TaskDecorator使用笔记

1. 定义TaskDecorator实例

   public class ContextCopyingDecorator implements TaskDecorator {
       @Override
       public Runnable decorate(Runnable runnable) {
           try {
               RequestAttributes context = RequestContextHolder.currentRequestAttributes();  
               Map<String,String> previous = MDC.getCopyOfContextMap(); 					  
               SecurityContext securityContext = SecurityContextHolder.getContext();	      
               return () -> {
                   try {
                       RequestContextHolder.setRequestAttributes(context);	
                       runnable.run();
                   } finally {
                       RequestContextHolder.resetRequestAttributes();		
                   }
               };
           } catch (IllegalStateException e) {
               return runnable;
           }
       }
   }
   

2. 线程池使用TaskDecorator

   @Configuration
   public class AppConfig {
       @Bean
       public TaskExecutor taskExecutor() {
           ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
           executor.setCorePoolSize(corePoolSize);
           executor.setMaxPoolSize(maxPoolSize);
           executor.setQueueCapacity(queueCapacity);
           executor.setThreadNamePrefix("MyExecutor-");
           // for passing in request scope context
           executor.setTaskDecorator(new ContextCopyingDecorator());
           executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
           executor.setWaitForTasksToCompleteOnShutdown(true);
           executor.initialize();
           return executor;
       }
   }
   

3. 存在问题

   从父线程取出的RequestContextHolder对象，此为持有线程上下文的request容器，将其设置到子线程中，按道理只要对象还存在强引用，
   就不会被销毁，但由于RequestContextHolder的特殊性，在父线程销毁的时候，会触发里面的resetRequestAttributes方法（即清除threadLocal里面的信息，
       即reques中的信息会被清除），此时即使RequestContextHolder这个对象还是存在，子线程也无法继续使用它获取request中的数据了
   

4. 完善思路

   既然是RequestContextHolder的特殊性，那我们就让绕过他的销毁清除，思路不变，还是继续使用threadLocal来传递我们需要使用到的变量，
   在父线程装饰前将所需变量取出来，然后在子线程中设置到threadLocal，业务使用的时候从threadLocal中取即可。
   

5. 完善代码

   public class ContextCopyingDecorator implements TaskDecorator {
       @Override
       public Runnable decorate(Runnable runnable) {
           try {
              HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();
               String ua = request.getHeader("user-agent");
               return () -> {
                   try {
                       ThreadLocalData.setUa(ua);
                       runnable.run();
                   } finally {
                       //清除线程threadLocal的值
                       ThreadLocalData.remove();
                   }
               };
           } catch (IllegalStateException e) {
               return runnable;
           }
       }
   }
   

6. 参考：https://www.jianshu.com/p/d000a0a44956