Java中Lombok @Value註解導致的variable not been initialized問題

背景

想要修改一個POJO類,在其中增加一個字段,但是增加以後就開始報錯:

  • 該類已經存在一個構造函數,為瞭不破壞該類原來的使用方式,於是重新寫瞭一個構造方法,之前的構造函數未改動。
  • 該類被Lombok的@Value註解修飾

解決

  • 報錯信息顯示,變量未被初始化。於是主要排查是否有被初始化。
  • 在重寫的構造方法中,我已經對該變量進行瞭初始化。
  • 不明所以,開始找不同,這個類中,唯一不熟悉的就是@Value註解,於是查看註解中的註釋:
/**
 * Generates a lot of code which fits with a class that is a representation of an immutable entity.
 *<p>
* Equivalent to {@code@Getter @FieldDefaults(makeFinal=true, level=AccessLevel.PRIVATE) @AllArgsConstructor @ToString @EqualsAndHashCode}.
 *<p>
* Complete documentation is found at<a href="<https://projectlombok.org/features/Value>" rel="external nofollow" >the project lombok features page for@Value</a>.
 *
 *@seelombok.Getter
*@seelombok.experimental.FieldDefaults
*@seelombok.AllArgsConstructor
*@seelombok.ToString
*@seelombok.EqualsAndHashCode
*@seelombok.Data
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface Value {
 /**
  * If you specify a static constructor name, then the generated constructor will be private, and
  * instead a static factory method is created that other classes can use to create instances.
  * We suggest the name: "of", like so:
  * 
  * <pre>
  *     public @Value(staticConstructor = "of") class Point { final int x, y; }
  * </pre>
  * 
  * Default: No static constructor, instead the normal constructor is public.
  * 
  * @return Name of static 'constructor' method to generate (blank = generate a normal constructor).
  */
 String staticConstructor() default "";
}

這個註解的作用是為一個不可變的實體類生成一系列與之匹配的代碼。效果等同於以下註解的組合:@Getter @FieldDefaults(makeFinal=true, level=AccessLevel.PRIVATE) @AllArgsConstructor @ToString @EqualsAndHashCode。

這其中有一個註解比較特殊,@FieldDefaults(makeFinal=true, level=AccessLevel.PRIVATE),見名知意,這是一個為字段設置默認屬性的註解,註解的屬性值中,標註瞭是否設置實例字段為final,訪問級別設置為private。

/**
 * Adds modifiers to each field in the type with this annotation.
 *<p>
* Complete documentation is found at<a href="<https://projectlombok.org/features/experimental/FieldDefaults>" rel="external nofollow" >the project lombok features page for@FieldDefaults</a>.
 *<p>
* If {@codemakeFinal} is {@codetrue}, then each (instance) field that is not annotated with {@code@NonFinal} will have the {@codefinal} modifier added.
 *<p>
* If {@codelevel} is set, then each (instance) field that is package private (i.e. no access modifier) and does not have the {@code@PackagePrivate} annotation will
 * have the appropriate access level modifier added.
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface FieldDefaults {
 AccessLevel level() default AccessLevel.NONE;
 boolean makeFinal() default false;
}

若makeFinal是true,則每個實例字段(被@NonFinal註解修飾的除外)都會被final修飾符修飾。
若level屬性有值,那麼每個包私有訪問的(即沒有訪問修飾符修飾)和沒有被@PackagePrivate註解修飾的實例字段都會被添加一個與屬性值對應的修飾符。

也就是說,@Value標記瞭此POJO類為不可能變的類,其所有沒有被@NonFinal註解修飾的成員變量,都會被final修飾

事情到瞭這裡,還是不知道問題在哪裡(Java基礎差)。繼續找解決辦法。

Google搜索找到此問答:

Lombok @Wither, @Value, @NoArgsConstructor, @AllArgsConstructor do not work together

回答中有一段對於Java final的描述:

A final variable can only be initialized once, either via an initializer or an assignment statement. It does not need to be initialized at the point of declaration: this is called a “blank final” variable. A blank final instance variable of a class must be definitely assigned in every constructor of the class in which it is declared; similarly, a blank final static variable must be definitely assigned in a static initializer of the class in which it is declared; otherwise, a compile-time error occurs in both cases.

翻譯如下:
一個final修飾的變量隻能通過初始化器或賦值語句初始化一次。它不需要在聲明處初始化:這被稱為“空白final”變量。類的空白final實例變量必須在聲明它的類的每個構造函數中確定賦值;同樣,空白final靜態變量必須在聲明它的類的靜態初始化器中明確賦值;否則,以上兩種情況下都會發生編譯錯誤。

真相大白,原因是在原來的構造器中沒有對新加入的字段進行初始化。至此,問題解決。

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