线程同步
synchronized:同步的
例:取钱
不做线程同步的场合,假设骗子和户主同时取钱,可能出现这种情况:
- 【骗子】取款2000:账户余额1000
- 【户主】取款2000:账户余额1000
- 结果是社会财富增加1000,银行不开心。
代码如下所示:
// 账户类
class Account {
private int accountBalance = 2000;
public void withdraw(String userName, int amount) {
System.out.println(userName + "===in===");
if (accountBalance >= amount) {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
accountBalance -= amount;// 取钱
System.out.println(userName + "取款" + amount + ",余额:" + accountBalance);
} else {
System.out.println(userName + "企图取款" + amount + ",但余额只有:" + accountBalance);
}
System.out.println(userName + "===out===");
}
}
class MyThread extends Thread {
@Override
public void run() {
取钱Demo.act.withdraw(getName(), 1000);
}
}
public class 取钱Demo {
static Account act = new Account();// 账户就一份
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
t1.setName("户主");
t2.setName("骗子");
t2.start();
t1.start();
}
}
想要银行开心,就需要对线程进行同步处理,避免出现重复取款的情况。
线程同步
方法一:锁对象
public void withdraw(String userName, int amount) {
synchronized (this) {
……
}
}
如果是静态方法,没有this,则是锁住【类名.class】
public static void withdraw(String userName, int amount) {
synchronized (Account.class) {
……
}
}
方法二:锁方法
当方法被调用时,调用线程必须获得当前对象的锁,否则将等待下去。
方法结束后,锁会被释放。
public synchronized void withdraw(String userName, int amount) {...}
方法三:ReentrantLock重入锁
ReentrantLock是java.util.concurrent.locks.Lock接口的一个实现类。(reentrant:[r?‘entr?nt]再进去)
一个可重入的互斥锁 Lock,它具有与使用 synchronized 方法和语句所访问的隐式监视器锁相同的一些基本行为和语义,但功能更强大(可以中断、定时)。
API文档上建议的用法:
建议总是 立即实践,使用 lock 块来调用 try,在之前/之后的构造中,最典型的代码如下:
class X {
private final ReentrantLock lock = new ReentrantLock();
// ...
public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock()
}
}
}
应用示例:
import java.util.concurrent.locks.ReentrantLock;
// 账户类
class Account {
private int accountBalance = 2000;
private ReentrantLock lock = new ReentrantLock();
public void withdraw(String userName, int amount) {
synchronized (Account.class) {
lock.lock();
try {
System.out.println(userName + "===in===");
if (accountBalance >= amount) {
Thread.sleep(500);
accountBalance -= amount;// 取钱
System.out.println(userName + "取款" + amount + ",余额:" + accountBalance);
} else {
System.out.println(userName + "企图取款" + amount + ",但余额只有:" + accountBalance);
}
System.out.println(userName + "===out===");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
练习:买票
(未做线程同步,请实现线程同步)
import java.util.Random;
public class 卖票 {
public static void main(String[] args) {
// 一个Runnable实例对象
SellTicket st = new SellTicket();
// 创建三个线程对象
Thread t1 = new Thread(st, "窗口1");
Thread t2 = new Thread(st, "窗口2");
Thread t3 = new Thread(st, "窗口3");
// 启动线程
t1.start();
t2.start();
t3.start();
}
}
class SellTicket implements Runnable {
// 定义票数
private int tickets = 100;
private void sell() {
if (tickets > 0) {
// 模拟售票过程
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
String name = Thread.currentThread().getName();
System.out.println(name + "正在出售第" + (tickets--) + "张票");
}
}
@Override
public void run() {
while (tickets > 0) {
sell();
// 模拟空闲过程
try {
Thread.sleep(new Random().nextInt(11) * 100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
原文地址:https://www.cnblogs.com/tigerlion/p/11179366.html
时间: 2024-10-01 22:10:36