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