競爭條件:兩個或是兩個以上的線程需要共享對同一數據的存取。如果兩個線程存取相同的對象,並且每一個線程都調用了一個修改該對象狀態的方法。可能會產生訛誤的對象,這一情況通常稱爲競爭條件。
如下程序,當多個線程同時更新銀行賬戶餘額,一段時間,錯誤不知不覺就出現了,總額要麼增加,要麼減少。問題在於這不是原子操作
假定兩個線程同時執行指令accounts[to] += amount;
該指令可能被處理如下:
1.將accounts[to]加載到寄存器。
2.增加amount。
3.將結果寫回accounts[to].
現在,假定第一個線程執行步驟1和2,然後,它被剝奪了運行權。假定第二個線程被喚醒並修改了accounts數組中的同一項。然後,第一個線程被喚醒並完成其第3步。
這樣,這一動作擦去了第二個線程所做的更新。於是,總金額不在正確。
package com.bing.jia;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class UnsynchBankTest{
public static final int NACCOUNTS = 100;//這家銀行有多少個賬號
public static final double INITIAL_BALANCE = 1000;//每個賬號開始的金額
public static void main(String[] args) {
Bank b = new Bank(NACCOUNTS, INITIAL_BALANCE);
int i;
for(i=0;i<NACCOUNTS;i++){
TransferRunnable r = new TransferRunnable(b, i, INITIAL_BALANCE);
Thread t = new Thread(r);
t.start();
}
}
}
class Bank extends Thread{
private final double[] accounts;//數組的長度爲銀行賬號數,數組的值爲銀行賬號的金額
public Bank(int n,double initialBalance){
accounts = new double[n];
for(int i=0;i<accounts.length;i++){
accounts[i] = initialBalance;
}
}
public void transfer(int from,int to,double amount){
if(accounts[from]<amount){
return;
}
System.out.println("Thread.currentThread():"+Thread.currentThread());
accounts[from] -= amount;
System.out.println("轉賬金額數amount:"+amount+" from:"+from+" to:"+to);
accounts[to] += amount;
System.out.println("getTotalBalance()賬戶剩餘金額:"+getTotalBalance());
}
public double getTotalBalance(){
double sum = 0;
for(double a : accounts){
sum += a;
}
return sum;
}
public int size(){
return accounts.length;
}
}
class TransferRunnable implements Runnable{
private Bank bank;
private int fromAccount;
private double maxAmount;
private int DELAY = 10;
public TransferRunnable(Bank b,int from,double max){
bank = b;
fromAccount = from;
maxAmount = max;
}
@Override
public void run() {
try {
while(true){
int toAccount = (int) (bank.size()*Math.random());//隨機選一個要收賬的賬號Math.random()返回0-1的小數
double amount = maxAmount*Math.random();//轉賬的金額數,始終不能比賬號總額大
bank.transfer(fromAccount, toAccount, amount);
Thread.sleep((int)(DELAY*Math.random()));
Thread.sleep(DELAY);
}
} catch (InterruptedException e) {
// TODO: handle exception
}
}
}