共享模型之无锁—

共享模型之无锁——Unsafe

本文主要是介绍共享模型之无锁——Unsafe，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

文章目录

概述
Unsafe CAS 操作

名字虽然叫Unsafe,但并不是线程不安全，而是因为他会操作内存，操作线程，不建议开发人员使用。

概述

Unsafe 对象提供了非常底层的，操作内存、线程的方法，Unsafe 对象不能直接调用，只能通过反射获得

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

Unsafe CAS 操作

我们使用Unsafe的cas操作，来线程安全的对Teacher对象的成员变量进行修改。
因为更底层，所有要更麻烦。

public class TestUnsafe {public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");  // getDeclaredField反射获得私有成员变量theUnsafe.setAccessible(true);   // 允许访问私有成员变量Unsafe unsafe = (Unsafe) theUnsafe.get(null);   //theUnsafe成员变量是静态的，静态成员变量从属于类，不从属于对象，所以直接传null值System.out.println(unsafe);// 1. 获取域的偏移地址long idOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("id"));long nameOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("name"));Teacher t = new Teacher();// 2. 执行 cas 操作unsafe.compareAndSwapInt(t, idOffset, 0, 1);unsafe.compareAndSwapObject(t, nameOffset, null, "张三");// 3. 验证System.out.println(t);}
}
@Data
class Teacher {volatile int id;volatile String name;
}

输出

sun.misc.Unsafe@4d7e1886
Teacher(id=1, name=张三)

使用自定义的 AtomicData 实现之前线程安全的原子整数 Account 实现

class AtomicData {private volatile int data;static final Unsafe unsafe;static final long DATA_OFFSET;static {unsafe = UnsafeAccessor.getUnsafe();try {// data 属性在 DataContainer 对象中的偏移量，用于 Unsafe 直接访问该属性DATA_OFFSET = unsafe.objectFieldOffset(AtomicData.class.getDeclaredField("data"));} catch (NoSuchFieldException e) {throw new Error(e);}}public AtomicData(int data) {this.data = data;}public void decrease(int amount) {int oldValue;while(true) {// 获取共享变量旧值，可以在这一行加入断点，修改 data 调试来加深理解oldValue = data;// cas 尝试修改 data 为 旧值 + amount，如果期间旧值被别的线程改了，返回 falseif (unsafe.compareAndSwapInt(this, DATA_OFFSET, oldValue, oldValue - amount)) {return;}}}public int getData() {return data;}
}

Account 实现

Account.demo(new Account() {AtomicData atomicData = new AtomicData(10000);@Overridepublic Integer getBalance() {return atomicData.getData();}@Overridepublic void withdraw(Integer amount) {atomicData.decrease(amount);}
});