java中的Unsafe类使用

Unsafe类存在sun.misc.Unsafe中可以直接读写内存、获得地址偏移值、锁定或释放线程。concurrentHashmap的1.7版本大量使用该类来提高提升Java运行效率。下面我们介绍一下其使用。

操作成员变量

public class test {
    public static Unsafe getUnsafe() {
        try {
            final Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
            unsafeField.setAccessible(true);
            // the unsafe instance
            return (Unsafe) unsafeField.get(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static void main(String[] args)throws Exception {
        Unsafe unsafe = getUnsafe();
        Dog dog = new Dog();
        // 获取name属性偏移量
        long nameOffset = unsafe.objectFieldOffset(Dog.class.getDeclaredField("name"));
        unsafe.putObject(dog, nameOffset, "李富貴");
        //获取age属性偏移量
        long ageOffset = unsafe.objectFieldOffset(Dog.class.getDeclaredField("age"));
        unsafe.putInt(dog, ageOffset,5);
        System.out.println(dog.toString());

    }

}
class Dog {
    private String name;
    private int age;

    @Override
    public String toString() {
        return "{\"Dog\":{"
                + "                        \"name\":\"" + name + "\""
                + ",                         \"age\":\"" + age + "\""
                + "}}";
    }
}

操作数组

public class test {
    public static Unsafe getUnsafe() {
        try {
            final Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
            unsafeField.setAccessible(true);
            // the unsafe instance
            return (Unsafe) unsafeField.get(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static void main(String[] args)throws Exception {
        Unsafe unsafe = getUnsafe();
        //帮助理解concurrentHashmap
        Dog[] dogs = new Dog[3];
        long sbase = unsafe.arrayBaseOffset(Dog[].class);
        int ts = unsafe.arrayIndexScale(Dog[].class);
        int sshift = 31 - Integer.numberOfLeadingZeros(ts);
        unsafe.putOrderedObject(dogs, (long)(0 << sshift) + sbase, new Dog("那傻狗",5));
        unsafe.putOrderedObject(dogs, (long)(1 << sshift) + sbase, new Dog("李富貴",5));
        unsafe.putOrderedObject(dogs, ts*2 + sbase, new Dog("哈士奇", 5));
        for(int i = 0 ; i < dogs.length ; i++){
            System.out.println(dogs[i].toString());
        }
    }

}
class Dog {
    private String name;
    private int age;

    public Dog(String name, int age){
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "{\"Dog\":{"
                + "                        \"name\":\"" + name + "\""
                + ",                         \"age\":\"" + age + "\""
                + "}}";
    }
}