1. Overview

1.概述

In this quick tutorial, we’ll take a look at JSR-330‘s @Size annotation, Hibernate‘s @Length annotation, and JPA‘s @Column‘s length attribute.

在这个快速教程中，我们将看看JSR-330的@Size注释，Hibernate的@Length注释。和JPA的@Column的length属性。

At first blush, these may seem the same, but they perform different functions.

乍一看，这些东西可能看起来是一样的，但它们的功能是不同的。

2. Origins

2.起源

Simply put, all of these annotations are meant to communicate the size of a field.

简单地说，所有这些注释都是为了传达一个字段的大小。

@Size and @Length are similar. We can use either annotation to validate the size of a field. The former is a Java-standard annotation, while the latter is specific to Hibernate.

@Size和@Length类似。我们可以使用这两种注解来验证一个字段的大小。前者是Java标准的注解，而后者是Hibernate特有的。

@Column, though, is a JPA annotation that we use to control DDL statements.

@Column是一个JPA注释，我们用它来控制DDL语句。

Now let’s go through each of them in detail.

现在让我们来详细了解一下它们中的每一个。

3. @Size

3.@大小

For validations, we’ll use @Size, a bean validation annotation. We’ll use the property middleName annotated with @Size to validate its value between the attributes min and max:

对于验证，我们将使用@Size，一个Bean验证注解。我们将使用带有@Size注解的属性middleName来验证其在属性min和max：之间的值。

public class User {

    // ...

    @Size(min = 3, max = 15)
    private String middleName;

    // ...

}

Most importantly, @Size makes the bean independent of JPA and its vendors, such as Hibernate. As a result, it’s more portable than @Length.

最重要的是，@Size使bean独立于JPA及其供应商，例如Hibernate。因此，它比@Length更容易移植。

4. @Length

4.@长度

As we previously mentioned, @Length is the Hibernate-specific version of @Size. We’ll enforce the range for lastName using @Length:

正如我们之前提到的，@Length是Hibernate特定版本的@Size。我们将使用@Length来强制执行lastName的范围。

@Entity
public class User {

    // ...
      
    @Length(min = 3, max = 15)
    private String lastName;

    // ...

}

5. @Column(length=value)

5.@Column(length=value)

@Column is quite different from the previous two annotations.

@Column与前两个注释完全不同。

We’ll use @Column to indicate specific characteristics of the physical database column. We’ll use the length attribute of the @Column annotation to specify the string-valued column length:

我们将使用@Column来表示物理数据库列的具体特征。我们将使用length属性的@Column注释来指定字符串值的列长度。

@Entity
public class User {

    @Column(length = 3)
    private String firstName;

    // ...

}

The resulting column will be generated as a VARCHAR(3), and trying to insert a longer string will result in an SQL error.

产生的列将被生成为VARCHAR(3),，试图插入一个更长的字符串将导致SQL错误。

Note that we’ll use @Column only to specify table column properties, as it doesn’t provide validations.

注意，我们将使用@Column来指定表的列属性，因为它不提供验证。

Of course, we can use @Column together with @Size to specify database column properties with bean validation:

当然，我们可以使用@Column和@Size来指定带有bean验证的数据库列属性。

@Entity
public class User {

    // ... 
    
    @Column(length = 5)
    @Size(min = 3, max = 5)
    private String city;

    // ...

}

6. Conclusion

6.结语

In this article, we learned about the differences between the @Size annotation, @Length annotation, and @Column‘s length attribute. Then we examined each separately within the areas of their use.

在这篇文章中，我们了解了@Size注释、@Length注释和@Column的length属性之间的区别。然后我们在它们的使用范围内分别进行了检查。

As always, the full source code of the examples is available over on GitHub.

一如既往，这些示例的完整源代码可在GitHub上获得over。