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Spring 實現(xiàn)給Bean屬性注入null值

更新時間：2021年08月28日 10:06:28 作者：xiaojia1100

這篇文章主要介紹了Spring 實現(xiàn)給Bean屬性注入null值的操作，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

給Bean屬性注入null值

空字符串值可以使用<value/>元素可用來表示。例如：

<bean class="ExampleBean">
  <property name="email"><value/></property>
</bean>

等同于Java代碼： exampleBean.setEmail("")。

null值則可以使用<null/>元素可用來表示。例如：

<bean class="ExampleBean">
  <property name="email"><null/></property>
</bean>

等同于Java代碼：exampleBean.setEmail(null)。

Spring注入bean 為null的相關(guān)問題

問題描述

有一個類注入發(fā)現(xiàn)里面的MongoLogInfoDAO始終為null

@Component("countFunction")
public class CountFunction implements AviatorFunction {    
    @Resource(name = "mongoLogInfoDAO")
    private MongoLogInfoDAO mongoLogInfoDAO;
 
    @Override
    public String getName() {
        return "count";
    }

原因是竟然是因為使用了 new 方式使用 CountFunction 導(dǎo)致類中的mongoLogInfoDao 無法注入

AviatorEvaluator.addFunction(new CountFunction());

這個是不對的，為啥？

這個對象是你new出來的，不是spring創(chuàng)建的當(dāng)然無法DI依賴注入

反之，如果你這個Bean是交易Spring IOC 容器管理的，那也應(yīng)該報錯NoSuchBeanDefinitionException異常

如何處理？

應(yīng)對這個問題的最好辦法是不要new

還有一種情況spring 注入的時候是null

問題描述

@Component("functionExecutor")
public class FunctionExecutor {
    protected static final Loggerx LOGGER = Loggerx.getLogger("system");
 
    @Resource(name = "mongoDAO")
    private MongoDAO mongoDAO;
 
    @Resource(name = "prism4001Producer")
    private SmartMQProducer prism4001Producer;
    
    @Resource(name = "whereGreaterThenFunction")
    private  WhereGreaterThenFunction whereGreaterThenFunction;
    
    @Resource(name = "countFunction")
    private  CountFunction countFunction;
    
    @Resource(name = "devideFunction")
    private  DevideFunction devideFunction;
    
    @Resource(name = "selectFunction")
    private  SelectFunction selectFunction;
    
    @Resource(name = "whereEqualFunction")
    private  WhereEqualFunction whereEqualFunction;
    
    @Resource(name = "timeFunction")
    private  TimeFunction timeFunction;
    
    @Resource(name = "whereLessThenFunction")
    private  WhereLessThenFunction whereLessThenFunction;
    
    @Resource(name = "whereGreaterThenEqualFunction")
    private  WhereGreaterThenEqualFunction whereGreaterThenEqualFunction;
    
    @Resource(name = "whereInFunction")
    private  WhereInFunction whereInFunction;
    
    @Resource(name = "greaterThenFunction")
    private  GreaterThenFunction greaterThenFunction;
    
    @Resource(name = "regexFunction")
    private  RegexFunction regexFunction;
    
    @Resource(name = "maxFunction")
    private  MaxFunction maxFunction;
    
    @Resource(name = "minFunction")
    private  MinFunction minFunction;
    
    @Resource(name = "equalFunction")
    private  EqualFunction equalFunction;
    
    @Resource(name = "lessThenEqualFunction")
    private  LessThenEqualFunction lessThenEqualFunction;
    
    @Resource(name = "lessThenFunction")
    private  LessThenFunction lessThenFunction;
    
    @Resource(name = "plusFunction")
    private  PlusFunction plusFunction;
    
    @Resource(name = "multiFunction")
    private  MultiFunction multiFunction;
    
    @Resource(name = "notEqualFunction")
    private  NotEqualFunction notEqualFunction;
    
    @Resource(name = "whereNotInFunction")
    private  WhereNotInFunction whereNotInFunction;
    
    @Resource(name = "sumFunction")
    private  SumFunction sumFunction;
    
    @Resource(name = "minusFunction")
    private  MinusFunction minusFunction;
    
   {
        AviatorEvaluator.addFunction(whereGreaterThenFunction);
        AviatorEvaluator.addFunction(countFunction);
        AviatorEvaluator.addFunction(devideFunction);
        AviatorEvaluator.addFunction(selectFunction);
        AviatorEvaluator.addFunction(whereEqualFunction);
        AviatorEvaluator.addFunction(timeFunction);
        AviatorEvaluator.addFunction(whereLessThenFunction);
        AviatorEvaluator.addFunction(whereGreaterThenEqualFunction);
        AviatorEvaluator.addFunction(whereInFunction);
        AviatorEvaluator.addFunction(greaterThenFunction);
        AviatorEvaluator.addFunction(regexFunction);
        AviatorEvaluator.addFunction(maxFunction);
        AviatorEvaluator.addFunction(minFunction);
        AviatorEvaluator.addFunction(equalFunction);
        AviatorEvaluator.addFunction(lessThenEqualFunction);
        AviatorEvaluator.addFunction(lessThenFunction);
        AviatorEvaluator.addFunction(plusFunction);
        AviatorEvaluator.addFunction(multiFunction);
        AviatorEvaluator.addFunction(notEqualFunction);
        AviatorEvaluator.addFunction(whereNotInFunction);
        AviatorEvaluator.addFunction(sumFunction);
        AviatorEvaluator.addFunction(minusFunction);
    }

容器加載這個functionExecutor這個bean的時候，走到靜態(tài)代碼塊發(fā)現(xiàn)注入的sumFunction ，minusFunction都是null

這個是為什么呢？

spring 執(zhí)行代碼塊的時候，其他的bean 還未注入，會導(dǎo)致注入的bean 是null