Unsafe CAS 操作

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
创建Unsafe对象的两种方式

/**
 *  Unsafe工具类，生成Unsafe对象的两种方式
 * @author hikktn
 */
public class UnsafeUtil {
    /**
     * 1. 通过反射获取Unsafe实例对象的字段
     * @return
     */
    public static final Unsafe getUnsafeInstance(){
        Unsafe unsafe = null;
        try {
            // 字段名可能会变化，比如在Android的实现中时"THE_ONE"，因此更推荐使用方式二
            // 通过反射得到Unsafe中的单例字段
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            // 设置该Field为可访问
            field.setAccessible(true);
            // 通过get方法得到Undafe实例对象，传入null是因为该Field为static的
            unsafe = (Unsafe) field.get(null);
        } catch(NoSuchFieldException e) {
            e.printStackTrace();
        } catch(IllegalAccessException e) {
            e.printStackTrace();
        }
        return unsafe;
    }
    /**
     * 2. 通过反射调用构造方法来生成Unsafe实例，推荐用这种方式
     * @param v
     * @return
     */
    public static final Unsafe getUnsafeInstance(Void v){
        Unsafe unsafe = null;
        try {
            Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
            unsafeConstructor.setAccessible(true);
            // 通过默认的私有函数构造Unsafe实例
            unsafe = unsafeConstructor.newInstance();
            return unsafe;
        } catch(NoSuchMethodException e) {
            e.printStackTrace();
        } catch(IllegalAccessException e) {
            e.printStackTrace();
        } catch(InstantiationException e) {
            e.printStackTrace();
        } catch(InvocationTargetException e) {
            e.printStackTrace();
        }
        return unsafe;
    }
}

创建实例Unsafe

public class UnsafeTest {
    public static final Unsafe getUnsafeInstance() {
        Unsafe unsafe = null;
        try {
            // 字段名可能会变化，比如在Android的实现中时"THE_ONE"，因此更推荐使用方式二
            // 通过反射得到Unsafe中的单例字段
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            // 设置该Field为可访问
            field.setAccessible(true);
            // 通过get方法得到Undafe实例对象，传入null是因为该Field为static的
            unsafe = (Unsafe) field.get(null);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return unsafe;
    }
    public static void main(String[] args) {
        Unsafe unsafe = getUnsafeInstance();
        System.out.println(unsafe);
        try {
            // 获取域的偏移地址
            long nameOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("name"));
            long idOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("id"));
            Teacher teacher = new Teacher();
            // 执行cas操作
            unsafe.compareAndSwapInt(teacher, idOffset, 0, 1);
            unsafe.compareAndSwapObject(teacher, nameOffset, null, "hikktn");
            System.out.println(teacher);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }
}
class Teacher {
    volatile String name;
    volatile int id;
    @Override
    public String toString() {
        return "Teacher{" +
                "name='" + name + '\'' +
                ", id=" + id +
                '}';
    }
}