实现自定义参数解析器在SpringBoot

SpringBoot的@RequestBody注入的实现流程

获取http请求中post的body部分，转换为String，再反序列化为一个对应的对象.

方法参数解析器的组合:
InvocableHandlerMethod#private HandlerMethodArgumentResolverComposite resolvers = new HandlerMethodArgumentResolverComposite();
->HandlerMethodArgumentResolverComposite#private final List<HandlerMethodArgumentResolver> argumentResolvers = new LinkedList<>();
对于每一种参数都有对应的参数解析器,通过调用
HandlerMethodArgumentResolverComposite#private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter)
来获取需要的参数解析器，获取到之后进行解析.

实现自定义注解加入SpringBoot的解析器之中

比如实现一个自定义的@MyRequestBody,使得在原有的解析并反序列化过程中，自动加入一个时间戳的参数.

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyRequestBody {
}

Spring之所以能够处理`@RequestBody`这个注解，是因为在启动过程中，执行查找参数解析器时

HandlerMethodArgumentResolverComposite#private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter)
获取到了@RequestBody对应的参数解析器，即
RequestResponseBodyMethodProcessor#public boolean supportsParameter(MethodParameter parameter);
所以只需要按照同样的方式在HandlerMethodArgumentResolverComposite中加入自己的参数解析器即可

先简单实现自定义参数解析器

public class MyHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver{
    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(MyRequestBody.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                                  NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        return null;
    }
}

spring是在这里实现参数解析器的获取的:

RequestMappingHandlerAdapter#private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers();
通过这个方法的这个判断可以加入自定义的参数解析器

// Custom arguments
if (getCustomArgumentResolvers() != null) {
    resolvers.addAll(getCustomArgumentResolvers());
}

且是在执行这个方法的时候实现的自定义解析器加入:
DelegatingWebMvcConfiguration#protected void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers)
即实现了org.springframework.web.servlet.config.annotation.WebMvcConfigurer接口
so
只要我们这样实现一个自己的Configuration即可被加入到DelegatingWebMvcConfiguration之中
实现:

@Configuration
public class MyConfiguration implements WebMvcConfigurer {
    @Override
    public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {}
}

以上这部分代码实现的是如何将自定义参数解析器注入，接下来实现自定义参数解析器

实现的思路是扩展原有的RequestResponseBodyMethodProcessor,为了不修改原有代码，采用装饰器模式实现:

public class RequestResponseBodyMethodProcessorDecorator implements HandlerMethodArgumentResolver{

    private RequestResponseBodyMethodProcessor delegate;

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(MyRequestBody.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                                  NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        Object resolvedArgument = delegate.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
        //实现对于Map类型的参数注入时间戳的功能
        if (parameter instanceof Map) {
            ((Map) parameter).put("timestamp", System.currentTimeMillis());
        }
        return resolvedArgument;
    }

}

难点来了，如何将自定义参数解析器加入`MyConfiguration`

不能直接使用@Autowired注入RequestResponseBodyMethodProcessor,
因为它并不是以一个Bean的方式被List<HandlerMethodArgumentResolver>注入的:
resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));

可行思路:获取到
RequestMappingHandlerAdapter#private HandlerMethodArgumentResolverComposite argumentResolvers;
后，将自己的参数解析器加入进去即可.
ps.可能由于时机等原因,RequestMappingHandlerAdapter不能使用@Autowired的字段注入方式获取
思路:采用延迟初始化的方式，在使用时再去获取.

MyConfiguration增加:

@Bean
public MyController.RequestResponseBodyMethodProcessorDecorator myProcessor() {
    return new MyController.RequestResponseBodyMethodProcessorDecorator();
}

RequestResponseBodyMethodProcessorDecorator修改为:

public class RequestResponseBodyMethodProcessorDecorator implements HandlerMethodArgumentResolver, ApplicationContextAware {

    private RequestResponseBodyMethodProcessor delegate;

    private ApplicationContext applicationContext;

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(MyRequestBody.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        //延迟获取delegate
        if (delegate == null) {
            delegate = (RequestResponseBodyMethodProcessor) applicationContext.getBean(RequestMappingHandlerAdapter.class)
                    .getArgumentResolvers().stream()
                    .filter(p -> p instanceof RequestResponseBodyMethodProcessor)
                    .findFirst().get();
        }
        Object resolvedArgument = delegate.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
        if (resolvedArgument instanceof Map) {
            ((Map) resolvedArgument).put("timestamp", System.currentTimeMillis());
        }
        return resolvedArgument;
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

自定义MyConfiguration注入自定义参数解析器实现:

@Configuration
public class MyConfiguration implements WebMvcConfigurer {

    @Override
    public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
        resolvers.add(myProcessor());
    }

    @Bean
    public RequestResponseBodyMethodProcessorDecorator myProcessor() {
        return new RequestResponseBodyMethodProcessorDecorator();
    }
}

到此，成功的实现了自定义注解的功能，成功在public void test(@MyRequestBody Map<String, Object> param)参数中加入了时间戳

实现`spring-boot-starter`

SpringBoot实现自动注入外部Bean的原理是:
扫描正确目录下的spring.factories这样的文件，
读取文件中的配置org.springframework.boot.autoconfigure.EnableAutoConfiguration,
注入其中所有的Bean.

同理在项目的resources/META-INF/spring.factories
中配置
org.springframework.boot.autoconfigure.EnableAutoConfiguration=MyConfiguration,
即可注入自定义的Bean，实现SpringBoot-starter

ArchGeass

Java Software Development Engineer