@CallerSensitivepublic static Unsafe getUnsafe() {//得到调用该方法的Class对象Class cc = Reflection.getCallerClass();//判断调用该方法的类是否是引导类加载器（bootstrap class loader）//如果不是的话，比如由AppClassLoader调用该方法，则抛出SecurityException异常if (cc.getClassLoader() != null)throw new SecurityException("Unsafe");//返回单例对象return theUnsafe;}

package concurrency;import java.lang.reflect.Field;
import sun.misc.Unsafe;
import sun.reflect.Reflection;public class Test {public static void main(String[] args) throws NoSuchFieldException,SecurityException, IllegalArgumentException, IllegalAccessException {// 通过反射得到theUnsafe对应的Field对象Field field = Unsafe.class.getDeclaredField("theUnsafe");// 设置该Field为可访问field.setAccessible(true);// 通过Field得到该Field对应的具体对象，传入null是因为该Field为static的Unsafe unsafe = (Unsafe) field.get(null);System.out.println(unsafe);}
}

    /** Allocate an instance but do not run any constructor. Initializes the class if it has not yet been. */public native Object allocateInstance(Class cls)throws InstantiationException;

package concurrency;import java.lang.reflect.Field;import sun.misc.Unsafe;
import sun.reflect.Reflection;class User {private String name = "";private int age = 0;public User() {this.name = "test";this.age = 22;}@Overridepublic String toString() {return name + ": " + age;}
}public class Test {public static void main(String[] args) throws NoSuchFieldException,SecurityException, IllegalArgumentException, IllegalAccessException, InstantiationException {// 通过反射得到theUnsafe对应的Field对象Field field = Unsafe.class.getDeclaredField("theUnsafe");// 设置该Field为可访问field.setAccessible(true);// 通过Field得到该Field对应的具体对象，传入null是因为该Field为static的Unsafe unsafe = (Unsafe) field.get(null);User user = (User) unsafe.allocateInstance(User.class);System.out.println(user); //dont invoke constructor, print null: 0
        User userFromNormal = new User();System.out.println(userFromNormal); //print test: 22
}
}

    public native void    putLong(Object o, long offset, long x);

void
sun::misc::Unsafe::putLong (jobject obj, jlong offset, jlong value)
{jlong *addr = (jlong *) ((char *) obj + offset);//计算要修改的数据的内存地址=对象地址+成员属性地址偏移量spinlock lock;//自旋锁，通过循环来获取锁， i386处理器需要加锁访问64位数据，如果是int，则不需要改行代码*addr = value;//往该内存地址位置直接写入数据
}

package concurrency;import java.lang.reflect.Field;import sun.misc.Unsafe;
import sun.reflect.Reflection;class User {private String name = "test"; private long id = 1;private int age = 2;private double height = 1.72;@Overridepublic String toString() {return name + "," + id + "," + age + "," + height;}
}public class Test {public static void main(String[] args) throws NoSuchFieldException,SecurityException, IllegalArgumentException, IllegalAccessException, InstantiationException {// 通过反射得到theUnsafe对应的Field对象Field field = Unsafe.class.getDeclaredField("theUnsafe");// 设置该Field为可访问field.setAccessible(true);// 通过Field得到该Field对应的具体对象，传入null是因为该Field为static的Unsafe unsafe = (Unsafe) field.get(null);User user = new User();System.out.println(user); //打印test,1,2,1.72
        Class userClass = user.getClass();Field name = userClass.getDeclaredField("name");Field id = userClass.getDeclaredField("id");Field age = userClass.getDeclaredField("age");Field height = userClass.getDeclaredField("height");//直接往内存地址写数据unsafe.putObject(user, unsafe.objectFieldOffset(name), "midified-name");unsafe.putLong(user, unsafe.objectFieldOffset(id),100l);unsafe.putInt(user, unsafe.objectFieldOffset(age), 101);unsafe.putDouble(user, unsafe.objectFieldOffset(height), 100.1);System.out.println(user);//打印midified-name,100,101,100.1
}
}

static inline bool
compareAndSwap (volatile jint *addr, jint old, jint new_val)
{jboolean result = false;spinlock lock;if ((result = (*addr == old)))*addr = new_val;return result;
}