@Value注解不能注入static属性怎么办呢？

@Value注解不能注入static属性

问题描述

在application.yml中：

constant:
 key: hello
 value: world

工具类ConstantHelper：

@Component
public class ConstantHelper {

  @Value("${
constant.value}
    ")
  private static String value;
    
  private static String key;

  public static String getValue() {
    
    return value;

  }

  public void setValue(String value) {
    
    ConstantHelper.value = value;

  }

  public static String getKey() {
    
    return key;

  }

  @Value("${
constant.key}
")
  public void setKey(String key) {
    
    ConstantHelper.key = key;

  }

}
    

测试类：

@RequestMapping("/getConfig")
public MapString, Object>
 getConfig(){
    
  MapString,Object>
     map = new HashMap>
    ();
    
  map.put("key", ConstantHelper.getKey());
    
  map.put("value",ConstantHelper.getValue());
    
  return map;

}

结果：

{
"value": null,
"key": "hello"
}

可以发现，@Value注解放在属性上注入值失败，而@Value放在setter方法上(注意，该方法也不能是静态方法)却能注入成功。为什么？？

剖析

答案就在AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata方法中,AutowiredAnnotationBeanPostProcessor主要处理了@Autowired和@Value注解等等：

private InjectionMetadata buildAutowiringMetadata(final Class?>
 clazz) {
    
  ListInjectionMetadata.InjectedElement>
     elements = new ArrayList>
    ();
    
  Class?>
     targetClass = clazz;

  do {
    
   final ListInjectionMetadata.InjectedElement>
     currElements = new ArrayList>
    ();
    
   ReflectionUtils.doWithLocalFields(targetClass, field ->
 {
    
    AnnotationAttributes ann = findAutowiredAnnotation(field);

    if (ann != null) {

     //here!!
     if (Modifier.isStatic(field.getModifiers())) {

      if (logger.isInfoEnabled()) {
    
       logger.info("Autowired annotation is not supported on static fields: " + field);

      }
    
      return;

     }
    
     boolean required = determineRequiredStatus(ann);
    
     currElements.add(new AutowiredFieldElement(field, required));

    }

   }
    );
    
   ReflectionUtils.doWithLocalMethods(targetClass, method ->
 {
    
    Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);

    if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
    
     return;

    }
    
    AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
    
    if (ann != null &
    &
 method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {

     //here!!
     if (Modifier.isStatic(method.getModifiers())) {

      if (logger.isInfoEnabled()) {
    
       logger.info("Autowired annotation is not supported on static methods: " + method);

      }
    
      return;

     }

     if (method.getParameterCount() == 0) {

      if (logger.isInfoEnabled()) {
    
       logger.info("Autowired annotation should only be used on methods with parameters: " +
         method);

      }

     }
    
     boolean required = determineRequiredStatus(ann);
    
     PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
    
     currElements.add(new AutowiredMethodElement(method, required, pd));

    }

   }
    );
    
   elements.addAll(0, currElements);
    
   targetClass = targetClass.getSuperclass();

  }
    
  while (targetClass != null &
    &
     targetClass != Object.class);
    
  return new InjectionMetadata(clazz, elements);

 }

The conceptual problem here is that annotation-driven injection happens for each bean instance. So we shouldn’t inject static fields or static methods there because that would happen for every instance of that class. The injection lifecycle is tied to the instance lifecycle, not to the class lifecycle. Bridging between an instance’s state and static accessor - if really desired - is up to the concrete bean implementation but arguably shouldn’t be done by the framework itself.

从源码上发现，理论上spring是可以对静态域注入的，只是spring没有这样做，它认为依赖注入发生的时段是在实例的生命周期，而不是类的生命周期

@Value(“${ 属性} “)注入被static修饰的属性

场景：

通过httpclient调用第三方接口的时候，ip和端口不确定

需求：

写一个工具类，可以动态配置ip和端口来修改调用的地址和端口，要求工具类方法可以静态调用。

问题描述

static 不能和注解并用，被static修饰的成员变量，无法通过@Value注解动态获取到

解决方案

通过注入到set方法实现属性动态赋值

application.yml配置：

key:
 box:
  ip: 192.168.1.166
  port: 9987

错误代码：

@Value("${
key.box.ip}
    ")
private static String ip ;

@Value("${
key.box.port}
    ")
private static String port;
    

这样写的话，你会发现拿到的结果还是null

正确代码：

import lombok.extern.slf4j.Slf4j;
    
import org.springframework.beans.factory.annotation.Value;
    
import org.springframework.stereotype.Component;
    
import java.util.Map;

/**
 * Created in 2021/6/29 15:07
 * @author 
 */
@Slf4j
@Component
public class KeyBoxHttpClientUtil {
    
    private static String ip ;
    
    private static String port;

    @Value("${
key.box.ip}
")
    public void setIP(String ip) {
    
        KeyBoxHttpClientUtil.ip = ip;

    }

    @Value("${
key.box.port}
")
    public void setPort(String port) {
    
        KeyBoxHttpClientUtil.port = port;

    }

}
    

Tips：调整代码之后，工具类必须使用@Component注解来修饰，否则依然无法获取到结果。

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