Unsafe使用案例

public class UseUnsafeClassDemo {

    static Unsafe unsafe = Unsafe.getUnsafe();
    private static long offset;
    private static volatile int state = 0;

    static {
        try {
            offset = unsafe.objectFieldOffset(UseUnsafeClassDemo.class.getField("state"));
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        UseUnsafeClassDemo useUnsafeClassDemo = new UseUnsafeClassDemo();
        boolean result = unsafe.compareAndSwapInt(useUnsafeClassDemo, offset, state, 1);
        System.out.println(result);

    }
}

运行结果：

报错原因：
当前应用使用AppClassLoader是无法获取Unsafe实例的，这是jvm出于安全考虑，只能由Bootstrap类加载器加载。

@CallerSensitive
public static Unsafe getUnsafe() {
     Class var0 = Reflection.getCallerClass();
     // 判断是否为BootstrapClassLoader
     if (!VM.isSystemDomainLoader(var0.getClassLoader())) {
         throw new SecurityException("Unsafe");
     } else {
         return theUnsafe;
     }
 }

利用反射获取

/**
 * 利用反射获取Unsafe
 */
public class ReflectUnsafeClassDemo {

    static Unsafe unsafe = null;
    static long offset = 0;
    private volatile int state = 0;

    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
            offset = unsafe.objectFieldOffset(ReflectUnsafeClassDemo.class.getDeclaredField("state"));
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        ReflectUnsafeClassDemo useUnsafeClassDemo = new ReflectUnsafeClassDemo();
        boolean result = unsafe.compareAndSwapInt(useUnsafeClassDemo, offset, 0, 1);
        System.out.println(result);

    }
}

运行结果：