用Unsafe实现AtomicInteger类

原理 AtomicInteger可以在并发情况下达到原子化更新,避免使用了synchronized,而且性能非常高。

AtomicInteger的底层是用Unsafe来实现的，这里我们手写一个AtomicInteger类，并且测试一下。 测试例子 启动 1000 个线程，每个线程做 -10 元 的操作，如果初始余额为 10000 那么正确的结果应当是 0。 以下代码在JDK1.8通过测试。

账户Account 类

interface Account {
    // 获取余额
    Integer getBalance();

    // 取款
    void withdraw(Integer amount);

    /**
     * 方法内会启动 1000 个线程，每个线程做 -10 元 的操作
     * 如果初始余额为 10000 那么正确的结果应当是 0
     */
    static void demo(Account account) {
        List<Thread> ts = new ArrayList<>();
        for (int i = 0; i < 1000; i++) {
            ts.add(new Thread(() -> {
                account.withdraw(10);
            }));
        }
        long start = System.nanoTime();
        ts.forEach(Thread::start);
        ts.forEach(t -> {
            try {
                t.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        long end = System.nanoTime();
        System.out.println(account.getBalance()
                + " cost: " + (end-start)/1000_000 + " ms");
    }
}

Account 实现类

class DiyAccount implements Account {

    private DiyAtomicInteger balance;

    public DiyAccount(int balance) {
        this.balance = new DiyAtomicInteger(balance);
    }

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

    @Override
    public void withdraw(Integer amount) {
        balance.decrement(amount);
    }
}

DiyAtomicInteger 类

class DiyAtomicInteger {
    private volatile int value; //实例操作的变量
    private static final long valueOffset;//偏移量
    private static final Unsafe UNSAFE;// unsafe实例对象

    static {
        UNSAFE = UnsafeAccessor.getUnsafe();
        try {
            valueOffset = UNSAFE.objectFieldOffset(DiyAtomicInteger.class.getDeclaredField("value"));
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }

    public int getValue() {
        return value;
    }

    public void decrement(int amount) {
        while (true) {
            int prev = this.value;
            int next = prev - amount;
            if (UNSAFE.compareAndSwapInt(this, valueOffset, prev, next)) {
                break;
            }
        }
    }

    public DiyAtomicInteger(int value) {
        this.value = value;
    }

}

测试

public class Test {
    public static void main(String[] args) {
        Account account = new DiyAccount(10000);
        Account.demo(account);
    }
}

输出

0 cost: 65 ms

其它——Unsafe工具类UnsafeAccessor

我们知道Unsafe 的构造方法是私有的，也没有get方法可以获取对象，所以我们只能通过反射的方式来实例化Unsafe 对象。

public final class Unsafe {
    private static final Unsafe theUnsafe;

    private Unsafe() {
    }
    ...
}

所以

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class UnsafeAccessor {
    private static final Unsafe unsafe;

    static {
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            unsafe = (Unsafe) theUnsafe.get(null);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new Error(e);
        }
    }

    public static Unsafe getUnsafe() {
        return unsafe;
    }
}