HarmonyOS 应用开发之TaskPool和Worker支持的序列化类型

1.返回值解析器介绍

返回值解析器用于解析Hanlder执行方法后的返回结果，例如将方法上标注有@ResponseBody注解的返回值解析成JSON、将方法返回的字符串作为视图名等

SpringMVC中默认的返回值解析器见RequestMappingHandlerAdapter#getDefaultReturnValueHandlers

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
    List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);
    handlers.add(new ModelAndViewMethodReturnValueHandler());
    handlers.add(new ModelMethodProcessor());
    handlers.add(new ViewMethodReturnValueHandler());
    handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),
                                                           this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));
    handlers.add(new StreamingResponseBodyReturnValueHandler());
    handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
                                               this.contentNegotiationManager, this.requestResponseBodyAdvice));
    handlers.add(new HttpHeadersReturnValueHandler());
    handlers.add(new CallableMethodReturnValueHandler());
    handlers.add(new DeferredResultMethodReturnValueHandler());
    handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
    handlers.add(new ModelAttributeMethodProcessor(false));
    handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
                                                        this.contentNegotiationManager, this.requestResponseBodyAdvice));
    handlers.add(new ViewNameMethodReturnValueHandler());
    handlers.add(new MapMethodProcessor());
    // 添加自定义的返回值解析器
    if (getCustomReturnValueHandlers() != null) {
        handlers.addAll(getCustomReturnValueHandlers());
    }
    // 兜底的解析器
    if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
        handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
    }
    else {
        handlers.add(new ModelAttributeMethodProcessor(true));
    }
    return handlers;
}

这些返回值解析器会被封装到HandlerMethodReturnValueHandlerComposite中，处理返回值时，会按照加入的顺序依次判断每个返回值解析器能否解析此任务，如果其中某个解析器能够解析此参数，则返回解析结果

2.自定义返回值解析器

在某些场景下，我们可以自定义返回值解析器，将响应的返回值转成我们希望的格式，假如我们想要将响应转换为yml格式，这时我们可以自定义注解，然后再自定义返回值解析器实现HandlerMethodReturnValueHandler接口，示例如下：

1）创建自定义注解@Yml及自定义返回值处理器YmlReturnResolver实现HandlerMethodReturnValueHandler接口

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Yml {
}

public class YmlReturnResolver implements HandlerMethodReturnValueHandler {
    // 判断返回值解析器是否支持
    @Override
    public boolean supportsReturnType(MethodParameter methodParameter) {
        Yml yml = methodParameter.getMethodAnnotation(Yml.class);
        return yml != null;
    }

    @Override
    public void handleReturnValue(Object o, MethodParameter methodParameter, ModelAndViewContainer modelAndViewContainer,
                                  NativeWebRequest nativeWebRequest) throws Exception {
        // 将返回值转换为yaml
        String str = new Yaml().dump(o);
        // 设置响应内容
        HttpServletResponse response = nativeWebRequest.getNativeResponse(HttpServletResponse.class);
        response.setContentType("text/plain;charset=utf-8");
        response.getWriter().print(str);
        // 设置请求已处理完毕
        modelAndViewContainer.setRequestHandled(true);
    }
}

2）配置类中加入自定义返回值解析器

@Configuration
@ComponentScan
public class WebConfig {
    @Bean
    public ServletWebServerFactory servletWebServerFactory() {
        return new TomcatServletWebServerFactory();
    }

    @Bean
    public DispatcherServlet dispatcherServlet() {
        return new DispatcherServlet();
    }

    @Bean
    public DispatcherServletRegistrationBean servletRegistrationBean(DispatcherServlet dispatcherServlet) {
        DispatcherServletRegistrationBean dispatcherServletRegistrationBean = new DispatcherServletRegistrationBean(dispatcherServlet, "/");
        dispatcherServletRegistrationBean.setLoadOnStartup(1);
        return dispatcherServletRegistrationBean;
    }

    @Bean
    public MyRequestMappingHandlerAdapter requestMappingHandlerAdapter() {
        MyRequestMappingHandlerAdapter handlerAdapter = new MyRequestMappingHandlerAdapter();
        // 添加自定义返回值解析器
        handlerAdapter.setCustomReturnValueHandlers(Arrays.asList(new YmlReturnResolver()));
        return handlerAdapter;
    }
}

3）创建控制器类及实体类

@Controller
public class Controller01 {
    @GetMapping("/test")
    @Yml
    public User test() {