java 異步查詢轉(zhuǎn)同步多種實(shí)現(xiàn)方式：循環(huán)等待，CountDownLatch，EventListen

Jochen 發(fā)布于2019-08-19 10:55 / 1665人閱讀

摘要：異步轉(zhuǎn)同步業(yè)務(wù)需求有些接口查詢反饋結(jié)果是異步返回的，無(wú)法立刻獲取查詢結(jié)果。正常處理邏輯觸發(fā)異步操作，然后傳遞一個(gè)唯一標(biāo)識(shí)。等到異步結(jié)果返回，根據(jù)傳入的唯一標(biāo)識(shí)，匹配此次結(jié)果。異步轉(zhuǎn)同步查詢空循環(huán)短暫等待。

異步轉(zhuǎn)同步 業(yè)務(wù)需求

有些接口查詢反饋結(jié)果是異步返回的，無(wú)法立刻獲取查詢結(jié)果。

正常處理邏輯

觸發(fā)異步操作，然后傳遞一個(gè)唯一標(biāo)識(shí)。

等到異步結(jié)果返回，根據(jù)傳入的唯一標(biāo)識(shí)，匹配此次結(jié)果。

如何轉(zhuǎn)換為同步

正常的應(yīng)用場(chǎng)景很多，但是有時(shí)候不想做數(shù)據(jù)存儲(chǔ)，只是想簡(jiǎn)單獲取調(diào)用結(jié)果。

即想達(dá)到同步操作的結(jié)果，怎么辦呢？

思路

發(fā)起異步操作

在異步結(jié)果返回之前，一直等待(可以設(shè)置超時(shí))

結(jié)果返回之后，異步操作結(jié)果統(tǒng)一返回

循環(huán)等待

LoopQuery.java

使用 query()，將異步的操作 remoteCallback() 執(zhí)行完成后，同步返回。

public class LoopQuery implements Async {

    private String result;

    private static final Logger LOGGER = LogManager.getLogger(LoopQuery.class.getName());

    @Override
    public String query(String key) {
        startQuery(key);
        new Thread(new Runnable() {
            @Override
            public void run() {
                remoteCallback(key);
            }
        }).start();

        final String queryResult = endQuery();
        LOGGER.info("查詢結(jié)果: {}", queryResult);
        return queryResult;
    }

    /**
     * 開始查詢
     * @param key 查詢條件
     */
    private void startQuery(final String key) {
        LOGGER.info("執(zhí)行查詢: {}", key);
    }

    /**
     * 遠(yuǎn)程的回調(diào)是等待是隨機(jī)的
     *
     * @param key 查詢條件
     */
    private void remoteCallback(final String key) {
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.result = key + "-result";
        LOGGER.info("remoteCallback set result: {}", result);
    }

    /**
     * 結(jié)束查詢
     * @return 返回結(jié)果
     */
    private String endQuery() {
        while (true) {
            if (null == result) {
                try {
                    TimeUnit.MILLISECONDS.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                return result;
            }
        }
    }
}

main()

public static void main(String[] args) {
    new LoopQuery().query("12345");
}

測(cè)試結(jié)果

18:14:16.491 [main] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - 執(zhí)行查詢: 12345
18:14:21.498 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - remoteCallback set result: 12345-result
18:14:21.548 [main] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - 查詢結(jié)果: 12345-result

CountDownLatch

AsyncQuery.java

使用 CountDownLatch 類達(dá)到同步的效果。

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class AsyncQuery {

    private static final Logger LOGGER = LogManager.getLogger(AsyncQuery.class.getName());

    /**
     * 結(jié)果
     */
    private String result;

    /**
     * 異步轉(zhuǎn)同步查詢
     * @param key
     */
    public void asyncQuery(final String key) {
        final CountDownLatch latch = new CountDownLatch(1);
        this.startQuery(key);

        new Thread(new Runnable() {
            @Override
            public void run() {
                LOGGER.info("遠(yuǎn)程回調(diào)線程開始");
                remoteCallback(key, latch);
                LOGGER.info("遠(yuǎn)程回調(diào)線程結(jié)束");
            }
        }).start();

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        this.endQuery();
    }

    private void startQuery(final String key) {
        LOGGER.info("執(zhí)行查詢: {}", key);
    }

    /**
     * 遠(yuǎn)程的回調(diào)是等待是隨機(jī)的
     * @param key
     */
    private void remoteCallback(final String key, CountDownLatch latch) {
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.result = key + "-result";
        latch.countDown();
    }

    private void endQuery() {
        LOGGER.info("查詢結(jié)果: {}", result);
    }

}

main()

public static void main(String[] args) {
    AsyncQuery asyncQuery = new AsyncQuery();
    final String key = "123456";
    asyncQuery.asyncQuery(key);
}

日志

18:19:12.714 [main] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 執(zhí)行查詢: 123456
18:19:12.716 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 遠(yuǎn)程回調(diào)線程開始
18:19:17.720 [main] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 查詢結(jié)果: 123456-result
18:19:17.720 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 遠(yuǎn)程回調(diào)線程結(jié)束

Spring EventListener

使用觀察者模式也可以。（對(duì)方案一的優(yōu)化）

此處結(jié)合 spring 進(jìn)行使用。

BookingCreatedEvent.java

定義一個(gè)傳輸屬性的對(duì)象。

public class BookingCreatedEvent extends ApplicationEvent {

    private static final long serialVersionUID = -1387078212317348344L;

    private String info;

    public BookingCreatedEvent(Object source) {
        super(source);
    }

    public BookingCreatedEvent(Object source, String info) {
        super(source);
        this.info = info;
    }

    public String getInfo() {
        return info;
    }
}

BookingService.java

說明：當(dāng) this.context.publishEvent(bookingCreatedEvent); 觸發(fā)時(shí)，
會(huì)被 @EventListener 指定的方法監(jiān)聽到。

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Service;

import java.util.concurrent.TimeUnit;

@Service
public class BookingService {

    @Autowired
    private ApplicationContext context;

    private volatile BookingCreatedEvent bookingCreatedEvent;

    /**
     * 異步轉(zhuǎn)同步查詢
     * @param info
     * @return
     */
    public String asyncQuery(final String info) {
        query(info);

        new Thread(new Runnable() {
            @Override
            public void run() {
                remoteCallback(info);
            }
        }).start();

        while(bookingCreatedEvent == null) {
            //.. 空循環(huán)
            // 短暫等待。
            try {
                TimeUnit.MILLISECONDS.sleep(1);
            } catch (InterruptedException e) {
                //...
            }
            //2. 使用兩個(gè)多帶帶的 event...

        }

        final String result = bookingCreatedEvent.getInfo();
        bookingCreatedEvent = null;
        return result;
    }

    @EventListener
    public void onApplicationEvent(BookingCreatedEvent bookingCreatedEvent) {
        System.out.println("監(jiān)聽到遠(yuǎn)程的信息: " + bookingCreatedEvent.getInfo());
        this.bookingCreatedEvent = bookingCreatedEvent;
        System.out.println("監(jiān)聽到遠(yuǎn)程消息后: " + this.bookingCreatedEvent.getInfo());
    }

    /**
     * 執(zhí)行查詢
     * @param info
     */
    public void query(final String info) {
        System.out.println("開始查詢: " + info);
    }

    /**
     * 遠(yuǎn)程回調(diào)
     * @param info
     */
    public void remoteCallback(final String info) {
        System.out.println("遠(yuǎn)程回調(diào)開始: " + info);

        try {
            TimeUnit.SECONDS.sleep(2);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 重發(fā)結(jié)果事件
        String result = info + "-result";
        BookingCreatedEvent bookingCreatedEvent = new BookingCreatedEvent(this, result);
        //觸發(fā)event
        this.context.publishEvent(bookingCreatedEvent);
    }
}

測(cè)試方法

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = SpringConfig.class)
public class BookServiceTest {

    @Autowired
    private BookingService bookingService;

    @Test
    public void asyncQueryTest() {
        bookingService.asyncQuery("1234");
    }

}

日志

2018-08-10 18:27:05.958  INFO  [main] com.github.houbb.spring.lean.core.ioc.event.BookingService:84 - 開始查詢:1234
2018-08-10 18:27:05.959  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:93 - 遠(yuǎn)程回調(diào)開始:1234
接收到信息: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:73 - 監(jiān)聽到遠(yuǎn)程的信息: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:75 - 監(jiān)聽到遠(yuǎn)程消息后: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:106 - 已經(jīng)觸發(fā)event
2018-08-10 18:27:07.964  INFO  [main] com.github.houbb.spring.lean.core.ioc.event.BookingService:67 - 查詢結(jié)果: 1234-result
2018-08-10 18:27:07.968  INFO  [Thread-1] org.springframework.context.support.GenericApplicationContext:993 - Closing org.springframework.context.support.GenericApplicationContext@5cee5251: startup date [Fri Aug 10 18:27:05 CST 2018]; root of context hierarchy

超時(shí)和空循環(huán) 空循環(huán)

空循環(huán)會(huì)導(dǎo)致 cpu 飆升

while(true) {
}

解決方式

while(true) {
    // 小睡即可
    TimeUnit.sleep(1);
}

超時(shí)編寫

不可能一直等待反饋，可以設(shè)置超時(shí)時(shí)間。

/**
 * 循環(huán)等待直到獲取結(jié)果
 * @param key key
 * @param timeoutInSeconds 超時(shí)時(shí)間
 * @param  泛型
 * @return 結(jié)果。如果超時(shí)則拋出異常
 */
public  T loopWaitForValue(final String key, long timeoutInSeconds) {
    long startTime = System.nanoTime();
    long deadline = startTime + TimeUnit.SECONDS.toNanos(timeoutInSeconds);
    //1. 如果沒有新回調(diào)，或者 key 對(duì)應(yīng)元素不存在。則一直循環(huán)
    while(ObjectUtil.isNull(map.get(key))) {
        try {
            TimeUnit.MILLISECONDS.sleep(5);
        } catch (InterruptedException e) {
            LOGGER.warn("Loop meet InterruptedException, just ignore it.", e);
        }
        // 超時(shí)判斷
        long currentTime = System.nanoTime();
        if(currentTime >= deadline) {
            throw new BussinessException(ErrorCode.READ_TIME_OUT);
        }
    }
    final T target = (T) map.get(key);
    LOGGER.debug("loopWaitForValue get value:{} for key:{}", JSON.toJSON(target), key);
    //2. 獲取到元素之后，需要移除掉對(duì)應(yīng)的值
    map.remove(key);
    return target;
}

代碼地址

loop

countdownlatch

spring-event-listener