Spring 实现给Bean属性注入null值

目录

• 给Bean属性注入null值
• Spring注入bean 为null的相关问题
  • 问题描述
  • 问题描述
  • 如何处理？

给Bean属性注入null值

空字符串值可以使用<value/>元素可用来表示。例如：

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

等同于Java代码： exampleBean.setEmail("")。

null值则可以使用<null/>元素可用来表示。例如：

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

等同于Java代码：exampleBean.setEmail(null)。

Spring注入bean 为null的相关问题

问题描述

有一个类注入 发现里面的MongoLogInfoDAO始终为null

@Component("countFunction")
public class CountFunction implements AviatorFunction {
    @Resource(name = "mongoLogInfoDAO")
    private MongoLogInfoDAO mongoLogInfoDAO;

    @Override
    public String getName() {
        return "count";
    }

原因是竟然是因为使用了 new 方式使用 CountFunction 导致类中的mongoLogInfoDao 无法注入

AviatorEvaluator.addFunction(new CountFunction());

这个是不对的， 为啥？

这个对象是你new出来的 ，不是spring创建的 当然无法DI依赖注入

反之，如果你这个Bean是交易Spring IOC 容器管理的，那也应该报错NoSuchBeanDefinitionException异常

如何处理？

应对这个问题的最好办法是 不要new

还有一种情况spring 注入的时候是null

问题描述

@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);
    }

容器加载这个functionExecutor这个bean的时候，走到静态代码块发现注入的sumFunction ，minusFunction都是null

这个是为什么呢？

spring 执行代码块的时候，其他的bean 还未注入，会导致注入的bean 是null

如何处理？

如果你想 当spring加载 bean 初始化完成之后，自动执行一些初始化方法

使用

@PostConstruct

注解可以实现

以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。

原文链接：https://blog.csdn.net/xiaojia1100/article/details/51595362