Java 多线程锁synchronized与Lock 银行账号案例

线程同步：

同一进程的多个线程共享同一块存储空间

解决冲突：

访问时，加入Synchronized与lock 锁机制，当一个线程获得排他锁，其他线程就必须等待释放

存在问题：

1，一个线程持有锁，会导致其他线程挂起
2，性能问题：加锁，释放锁，会导致较多上下文切换和调度延时
3，优先级倒挂：较高优先级的等待较低优先级的锁
4，sleep不会释放锁

synchronized代码——银行账号案例

package com.chenxb.myThread;import java.math.BigDecimal;public class ThreadSynchronized {public static void main(String[] args) {//初始化账号金额Account account = new Account("100000000000000", new BigDecimal("10000"));//取钱1GetMoney getMoney = new GetMoney(account, new BigDecimal("5000"));new Thread(getMoney).start();//取钱2GetMoney getMoney2 = new GetMoney(account, new BigDecimal("1000"));new Thread(getMoney2).start();//取钱3GetMoney getMoney3 = new GetMoney(account, new BigDecimal("8000"));new Thread(getMoney3).start();}
}//银行账号
class Account {String name;BigDecimal money;public Account(String name, BigDecimal money) {this.name = name;this.money = money;}
}//取钱
class GetMoney implements Runnable {Account account;BigDecimal getMoney;public GetMoney(Account account, BigDecimal getMoney) {this.account = account;this.getMoney = getMoney;}@Overridepublic void run() {//synchronized要锁块,锁的对象就是变化的量（需要增删改的对象）synchronized (account) {// -1,表示account.money小于getMoney；if (account.money.compareTo(getMoney) == -1) {System.out.println("账号金额不够");return;}try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}account.money = account.money.subtract(getMoney);System.out.println(account.money);}}
}

Lock代码——银行账号案例

package com.chenxb.myThread;import java.math.BigDecimal;
import java.util.concurrent.locks.ReentrantLock;public class ThreadLock {public static void main(String[] args) {//注意锁的位置，如果放run()里面，下面new对象时会被覆盖final ReentrantLock lock = new ReentrantLock();//初始化账号金额MyAccount myAccount = new MyAccount("100000000000000", new BigDecimal("10000"));//取钱1GetMyMoney getMyMoney = new GetMyMoney(myAccount, new BigDecimal("5000"), lock);new Thread(getMyMoney).start();//取钱2GetMyMoney getMyMoney2 = new GetMyMoney(myAccount, new BigDecimal("1000"), lock);new Thread(getMyMoney2).start();//取钱3GetMyMoney getMyMoney3 = new GetMyMoney(myAccount, new BigDecimal("8000"), lock);new Thread(getMyMoney3).start();}
}//银行账号
class MyAccount {String name;BigDecimal money;public MyAccount(String name, BigDecimal money) {this.name = name;this.money = money;}
}//取钱
class GetMyMoney implements Runnable {MyAccount myAccount;BigDecimal getMoney;ReentrantLock lock;//定义Lock锁public GetMyMoney(MyAccount myAccount, BigDecimal getMoney, ReentrantLock lock) {this.myAccount = myAccount;this.getMoney = getMoney;this.lock = lock;}@Overridepublic void run() {lock.lock();try {// -1,表示account.money小于getMoney；if (myAccount.money.compareTo(getMoney) == -1) {System.out.println("账号金额不够");return;}try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}myAccount.money = myAccount.money.subtract(getMoney);System.out.println(myAccount.money);} finally {lock.unlock();}}
}