tomcat管理线程数量有限，当达到一定请求数量时，无法继续接受请求，使用多线程的方式，可以调用一个异步线程来执行。

执行逻辑如下图，tomcat就收http请求，调用一个副线程进行处理，副线程处理后，将结果返回给主线程。在副线程处理整个业务逻辑的过程中，主线程可以空闲出来，去处理其他请求。使得服务器的吞吐量可以有一个很大的提升。

用同步方式和异步方式编写两个请求

package com.ustc.reed.controller.async;
 
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
/**
 * @author xuanxuan
 *
 */
import java.util.concurrent.Callable;
 
@RestController
public class AsyncController {
    private static Logger logger  = LoggerFactory.getLogger(AsyncController.class);
 
    @GetMapping("/sync")
    public String sync() throws Exception{
        logger.info("主线程开始");
        Thread.sleep(1000);
 
        logger.info("主线程结束");
        return "success";
    }
 
    @GetMapping("/async")
    public Callable<String> async() throws Exception{
        logger.info("主线程开始");
        Callable<String> result = new Callable<String>() {
            @Override
            public String call() throws Exception {
                logger.info("副线程开始");
				Thread.sleep(1000);
				logger.info("副线程返回");
				return "success";
            }
        };
        logger.info("主线程结束");
        return result;
    }
}

在浏览器中输入http://localhost:8082/reed/sync

在浏览器中输入http://localhost:8082/reed/async

在实际开发中，可能遇到接收请求和响应请求不是同一个线程的场景。如下图，此时使用callable无法满足业务需求。

可以使用 DeferredResult来处理此类业务场景。

以下单处理场景为例，应用服务器1的线程1收到下单请求，将下单信息发送给消息队列。应用服务器2消费消息，进行下单处理。下单完成后，将结果返回给消息队列，应用服务器有另外一个线程2来监听消息队列，当发现有订单处理结果的消息，根据消息的结果返回HTTP响应。

线程1和线程2是完全隔离的，谁也不知道对方的存在。

限于篇幅，新建一个MockQueue，来模拟下单处理。

package com.ustc.reed.service.sync;
 
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;
/**
 * created by xuanxuan on 2019/1/6
 */
 
@Component
public class MockQueue {
    private String placeOrder;   //下单消息
 
    private String completeOrder;   //下单成功消息
 
    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;
    }
}

每个订单号会有一个处理结果。DeferredResultHolder可以在图示的线程1和线程2之间传递DeferredResult这个对象map的key可以理解为订单号。

package com.ustc.reed.service.sync;
 
import org.springframework.stereotype.Component;
import org.springframework.web.context.request.async.DeferredResult;
 
import java.util.HashMap;
import java.util.Map;
 
/**
 * created by xuanxuan on 2019/1/6
 */
@Component
public class DeferredResultHolder {
 
    private Map<String,DeferredResult<String>> map = new HashMap<>();
 
    public Map<String, DeferredResult<String>> getMap() {
        return map;
    }
 
    public void setMap(Map<String, DeferredResult<String>> map) {
        this.map = map;
    }
}

监听下单是否完成，完成则返回订单结果。

package com.ustc.reed.service.sync;
 
import org.apache.commons.lang3.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;
 
/**
 * created by xuanxuan on 2019/1/6
 */
 
@Component
public class QueueListener implements ApplicationListener<ContextRefreshedEvent> {
 
    private Logger logger = LoggerFactory.getLogger(QueueListener.class);
 
    @Autowired
    private MockQueue mockQueue;
 
    @Autowired
    private DeferredResultHolder deferredResultHolder;
 
    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
 
        new Thread(
                new Runnable() {
                    @Override
                    public void run() {
                        while (true){
 
                            if(StringUtils.isNotBlank(mockQueue.getCompleteOrder())){
                                String orderNumber = mockQueue.getCompleteOrder();
                                logger.info("返回订单结果："+orderNumber);
                                deferredResultHolder.getMap().get(orderNumber).setResult("place order success");
                                mockQueue.setCompleteOrder(null);
                            }else {
                                try {
                                    Thread.sleep(100);
                                } catch (InterruptedException e) {
                                    e.printStackTrace();
                                }
                            }
 
                        }
                    }
                }
 
        ).start();
 
    }
}

请求代码如下。

package com.ustc.reed.controller.async;
 
 
import com.ustc.reed.service.sync.DeferredResultHolder;
import com.ustc.reed.service.sync.MockQueue;
import org.apache.commons.lang3.RandomStringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.context.request.async.DeferredResult;
/**
 * @author xuanxuan
 *
 */
import java.util.concurrent.Callable;
 
@RestController
public class AsyncController {
    private static Logger logger  = LoggerFactory.getLogger(AsyncController.class);
 
    @Autowired
    private MockQueue mockQueue;
 
    @Autowired
    private DeferredResultHolder deferredResultHolder;
 
   
    @GetMapping("/mqasync")
    public DeferredResult<String> mqasync() 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;
 
    }
}

在浏览器中输入http://localhost:8082/reed/mqasync ，模拟出编号为05804777的订单的处理过程，结果如下。