Java并发编程 - 锁Lock 取款机场景实例（转）
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Lock是一个接口，ReentrantLock是它的实现类，下面通过“取款机案例”来剖析它的4个常用方法。
1.爸爸妈妈同时在ATM上登录取款（不加任何锁）
public class Bank {
private static double money = 10000;
public void login(Thread currentUserThread) {
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额 : " + money);
}
public void logout() {
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额 : " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款 : " + withdrawMoney + " 当前银行余额 : " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动妈妈线程
Thread motherThread = new Thread("妈妈") {
public void run() {
try {
//妈妈登录
bank.login(Thread.currentThread());
//过5秒取 10000
Thread.sleep(5000);
bank.withdraw(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
motherThread.start();
}
}
结果：
妈妈 登录进入银行 当前银行余额 : 10000.0 爸爸 登录进入银行 当前银行余额 : 10000.0 爸爸 取款 : 10000.0 当前银行余额 : 0.0 妈妈 当前银行余额 : 0.0 余额不够
妈妈登录显示银行余额为10000，但是当她取钱时却显示“余额不足”，产生了数据不一致。
2.同一时刻，爸爸或妈妈线程只能有一个能够登录银行取款（获取lock()），另外一个线程需要等待, 直到unlock()释放锁
public class Bank {
private static double money = 10000;
private Lock lock = new ReentrantLock();
public void login(Thread currentUserThread) {
lock.lock();//登录加锁
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额 : " + money);
}
public void logout() {
lock.unlock();//退出释放锁
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额 : " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款 : " + withdrawMoney + " 当前银行余额 : " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
//爸爸退出
bank.logout();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动妈妈线程
Thread motherThread = new Thread("妈妈") {
public void run() {
try {
//妈妈登录
bank.login(Thread.currentThread());
//过5秒取 10000
Thread.sleep(5000);
bank.withdraw(10000);
//妈妈退出
bank.logout();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
motherThread.start();
}
}
结果：
爸爸 登录进入银行 当前银行余额 : 10000.0 爸爸 取款 : 10000.0 当前银行余额 : 0.0 爸爸 退出银行 妈妈 登录进入银行 当前银行余额 : 0.0 妈妈 当前银行余额 : 0.0 余额不够 妈妈 退出银行
lock() 和unlock()成对出现，在login(Thread currentUserThread) 登录方法中调用 lock() ,在 logout()退出方法中调用了unlock()。
也就是说，Lock类的锁机制允许在不同的方法中加锁和解锁，而synchronized关键字只能在同一个方法中加锁和解锁。
3. 通过tryLock()判断是否可以获得锁, 能获得锁返回true，否则返回false
public class Bank {
private static double money = 10000;
private Lock lock = new ReentrantLock();
public void login(Thread currentUserThread) {
//判断是否已经有线程登录
if (!lock.tryLock()) {
System.out.println(Thread.currentThread().getName() + " 有人已经登录进入银行 请稍等");
} else {
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额: " + money);
}
}
public void logout() {
lock.unlock();//退出释放锁
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额: " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款: " + withdrawMoney + " 当前银行余额: " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
//爸爸退出
bank.logout();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动妈妈线程
Thread motherThread = new Thread("妈妈") {
public void run() {
//妈妈登录
bank.login(Thread.currentThread());
}
};
motherThread.start();
}
}
结果：
妈妈 有人已经登录进入银行 请稍等 爸爸 登录进入银行 当前银行余额: 10000.0 爸爸 取款: 10000.0 当前银行余额: 0.0 爸爸 退出银行
4. 通过tryLock(long time,TimeUnit timeUnit)设置一段时间后重新进入
public class Bank {
private static double money = 10000;
private Lock lock = new ReentrantLock();
public void login(Thread currentUserThread) {
//如果登录不成功，10秒后再重新尝试获得锁
try {
if (!lock.tryLock(10, TimeUnit.SECONDS)) {
System.out.println(Thread.currentThread().getName() + " 有人已经登录进入银行，请稍等");
} else {
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额: " + money);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void logout() {
lock.unlock();//退出释放锁
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额: " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款: " + withdrawMoney + " 当前银行余额: " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
//爸爸退出
bank.logout();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动妈妈线程
Thread motherThread = new Thread("妈妈") {
public void run() {
//妈妈登录
bank.login(Thread.currentThread());
}
};
motherThread.start();
}
}
结果妈妈10秒后重新尝试得到锁登录进入了银行 ：
爸爸 登录进入银行 当前银行余额: 10000.0 爸爸 取款: 10000.0 当前银行余额: 0.0 爸爸 退出银行 妈妈 登录进入银行 当前银行余额: 0.0
5.如果爸爸线程调用 lock() 获得锁以后，没有unlock()释放锁，妈妈线程将会一直等待产生死锁，即使调用 interrupt()中断也没有作用。
public class Bank {
private static double money = 10000;
private Lock lock = new ReentrantLock();
public void login(Thread currentUserThread) {
lock.lock();
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额 : " + money);
}
public void logout() {
lock.unlock();//退出释放锁
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额: " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款: " + withdrawMoney + " 当前银行余额: " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动妈妈线程
Thread motherThread = new Thread("妈妈") {
public void run() {
try {
Thread.sleep(1000);
bank.login(Thread.currentThread());
} catch (InterruptedException e) {
System.out.println(Thread.currentThread().getName()+" 登录超时被中断");
}
}
};
motherThread.start();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
motherThread.interrupt();
}
}
结果：
妈妈线程一直等待爸爸线程释放锁，结果造成死锁。妈妈线程调用motherThread.interrupt(); 中断也不起做用。
6. 在登录获得锁时，调用lockInterruptibly()，允许中断等待的线程，使motherThread.interrupt()可用。
public class Bank {
private static double money = 10000;
private Lock lock = new ReentrantLock();
public void login(Thread currentUserThread) throws InterruptedException {
lock.lockInterruptibly();//运行等待线程被中断
System.out.println(Thread.currentThread().getName() + " 登录进入银行" + " 当前银行余额 : " + money);
}
public void logout() {
lock.unlock();//退出释放锁
System.out.println(Thread.currentThread().getName() + " 退出银行");
}
public double withdraw(double withdrawMoney) {
if (this.money < withdrawMoney) {
System.out.println(Thread.currentThread().getName() + " 当前银行余额: " + this.money + " 余额不够");
return 0;
}
this.money -= withdrawMoney;
System.out.println(Thread.currentThread().getName() + " 取款: " + withdrawMoney + " 当前银行余额: " + this.money);
return withdrawMoney;
}
}
public class TestLock {
public static void main(String[] args) {
final Bank bank = new Bank();
//启动爸爸线程
Thread fatherThread = new Thread("爸爸") {
public void run() {
try {
//爸爸登录
bank.login(Thread.currentThread());
//过2秒取10000
Thread.sleep(2000);
bank.withdraw(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
fatherThread.start();
// 启动 妈妈 线程
Thread motherThread = new Thread("妈妈") {
public void run() {
try {
Thread.sleep(1000);
bank.login(Thread.currentThread());
} catch (InterruptedException e) {
System.out.println(Thread.currentThread().getName()+" 登录超时被中断");
}
}
};
motherThread.start();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
motherThread.interrupt();
}
}
结果：
爸爸 登录进入银行 当前银行余额 : 10000.0 妈妈 登录超时被中断 爸爸 取款: 10000.0 当前银行余额: 0.0
转自：http://www.verejava.com/?id=17236703718463
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