Java Spring Bean 依赖注入，conditionalOnBean

Question

Attempting to create a class that will be used by JavaConfig as a source of bean definitions.

@Configuration
public class MyClass {
    @Bean
    @ConditionalOnProperty(name = "property")
    public A fuction1() {
       doSomething1(); // may return an exception
    }

    @Bean
    @ConditionalOnMissingBean(name = "fuction1")
    public A fuction2() {
        doSomething2();
    }

    @Bean
    public B fuction3(A a) {
        doSomething3(a);
    }
}

The third bean definition has the error "could not autowire. There is more than one bean of A type." How do I tell Spring to try to autowire the first A and if it is missing then to try the second A, ie following the conditional process described. Hopefully that makes sense. Thanks!

Answer 1

You are defining two bean with same name hence, autowiring issue is obvious. Try using @Primary on the bean you want to give priority.

Answer 2

How about this approach?

@Configuration
public class MyClass {
    @Bean
    @ConditionalOnProperty(name = "property", havingValue="true")
    public A fuction1() {
       doSomething1(); // may return an exception
    }

    @Bean
    @ConditionalOnProperty(name = "property", havingValue="false")
    public A fuction2() {
        doSomething2();
    }

    @Bean
    public B fuction3(A a) {
        doSomething3(a);
    }
}

Answer 3

Maybe this config will be helpful:

@Configuration
public class MyClass {
    @Bean
    @ConditionalOnProperty(name = "property")
    public A fuction1() {
        try {
            doSomething1(); // may return an exception
        } catch(Exception e) {
            return null;
        }
    }

    @Bean
    @ConditionalOnMissingBean(name = "fuction1")
    public A fuction2() {
        doSomething2();
    }

    @Bean
    public B fuction3(@Autowired(required = false) @Qualifier("function1") A a1, @Qualifier("function2") A a2) {
        if (a1 == null) {
            doSomething3(a2);
        } else {
            doSomething3(a1);
        }
    }
}

Answer 4

The sample code you've provided is conceptually correct. Of course there are compilation errors but annotations are correct.

Here is a sample that compiles and works.

@Configuration
public class ConditionalConfiguration {

    public static class A {

        private final String name;

        public A(String name) {
            this.name = name;
        }
    }

    public static class B {

        private final A a;

        public B(A a) {
            this.a = a;
        }
    }

    @Bean
    @ConditionalOnProperty(name = "a1-property")
    public A a1() {
        return new A("a1");
    }

    @Bean
    @ConditionalOnMissingBean(A.class)
    public A a2() {
        return new A("a2");
    }

    @Bean
    public B b(A a) {
        System.out.println("########## " + a.name);
        return new B(a);
    }
}

When the property a1-property is passed to the application

./gradlew clean bootRun -i -Da1-property
########## a1

the bean a1 is created.

When the property is missing

./gradlew clean bootRun -i
########## a2

The bean a2 is created.

Make sure to configure Gradle to pass system properties to the bootRun task by adding in the build.gradle

bootRun {
    systemProperties System.properties
}

To be decoupled from a1 bean name, condition on bean type is used: @ConditionalOnMissingBean(A.class) instead of @ConditionalOnMissingBean(name = "a1") .

If you expect an exception during bean creation it is another case. When factory method annotated with @Bean throws an exception, Spring application crashes with BeanInstantiationException .

So, exceptions should be handled in the factory method:

@Bean
@ConditionalOnProperty(name = "a1-property")
public A a1() {
    try {
        //return new A("a1");
        throw new RuntimeException("Sample exception");
    } catch (Exception e) {
        return fallbackA();
    }
}

@Bean
@ConditionalOnMissingBean(A.class)
public A a2() {
    return fallbackA();
}

private A fallbackA() {
    return new A("a2");
}