Shulou(Shulou.com)06/01 Report--

This article mainly introduces how to use Lookup annotations in Spring, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let Xiaobian take you to understand.

We know that a single abstract class in a spring container cannot be a bean, so is there any way? At this point we can use Lookup annotations, we can take a look at the spring scan bean part of the logic. We know that in order to become a bean in spring, it must be a BeanDefinition object, and if an abstract class does not have a method that contains Lookup annotations, it will be excluded from the spring scan.

/ * 1. Determine whether it is an independent class, while a non-static inner class cannot generate bean. * 2. Determine whether it is an interface or abstract class (there is a special case). If it is an abstract class, you can also generate bean * Determine whether the given bean definition qualifies as candidate if it is an abstract class, but there is a @ lookup annotation on it. *

The default implementation checks whether the class is not an interface * and not dependent on an enclosing class. *

Can be overridden in subclasses. * @ param beanDefinition the bean definition to check * @ return whether the bean definition qualifies as a candidate component * / protected boolean isCandidateComponent (AnnotatedBeanDefinition beanDefinition) {AnnotationMetadata metadata = beanDefinition.getMetadata (); return (metadata.isIndependent () & & (metadata.isConcrete ()) | (metadata.isAbstract () & & metadata.hasAnnotatedMethods (Lookup.class.getName ()) }

Let's verify that Lookup annotations are not used.

@ Componentpublic abstract class C1 {}

The running result reported an error.

Write a random method in the abstract class, and then add Lookup annotations to the method

@ Componentpublic abstract class C1 {@ Lookup public void a () {}}

Run the result, output normally, generate a new class through the cglib agent

But it is rarely used in this way, and another scenario may be used. Use another bean object in a singleton bean, but you want to return a different object each time. However, when spring injects bean into the container, scope defaults to singleton mode, which means that only one instance can be created in the entire application. When scope is of type PROTOTYPE, a new bean instance is automatically created each time it is injected. But when a singleton bean references a bean of type PROTOTYPE, the bean of type PROTOTYPE also becomes a singleton.

@ Componentpublic class D3 {@ Autowired private E4 e4; public void a () {System.out.println (this.e4);} @ Componentpublic class E4 {}

If you output the result, you can see that the object printed out each time is the same.

Use Lookup annotations

@ Componentpublic class D3 {public void a () {System.out.println (this.a1 ());} @ Lookup public E4 A1 () {return null;}}

Run the output result, the result of each output has been different, has reached our requirements

What is the cause of this? Also, our A1 method returns empty, but why does the output also have a value?

Because spring overrides the method when it encounters a method marked with Lookup annotations and returns the result, it doesn't matter whether we define the method with or without a return value.

Thank you for reading this article carefully. I hope the article "how to use Lookup annotations in Spring" shared by the editor will be helpful to you. At the same time, I also hope you will support us and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.