Java多线程中关于一个计数器问题

杨佳名

本帖最后由 hello_csu 于 2014-12-2 12:10 编辑

问题描述：
在看书中遇到如下一个仿真例子：小区委员会苏，希望了解每天通过各个大门进入公园的总人数，每个大门都安装了一个计数器，当人通过事后计数器就会递增，其递增，也表示说总人数的共享计数器也会递增。
需求：现在需要统计出来每个门进入的人数和总人数。
其设计思路：
采用多线程机制，对每个门通过的人数采用一个线程去实现,且通过让其之间共享一个Count计数器去统计总人数。
代码设计如下

/**
*
*/
/**
* @author bobwang
*
*/
package OrnamentalGarden;
import java.util.concurrent.*;
import java.util.*;
import java.lang.*;
/**
* Descriptsion:Class Count for counting the total pepole.
*/
class Count
{
private int count =0 ;
private Random rand = new Random(47);
public synchronized int increment() {
int temp = count;
if(rand.nextBoolean())
Thread.yield();
return (count= ++temp);
}
public synchronized int getvalue() {
return count;
}
}
/**
* Decription:Class Entrance for counting the people that enters this entrance.
*/
class Entrance implements Runnable {
//attribute area
private static Count count = new Count();
private static List<Entrance> entrances = new ArrayList<Entrance>();
private int number = 0;
private int id;
private static volatile boolean canceled = false;
//static method area
public static void cancel() {
canceled = true;
}
public static int getTotalCount() {
return count.getvalue();
}
public static int sumEntrances() {
int sum =0;
for(Entrance e : entrances){
sum +=e.getValue();
}
return sum;
}
//Contructor area
public Entrance (int id) {
this.id = id;
entrances.add(this);
}
//Method area
public synchronized int getValue(){
return number;
}
public String toString(){
return "Entrance"+ id+":"+getValue();
}
//Override from interface Runnable
public void run() {
while(!canceled)
{
synchronized(this)
{
number++;
}
System.out.println(this+"Total:"+count.increment());
try {
TimeUnit.MICROSECONDS.sleep(100);
}
catch(InterruptedException e) {
System.out.println("sleep Interrupted");
}
}
System.out.println("Stopping"+this);
}
}
public class OrnamentalGarden {
public static void main(String[] args) throws Exception {
ExecutorService exec = Executors.newCachedThreadPool();
for(int i=0;i<5;i++)
{
exec.execute(new Entrance(i));
}
TimeUnit.MILLISECONDS.sleep(20);
Entrance.cancel();
exec.shutdown();
if(!exec.awaitTermination(250, TimeUnit.MILLISECONDS))
{
System.out.println("Some task was not closed.");
}
System.out.println("Total:"+Entrance.getTotalCount());
System.out.println("Total:"+Entrance.sumEntrances());
}
}

复制代码

Output Result：
Entrance0:1Total:1

Entrance1:1Total:2

Entrance2:1Total:3

Entrance3:1Total:4

......

Entrance4:3Total:17

Entrance2:4Total:18

Entrance2:5Total:19

Entrance1:5Total:20

Entrance4:4Total:23

Entrance3:4Total:21

Entrance0:5Total:22

Entrance4:5Total:24

Entrance2:6Total:27

Entrance1:21Total:100

Entrance0:21Total:101

Entrance4:20Total:102

Entrance3:20Total:99

StoppingEntrance2:20

StoppingEntrance1:21

StoppingEntrance3:20

StoppingEntrance4:20

StoppingEntrance0:21

Total:102

疑问点：现在Entrance4:4Total:23中是实现监控说每个门都通过的人数和这个时间点总人数，但依照我的理解Total的数目应该是一个递增的趋势值，但是从log输出来看，运行过程中是个比较随机的值，为什么会出现忽大忽小？

谁能帮解答一些，特别感谢。

hello_csu · hello_csu

感谢kernel楼主，你这样子一解释，我豁然明白了，非常谢谢！

kerner · kerner

朋友你代码写的真好，发现你发的帖子都不错，有营养。
线程同步有问题，你是保证了计数器同步，但是没有保证5个线程先后同步问题，你是用了5个不同的锁。
稍微改了一下你的代码。

package myfirst;
import java.util.concurrent.*;
import java.util.*;
import java.lang.*;
/**
* Descriptsion:Class Count for counting the total pepole.
*/
class Count
{
private int count =0 ;
private Random rand = new Random(47);
public synchronized int increment() {
int temp = count;
if(rand.nextBoolean())
Thread.yield();
return (count= ++temp);
}
public synchronized int getvalue() {
return count;
}
}
/**
* Decription:Class Entrance for counting the people that enters this entrance.
*/
class Entrance implements Runnable {
//attribute area
private static Count count = new Count();
private static List<Entrance> entrances = new ArrayList<Entrance>();
private int number = 0;
private int id;
private static volatile boolean canceled = false;
//static method area
public static void cancel() {
canceled = true;
}
public static int getTotalCount() {
return count.getvalue();
}
public static int sumEntrances() {
int sum =0;
for(Entrance e : entrances){
sum +=e.getValue();
}
return sum;
}
//Contructor area
public Entrance (int id) {
this.id = id;
entrances.add(this);
}
//Method area
public int getValue(){
synchronized(this.getClass()) {
return number;
}
}
public String toString(){
return "Entrance"+ id+":"+getValue();
}
//Override from interface Runnable
public void run() {
while(!canceled)
{
synchronized(this.getClass())
{
number++;
System.out.println(this+"Total:"+count.increment());
}
try {
TimeUnit.MICROSECONDS.sleep(100);
}
catch(InterruptedException e) {
System.out.println("sleep Interrupted");
}
}
System.out.println("Stopping"+this);
}
}
public class OrnametalGarden {
public static void main(String[] args) throws Exception {
ExecutorService exec = Executors.newCachedThreadPool();
for(int i=0;i<5;i++)
{
exec.execute(new Entrance(i));
}
TimeUnit.MILLISECONDS.sleep(20);
Entrance.cancel();
exec.shutdown();
if(!exec.awaitTermination(250, TimeUnit.MILLISECONDS))
{
System.out.println("Some task was not closed.");
}
System.out.println("Total:"+Entrance.getTotalCount());
System.out.println("Total:"+Entrance.sumEntrances());
}
}

复制代码

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Java多线程中关于一个计数器问题

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