Spring注释：当类是@Autowired时，为什么@Required不起作用

Question

When I have a class as follows:

public class MyConfig {
    private Integer threshold;

    @Required
    public void setThreshold(Integer threshold) { this.threshold = threshold; }
}

And I use it as follows:

public class Trainer {
    @Autowired
    private MyConfig configuration;

    public void setConfiguration(MyConfig configuration) { this.configuration = configuration; }
}

And initialize the Trainer in the xml context as follows:

<bean id="myConfiguration" class="com.xxx.config.MyConfig">
        <!--<property name="threshold" value="33"/>-->
</bean>

For some reason the @Required annotation doesn't apply, and The context starts without a problem (It should have thrown an exception saying the field threshold is required...).

Why is that??

Answer 1

I think you might have missed a configuration.

Simply applying the @Required annotation will not enforce the property checking, you also need to register an RequiredAnnotationBeanPostProcessor to aware of the @Required annotation in bean configuration file.

The RequiredAnnotationBeanPostProcessor can be enabled in two ways.

1. Include <context:annotation-config/>

   Add Spring context and in bean configuration file.

    <beans ... xmlns:context="http://www.springframework.org/schema/context" ... http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd" > ... <context:annotation-config /> ... </beans> 

2. Include RequiredAnnotationBeanPostProcessor

   Include ' RequiredAnnotationBeanPostProcessor ' directly in bean configuration file.

 <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd"> <bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>