测试Vm配置hashCode值

测试Vm配置hashCode值

程序启动时配置参数。
VM arguments:
-XX:hashCode=4

可选范围0~4，其他值都是else

look out又是一张拼图
native 方式表示是本地程序c语言实现的，不是java实现。

这里是内部实现
jdk: 源码
https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/a3f01f9da231/src/share/vm/runtime/synchronizer.cpp

java程序

package demo;

public class HashCode {

	public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "world";
        String s3 = "helloworld";
        String s4 = s1+s2;
        System.out.println(s3==s4);
        System.out.println(s3.hashCode());
        System.out.println(s4.hashCode());
        System.out.println(System.identityHashCode(s3));
        System.out.println(System.identityHashCode(s4));
	}
	
}

默认配置和hashCode=5 else块代码一致结果

false
-1524582912
-1524582912
1829164700
2018699554

-XX:hashCode=0
This form uses an unguarded global Park-Miller RNG,

false
-1524582912
-1524582912
401056976
1760911346

-XX:hashCode=1
以后在其他client程序或者socket也许会见到
// This variation has the property of being stable (idempotent)
// between STW operations. This can be useful in some of the 1-0
// synchronization schemes.

false
-1524582912
-1524582912
1778440328
1778440415

-XX:hashCode=2
for sensitivity testing

false
-1524582912
-1524582912
1
1

-XX:hashCode=3

false
-1524582912
-1524582912
24
25

-XX:hashCode=4

false
-1524582912
-1524582912
1801201000
1801201600

-XX:hashCode=5 else块

false
-1524582912
-1524582912
1829164700
2018699554