记一次SpringBoot国际化原理分析

关键字：LocaleContextHolder、LocaleContext、LocaleResolver
第一步：Web服务器第一次接收请求时会初始化国际化策略

DispatcherServlet初始化

/**
   * 初始化
   */
  protected void initStrategies(ApplicationContext context) {
      initMultipartResolver(context);
      //重点初始化国际化
      initLocaleResolver(context);
      initThemeResolver(context);
      initHandlerMappings(context);
      initHandlerAdapters(context);
      initHandlerExceptionResolvers(context);
      initRequestToViewNameTranslator(context);
      initViewResolvers(context);
      initFlashMapManager(context);
  }
  
  /**
   * 给DispatcherServlet属性localeResolver赋值
   * 便于后续每次请求使用，可以自定义LocaleResolver的实现类
   */
  private void initLocaleResolver(ApplicationContext context) {
      try {
          //从spring容器中获取LocaleResolver的实例（自定义的话要重写这个接口，并将其注册到Spring,注意bean的名字：localeResolver）
          //LOCALE_RESOLVER_BEAN_NAME为DispatcherServlet#LOCALE_RESOLVER_BEAN_NAME = "localeResolver";
          this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
          if (logger.isTraceEnabled()) {
              logger.trace("Detected " + this.localeResolver);
          }
          else if (logger.isDebugEnabled()) {
              logger.debug("Detected " + this.localeResolver.getClass().getSimpleName());
          }
      }
      catch (NoSuchBeanDefinitionException ex) {
          // We need to use the default.
          this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
          if (logger.isTraceEnabled()) {
              logger.trace("No LocaleResolver '" + LOCALE_RESOLVER_BEAN_NAME +
                      "': using default [" + this.localeResolver.getClass().getSimpleName() + "]");
          }
      }
  }

this.localeResolver赋值是关键一步，下面我们自定义一个LocaleResolver实现类MessageLocaleResolver：
特别注意：请求头中传的值是zh-CN，不是下划线zh_CN
重要说三遍：
是zh-CN不是zh_CN
是zh-CN不是zh_CN
是zh-CN不是zh_CN
.......

/**
 * 国际化解析器
 * bean的名字必须为localeResolver
 * @author 匿名者
 * @since 2021-01-24
 * @see DispatcherServlet#LOCALE_RESOLVER_BEAN_NAME
 * @see DispatcherServlet#initLocaleResolver(org.springframework.context.ApplicationContext)
 * @see Request#getLocale()
 */
public class MessageLocaleResolver implements LocaleResolver {
    private String localeHeader = "locale";
    @Override
    public Locale resolveLocale(HttpServletRequest request) {
        //从请求头中获取语言标识，类似zh-CN，注意中间要用"-"
        String language = request.getHeader(localeHeader);
        //当为空时,采用默认的
        if(StringUtils.isBlank(language)) {
            return Locale.getDefault();
        }
        Locale locale = Locale.forLanguageTag(language);
        return locale;
    }
    @Override
    public void setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale) {
    }

第二步：后续每次请求都会调用 buildLocaleContext(request);

1、将“第一步”得到的this.localeResolver（自定义MessageLocaleResolver的实例），调用其resolveLocale方法返回LocaleContext（函数式接口）对象
注意：每次请求都会调用MessageLocaleResolver#resolveLocale，然后将LocaleContext放入LocaleContextHolder中

/**
  * this.localeResolver已经被第一步赋值，即this.localeResolver等于spring中实现LocaleResolver接口的实例
  */
 @Override
 protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
     LocaleResolver lr = this.localeResolver;
     if (lr instanceof LocaleContextResolver) {
         return ((LocaleContextResolver) lr).resolveLocaleContext(request);
     }
     else {
         //调用自定义LocaleResolver的实现类resolveLocale方法，从请求中获取国际化标识
         return () -> (lr != null ? lr.resolveLocale(request) : request.getLocale());
     }
 }

2、将上一步方法返回的LocaleContext放入LocaleContextHolder中，LocaleContextHolder内部有两个ThreadLocal属性，所以同一个请求所在线程都可以通过LocaleContextHolder.getLocale()方法获取当前国际化标识
LocaleContextHolder类的两个属性
private static final ThreadLocal localeContextHolder = new NamedThreadLocal<>("LocaleContext");
//子类可以获取ThreadLocal中值
private static final ThreadLocal inheritableLocaleContextHolder = new NamedInheritableThreadLocal<>("LocaleContext");
上述方法调用流程如下（每次请求都会调用）：

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        //********调用org.springframework.web.servlet.DispatcherServlet#buildLocaleContext，返回LocaleContext(返回的是函数式接口的实现)
        LocaleContext localeContext = buildLocaleContext(request);
        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
    
        //**********将buildLocaleContext方法返回的localeContext，放入LocaleContextHolder上下文中
        initContextHolders(request, localeContext, requestAttributes);
        try {
            doService(request, response);
        }
        catch (ServletException | IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }
        finally {
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }
            logResult(request, response, failureCause, asyncManager);
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

最后：国际化标识使用：
1、配置文件：指定国际化文件所在目录

spring:
  messages:
    basename: i18n/messages

2、编写根据编号获取具体国际化内容（一般在全局异常处理时根据异常code获取相应的国际化信息返回给前端）

/**
 * 获取国际化信息
 * @author 匿名者
 * @since 2021-01-12
 */
public class MessageSourceHolder {
    @Autowired
    private MessageSource messageSource;
    /**
     * 根据编号获取
     * @param code
     * @return
     */
    public String getMessage(String code) {
        return messageSource.getMessage(code, null,LocaleContextHolder.getLocale());
    }
    /**
     * 根据编号获取，并解析占位符
     * @param code
     * @param args
     * @return
     */
    public String getMessage(String code, Object[] args) {
        return messageSource.getMessage(code, args,LocaleContextHolder.getLocale());
    }
}

上述简单介绍了国际化的语言识别流程，大家可根据需要自己定制localeResolver，如果有发现问题，请及时告知