Java中的@interface默认声明用法 [英] @interface default declaration usage in Java

问题描述

我刚刚发现了这个功能.

I have just discovered this feature.

使用@interface"语法声明接口允许您放置默认值.

Declaring an interface using the "@interface" syntax allows you to put a default value.

public @interface HelloWorld { 
     public String sayHello() default "hello world";
}

这对我来说是新事物.假设如何使用该默认值.

This is something new for me. How is that default value suppose to be used.

我找不到对它的引用，因为在 Java 1.5 中添加@"之前，www 充满了 Java 接口文档(它是在 .5 还是在 .4 中?)

I cannot find references to that, because the www is full of java interface documents prior to "@" addition in Java 1.5 ( was it on .5 or in .4? )

编辑

感谢您的回答_{(我有点接近注释"，因为我已经使用了标签):P}

我知道我应该在几年前读过那份文件！！！...让我们看看...

I knew I should've read that document years ago!!!... let's see...

许多 API 需要大量样板代码.对于....

推荐答案

您刚刚编写了一个 注释.

特别是关于 default 语句:使用它是因为注释和接口不能有构造函数，所以这是为注释属性设置默认值的唯一方法.来自 Java 语言规范:

Regarding the default statement in particular: This is used because annotations and interfaces can't have constructors, so this is the only way to have a default value for an annotation attribute. From the Java Language Specification:

注解类型元素可能具有为其指定的默认值.这是通过在其(空)参数列表后面加上关键字 default 和元素的默认值来完成的.

An annotation type element may have a default value specified for it. This is done by following its (empty) parameter list with the keyword default and the default value of the element.

在读取注释时动态应用默认值；默认值不会编译为注释.因此，即使在进行更改之前编译的类中，更改默认值也会影响注释(假设这些注释缺少默认元素的显式值).

Defaults are applied dynamically at the time annotations are read; default values are not compiled into annotations. Thus, changing a default value affects annotations even in classes that were compiled before the change was made (presuming these annotations lack an explicit value for the defaulted element).

我注意到 java.lang.annotation 使用默认值.

I note that none of the annotations in java.lang.annotation use default values, though.

用法:您有一个带有属性 sayHello 的注释 @HelloWorld.你可以把它放在这样的类上:

Usage: You have an annotation @HelloWorld with an attribute sayHello. You could put it on a class like this:

@HelloWorld(sayHello="Hi")
public class MyClass {
}

既然你有一个默认值，你就可以把

Since you have a default value, you could just put

@HelloWorld
public class MyClass {
}

(请注意，文档说，在具有单个元素的注释中，该元素应命名为 value"；我相信这样做的唯一原因是您可以只编写 @HelloWorld("Hi") 无需命名参数.)

(Note that the document says, "In annotations with a single element, the element should be named value"; I believe the only reason to do this is that you could just write @HelloWorld("Hi") without having to name the parameter.)

如所写，您的注解可用于任何有效的程序元素(包括方法和变量声明).您可以使用 @Target 更改此设置 注释.

As written, your annotation can be used on any valid program element (including methods and variable declarations). You can change this with the @Target annotation.

最后，设置RetentionPolicy 让您决定是编译器丢弃注释、VM 丢弃注释还是始终保留注释.

Finally, setting the RetentionPolicy lets you decide if the annotation should be discarded by the compiler, discarded by the VM, or kept always.

两个可能也很有趣的包:javax.annotation 和 javax.annotation.processing.和 这里是一个使用注解处理进行源码分析的例子.

Two packages that might also be interesting: javax.annotation and javax.annotation.processing. And here is an example of using annotation processing for source code analysis.

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