Java线程安全的计数器简单实现代码示例

package com.hikvision.cms.rvs.common.util;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
 * Created by lihong10 on 2017/8/9.
 * 一个循环计数器，每天从1开始计数，隔天重置为1。
 * 可以创建一个该类的全局对象，然后每次使用时候调用其get方法即可，可以保证线程安全性
 */
public class CircularCounter {
  private static final AtomicReferenceFieldUpdater<CircularCounter, AtomicInteger> valueUpdater =
      AtomicReferenceFieldUpdater.newUpdater(CircularCounter.class, AtomicInteger.class, "value");
  //保证内存可见性
  private volatile String key;
  //保证内存可见性
  private volatile AtomicInteger value;
  private static final String DATE_PATTERN = "yyyy-MM-dd";
  public CircularCounter() {
    /**
     * 这里将key设置为getCurrentDateString() + "sssssssssss" 是为了测试addAndGet()方法中日期发生变化的情况
     * 正常使用应该将key初始化为getCurrentDateString()
     */
    this.key = getCurrentDateString() + "sssssssssss";
    this.value = new AtomicInteger(0);
  }
  /**
   * 获取计数器加1以后的值
   *
   * @return
   */
  public Integer addAndGet() {
    AtomicInteger oldValue = value;
    AtomicInteger newInteger = new AtomicInteger(0);
    int newVal = -1;
    String newDateStr = getCurrentDateString();
    //日期一致，计数器加1后返回
    if (isDateEquals(newDateStr)) {
      newVal = add(1);
      return newVal;
    }
    //日期不一致，保证有一个线程重置技术器
    reSet(oldValue, newInteger, newDateStr);
    this.key = newDateStr;
    //重置后加1返回
    newVal = add(1);
    return newVal;
  }
  /**
   * 获取计数器的当前值
   * @return
   */
  public Integer get() {
    return value.get();
  }
  /**
   * 判断当前日期与老的日期（也即key成员变量记录的值）是否一致
   *
   * @return
   */
  private boolean isDateEquals(String newDateStr) {
    String oldDateStr = key;
    if (!isBlank(oldDateStr) && oldDateStr.equals(newDateStr)) {
      return true;
    }
    return false;
  }
  /**
   * 如果日期发生变化，重置计数器，也即将key设置为当前日期，并将value重置为0，重置后才能接着累加，
   */
  private void reSet(AtomicInteger oldValue, AtomicInteger newValue, String newDateStr) {
    if(valueUpdater.compareAndSet(this, oldValue, newValue)) {
      System.out.println("线程" + Thread.currentThread().getName() + "发现日期发生变化");
    }
  }
  /**
   * 获取当前日期字符串
   *
   * @return
   */
  private String getCurrentDateString() {
    Date date = new Date();
    String newDateStr = new SimpleDateFormat(DATE_PATTERN).format(date);
    return newDateStr;
  }
  /**
   * 计数器的值加1。采用CAS保证线程安全性
   *
   * @param increment
   */
  private int add(int increment) {
    return value.addAndGet(increment);
  }
  public static boolean isBlank(CharSequence cs) {
    int strLen;
    if(cs != null && (strLen = cs.length()) != 0) {
      for(int i = 0; i < strLen; ++i) {
        if(!Character.isWhitespace(cs.charAt(i))) {
          return false;
        }
      }
      return true;
    } else {
      return true;
    }
  }
  public static void test() {
    CircularCounter c = new CircularCounter();
    AtomicInteger count = new AtomicInteger(0);
    List<Thread> li = new ArrayList<Thread>();
    int size = 10;
    CountDownLatch latch1 = new CountDownLatch(1);
    CountDownLatch latch2 = new CountDownLatch(size);
    for (int i = 0; i < size; i++) {
      Thread t = new Thread(new CounterRunner(c, latch1, latch2, count), "thread-" + i);
      li.add(t);
      t.start();
    }
    System.out.println("start");
    latch1.countDown();
    try {
      latch2.await();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    System.out.println(count.get());
    System.out.println(c.get());
    if(count.get() == c.get()) {
      System.out.println("该计数器是线程安全的！！！");
    }
  }
  public static void main(String... args) {
    for(int i = 0; i < 15; i++) {
      test();
    }
  }
}
/**
 * 测试使用的Runnable对象
 */
class CounterRunner implements Runnable {
  private CircularCounter counter;
  private CountDownLatch latch1;
  private CountDownLatch latch2;
  private AtomicInteger count;
  public CounterRunner(CircularCounter counter, CountDownLatch latch1, CountDownLatch latch2, AtomicInteger count) {
    this.latch1 = latch1;
    this.latch2 = latch2;
    this.counter = counter;
    this.count = count;
  }
  @Override
  public void run() {
    try {
      latch1.await();
      System.out.println("****************");
      for (int i = 0; i < 20; i++) {
        counter.addAndGet();
        count.addAndGet(1);
      }
      latch2.countDown();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}