【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}

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)

    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

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()

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

 /**
     * 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