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解決@Value注解不能注入static修飾的屬性問題

 更新時間：2022年07月08日 14:18:38   作者：iiaythi  
這篇文章主要介紹了解決@Value注解不能注入static修飾的屬性問題，具有很好的參考價值，希望對大家有所幫助。如有錯誤或未考慮完全的地方，望不吝賜教

@Value注解不能注入static屬性

問題描述

在application.yml中：

constant:
? key: hello
? value: world

工具類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;
? ? }
}

測試類：

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

結(jié)果：

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

可以發(fā)現(xiàn)，@Value注解放在屬性上注入值失敗，而@Value放在setter方法上(注意，該方法也不能是靜態(tài)方法)卻能注入成功。為什么？？

剖析

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

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

從源碼上發(fā)現(xiàn)，理論上spring是可以對靜態(tài)域注入的，只是spring沒有這樣做，它認(rèn)為依賴注入發(fā)生的時段是在實例的生命周期，而不是類的生命周期

@Value(“${屬性}“)注入被static修飾的屬性

場景：

通過httpclient調(diào)用第三方接口的時候，ip和端口不確定

需求：

寫一個工具類，可以動態(tài)配置ip和端口來修改調(diào)用的地址和端口，要求工具類方法可以靜態(tài)調(diào)用。

問題描述

 static 不能和注解并用，被static修飾的成員變量，無法通過@Value注解動態(tài)獲取到

解決方案

通過注入到set方法實現(xiàn)屬性動態(tài)賦值

application.yml配置：

key:
? box:
? ? ip: 192.168.1.166
? ? port: 9987

錯誤代碼：

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

這樣寫的話，你會發(fā)現(xiàn)拿到的結(jié)果還是null

正確代碼：

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：調(diào)整代碼之后，工具類必須使用@Component注解來修飾，否則依然無法獲取到結(jié)果。

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。

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