1. Overview

1.概述

In this tutorial, we’ll see how we can inject a value from a Java properties file to a static field with Spring.

在本教程中，我们将看到如何使用Spring将Java属性文件中的一个值注入到静态字段。

2. Problem

2 问题

To begin with, let’s imagine that we set a property to a properties file:

首先，让我们想象一下，我们将一个属性设置到一个属性文件中。

name = Inject a value to a static field

Afterward, we want to inject its value to an instance variable.

之后，我们要把它的值注入到一个实例变量中。

That usually can be done by using the @Value annotation on an instance field:

这通常可以通过在实例字段上使用@Value注释来实现。

@Value("${name}")
private String name;

However, when we try to apply it to a static field, we’ll find that it will still be null:

然而，当我们试图将它应用于一个静态字段时，我们会发现它仍然是null：。

@Value("${name}")
private static String NAME_NULL;

That’s because Spring doesn’t support @Value on static fields.

这是因为Spring不支持静态字段的@Value 。

Now, truthfully, this is an odd position for our code to be in, and we ought to first consider refactoring. But, let’s see how we can make this work.

现在，说实话，我们的代码处于这样一个奇怪的位置，我们应该首先考虑重构。但是，让我们来看看我们如何能让它发挥作用。

3. Solution

3.解决办法

First, let’s declare the static variable we want to inject NAME_STATIC.

首先，让我们声明我们要注入的静态变量NAME_STATIC。

Afterward, we’ll create a setter method, called setNameStatic and annotate it with the @Value annotation:

之后，我们将创建一个setter方法，叫做setNameStatic，并用@Value注解来注释它。

@RestController
public class PropertyController {

    @Value("${name}")
    private String name;

    private static String NAME_STATIC;

    @Value("${name}")
    public void setNameStatic(String name){
        PropertyController.NAME_STATIC = name;
    }
}

Let’s try and make sense of what’s happening above.

让我们试着把上面发生的事情弄明白。

First, PropertyController, which is a RestController, is being initialized by Spring.

首先，PropertyController是一个RestController，正在被Spring初始化。

Afterward, Spring searches for the Value annotated fields and methods.

之后，Spring搜索Value注释的字段和方法。

Spring uses dependency injection to populate the specific value when it finds the @Value annotation. However, instead of handing the value to the instance variable, it’s handed to the implicit setter instead. This setter then handles the population of our NAME_STATIC value.

Spring使用依赖注入来填充特定的值，当它找到@Value 注释时。然而，我们并没有将该值交给实例变量，而是将其交给了隐式设置器。

4. Conclusion

4.总结

In this short tutorial, we’ve looked at how to inject a value from a properties file into a static variable. This is a route we can consider when our attempts to refactor fail.

在这个简短的教程中，我们研究了如何将属性文件中的一个值注入静态变量中。当我们尝试重构失败时，这是一条我们可以考虑的途径。

As always, the code is available over on GitHub.

像往常一样，代码可在GitHub上获得。