在Spring XML中使用Java8 Lambda函数

Question

Let's say I have this class:

public class FooToBarTransformer {
    public Bar transform(Foo foo) {
        // do some cool stuff
    }
}

And I want to consume it as a Function in some other class:

public class Thing {
    public Thing(Function<Foo, Bar> f) {
        this.converter = f;
    }
}

Now, if I'm instantiating a Thing through Java, I'd do it with Java8 Lambdas like this:

FooToBarTransformer transformer = new FooToBarTransformer();
new Thing((foo) -> transformer.transform(foo));
// or new Thing(transformer::transform);

My problem, is that I need to create a Thing though spring .

<bean id="fooToBarTransformer" class="com.mypackage.FooToBarTransformer"/>
<bean id="theThing" class="com.mypackage.Thing">
    <constructor-arg index="0" ????????? />
</bean>

Now, there are a few possible workarounds I've considered to make this easier:

1. if FooToBarTransformer implemented Function , then it would just be a simple ref="fooToBarTransformer"
2. I could create a different interface that FooToBarTransformer implements and change Thing to take an instance of that interface instead of Function . For the purpose of this question, neither of those are options.

Based on some other ways I've seen of doing executions in the spring xml, I've tried value="#{(foo) -> fooToBarTransformer.transform(foo)}" and value="#{fooToBarTransformer::transform}" but spring choked on this.

The best option I've come up with so far is to provide a translation function in code:

public Function<Foo, Bar> toFunction() {
    return transformer::transform;
}

and reference it in spring with value="#{fooToBarTransformer.toFunction()}" , but this seems rather hokey.

Is there a better way to do this?

Answer 1

I think what you need to do is to switch to Java Config ( http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#beans-java ) In the meantime, if you can't switch, you can still pull it off by combining methods. In you're XML configuration file add:

<bean class="com.myapp.config.FooBarConfiguration"/>

Then, you create class com.myapp.config.FooBarConfiguration like this:

@Configuration
public class FooBarConfiguration {
    @Bean
    public FooToBarTransformer fooTransformer() {
        return new FooToBarTransformer();
    }

    @Bean
    public Thing theThing(FooToBarTransformer fooTransformer) {
        return new Thing(fooTransformer::transform);
    }
}

Just make sure that ConfigurationClassPostProcessor is a registered post-processor in your Spring context by either having:

<context:annotation-config>

Or

<context:component-scan>

Or by manually adding the post processor as a bean:

<bean class="org.springframework.context.annotation.ConfigurationClassPostProcessor"/>

This should work from Spring 3.0 and above according with http://docs.spring.io/spring/docs/current/javadoc-api/index.html?org/springframework/context/annotation/ConfigurationClassPostProcessor.html . Not sure what version you're using.

Answer 2

Try to use factory pattern. See org.springframework.beans.factory.FactoryBean. Also spring has expression language.