Day18 - HttpMessageConverter

15th鐵人賽

James

2023-10-04 00:04:23

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Day18 - HttpMessageConverter

前言

昨日我們談到了content negotiation讓我們可以透過Accept Header Strategy/URL Parameter Strategy可以取得不一樣的數據類型，那麼它背後的原理是什麼呢?如果今天我還想要再加入其他的數據類型是否可以做得到呢?這就是今天我們要討論的

專案建立

create module

maven

<!--加入對yaml的依賴-->
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-yaml</artifactId>
</dependency>

HttpMessageConverter

原理

簡單的來說HttpMessageConverter就是作為Spring MVC與Request/Response中間橋梁，透過它做訊息的轉換讓他們可以互相溝通

標註@ResponseBody會由HttpMessageConverter處理
HttpMessageConverter會先進行內容協，看哪個MessageConverter support，預設的MessageConverter如下

自訂義HttpMessageConverter

自訂義的MyYamlHttpMessageConverter

public class MyYamlHttpMessageConverter extends AbstractHttpMessageConverter<Object> {

    private ObjectMapper objectMapper = null; //把物件轉成yaml

    public MyYamlHttpMessageConverter(){
        super(new MediaType("text","yaml", Charset.forName("UTF-8")));
        //告訴Spring Boot符合哪種類型
        YAMLFactory factory = new YAMLFactory()
                .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER);
        this.objectMapper = new ObjectMapper(factory);
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return true;
    }
    @Override  //@RequestBody
    protected Object readInternal(Class<?> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        return null;
    }
    @Override //@ResponseBody 把物件寫出去
    protected void writeInternal(Object methodReturnValue, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        try(OutputStream os = outputMessage.getBody()){
            this.objectMapper.writeValue(os,methodReturnValue);
        }
    }
}

配置類

@Configuration
public class MyHttpMessageConvertersConfiguration  {
    @Bean
    public WebMvcConfigurer webMvcConfigurer(){
        return new WebMvcConfigurer() {
            @Override //配置一個能把對象轉為yaml的messageConverter
            public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
                converters.add(new MyYamlHttpMessageConverter());
            }
        };
    }
}

Controller

@RestController
public class HelloController {

    @GetMapping("/emp")
    public Employee emp(){
        return new Employee("james","2023100401","IT","A01");
    }
}

Result