详解JAVA如何实现乐观锁以及CAS机制_java

public class AccountUnsafe {
// 余额
private Integer balance;

public AccountUnsafe(Integer balance) {
this.balance = balance;
}

@Override
public Integer getBalance() {
return balance;
}

@Override
public void withdraw(Integer amount) {
balance -= amount;
}
}

public static void main(String[] args) {
// 账户10000元
AccountUnsafe account = new AccountUnsafe(10000);
List ts = new ArrayList<>();
long start = System.nanoTime();
// 1000个线程，每次取10元
for (int i = 0; i <1000; i++) {
ts.add(new Thread(() -> {
account.withdraw(10);
}));
}
ts.forEach(Thread::start);
ts.forEach(t -> {
try {
t.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
long end = System.nanoTime();
// 打印账户余额和花费时间
log.info("账户余额：{}, 花费时间: {}", account.getBalance(), (end-start)/1000_000 + " ms");
}

@Slf4j(topic = "a.AccountCAS")
public class AccountCAS {
// 余额
private volatile int balance;
// Unsafe对象
static final Unsafe unsafe;
// balance 字段的偏移量
static final long BALANCE_OFFSET;
static {
try {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
unsafe = (Unsafe) theUnsafe.get(null);
// balance 属性在 AccountCAS 对象中的偏移量，用于 Unsafe 直接访问该属性
BALANCE_OFFSET = unsafe.objectFieldOffset(AccountCAS.class.getDeclaredField("balance"));
} catch (NoSuchFieldException | IllegalAccessException e) {
throw new Error(e);
}
}
public AccountCAS(Integer balance) {
this.balance = balance;
}
public int getBalance() {
return balance;
}
public void withdraw(Integer amount) {
// 自旋
while (true) {
// 获取老的余额
int oldBalance = balance;
// 获取新的余额
int newBalance = oldBalance - amount;
// 更新余额，BALANCE_OFFSET表示balance属性的偏移量， 返回true表示更新成功， false更新失败，继续更新
if(unsafe.compareAndSwapInt(this, BALANCE_OFFSET, oldBalance, newBalance)) {
return;
}
}
}
public static void main(String[] args) {
// 账户10000元
AccountCAS account = new AccountCAS(10000);
List ts = new ArrayList<>();
long start = System.nanoTime();
// 1000个线程，每次取10元
for (int i = 0; i <1000; i++) {
ts.add(new Thread(() -> {
account.withdraw(10);
}));
}
ts.forEach(Thread::start);
ts.forEach(t -> {
try {
t.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
long end = System.nanoTime();
// 打印账户余额和花费时间
log.info("账户余额：{}, 花费时间: {}", account.getBalance(), (end-start)/1000_000 + " ms");
}
}