对synchronized关键字的理解
先看两个线程同时访问一个对象的例子.
public class Account {
private String accountNo;
private double balance;
public Account() {
}
public Account(String accountNo, double balance) {
this.accountNo = accountNo;
this.balance = balance;
}
public void setAccountNo(String accountNo) {
this.accountNo = accountNo;
}
public String getAccountNo() {
return this.accountNo;
}
public void setBalance(double balance) {
this.balance = balance;
}
public double getBalance() {
return this.balance;
}
}
public class DrawThread extends Thread {
private Account account;
private double drawAmount;
public DrawThread(String name, Account account,
double drawAmount) {
super(name);
this.account = account;
this.drawAmount = drawAmount;
}
@Override
public void run() {
if (account.getBalance() >= drawAmount) {
System.out.println(getName() +
"取钱成功!吐出钞票:" + drawAmount);
account.setBalance(account.getBalance() - drawAmount);
System.out.println("\t余额为: " + account.getBalance());
} else {
System.out.println(getName() + "取钱失败!余额不足!");
}
}
}
public class TestDraw {
public static void main(String[] args) {
Account acct = new Account("1234567", 1000);
new DrawThread("甲", acct, 800).start();
new DrawThread("乙", acct, 800).start();
}
}
类Account有2个成员变量,帐户名字和余额.DrawThread中的取钱逻辑很简单.先判断余额够不够,如果够,正常取钱,不够的话报余额不足.在TestDraw同时开2个线程取钱,这时候可能会有问题,线程甲判断出还能取钱,准备执行account.setBalance时放弃了处理器,线程乙获得了处理器,判断钱够不够.因为之前线程甲还没有修改balance的值,所以认为可以取钱.所以线程乙取了钱,之后放弃处理器.线程甲之前已经判断过可以取钱,就执行取钱逻辑,这时候帐户余额就变成负的了.多次运行该程序,肯定会出现不正常的现象.
synchronized就是为了解决类似的问题.把DrawThread改成如下形式:
public class DrawThread extends Thread {
private Account account;
private double drawAmount;
public DrawThread(String name, Account account,
double drawAmount) {
super(name);
this.account = account;
this.drawAmount = drawAmount;
}
@Override
public void run() {
synchronized (account) {
if (account.getBalance() >= drawAmount) {
System.out.println(getName() +
"取钱成功!吐出钞票:" + drawAmount);
account.setBalance(account.getBalance() - drawAmount);
System.out.println("\t余额为: " + account.getBalance());
} else {
System.out.println(getName() + "取钱失败!余额不足!");
}
}
}
}
synchronized (account){}表示锁住account对象,一个对象访问account时,另外一个对象不能访问.这就保证了线程安全.