1. Overview

In this quick tutorial, we’ll take a look at Spring's method-level dependency injection support, via the @Lookup annotation.

2. Why @Lookup?

A method annotated with @Lookup tells Spring to return an instance of the method's return type when we invoke it.

Essentially, Spring will override our annotated method and use our method’s return type and parameters as arguments to BeanFactory#getBean.

@Lookup is useful for:

• Injecting a prototype-scoped bean into a singleton bean (similar to Provider)
• Injecting dependencies procedurally

Note also that @Lookup is the Java equivalent of the XML element lookup-method.

3. Using @Lookup

3.1. Injecting prototype-scoped Bean Into a Singleton Bean

If we happen to decide to have a prototype Spring bean, then we are almost immediately faced with the problem of how will our singleton Spring beans access these prototype Spring beans?

Now, Provider is certainly one way, though @Lookup is more versatile in some respects.

First, let’s create a prototype bean that we will later inject into a singleton bean:

@Component
@Scope("prototype")
public class SchoolNotification {// ... prototype-scoped state
}

And if we create a singleton bean that uses @Lookup:

@Component
public class StudentServices {// ... member variables, etc.@Lookuppublic SchoolNotification getNotification() {return null;}// ... getters and setters
}

Using @Lookup, we can get an instance of SchoolNotification through our singleton bean:

@Test
public void whenLookupMethodCalled_thenNewInstanceReturned() {// ... initialize contextStudentServices first = this.context.getBean(StudentServices.class);StudentServices second = this.context.getBean(StudentServices.class);assertEquals(first, second); assertNotEquals(first.getNotification(), second.getNotification()); 
}

Note that in StudentServices, we left the getNotification method as a stub.

This is because Spring overrides the method with a call to beanFactory.getBean(StudentNotification.class), so we can leave it empty.

3.2. Injecting Dependencies Procedurally

Still more powerful, though, is that @Lookup allows us to inject a dependency procedurally, something that we cannot do with Provider.

Let’s enhance StudentNotification with some state:

@Component
@Scope("prototype")
public class SchoolNotification {@Autowired Grader grader;private String name;private Collection marks;public SchoolNotification(String name) {// ... set fields}// ... getters and setterspublic String addMark(Integer mark) {this.marks.add(mark);return this.grader.grade(this.marks);}
}

Now, it is dependent on some Spring context and also additional context that we will provide procedurally.

We can then add a method to StudentServices that takes student data and persists it:

public abstract class StudentServices {private Map notes = new HashMap<>();@Lookupprotected abstract SchoolNotification getNotification(String name);public String appendMark(String name, Integer mark) {SchoolNotification notification = notes.computeIfAbsent(name, exists -> getNotification(name)));return notification.addMark(mark);}
}

At runtime, Spring will implement the method in the same way, with a couple of additional tricks.

First, note that it can call a complex constructor as well as inject other Spring beans, allowing us to treat SchoolNotification a bit more like a Spring-aware method.

It does this by implementing getSchoolNotification with a call to beanFactory.getBean(SchoolNotification.class, name).

Second, we can sometimes make the @Lookup-annotated method abstract, like the above example.

Using abstract is a bit nicer-looking than a stub, but we can only use it when we don't component-scan or @Bean-manage the surrounding bean:

@Test
public void whenAbstractGetterMethodInjects_thenNewInstanceReturned() {// ... initialize contextStudentServices services = context.getBean(StudentServices.class);    assertEquals("PASS", services.appendMark("Alex", 89));assertEquals("FAIL", services.appendMark("Bethany", 78));assertEquals("PASS", services.appendMark("Claire", 96));
}

With this setup, we can add Spring dependencies as well as method dependencies to SchoolNotification.

4. Limitations

Despite @Lookup‘s versatility, there are a few notable limitations:

• @Lookup-annotated methods, like getNotification, must be concrete when the surrounding class, like Student, is component-scanned. This is because component scanning skips abstract beans.
• @Lookup-annotated methods won’t work at all when the surrounding class is @Bean-managed.

In those circumstances, if we need to inject a prototype bean into a singleton, we can look to Provider as an alternative.

参考：
@Lookup Annotation in Spring