ManyToOne
    private User author;
    @ManyToOne
    private Topic topic;

@Entity
public class User implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value = "true")
    private String key;

    @Unique
    private String username;

@Entity
public class Topic implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
    private String key;
    @ManyToOne(cascade = CascadeType.ALL)
    private User author;
    @OneToMany(cascade = CascadeType.ALL)
    private List<Comment> comments;

是否可以在不手动使用KeyFactory的情况下生成字符串ID？如果是，我的代码有什么问题

---

谢谢

我使用

表格生成器

。无论您想要什么

id样式，它都很有用。比方说，即使您希望获得
Group
id，比如
GRP000001
和
GRP0000500
等。。
您必须在实体中使用属性注入，而不是字段注入。它基于setter ID方法。如果
EntityManager
generate生成的id为201，则EntityManager将在setter注入中调用
setId（）。如果是，id将是
GRP0000201

我的例子是：

@Entity
@TableGenerator(name = "GROUP_GEN", table = "ID_GEN", pkColumnName = "GEN_NAME", 
                valueColumnName = "GEN_VAL", pkColumnValue = "GROUP_GEN", allocationSize = 1)
@Access(value = AccessType.FIELD)
public class Group implements Serializable {
    @Transient
    private String id;
    private String name;
    //name getter and setter
    public void setId(String id) {
        if(id != null) {
            this.id = Utils.formatId(id, "GRP", 10);    
        }
    }

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "GROUP_GEN")
    @Access(value = AccessType.PROPERTY)
    public String getId() {
        return id;
    }
}

Utils.java

public class Utils {
    /**
     * E.g.
     * Input: id=523, prefix="AAA", maxLength=15
     * Output: AAA000000000523
     */
    public static String formatId(String id, String prefix, int maxLength) {
        if (!id.startsWith(prefix)) {
            int length = id.length() + prefix.length();
            for (; (maxLength - length) > 0; length++) {
                id = '0' + id;
            }
            id = prefix + id;
        }
        return id;
    }
}

IIRC身份策略是生成数字（或密钥）ID。
如果您使用的是JDO，那么可以使用自动生成的UUID样式的ID。
看