TaskDecorator使用笔记

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;
        }
    }
}

@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;
    }
}

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

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。

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;
        }
    }
}