How can I understand whether a bean exists in runtime?

Question

For example, I have a class

public class Car{
     private Motor motor;

     public void setMotor(Motor motor){
          this.motor = motor;
     }   
}

My bean looks like

<bean id="car" class="Car">
    <property name="motor" ref="${motorProvider.getAvailableMotor()}"/>
</bean>

This method: motorProvider.getAvailableMotor() returns a bean name(string), of which motor I should use.

But there can be a situation when such bean(with such name) is not created. How can I check it?

What is the point to know whether the bean is in the context? Can you do something useful when it doesn't exist? — Aaron Digulla, Feb 06 '13 at 08:23
@AaronDigulla If it is not exist, then I will set something default value. — Mary Ryllo, Feb 06 '13 at 08:25

Aaron Digulla · Accepted Answer · 2017-11-28T13:08:52.047

23

There are several patterns how to do this. Here is one I often use:

public class Car{
     private Motor motor;

     @Autowired
     private ApplicationContext applicationContext;

     @PostConstruct
     public void init() {
        try {
            motor = applicationContext.getBean( Motor.class );
        } catch( NoSuchBeanDefinitionException e ) {
            motor = new DefaultMotor();
        }
     }
}

Note you could also call applicationContext.containsBeanDefinition(name) but that would search your context twice (once in containsBeanDefinition() and then second time when you call getBean()) so catching the exception is usually faster.

Since we catch a specific exception that says "bean doesn't exist", using if/else has almost no advantage anymore.

edited Nov 28 '17 at 13:08

answered Feb 06 '13 at 08:25

Aaron Digulla

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It looks interesting, I will try it – Mary Ryllo Feb 06 '13 at 08:29
2

I recommend using this instead of validating by throwing an exception if(appContext.containsBeanDefinition(w.getName())); – rolling stone Oct 10 '17 at 08:52
@Sridhar Than I have to search the context twice for the bean. This is more expensive (the larger the context is), than handling the exception. – Aaron Digulla Oct 18 '17 at 15:37
@aaron in that case you can try to use containsBeanDefinition – rolling stone Nov 13 '17 at 16:38

score 3 · Answer 2 · edited May 23 '17 at 12:10

3

SPeL; something like:

<property name="motor" value="#(if(${motorProvider} != null) ${motorProvider.getAvailableMotor()})"/>

I think it was discussed also here: Spring - set a property only if the value is not null . As they said before for more information see: http://static.springsource.org/spring/docs/3.0.5.RELEASE/reference/expressions.html

edited May 23 '17 at 12:10

Community

1
1

answered Feb 06 '13 at 07:51

Liviu Stirb

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I think, you don't don't understand me right) I don't want to check something on null, I want to understand is there such bean in my context. Because if such bean is not created, spring will cause an exception – Mary Ryllo Feb 06 '13 at 07:54

How can I understand whether a bean exists in runtime?

2 Answers2