Object.hashCode()与System.identityHashCode()区别

前言

今天无意中阅读SpringBoot的源码，发下了System.identityHashCode()的使用，不了解其作用，所以查阅资料学习，特记录下来。

hashCode 与 identityHashCode

Object.hashCode()和System.identityHashCode()的实现，原理是一致的，底层都是使用同一套算法得到具体的hash值。但是呢，Object中的hashCode()可以被子类重写，例如String，Integer等都重写了hashCode()函数。这样就会出现计算的hashCode值和System.identityHashCode()不一样。

public static void main(String[] args) {

    String str1 = new String("123");
    String str2 = new String("123");
    Object obj = new Object();

    System.out.println(System.identityHashCode(str1));
    System.out.println(System.identityHashCode(str2));
    System.out.println(System.identityHashCode(obj));

    System.out.println("----------------------compare---------------------");

    System.out.println(str1.hashCode());
    System.out.println(str2.hashCode());
    System.out.println(obj.hashCode());

}

运行结果：

"C:\Program Files\Java\jdk1.8.0_65\bin\java"......
1830908236
277630005
1288354730
----------------------compare----------------------
48690
48690
1288354730

Process finished with exit code 0

使用场景

可用于判断某个对象是否已经被处理过了，具体参考SpringBoot中ConfigurationClassPostProcessor类的实现：

public class ConfigurationClassPostProcessor implements 
BeanDefinitionRegistryPostProcessor,
		PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
		
	private final Set<Integer> registriesPostProcessed = new HashSet<Integer>();
	
	......................省略部分代码...............................
	
	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
	    // 根据对象所在内存地址生成唯一标识
		int registryId = System.identityHashCode(registry);
		if (this.registriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
		}
		if (this.factoriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanFactory already called on this post-processor against " + registry);
		}
		// 标记该对象已被处理
		this.registriesPostProcessed.add(registryId);
        
        // 处理该对象registry
		processConfigBeanDefinitions(registry);
	}
	
	......................省略部分代码...............................
  
}