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}
我要评论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？
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()
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
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