一致性hash算法

package hash;

import java.io.UnsupportedEncodingException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.*;

/**
* Created by IntelliJ IDEA.
* User: test
* Date: 12-5-24
* Time: 下午5:37
* To change this template use File | Settings | File Templates.
*/
public class ConsistencyHash {
private TreeMap<Long,Object> nodes = null;
//真实服务器节点信息
private List<Object> shards = new ArrayList();
//设置虚拟节点数目
private int VIRTUAL_NUM = 4;

/**
* 初始化一致环
*/
public void init() {
shards.add("192.168.0.0-服务器0");
shards.add("192.168.0.1-服务器1");
shards.add("192.168.0.2-服务器2");
shards.add("192.168.0.3-服务器3");
shards.add("192.168.0.4-服务器4");

nodes = new TreeMap<Long,Object>();
for(int i=0; i<shards.size(); i++) {
Object shardInfo = shards.get(i);
for(int j=0; j<VIRTUAL_NUM; j++) {
nodes.put(hash(computeMd5("SHARD-" + i + "-NODE-" + j),j), shardInfo);
}
}
}

/**
* 根据key的hash值取得服务器节点信息
* @param hash
* @return
*/
public Object getShardInfo(long hash) {
Long key = hash;
SortedMap<Long, Object> tailMap=nodes.tailMap(key);
if(tailMap.isEmpty()) {
key = nodes.firstKey();
} else {
key = tailMap.firstKey();
}
return nodes.get(key);
}

/**
* 打印圆环节点数据
*/
public void printMap() {
System.out.println(nodes);
}

/**
* 根据2^32把节点分布到圆环上面。
* @param digest
* @param nTime
* @return
*/
public long hash(byte[] digest, int nTime) {
long rv = ((long) (digest[3+nTime*4] & 0xFF) << 24)
| ((long) (digest[2+nTime*4] & 0xFF) << 16)
| ((long) (digest[1+nTime*4] & 0xFF) << 8)
| (digest[0+nTime*4] & 0xFF);

return rv & 0xffffffffL; /* Truncate to 32-bits */
}

/**
* Get the md5 of the given key.
* 计算MD5值
*/
public byte[] computeMd5(String k) {
MessageDigest md5;
try {
md5 = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException e) {
throw new RuntimeException("MD5 not supported", e);
}
md5.reset();
byte[] keyBytes = null;
try {
keyBytes = k.getBytes("UTF-8");
} catch (UnsupportedEncodingException e) {
throw new RuntimeException("Unknown string :" + k, e);
}

md5.update(keyBytes);
return md5.digest();
}

public static void main(String[] args) {
Random ran = new Random();
ConsistencyHash hash = new ConsistencyHash();
hash.init();
hash.printMap();
//循环50次，是为了取50个数来测试效果，当然也可以用其他任何的数据来测试
for(int i=0; i<50; i++) {
System.out.println(hash.getShardInfo(hash.hash(hash.computeMd5(String.valueOf(i)),ran.nextInt(hash.VIRTUAL_NUM))));
}
}

}