多线程在微服务API统计和健康检查中的使用

API统计

在服务调用的时候，统计每个接口的调用次数，从而做到对接口的限流或统计。

在下面的代码中，使用了多线程的方式进行统计，主要使用了如下概念

• 线程池 Executor
• ConcurrentHashMap
• CountDownLatch

其中列举了四种实现方式

• 1 使用ConcurrentHashMap统计：不过该方法存在问题，统计的increase不是线程安全的，所以得到的结果不对
• 2 使用CAS理念对ConcurrentHashMap进行改进，从而解决自增方法increase的问题
• 3 使用Google的AtomicLongMap，原理同CAS一致，代码量小，比较优雅
• 4 对HashMap加锁ReentrantReadWriteLock

使用ConcurrentHashMap统计

package concurrent;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * Java 并发实践- ConcurrentHashMap 与 CAS * API调用次数统计 * 涉及概念： 多线程/线程池/ConcurrentHashMap/CountDownLatch * @author billjiang  * @createTime 2017-08-04 */public class CounterDemo {    private final Map<String, Long> urlCounter = new ConcurrentHashMap<>();    /**     * 接口调用次数，此方法存在问题,ConcurrentHashMap的原子方法是同步的，但increase方法没有同步     * @param url     * @return     */    public long increase(String url) {        Long oldValue=urlCounter.get(url);        Long newValue=(oldValue==null)?1l:oldValue+1;        urlCounter.put(url,newValue);        return newValue;    }    //获取调用次数    public long getCount(String url){        return urlCounter.get(url);    }    public static void main(String[] args) {        ExecutorService executorService= Executors.newFixedThreadPool(10);        final CounterDemo counterDemo=new CounterDemo();        int callTime=100000;        final String url="http://localhost:8082/test";        CountDownLatch countDownLatch=new CountDownLatch(callTime);        //模拟并发情况下的接口调用统计        for (int i = 0; i < callTime; i++) {            executorService.execute(new Runnable() {                @Override                public void run() {                    counterDemo.increase2(url);                    countDownLatch.countDown();                }            });        }        try{            countDownLatch.await();        }catch (InterruptedException e){            e.printStackTrace();        }        executorService.shutdown();        //等待所有线程统计完成后输出调用次数        System.out.println("调用次数："+counterDemo.getCount(url));    }}

从结果上看，使用ConcurrentHashMap存在问题，没有输出预期结果，这是因为ConcurrentHashMap虽然是线程安全的，不过它的线程安全指的是get和put等原子方法。而方法increase却不是线程安全的，当然可以通过对increase方法加锁（使用synchonized关键字），不过synchonized是悲观锁，其他线程要挂起等待，影响性能。可以使用类似乐观锁CAS对increase改进。

使用CAS对increase方法改进

关于CAS，可参考这篇文章：

深入浅出Java并发包—CAS机制

改进后的increase方法如下：

  /**     * CAS 乐观锁/自旋     * @param url     * @return     */    public long increase2(String url){        Long oldValue,newValue;        while(true){            oldValue=urlCounter.get(url);            if(oldValue==null){                newValue=1l;                //初始化成功，退出循环                if(urlCounter.putIfAbsent(url,1l)==null)                    break;                //如果初始化失败，说明其他线程已经初始化了            }else{                newValue=oldValue+1;                //+1成功，退出循环                if(urlCounter.replace(url,oldValue,newValue)){                    break;                    //如果+1失败，则说明其他线程已经修改过了旧值                }            }        }        return newValue;    }

不过还有更简单的方法，就是使用AtomicLongMap

使用Google的AtomicLongMap

AtomicLongMap<String> urlCounter3 = AtomicLongMap.create(); //线程安全，支持并发public long increase3(String url){     return urlCounter3.incrementAndGet(url);}

传统做法，对HashMap加锁

 Map<String, Integer> map = new HashMap<String, Integer>(); //线程不安全 ReentrantReadWriteLock lock = new ReentrantReadWriteLock(); //为map2增加并发锁 public long increase4(String url){    //对map2添加写锁，可以解决线程并发问题        lock.writeLock().lock();    try{        if(map.containsKey(key)){            map.put(key, map.get(key)+1);        }else{            map.put(key, 1);        }    }catch(Exception ex){        ex.printStackTrace();    }finally{        lock.writeLock().unlock();    } }

上文中提到的CountDownLatch的概念可参考：

什么时候使用CountDownLatch

健康检查

场景：服务注册中心需要定时对服务提供者进行心跳检测，即定时调用服务提供者的特定借口，如果返回正常状态吗，则认为服务正常，否则，认为服务提供者异常，在注册中心显示为Down状态，如Consul的服务健康检查机制与之类似。

下面使用CountDownLatch和线程池模拟这种实现。

思路

首先定义一个应用程序启动类，它开始时启动了n个线程类，这些线程将检查外部系统并通知闭锁，并且启动类一直在闭锁上等待着。一旦验证和检查了所有外部服务，那么启动类恢复执行。

实现

BaseHealthChecker：基础健康检查类，实现Runable接口，包含CountDownLatch, ServiceName(服务名称),ServiceUp(服务状态)，其中verifyService 为具体继承该类的子类要实现的方法。

package concurrent.health;import java.util.concurrent.CountDownLatch;public abstract class BaseHealthChecker implements Runnable {    private CountDownLatch countDownLatch;    private String serviceName;    private boolean serviceUp;    public BaseHealthChecker(String serviceName,CountDownLatch countDownLatch){        super();        this.serviceName=serviceName;        this.countDownLatch=countDownLatch;        this.serviceUp=false;    }    @Override    public void run() {        try{            verifySerivce();            serviceUp=true;        }catch (Throwable t){            t.printStackTrace(System.err);            serviceUp=false;        }finally {            if(countDownLatch!=null)                countDownLatch.countDown();        }    }    public String getServiceName() {        return serviceName;    }    public boolean isServiceUp() {        return serviceUp;    }    //this method need to be implemented by all specific service checker    public abstract void verifySerivce();}

DatabaseHealthChecker： 数据库健康检查类

package concurrent.health;import java.util.concurrent.CountDownLatch;public class DataBaseHealthChecker extends BaseHealthChecker {    public DataBaseHealthChecker(CountDownLatch countDownLatch) {        super("database service", countDownLatch);    }    @Override    public void verifySerivce() {        System.out.println("Checking " + this.getServiceName());        try {            Thread.sleep(7000);        } catch (InterruptedException e) {            e.printStackTrace();        }        System.out.println(this.getServiceName() + " is UP");    }}

FileHealthChecker：文件服务健康检查（UserHealthChecker类似）

package concurrent.health;import java.util.concurrent.CountDownLatch;public class FileHealthChecker extends BaseHealthChecker {    public FileHealthChecker(CountDownLatch countDownLatch) {        super("file service", countDownLatch);    }    @Override    public void verifySerivce() {        System.out.println("Checking " + this.getServiceName());        try {            Thread.sleep(7000);        } catch (InterruptedException e) {            e.printStackTrace();        }        System.out.println(this.getServiceName() + " is UP");    }}

ApplicationStartupUtil：服务注册中心调用发起方的主类，在系统启动的时候发起健康检测请求。

package concurrent.health;import java.util.ArrayList;import java.util.List;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class ApplicationStartupUtil {    //list of service checker    private static List<BaseHealthChecker> checkers;    //this latch will be used to wait on    private static CountDownLatch countDownLatch;    //singleton    private ApplicationStartupUtil() {    }    private static ApplicationStartupUtil applicationStartupUtil = new ApplicationStartupUtil();    public static ApplicationStartupUtil getInstance() {        return applicationStartupUtil;    }    public static boolean checkExternalServices() throws InterruptedException {        //init the latch with the number of service checks        countDownLatch = new CountDownLatch(3);        //add all service checks into the list        checkers = new ArrayList<>();        checkers.add(new DataBaseHealthChecker(countDownLatch));        checkers.add(new UserHealthChecker(countDownLatch));        checkers.add(new FileHealthChecker(countDownLatch));        //start service checks using executor framework        ExecutorService executor = Executors.newFixedThreadPool(checkers.size());        for (BaseHealthChecker checker : checkers) {            executor.execute(checker);        }        //now wait all services checked        countDownLatch.await();        //service checkers are finished and now proceed startup        for (BaseHealthChecker checker : checkers) {            if (!checker.isServiceUp()) {                return false;            }        }        return true;    }}

测试

测试方法

package concurrent.health;public class TestMain {    public static void main(String[] args) {        boolean result = false;        try {            result = ApplicationStartupUtil.checkExternalServices();        } catch (Exception ex) {            ex.printStackTrace();        }        System.out.println("External services validation completed !! Result was :: " + result);    }}

结果

Checking database serviceChecking file serviceChecking user servicedatabase service is UPuser service is UPfile service is UPExternal services validation completed !! Result was :: true

本文参考了什么时候使用CountDownLatch