spring学习笔记（七）HttpMessageConverter

spring学习笔记（七）HttpMessageConverter

• 1. HttpMessageConverter的加载
• 2. 从StringMessageConverter探究消息转换器的原理

1. HttpMessageConverter的加载

• 首先我们找到WebMvcAutoConfiguration这个类

  在其中我们可以看到这么一段代码

  @Configuration
  @Import({WebMvcAutoConfiguration.EnableWebMvcConfiguration.class})
  @EnableConfigurationProperties({WebMvcProperties.class, ResourceProperties.class})
  @Order(0)    //表示最先加载
  public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer, ResourceLoaderAware {
          private static final Log logger = LogFactory.getLog(WebMvcConfigurer.class);
          private final ResourceProperties resourceProperties;
          private final WebMvcProperties mvcProperties;
          private final ListableBeanFactory beanFactory;
          private final ObjectProvider<HttpMessageConverters> messageConvertersProvider;
          final WebMvcAutoConfiguration.ResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer;
          private ResourceLoader resourceLoader;
  
          public WebMvcAutoConfigurationAdapter(ResourceProperties resourceProperties, WebMvcProperties mvcProperties, ListableBeanFactory beanFactory, ObjectProvider<HttpMessageConverters> messageConvertersProvider, ObjectProvider<WebMvcAutoConfiguration.ResourceHandlerRegistrationCustomizer> resourceHandlerRegistrationCustomizerProvider) {
              this.resourceProperties = resourceProperties;
              this.mvcProperties = mvcProperties;
              this.beanFactory = beanFactory;
              this.messageConvertersProvider = messageConvertersProvider;
              this.resourceHandlerRegistrationCustomizer = (WebMvcAutoConfiguration.ResourceHandlerRegistrationCustomizer)resourceHandlerRegistrationCustomizerProvider.getIfAvailable();
          }
  
          public void setResourceLoader(ResourceLoader resourceLoader) {
              this.resourceLoader = resourceLoader;
          }
  
          public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {          this.messageConvertersProvider.ifAvailable((customConverters) -> {
              // 添加自定义的converter
                  converters.addAll(customConverters.getConverters());
              });
          }
       // 省略一部分不重要的代码
  	.....
  

• 上面我们要分析的核心代码就是configureMessageConverters（List<HttpMessageConverter<?>> converters)这个方法，首先我要知道这个方法的入参是什么，在哪里初始化的。

• 我们可以看到在这个类上有一个@import注解

  @Import({WebMvcAutoConfiguration.EnableWebMvcConfiguration.class})

• 我们可以跟一下这个类----》EnableWebMvcConfiguration

• 我们可以发现这个类继承了WebMvcConfigurationSupport，核心的关于messageConverter的代码如下：

  protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
          StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
          stringHttpMessageConverter.setWriteAcceptCharset(false);
          messageConverters.add(new ByteArrayHttpMessageConverter());
          messageConverters.add(stringHttpMessageConverter);
          messageConverters.add(new ResourceHttpMessageConverter());
          messageConverters.add(new ResourceRegionHttpMessageConverter());
  
          try {
              messageConverters.add(new SourceHttpMessageConverter());
          } catch (Throwable var4) {
          }
  
          messageConverters.add(new AllEncompassingFormHttpMessageConverter());
          if (romePresent) {
              messageConverters.add(new AtomFeedHttpMessageConverter());
              messageConverters.add(new RssChannelHttpMessageConverter());
          }
  
          Jackson2ObjectMapperBuilder builder;
          if (jackson2XmlPresent) {
              builder = Jackson2ObjectMapperBuilder.xml();
              if (this.applicationContext != null) {
                  builder.applicationContext(this.applicationContext);
              }
  
              messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
          } else if (jaxb2Present) {
              messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
          }
  
          if (jackson2Present) {
              builder = Jackson2ObjectMapperBuilder.json();
              if (this.applicationContext != null) {
                  builder.applicationContext(this.applicationContext);
              }
  
              messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
          } else if (gsonPresent) {
              messageConverters.add(new GsonHttpMessageConverter());
          } else if (jsonbPresent) {
              messageConverters.add(new JsonbHttpMessageConverter());
          }
  
          if (jackson2SmilePresent) {
              builder = Jackson2ObjectMapperBuilder.smile();
              if (this.applicationContext != null) {
                  builder.applicationContext(this.applicationContext);
              }
  
              messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
          }
  
          if (jackson2CborPresent) {
              builder = Jackson2ObjectMapperBuilder.cbor();
              if (this.applicationContext != null) {
                  builder.applicationContext(this.applicationContext);
              }
  
              messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
          }
  
      }
  

• 从这段代码中我们可以知道

  1. spring中默认添加了几个消息转换器，比如ByteArrayHttpMessageConverter，stringHttpMessageConverter，ResourceHttpMessageConverter等等
  2. 会根据Jackson，Gson等添加对应的小心转换器

2. 从StringMessageConverter探究消息转换器的原理

public class StringHttpMessageConverter extends AbstractHttpMessageConverter<String> {
    // 默认字符集
    public static final Charset DEFAULT_CHARSET;
    @Nullable
    private volatile List<Charset> availableCharsets;
    private boolean writeAcceptCharset;

    public StringHttpMessageConverter() {
        this(DEFAULT_CHARSET);
    }

    public StringHttpMessageConverter(Charset defaultCharset) {
        super(defaultCharset, new MediaType[]{MediaType.TEXT_PLAIN, MediaType.ALL});
        this.writeAcceptCharset = true;
    }

    public void setWriteAcceptCharset(boolean writeAcceptCharset) {
        this.writeAcceptCharset = writeAcceptCharset;
    }

    public boolean supports(Class<?> clazz) {
        return String.class == clazz;
    }

    protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
        Charset charset = this.getContentTypeCharset(inputMessage.getHeaders().getContentType());
        return StreamUtils.copyToString(inputMessage.getBody(), charset);
    }

    protected Long getContentLength(String str, @Nullable MediaType contentType) {
        Charset charset = this.getContentTypeCharset(contentType);
        return (long)str.getBytes(charset).length;
    }

    protected void writeInternal(String str, HttpOutputMessage outputMessage) throws IOException {
        if (this.writeAcceptCharset) {
            outputMessage.getHeaders().setAcceptCharset(this.getAcceptedCharsets());
        }

        Charset charset = this.getContentTypeCharset(outputMessage.getHeaders().getContentType());
        StreamUtils.copy(str, charset, outputMessage.getBody());
    }

    protected List<Charset> getAcceptedCharsets() {
        List<Charset> charsets = this.availableCharsets;
        if (charsets == null) {
            charsets = new ArrayList(Charset.availableCharsets().values());
            this.availableCharsets = (List)charsets;
        }

        return (List)charsets;
    }

    private Charset getContentTypeCharset(@Nullable MediaType contentType) {
        if (contentType != null && contentType.getCharset() != null) {
            return contentType.getCharset();
        } else if (contentType != null && contentType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
            return StandardCharsets.UTF_8;
        } else {
            Charset charset = this.getDefaultCharset();
            Assert.state(charset != null, "No default charset");
            return charset;
        }
    }

    static {
        // 默认字符集为ISO-8859-1
        DEFAULT_CHARSET = StandardCharsets.ISO_8859_1;
    }
}

​ 上面代码的核心就在两个读写的方法上，分别是将字符串转换为httpMessage,跟将httpMessage转换为字符串

// 读方法
protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
        Charset charset =
   // 根据请求头中的信息，得到编码类型并转换
   this.getContentTypeCharset(inputMessage.getHeaders().getContentType());
        return StreamUtils.copyToString(inputMessage.getBody(), charset);
}

// 写方法
protected void writeInternal(String str, HttpOutputMessage outputMessage) throws IOException {
        if (this.writeAcceptCharset) {
            outputMessage.getHeaders().setAcceptCharset(this.getAcceptedCharsets());
        }

        Charset charset =
         // 根据响应头中的编码类型，将字符串转换为httpMessage消息
            this.getContentTypeCharset(outputMessage.getHeaders().getContentType());
        StreamUtils.copy(str, charset, outputMessage.getBody());
    }