【Java】 hashcode()和System.identityHashCode()

hashcode()和System.identityHashCode()

openjdk8: http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/5b86f66575b7

最近在看Spring源码的过程中看到这么一行

@{link org.springframework.context.support.AbstractApplicationContext}

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public AbstractApplicationContext() {
        this.logger = LogFactory.getLog(this.getClass());
        this.id = ObjectUtils.identityToString(this);
        this.displayName = ObjectUtils.identityToString(this);
        this.beanFactoryPostProcessors = new ArrayList();
        this.active = new AtomicBoolean();
        this.closed = new AtomicBoolean();
        this.startupShutdownMonitor = new Object();
        this.applicationListeners = new LinkedHashSet();
        this.resourcePatternResolver = this.getResourcePatternResolver();
}

在初始化Context时设置 id 和 displayName名字的时候 ObjectUtils.identityToString(this)

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public static String identityToString(Object obj) {
    return obj == null ? "" : obj.getClass().getName() + "@" + getIdentityHexString(obj);
}

public static String getIdentityHexString(Object obj) {
    return Integer.toHexString(System.identityHashCode(obj));
}

可以看到Spring的做法是：类名 + @ + 16进制的字符串

所以System.identityHashCode()是什么？

hashcode()和System.identityHashCode()对比

来看个实例

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public class OK {
    public static void main(String[] args) {
        OK ok1 = new OK();
        OK ok2 = new OK();

        System.out.println("ok1 - hashCode : " + ok1.hashCode());// ok1 - hashCode : 1554874502
        System.out.println("ok2 - hashCode : " + ok2.hashCode());// ok2 - hashCode : 1846274136


        System.out.println("ok1 - System.identityHashCode : " + System.identityHashCode(ok1)); //ok1 - System.identityHashCode : 1554874502
        System.out.println("ok2 - System.identityHashCode : " + System.identityHashCode(ok2));//ok2 - System.identityHashCode : 1846274136
    }
}

从结果上来看，相同对象的hashCode()和System.identityHashCode()是一致的

接下来，我们覆盖下hashCode()

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public class OK {

    @Override
    public int hashCode() {
        return 1;
    }

    public int getSuperHashCode(){
        return super.hashCode();
    }

    public static void main(String[] args) {
        OK ok1 = new OK();
        OK ok2 = new OK();

        System.out.println("ok1 - hashCode : " + ok1.hashCode()); // ok1 - hashCode : 1
        System.out.println("ok2 - hashCode : " + ok2.hashCode()); // ok2 - hashCode : 1


        System.out.println("ok1 - System.identityHashCode : " + System.identityHashCode(ok1));//ok1 - System.identityHashCode : 1554874502
        System.out.println("ok2 - System.identityHashCode : " + System.identityHashCode(ok2));//ok2 - System.identityHashCode : 1846274136

        System.out.println("ok1 - SuperHashCode : " + ok1.getSuperHashCode());//ok1 - SuperHashCode : 1554874502
        System.out.println("ok2 - SuperHashCode : " + ok2.getSuperHashCode());//ok2 - SuperHashCode : 1846274136


    }
}

可以看到，如果重载了hashCode()方法，而又想获未重载之前的object.hashCode(),则可以使用System.identityHashCode()

深入System.identityHashCode()

openJDK8： http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/5b86f66575b7

关于System.identityHashCode()里面的声明是这样的

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/**
    * Returns the same hash code for the given object as
    * would be returned by the default method hashCode(),
    * whether or not the given object's class overrides
    * hashCode().
    * The hash code for the null reference is zero.
    *
    * @param x object for which the hashCode is to be calculated
    * @return  the hashCode
    * @since   JDK1.1
    */
   public static native int identityHashCode(Object x);

对于源码中的解读可以参考 hashCode和identityHashCode底层是怎么生成的