放一个注释@Lob与否有什么区别？

Question

我的数据库中有一个请求表。这里是它的结构

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通常我用手创建实体，但这次我决定从数据库生成它。这是我得到的

@Entity
@Table(name = "request")
public class Request
{
    @Id
    @Column(name = "id", nullable = false)
    private Integer id;

    @Lob
    @Column(name = "body")
    private String body;

    @Column(name = "description", length = 10000)
    private String description;

    @Lob
    @Column(name = "headers")
    private String headers;

    @Column(name = "http_method", nullable = false, length = 20)
    private String httpMethod;

    @Column(name = "name", nullable = false, length = 100)
    private String name;

    @Lob
    @Column(name = "path_variables")
    private String pathVariables;

    @Column(name = "port")
    private Integer port;

    @Lob
    @Column(name = "query_params")
    private String queryParams;

    @Lob
    @Column(name = "url", nullable = false)
    private String url;

我注意到出现了一个奇怪的注释@Lob。我以前从未见过她。我决定用谷歌搜索它的意思以及为什么要放它。我找到了很多答案，他们都说需要这个注释，以便Hibernate理解这里可能有一个大对象。

但在我看来，这个注释没有任何作用。毕竟，不管是否值得，一切都是一样的？

它是干什么用的？

Answer 1

LOB or Large OBject refers to a variable length datatype for storing large objects.

This datatype has two variants as follows :
CLOB – Character Large Object will store large text data
BLOB – Binary Large Object is for storing binary data like image, audio, or video.

When we have the string field in a entity class, The hibernate framework will create the table with string fields as varchar datatype with maximum value of 255 (ie:  varchar(255)) by default

but , If we add the @LOB annotation to string field it will be changed as longtext from the varchar datatype in table, then we can able to store the large amount of character to specific column  .

For Example , to  Create a  materialized mappings , You just need an attribute of type String or byte[] and annotate it with  @Lob annotation.

@Entity
public class Book {
 
    @Id
    @GeneratedValue
    private Long id;
 
    private String title;
     
    @Lob
    private String content;
     
    @Lob
    private byte[] cover;
 
    ...
}

结论：@Lob注释用于将大值的字段/属性映射到相应的数据库支持的大对象类型。

Lob可以是二进制类型或字符类型。