概述
Jedis是Redis官方推荐的Java客户端，更多Redis的客户端可以参考Redis官网客户端列表。当业务的数据量非常庞大时，需要考虑将数据存储到多个缓存节点上，如何定位数据应该存储的节点，一般用的是一致性哈希算法。Jedis在客户端角度实现了一致性哈希算法，对数据进行分片，存储到对应的不同的redis实例中。
Jedis对Sharded的实现主要是在ShardedJedis.java和ShardedJedisPool.java中。本文主要介绍ShardedJedis的实现，ShardedJedisPool是基于apache的common-pool2的对象池实现。
继承关系
ShardedJedis--->BinaryShardedJedis--->Sharded <Jedis, JedisShardInfo>
构造函数
查看其构造函数
public ShardedJedis(List<JedisShardInfo> shards, Hashing algo, Pattern keyTagPattern) { super(shards, algo, keyTagPattern); } 构造器参数解释：
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shards是一个JedisShardInfo的列表，一个JedisShardedInfo类代表一个数据分片的主体。
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algo是用来进行数据分片的算法
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keyTagPattern，自定义分片算法所依据的key的形式。例如，可以不针对整个key的字符串做哈希计算，而是类似对thisisa{key}中包含在大括号内的字符串进行哈希计算。
JedisShardInfo是什么样的？
public class JedisShardInfo extends ShardInfo<Jedis> { public String toString() { return host + ":" + port + "*" + getWeight(); } private int connectionTimeout; private int soTimeout; private String host; private int port; private String password = null; private String name = null; // Default Redis DB private int db = 0; public String getHost() { return host; } public int getPort() { return port; } public JedisShardInfo(String host) { super(Sharded.DEFAULT_WEIGHT); URI uri = URI.create(host); if (JedisURIHelper.isValid(uri)) { this.host = uri.getHost(); this.port = uri.getPort(); this.password = JedisURIHelper.getPassword(uri); this.db = JedisURIHelper.getDBIndex(uri); } else { this.host = host; this.port = Protocol.DEFAULT_PORT; } } public JedisShardInfo(String host, String name) { this(host, Protocol.DEFAULT_PORT, name); } public JedisShardInfo(String host, int port) { this(host, port, 2000); } public JedisShardInfo(String host, int port, String name) { this(host, port, 2000, name); } public JedisShardInfo(String host, int port, int timeout) { this(host, port, timeout, timeout, Sharded.DEFAULT_WEIGHT); } public JedisShardInfo(String host, int port, int timeout, String name) { this(host, port, timeout, timeout, Sharded.DEFAULT_WEIGHT); this.name = name; } public JedisShardInfo(String host, int port, int connectionTimeout, int soTimeout, int weight) { super(weight); this.host = host; this.port = port; this.connectionTimeout = connectionTimeout; this.soTimeout = soTimeout; } public JedisShardInfo(String host, String name, int port, int timeout, int weight) { super(weight); this.host = host; this.name = name; this.port = port; this.connectionTimeout = timeout; this.soTimeout = timeout; } public JedisShardInfo(URI uri) { super(Sharded.DEFAULT_WEIGHT); if (!JedisURIHelper.isValid(uri)) { throw new InvalidURIException(String.format( "Cannot open Redis connection due invalid URI. %s", uri.toString())); } this.host = uri.getHost(); this.port = uri.getPort(); this.password = JedisURIHelper.getPassword(uri); this.db = JedisURIHelper.getDBIndex(uri); } @Override public Jedis createResource() { return new Jedis(this); } /** * 省略setters和getters **/ } 可见JedisShardInfo包含了一个redis节点ip地址，端口号，name，密码等等相关信息。要构造一个ShardedJedis，提供一个或多个JedisShardInfo。
最终构造函数的实现在其父类Sharded里面
public Sharded(List<S> shards, Hashing algo, Pattern tagPattern) { this.algo = algo; this.tagPattern = tagPattern; initialize(shards); } 哈希环的初始化
Sharded类里面维护了一个TreeMap，基于红黑树实现，用来盛放经过一致性哈希计算后的redis节点，另外维护了一个LinkedHashMap，用来保存ShardInfo与Jedis实例的对应关系。
定位的流程如下
先在TreeMap中找到对应key所对应的ShardInfo，然后通过ShardInfo在LinkedHashMap中找到对应的Jedis实例。
Sharded类对这些实例变量的定义如下所示：
public static final int DEFAULT_WEIGHT = 1; private TreeMap<Long, S> nodes; private final Hashing algo; private final Map<ShardInfo<R>, R> resources = new LinkedHashMap<ShardInfo<R>, R>(); /** * The default pattern used for extracting a key tag. The pattern must have * a group (between parenthesis), which delimits the tag to be hashed. A * null pattern avoids applying the regular expression for each lookup, * improving performance a little bit is key tags aren't being used. */ private Pattern tagPattern = null; // the tag is anything between {} public static final Pattern DEFAULT_KEY_TAG_PATTERN = Pattern.compile("\\{(.+?)\\}"); 接下来看其构造函数中的initialize方法
private void initialize(List<S> shards) { nodes = new TreeMap<Long, S>(); for (int i = 0; i != shards.size(); ++i) { final S shardInfo = shards.get(i); if (shardInfo.getName() == null) for (int n = 0; n < 160 * shardInfo.getWeight(); n++) { nodes.put(this.algo.hash("SHARD-" + i + "-NODE-" + n), shardInfo); } else for (int n = 0; n < 160 * shardInfo.getWeight(); n++) { nodes.put( this.algo.hash(shardInfo.getName() + "*" + shardInfo.getWeight() + n), shardInfo); } resources.put(shardInfo, shardInfo.createResource()); } } 可以看到，它对每一个ShardInfo通过一定规则计算其哈希值，然后存到TreeMap中，这里它实现了一致性哈希算法中虚拟节点的概念，因为我们可以看到同一个ShardInfo不止一次被放到TreeMap中，数量是，权重*160。
增加了虚拟节点的一致性哈希有很多好处，能避免数据在redis节点间分布不均匀。
然后，在LinkedHashMap中放入ShardInfo以及其对应的Jedis实例，通过调用其自身的createSource()来得到jedis实例。
数据定位
从ShardedJedis的代码中可以看到，无论进行什么操作，都要先根据key来找到对应的Redis，然后返回一个可供操作的Jedis实例。
例如其set方法：
public String set(String key, String value) { Jedis j = getShard(key); return j.set(key, value); } 而getShard方法则在Sharded.java中实现，其源代码如下所示：
public R getShard(byte[] key) { return resources.get(getShardInfo(key)); } public R getShard(String key) { return resources.get(getShardInfo(key)); } public S getShardInfo(byte[] key) { SortedMap<Long, S> tail = nodes.tailMap(algo.hash(key)); if (tail.isEmpty()) { return nodes.get(nodes.firstKey()); } return tail.get(tail.firstKey()); } public S getShardInfo(String key) { return getShardInfo(SafeEncoder.encode(getKeyTag(key))); } 可以看到，先通过getShardInfo方法从TreeMap中获得对应的ShardInfo，然后根据这个ShardInfo就能够再LinkedHashMap中获得对应的Jedis实例了。