spring 如何将@Value 定义为可选
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How to define @Value as optional
提问by user1052610
I have the following in a Spring bean:
我在 Spring bean 中有以下内容：
@Value("${myValue}")
private String value;
The value is correctly injected. However, the variable needs to be optional, it is passed in as a command line parameter (which is then added to the Spring context using a SimpleCommandLinePropertySource), and this argument will not always exist.
该值已正确注入。但是，该变量需要是可选的，它作为命令行参数传入（然后使用SimpleCommandLinePropertySource添加到 Spring 上下文中），并且此参数不会始终存在。
I have tried both the following in order to provide a default value:
为了提供默认值，我尝试了以下两种方法：
@Value("${myValue:}")
@Value("${myValue:DEFAULT}")
but in each case, the default argument after the colon is injected even when there is an actual value - this appears override what Spring should inject.
但在每种情况下，即使存在实际值，也会注入冒号后的默认参数 - 这似乎覆盖了 Spring 应该注入的内容。
What is the correct way to specify that @Valueis not required?
指定不需要@Value的正确方法是什么？
Thanks
谢谢
回答by Andy Brown
What is the correct way to specify that @Value is not required?
指定不需要@Value 的正确方法是什么？
Working on the assumption that by 'not required' you mean nullthen...
假设“不需要”你的意思是null......
You have correctly noted that you can supply a default value to the right of a :character. Your example was @Value("${myValue:DEFAULT}").
您已经正确地注意到可以在:字符右侧提供默认值。你的例子是@Value("${myValue:DEFAULT}").
You are not limited to plain strings as default values. You can use SPELexpressions, and a simple SPEL expression to return nullis:
您不仅限于将纯字符串作为默认值。您可以使用SPEL表达式，返回的简单 SPEL 表达式null为：
@Value("${myValue:#{null}}")
回答by alonso_50
If you are using Java 8, you can take advantage of its java.util.Optionalclass.
You just have to declare the variable following this way:
如果您使用的是 Java 8，则可以利用它的java.util.Optional类。您只需按照以下方式声明变量：
@Value("${myValue:#{null}}")
private Optional<String> value;
Then, you can check whether the value is defined or not in a nicer way:
然后，您可以以更好的方式检查该值是否已定义：
if (value.isPresent()) {
// do something cool
}
Hope it helps!
希望能帮助到你！
