Using a TaskDecorator to copy MDC data to @Async threads

在Spring中使用線程池執行異步任務，經過TaskDecorator進行父線程向子線程傳遞參數 （同一個線程的實現方案正常是用ThreadLocal）

如下例子就是經過TaskDecorator實現日誌TraceID傳遞

@Component
public class MdcFilter extends GenericFilterBean {
 
  @Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
      throws IOException, ServletException {
    try {
      MDC.put("mdcData", "[userId:Duke]");
      chain.doFilter(request, response);
    } finally {
      MDC.clear();
    }
  }
}




@EnableAsync(proxyTargetClass = true)
@SpringBootApplication
public class Application extends AsyncConfigurerSupport {
 
  @Override
  public Executor getAsyncExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setTaskDecorator(new MdcTaskDecorator());
    executor.initialize();
    return executor;
  }
 
  public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
  }
}




class MdcTaskDecorator implements TaskDecorator {
 
  @Override
  public Runnable decorate(Runnable runnable) {
    // Right now: Web thread context !
    // (Grab the current thread MDC data)
    Map<String, String> contextMap = MDC.getCopyOfContextMap();
    return () -> {
      try {
        // Right now: @Async thread context !
        // (Restore the Web thread context's MDC data)
        MDC.setContextMap(contextMap);
        runnable.run();
      } finally {
        MDC.clear();
      }
    };
  }
}