官方文档中说DeferredResult和Callable都是为了异步生成返回值提供基本的支持。简单来说就是一个请求进来，如果你使用了DeferredResult或者Callable，在没有得到返回数据之前，DispatcherServlet和所有Filter就会退出Servlet容器线程，但响应保持打开状态，一旦返回数据有了，这个DispatcherServlet就会被再次调用并且处理，以异步产生的方式，向请求端返回值。
这么做的好处就是请求不会长时间占用服务连接池，提高服务器的吞吐量。

Callable

@GetMapping("/callable")
    public Callable<String> testCallable() throws InterruptedException {
        log.info("主线程开始！");
        Callable<String> result = new Callable<String>() {

            @Override
            public String call() throws Exception {
                log.info("副线程开始！");
                Thread.sleep(1000);
                log.info("副线程结束！");
                return "SUCCESS";
            }

        };
        log.info("主线程结束！");
        return result;
    }

输出的结果如下:

 主线程开始!
 主线程结束!
 副线程开始!
 副线程结束!

主线程会提前返回，可以处理其他请求，等待有结果之后再输出结果

DeferredResult

一旦启用了异步请求处理功能 ，控制器就可以将返回值包装在DeferredResult，控制器可以从不同的线程异步产生返回值。优点就是可以实现两个完全不相干的线程间的通信。
我们模拟如下场景:

由于消息队列和应用2的部分太繁琐，我们使用一个类来代替

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

@Component
public class MockQueue {

    private String placeOrder;

    private String completeOrder;

    private Logger logger = LoggerFactory.getLogger(getClass());

    public String getPlaceOrder() {
        return placeOrder;
    }

    public void setPlaceOrder(String placeOrder) throws Exception {
        new Thread(() -> {
            logger.info("接到下单请求, " + placeOrder);
            try {
                Thread.sleep(1000);
            } catch (Exception e) {
                e.printStackTrace();
            }
            this.completeOrder = placeOrder;
            logger.info("下单请求处理完毕," + placeOrder);
        }).start();
    }

    public String getCompleteOrder() {
        return completeOrder;
    }

    public void setCompleteOrder(String completeOrder) {
        this.completeOrder = completeOrder;
    }

}

定义一个Controller即线程1的部分

import org.apache.commons.lang.RandomStringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.context.request.async.DeferredResult;

@RestController
public class AsyncController {

    @Autowired
    private MockQueue mockQueue;

    @Autowired
    private DeferredResultHolder deferredResultHolder;

    private Logger logger = LoggerFactory.getLogger(getClass());

    @RequestMapping("/order")
    public DeferredResult<String> order() throws Exception {
        logger.info("主线程开始");

        String orderNumber = RandomStringUtils.randomNumeric(8);
        mockQueue.setPlaceOrder(orderNumber);

        DeferredResult<String> result = new DeferredResult<>();
        deferredResultHolder.getMap().put(orderNumber, result);

        return result;
    }
}

定义一个类，用来线程1和线程2之间通信的，使用@Component默认为单例，方便通信。

import java.util.HashMap;
import java.util.Map;

import org.springframework.stereotype.Component;
import org.springframework.web.context.request.async.DeferredResult;

@Component
public class DeferredResultHolder {

    private Map<String, DeferredResult<String>> map = new HashMap<String, DeferredResult<String>>();

    public Map<String, DeferredResult<String>> getMap() {
        return map;
    }

    public void setMap(Map<String, DeferredResult<String>> map) {
        this.map = map;
    }

}

定义一个类来监听订单是否处理完，如果处理完了的话就设置deferredResultHolder中的DeferredResult的值，就会返回结果了。

import org.apache.commons.lang.StringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.stereotype.Component;
@Component
public class QueueListener implements ApplicationListener<ContextRefreshedEvent> {

    @Autowired
    private MockQueue mockQueue;

    @Autowired
    private DeferredResultHolder deferredResultHolder;

    private Logger logger = LoggerFactory.getLogger(getClass());

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        new Thread(() -> {
            while (true) {

                if (StringUtils.isNotBlank(mockQueue.getCompleteOrder())) {

                    String orderNumber = mockQueue.getCompleteOrder();
                    logger.info("返回订单处理结果:"+orderNumber);
                    deferredResultHolder.getMap().get(orderNumber).setResult("place order success");
                    mockQueue.setCompleteOrder(null);

                }else{
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

            }
        }).start();
    }
}