Simple code example for Java thread-safe counter

This article mainly introduces Java thread-safe counter simple implementation code examples, which has certain reference value. Friends who need it can learn about it.

A piece of business code I was working on a few days ago required a variable to increment from 1 every day. For this purpose, I simply encapsulated a thread-safe counter, which allows a variable to increment from 1 every day. Of course, if the project is restarted during operation, even if the date is still the same day, the count will still start from 1. Therefore, it is more reliable to store the counter value in the database, but this does not affect the value of this code. It is posted now for the reference of those in need.

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();
    }
  }
}

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