代码中实际运用memcached——java

以下文章取自：http://jameswxx.iteye.com/blog/1168711

memcached的java客户端有好几种，http://code.google.com/p/memcached/wiki/Clients 罗列了以下几种

spymemcached

    * http://www.couchbase.org/code/couchbase/java
          o An improved Java API maintained by Matt Ingenthron and others at Couchbase.
          o Aggressively optimised, ability to run async, supports binary protocol, support Membase and Couchbase features, etc. See site for details.

Java memcached client

    * http://www.whalin.com/memcached
          o A Java API is maintained by Greg Whalin from Meetup.com.

More Java memcached clients

    * http://code.google.com/p/javamemcachedclient
    * http://code.google.com/p/memcache-client-forjava
    * http://code.google.com/p/xmemcached

Integrations

    * http://code.google.com/p/simple-spring-memcached
    * http://code.google.com/p/memcached-session-manager

我看的是第二个：Java memcached client源码，代码很简洁，一共只有9个类，最主要的有以下三个
MemcachedClient.java     客户端，负责提供外出程序接口，如get/set方法等等

SockIOPool.java          一个自平衡的连接池
NativeHandler.java       负责部分数据类型的序列化

它包含以下几个部分
1：key的服务端分布
2：数据序列化和压缩
3：连接池(连接方式和池的动态自动平衡)
4：failover和failback机制
5：和memcached服务器的通讯协议 
关于这几个点，我从key的set/get说起，会贯穿上面列举的4个部分。这个文章写下来，本来是作为一个笔记，思维比较跳跃，可能不是很连贯，如有疑问，欢迎站内交流。这个client的代码

很简洁明了，我也没有加过多注释，只是理了一个脉络。

从客户端自带的测试代码开始

package com.meetup.memcached.test;
import com.meetup.memcached.*;
import org.apache.log4j.*;

public class TestMemcached  {
    public static void main(String[] args) {
        BasicConfigurator.configure();
        String[] servers = { "127.0.0.1:12000"};
        SockIOPool pool = SockIOPool.getInstance();
        pool.setServers( servers );
        pool.setFailover( true );//故障转移
     pool.setInitConn( 10 ); //初始化连接为10
        pool.setMinConn( 5 );//最小连接为5
        pool.setMaxConn( 250 );//最大连接为250
        pool.setMaintSleep( 30 );//平衡线程休眠时间为30ms
        pool.setNagle( false );//Nagle标志为false
        pool.setSocketTO( 3000 );//响应超时时间为3000ms
        pool.setAliveCheck( true );//需要可用状态检查
     //初始化连接池，默认名称为"default"
        pool.initialize();
        //新建一个memcached客户端，如果没有给名字
     MemcachedClient mcc = new MemcachedClient();

        // turn off most memcached client logging:
        com.meetup.memcached.Logger.getLogger( MemcachedClient.class.getName() ).setLevel( com.meetup.memcached.Logger.LEVEL_WARN );

        for ( int i = 0; i < 10; i++ ) {
            boolean success = mcc.set( "" + i, "Hello!" );
            String result = (String)mcc.get( "" + i );
            System.out.println( String.format( "set( %d ): %s", i, success ) );
            System.out.println( String.format( "get( %d ): %s", i, result ) );
        }

        System.out.println( "
	 -- sleeping --
" );
        try { Thread.sleep( 10000 ); } catch ( Exception ex ) { }

        for ( int i = 0; i < 10; i++ ) {
            boolean success = mcc.set( "" + i, "Hello!" );
            String result = (String)mcc.get( "" + i );
            System.out.println( String.format( "set( %d ): %s", i, success ) );
            System.out.println( String.format( "get( %d ): %s", i, result ) );
        }
    }
}

 
以上代码大概做了这几件事情：
初始化一个连接池
新建一个memcached客户端
set一个key/value
get一个key，并且打印出value
这是我们实际应用中很常见的场景。

连接池的创建和初始化 
        连接池SockIOPool是非常重要的部分，它的好坏直接决定了客户端的性能。SockIOPool用一个HashMap持有多个连接池对象，连接池以名称作为标识，默认为"default"。看看

SockIOPool的getInstance方法就知道了。

public static SockIOPool getInstance() {
      return getInstance("default");
  }

  public static synchronized SockIOPool getInstance(String poolName) {
      if (pools.containsKey(poolName)) return pools.get(poolName);

      SockIOPool pool = new SockIOPool();
      pools.put(poolName, pool);

      return pool;
  }

连接池实例化完成后，还需要初始化，看看pool.initialize()做了什么：

public void initialize() {
   //这里以自身作为同步锁，防止被多次初始化
   synchronized (this) {
   // 如果已经被初始化了则终止初始化过程
   if (initialized && (buckets != null || consistentBuckets != null) && (availPool != null)&& (busyPool != null)) {
      log.error("++++ trying to initialize an already initialized pool");
      return;
   }
    <span style="color: #ff0000;">  // 可用连接集合</span>
      availPool = new HashMap<String, Map<SockIO, Long>>(servers.length * initConn);
     //工作连接集合
     busyPool = new HashMap<String, Map<SockIO, Long>>(servers.length * initConn);
     // 不可用连接集合
     deadPool = new IdentityHashMap<SockIO, Integer>();
    hostDeadDur = new HashMap<String, Long>();
   hostDead = new HashMap<String, Date>();
   maxCreate = (poolMultiplier > minConn) ? minConn : minConn / poolMultiplier;
   if (log.isDebugEnabled()) {
      log.debug("++++ initializing pool with following settings:");
      log.debug("++++ initial size: " + initConn);
      log.debug("++++ min spare   : " + minConn);
      log.debug("++++ max spare   : " + maxConn);
   }
   if (servers == null || servers.length <= 0) {
      log.error("++++ trying to initialize with no servers");
      throw new IllegalStateException("++++ trying to initialize with no servers");
   }
   // initalize our internal hashing structures
   if (this.hashingAlg == CONSISTENT_HASH) populateConsistentBuckets();
   else populateBuckets();
   // mark pool as initialized
   this.initialized = true;
   // start maint thread
   if (this.maintSleep > 0) this.startMaintThread();
  }
}

 
 

连接池的关闭

很简单，只是重置清空相关参数而已

public void shutDown() {
        synchronized (this) {
            if (log.isDebugEnabled()) log.debug("++++ SockIOPool shutting down...");

            if (maintThread != null && maintThread.isRunning()) {
                // stop the main thread
                stopMaintThread();

                // wait for the thread to finish
                while (maintThread.isRunning()) {
                    if (log.isDebugEnabled()) log.debug("++++ waiting for main thread to finish run +++");
                    try {
                        Thread.sleep(500);
                    } catch (Exception ex) {
                    }
                }
            }

            if (log.isDebugEnabled()) log.debug("++++ closing all internal pools.");
            closePool(availPool);
            closePool(busyPool);
            availPool = null;
            busyPool = null;
            buckets = null;
            consistentBuckets = null;
            hostDeadDur = null;
            hostDead = null;
            maintThread = null;
            initialized = false;
            if (log.isDebugEnabled()) log.debug("++++ SockIOPool finished shutting down.");
        }
    }

连接池的自动平衡
SockIOPool的initialize()方法最后有这么一行代码

// start maint thread
if (this.maintSleep > 0) this.startMaintThread();
 

 这是在初始化完成后，启动线程池平衡线程

  

protected void startMaintThread() {
      if (maintThread != null) {
          if (maintThread.isRunning()) {
              log.error("main thread already running");
          } else {
              maintThread.start();
          }
      } else {
          maintThread = new MaintThread(this);
          maintThread.setInterval(this.maintSleep);
          maintThread.start();
      }
  }

  

MaintThread的run方法

public void run() {
   this.running = true;
   while (!this.stopThread) {
       try {
           Thread.sleep(interval);
           // if pool is initialized, then
           // run the maintenance method on itself
           if (pool.isInitialized()) pool.selfMaint();
       } catch (Exception e) {
           break;
       }
   }
   this.running = false;

 
 其实最终的平衡方法是SockIOPool.selfMaint()

  

protected void selfMaint() {
        if (log.isDebugEnabled()) log.debug("++++ Starting self maintenance....");

        // go through avail sockets and create sockets
        // as needed to maintain pool settings
        Map<String, Integer> needSockets = new HashMap<String, Integer>();

        synchronized (this) {
            // 先统计每个服务器实例的可用连接是否小于最小可用连接数
        for (Iterator<String> i = availPool.keySet().iterator(); i.hasNext();) {
                String host = i.next();
                Map<SockIO, Long> sockets = availPool.get(host);

                if (log.isDebugEnabled()) log.debug("++++ Size of avail pool for host (" + host + ") = "
                                                    + sockets.size());

                // if pool is too small (n < minSpare)
                if (sockets.size() < minConn) {
                    // need to create new sockets
                    int need = minConn - sockets.size();
                    needSockets.put(host, need);
                }
            }
        }

        // 如果小于最小可用连接数，则要新建增加可用连接
     Map<String, Set<SockIO>> newSockets = new HashMap<String, Set<SockIO>>();

        for (String host : needSockets.keySet()) {
            Integer need = needSockets.get(host);

            if (log.isDebugEnabled()) log.debug("++++ Need to create " + need + " new sockets for pool for host: "
                                                + host);

            Set<SockIO> newSock = new HashSet<SockIO>(need);
            for (int j = 0; j < need; j++) {
                SockIO socket = createSocket(host);
                if (socket == null) break;
                newSock.add(socket);
            }
            newSockets.put(host, newSock);
        }

        // synchronize to add and remove to/from avail pool
        // as well as clean up the busy pool (no point in releasing
        // lock here as should be quick to pool adjust and no
        // blocking ops here)
        synchronized (this) {
            //将新建的连接添加到可用连接集合里
       for (String host : newSockets.keySet()) {
                Set<SockIO> sockets = newSockets.get(host);
                for (SockIO socket : sockets)
                    addSocketToPool(availPool, host, socket);
            }

            for (Iterator<String> i = availPool.keySet().iterator(); i.hasNext();) {
                String host = i.next();
                Map<SockIO, Long> sockets = availPool.get(host);
                if (log.isDebugEnabled()) log.debug("++++ Size of avail pool for host (" + host + ") = "
                                                    + sockets.size());

                //如果可用连接超过了最大连接数，则要关闭一些
          if (sockets.size() > maxConn) {
                    // need to close down some sockets
                    int diff = sockets.size() - maxConn;
                    int needToClose = (diff <= poolMultiplier) ? diff : (diff) / poolMultiplier;

                    if (log.isDebugEnabled()) log.debug("++++ need to remove " + needToClose
                                                        + " spare sockets for pool for host: " + host);

                    for (Iterator<SockIO> j = sockets.keySet().iterator(); j.hasNext();) {
                        if (needToClose <= 0) break;

                        // remove stale entries
                        SockIO socket = j.next();
                        long expire = sockets.get(socket).longValue();

                        // 这里回收可用连接池的闲置连接,连接设置到可用连接池里时,expire设置为当前时间。如果 (expire + maxIdle) < System.currentTimeMillis()为true，则表
明，该连接在可用连接池呆得太久了，需要回收
               if ((expire + maxIdle) < System.currentTimeMillis()) {
                            if (log.isDebugEnabled()) log.debug("+++ removing stale entry from pool as it is past its idle timeout and pool is over max spare");

                            // remove from the availPool
                            deadPool.put(socket, ZERO);
                            j.remove();
                            needToClose--;
                        }
                    }
                }
            }

            //清理正在工作的连接集合
        for (Iterator<String> i = busyPool.keySet().iterator(); i.hasNext();) {
                String host = i.next();
                Map<SockIO, Long> sockets = busyPool.get(host);
                if (log.isDebugEnabled()) log.debug("++++ Size of busy pool for host (" + host + ")  = "
                                                    + sockets.size());
                // loop through all connections and check to see if we have any hung connections
                for (Iterator<SockIO> j = sockets.keySet().iterator(); j.hasNext();) {
                    // remove stale entries
                    SockIO socket = j.next();
                    long hungTime = sockets.get(socket).longValue();
                    //如果工作时间超过maxBusyTime，则也要回收掉，超过maxBusyTime，可能是服务器响应时间过长
             if ((hungTime + maxBusyTime) < System.currentTimeMillis()) {
                        log.error("+++ removing potentially hung connection from busy pool ... socket in pool for "
                                  + (System.currentTimeMillis() - hungTime) + "ms");

                        // remove from the busy pool
                        deadPool.put(socket, ZERO);
                        j.remove();
                    }
                }
            }
        }

        // 最后清理不可用连接集合
     Set<SockIO> toClose;
        synchronized (deadPool) {
            toClose = deadPool.keySet();
            deadPool = new IdentityHashMap<SockIO, Integer>();
        }

        for (SockIO socket : toClose) {
            try {
                socket.trueClose(false);
            } catch (Exception ex) {
                log.error("++++ failed to close SockIO obj from deadPool");
                log.error(ex.getMessage(), ex);
            }

            socket = null;
        }

        if (log.isDebugEnabled()) log.debug("+++ ending self maintenance.");
    }

 
 

key的服务器端分布

初始化方法其实就是根据每个服务器的权重，建立一个服务器地址集合，如果选择了一致性哈希，则对服务器地址进行一致性哈希分布，一致性哈希算法比较简单，如果不了解的同学，可以

自行google一下，initialize() 方法里有这段代码：

//一致性哈希

if (this.hashingAlg == CONSISTENT_HASH){
  populateConsistentBuckets();
}else populateBuckets();

 
 看看populateConsistentBuckets()方法

// 用一致性哈希算法将服务器分布在一个2的32次方的环里，服务器的分布位置<=servers.length*40*4

private void populateConsistentBuckets() {
    if (log.isDebugEnabled()) log.debug("++++ initializing internal hashing structure for consistent hashing");

    // store buckets in tree map
    this.consistentBuckets = new TreeMap<Long, String>();
    MessageDigest md5 = MD5.get();
    if (this.totalWeight <= 0 && this.weights != null) {
        for (int i = 0; i < this.weights.length; i++)
            this.totalWeight += (this.weights[i] == null) ? 1 : this.weights[i];
    } else if (this.weights == null) {
        this.totalWeight = this.servers.length;
    }

    for (int i = 0; i < servers.length; i++) {
       int thisWeight = 1;
       if (this.weights != null && this.weights[i] != null) thisWeight = this.weights[i];

      //这个值永远小于40 * this.servers.length,因为thisWeight/totalWeight永远小于1

 double factor = Math.floor(((double) (40 * this.servers.length * thisWeight)) / (double) this.totalWeight);

  //服务器的分布位置为factor*4,factor<=40*this.servers.length,所以服务器的分布位置& lt;=40*this.servers.length*4。
for (long j = 0; j < factor; j++) {
      //md5值的二进制数组为16位
   byte[] d = md5.digest((servers[i] + "-" + j).getBytes());
       //16位二进制数组每4位为一组，每组第4个值左移24位，第三个值左移16位，第二个值左移8位，第一个值不移位。进行或运算，得到一个小于2的32 次方的long值。
    for (int h = 0; h < 4; h++) {
            Long k = ((long) (d[3 + h * 4] & 0xFF) << 24) | ((long) (d[2 + h * 4] & 0xFF) << 16)
                         | ((long) (d[1 + h * 4] & 0xFF) << 8) | ((long) (d[0 + h * 4] & 0xFF));
                consistentBuckets.put(k, servers[i]);
                if (log.isDebugEnabled()) log.debug("++++ added " + servers[i] + " to server bucket");
         }
   }

   // create initial connections
   if (log.isDebugEnabled()) log.debug("+++ creating initial connections (" + initConn + ") for host: "
                                            + servers[i]);

   //创建连接
 for (int j = 0; j < initConn; j++) {
       SockIO socket = createSocket(servers[i]);
       if (socket == null) {
            log.error("++++ failed to create connection to: " + servers[i] + " -- only " + j + " created.");
             break;
       }

       //添加到可用连接池
   addSocketToPool(availPool, servers[i], socket);
       if (log.isDebugEnabled()) log.debug("++++ created and added socket: " + socket.toString()
                                                + " for host " + servers[i]);
   }
}

    

   如果不是一致性哈希，则只是普通分布，很简单，只是根据权重将服务器地址放入buckets这个List里

private void populateBuckets() {
        if (log.isDebugEnabled()) log.debug("++++ initializing internal hashing structure for consistent hashing");

        // store buckets in tree map
        this.buckets = new ArrayList<String>();

        for (int i = 0; i < servers.length; i++) {
            if (this.weights != null && this.weights.length > i) {
                for (int k = 0; k < this.weights[i].intValue(); k++) {
                    this.buckets.add(servers[i]);
                    if (log.isDebugEnabled()) log.debug("++++ added " + servers[i] + " to server bucket");
                }
            } else {
                this.buckets.add(servers[i]);
                if (log.isDebugEnabled()) log.debug("++++ added " + servers[i] + " to server bucket");
            }

            // create initial connections
            if (log.isDebugEnabled()) log.debug("+++ creating initial connections (" + initConn + ") for host: "
                                                + servers[i]);

            for (int j = 0; j < initConn; j++) {
                SockIO socket = createSocket(servers[i]);
                if (socket == null) {
                    log.error("++++ failed to create connection to: " + servers[i] + " -- only " + j + " created.");
                    break;
                }

                //新建连接后，加入到可用连接集合里
           addSocketToPool(availPool, servers[i], socket);
                if (log.isDebugEnabled()) log.debug("++++ created and added socket: " + socket.toString()
                                                    + " for host " + servers[i]);
            }
        }
    }

 
 

如何创建socket连接

在上面的private void populateBuckets()方法里，createSocket(servers[i])是创建到服务器的连接，看看这个方法

protected SockIO createSocket(String host) {
SockIO socket = null;
//hostDeadLock是一个可重入锁,它的变量声明为


private final ReentrantLock             hostDeadLock    = new ReentrantLock();
hostDeadLock.lock();
try {
  //hostDead.containsKey(host)为true表示曾经连接过该服务器，但没有成功。
  //hostDead是一个HashMap，key为服务器地址，value为当时连接不成功的时间
  //hostDeadDur是一个HashMap，key为服务器地址，value为设置的重试间隔时间

   if (failover && failback && hostDead.containsKey(host) && hostDeadDur.containsKey(host)) {
       Date store = hostDead.get(host);
       long expire = hostDeadDur.get(host).longValue();

      if ((store.getTime() + expire) > System.currentTimeMillis()) return null;
   }
 } finally {
    hostDeadLock.unlock();
  }


try {
     socket = new SockIO(this, host, this.socketTO, this.socketConnectTO, this.nagle);
     if (!socket.isConnected()) {
               log.error("++++ failed to get SockIO obj for: " + host + " -- new socket is not connected");
               deadPool.put(socket, ZERO);
               socket = null;
     }
 } catch (Exception ex) {
           log.error("++++ failed to get SockIO obj for: " + host);
           log.error(ex.getMessage(), ex);
           socket = null;
 }

  // if we failed to get socket, then mark
  // host dead for a duration which falls off
  hostDeadLock.lock();
  try {
         //到了这里,socket仍然为null，说明这个server悲剧了，无法和它创建连接，则要把该server丢到不可用的主机集合里
      if (socket == null) {
               Date now = new Date();
               hostDead.put(host, now);

               //如果上次就不可用了，到期了仍然不可用，就要这次的不可用时间设为上次的2倍，否则初始时长为1000ms
               long expire = (hostDeadDur.containsKey(host)) ? (((Long) hostDeadDur.get(host)).longValue() * 2) : 1000;

               if (expire > MAX_RETRY_DELAY) expire = MAX_RETRY_DELAY;

               hostDeadDur.put(host, new Long(expire));
               if (log.isDebugEnabled()) log.debug("++++ ignoring dead host: " + host + " for " + expire + " ms");

               // 既然这个host都不可用了，那与它的所有连接当然要从可用连接集合"availPool"里删除掉
         clearHostFromPool(availPool, host);
           } else {
               if (log.isDebugEnabled()) log.debug("++++ created socket (" + socket.toString() + ") for host: " + host);
               //连接创建成功，如果上次不成功，那么这次要把该host从不可用主机集合里删除掉
          if (hostDead.containsKey(host) || hostDeadDur.containsKey(host)) {
                   hostDead.remove(host);
                   hostDeadDur.remove(host);
               }
           }
       } finally {
           hostDeadLock.unlock();
       }

       return socket;
   }

  

   SockIO构造函数

public SockIO(SockIOPool pool, String host, int timeout, int connectTimeout, boolean noDelay)
                                                                                               throws IOException,
                                                                                               UnknownHostException {
      this.pool = pool;
      String[] ip = host.split(":");
      // get socket: default is to use non-blocking connect
      sock = getSocket(ip[0], Integer.parseInt(ip[1]), connectTimeout);
      if (timeout >= 0) this.sock.setSoTimeout(timeout);
      // testing only
      sock.setTcpNoDelay(noDelay);
      // wrap streams
      in = new DataInputStream(new BufferedInputStream(sock.getInputStream()));
      out = new BufferedOutputStream(sock.getOutputStream());
      this.host = host;
  }

 
 

    getSocket方法

protected static Socket getSocket(String host, int port, int timeout) throws IOException {
         SocketChannel sock = SocketChannel.open();
         sock.socket().connect(new InetSocketAddress(host, port), timeout);
         return sock.socket();
 }

  可以看到，socket连接是用nio方式创建的。

 新建MemcachedClient 
MemcachedClient mcc = new MemcachedClient();新建了一个memcached客户端，看看构造函数，没作什么，只是设置参数而已。

/**
  * Creates a new instance of MemCachedClient.
  */
 public MemcachedClient() {
     init();
 }


 private void init() {
     this.sanitizeKeys       = true;
     this.primitiveAsString  = false;
     this.compressEnable     = true;
     this.compressThreshold  = COMPRESS_THRESH;
     this.defaultEncoding    = "UTF-8";
     this.poolName           = ( this.poolName == null ) ? "default" : this.poolName;

     // get a pool instance to work with for the life of this instance
     this.pool               = SockIOPool.getInstance( poolName );
 }

 set方法如何工作

到此memcached客户端初始化工作完成。再回到测试类TestMemcached，看看for循环里的

boolean success = mcc.set( ""  + i, "Hello!" );
 String result = (String)mcc.get( "" + i );
 初始化后，就可以set，get了。看看set是怎么工作的。

/**
     * Stores data on the server; only the key and the value are specified.
     *
     * @param key key to store data under
     * @param value value to store
     * @return true, if the data was successfully stored
     */
    public boolean set( String key, Object value ) {
        return set( "set", key, value, null, null, primitiveAsString );
    }


    //这个set方法比较长
   private boolean set( String cmdname, String key, Object value, Date expiry, Integer hashCode, boolean asString ) {
        if ( cmdname == null || cmdname.trim().equals( "" ) || key == null ) {
            log.error( "key is null or cmd is null/empty for set()" );
            return false;
        }

        try {
            key = sanitizeKey( key );
        }
        catch ( UnsupportedEncodingException e ) {
            // if we have an errorHandler, use its hook
            if ( errorHandler != null )
                errorHandler.handleErrorOnSet( this, e, key );
            log.error( "failed to sanitize your key!", e );
            return false;
        }

        if ( value == null ) {
            log.error( "trying to store a null value to cache" );
            return false;
        }

        // get SockIO obj
        SockIOPool.SockIO sock = pool.getSock( key, hashCode );

        if ( sock == null ) {
            if ( errorHandler != null )
                errorHandler.handleErrorOnSet( this, new IOException( "no socket to server available" ), key );
            return false;
        }

        if ( expiry == null )
            expiry = new Date(0);

        // store flags
        int flags = 0;

        // byte array to hold data
        byte[] val;

    //这些类型自己序列化，否则由java序列化处理
   if ( NativeHandler.isHandled( value ) ) {
            if ( asString ) {
                //如果是字符串，则直接getBytes
                try {
                    if ( log.isInfoEnabled() )
                        log.info( "++++ storing data as a string for key: " + key + " for class: " + value.getClass().getName() );
                    val = value.toString().getBytes( defaultEncoding );
                }
                catch ( UnsupportedEncodingException ue ) {
                    // if we have an errorHandler, use its hook
                    if ( errorHandler != null )
                        errorHandler.handleErrorOnSet( this, ue, key );
                    log.error( "invalid encoding type used: " + defaultEncoding, ue );
                    sock.close();
                    sock = null;
                    return false;
                }
            }
            else {
                try {
                    if ( log.isInfoEnabled() )
                        log.info( "Storing with native handler..." );
                    flags |= NativeHandler.getMarkerFlag( value );
                    val    = NativeHandler.encode( value );
                }
                catch ( Exception e ) {
                    // if we have an errorHandler, use its hook
                    if ( errorHandler != null )
                        errorHandler.handleErrorOnSet( this, e, key );
                    log.error( "Failed to native handle obj", e );

                    sock.close();
                    sock = null;
                    return false;
                }
            }
        }
        else {
            // 否则用java的序列化
        try {
                if ( log.isInfoEnabled() )
                    log.info( "++++ serializing for key: " + key + " for class: " + value.getClass().getName() );
                ByteArrayOutputStream bos = new ByteArrayOutputStream();
                (new ObjectOutputStream( bos )).writeObject( value );
                val = bos.toByteArray();
                flags |= F_SERIALIZED;
            }
            catch ( IOException e ) {
                // if we have an errorHandler, use its hook
                if ( errorHandler != null )
                    errorHandler.handleErrorOnSet( this, e, key );

                // if we fail to serialize, then
                // we bail
                log.error( "failed to serialize obj", e );
                log.error( value.toString() );

                // return socket to pool and bail
                sock.close();
                sock = null;
                return false;
            }
        }

        //压缩内容
     if ( compressEnable && val.length > compressThreshold ) {
            try {
                if ( log.isInfoEnabled() ) {
                    log.info( "++++ trying to compress data" );
                    log.info( "++++ size prior to compression: " + val.length );
                }
                ByteArrayOutputStream bos = new ByteArrayOutputStream( val.length );
                GZIPOutputStream gos = new GZIPOutputStream( bos );
                gos.write( val, 0, val.length );
                gos.finish();
                gos.close();

                // store it and set compression flag
                val = bos.toByteArray();
                flags |= F_COMPRESSED;

                if ( log.isInfoEnabled() )
                    log.info( "++++ compression succeeded, size after: " + val.length );
            }
            catch ( IOException e ) {
                // if we have an errorHandler, use its hook
                if ( errorHandler != null )
                    errorHandler.handleErrorOnSet( this, e, key );
                log.error( "IOException while compressing stream: " + e.getMessage() );
                log.error( "storing data uncompressed" );
            }
        }

        // now write the data to the cache server
        try {
             //按照memcached协议组装命令
        String cmd = String.format( "%s %s %d %d %d
", cmdname, key, flags, (expiry.getTime() / 1000), val.length );
            sock.write( cmd.getBytes() );
            sock.write( val );
            sock.write( "
".getBytes() );
            sock.flush();

            // get result code
            String line = sock.readLine();
            if ( log.isInfoEnabled() )
                log.info( "++++ memcache cmd (result code): " + cmd + " (" + line + ")" );

            if ( STORED.equals( line ) ) {
                if ( log.isInfoEnabled() )
                    log.info("++++ data successfully stored for key: " + key );
                sock.close();
                sock = null;
                return true;
            }
            else if ( NOTSTORED.equals( line ) ) {
                if ( log.isInfoEnabled() )
                    log.info( "++++ data not stored in cache for key: " + key );
            }
            else {
                log.error( "++++ error storing data in cache for key: " + key + " -- length: " + val.length );
                log.error( "++++ server response: " + line );
            }
        }
        catch ( IOException e ) {

            // if we have an errorHandler, use its hook
            if ( errorHandler != null )
                errorHandler.handleErrorOnSet( this, e, key );

            // exception thrown
            log.error( "++++ exception thrown while writing bytes to server on set" );
            log.error( e.getMessage(), e );

            try {
                sock.trueClose();
            }
            catch ( IOException ioe ) {
                log.error( "++++ failed to close socket : " + sock.toString() );
            }

            sock = null;
        }

        //用完了，就要回收哦，sock.close()不是真正的关闭，只是放入到可用连接集合里。
       if ( sock != null ) {
            sock.close();
            sock = null;
        }
        return false;
    }

 通过set方法向服务器设置key和value，涉及到以下几个点
数据的压缩和序列化 （如果是get方法，则和set方法基本是相反的）
为key分配服务器 对于一些常用类型，采用自定义的序列化，具体要看NativeHander.java，这个类比较简单，有兴趣可以自己看看

public static boolean isHandled( Object value ) {
       return (
           value instanceof Byte            ||
           value instanceof Boolean         ||
           value instanceof Integer         ||
           value instanceof Long            ||
           value instanceof Character       ||
           value instanceof String          ||
           value instanceof StringBuffer    ||
           value instanceof Float           ||
           value instanceof Short           ||
           value instanceof Double          ||
           value instanceof Date            ||
           value instanceof StringBuilder   ||
           value instanceof byte[]
           )
       ? true
       : false;
   }

 其他类型则用java的默认序列化

为key选择服务器 
SockIOPool.SockIO sock = pool.getSock( key, hashCode );就是为key选择服务器

public SockIO getSock(String key, Integer hashCode) {
        if (log.isDebugEnabled()) log.debug("cache socket pick " + key + " " + hashCode);
        if (!this.initialized) {
            log.error("attempting to get SockIO from uninitialized pool!");
            return null;
        }

        // if no servers return null
        if ((this.hashingAlg == CONSISTENT_HASH && consistentBuckets.size() == 0)
            || (buckets != null && buckets.size() == 0)) return null;

        // if only one server, return it
        if ((this.hashingAlg == CONSISTENT_HASH && consistentBuckets.size() == 1)
            || (buckets != null && buckets.size() == 1)) {
            SockIO sock = (this.hashingAlg == CONSISTENT_HASH) ? getConnection(consistentBuckets.get(consistentBuckets.firstKey())) : getConnection(buckets.get(0));
            if (sock != null && sock.isConnected()) {
                if (aliveCheck) {//健康状态检查

                    if (!sock.isAlive()) {
                        sock.close();
                        try {
                            sock.trueClose();//有问题，真的关闭socket

                        } catch (IOException ioe) {
                            log.error("failed to close dead socket");
                        }
                        sock = null;
                    }
                }
            } else {//连接不正常，放入不可用连接集合里
          if (sock != null) {
                    deadPool.put(sock, ZERO);
                    sock = null;
                }
            }

            return sock;
        }

        Set<String> tryServers = new HashSet<String>(Arrays.asList(servers));
        // get initial bucket
        long bucket = getBucket(key, hashCode);
        String server = (this.hashingAlg == CONSISTENT_HASH) ? consistentBuckets.get(bucket) : buckets.get((int) bucket);

        while (!tryServers.isEmpty()) {
            // try to get socket from bucket
            SockIO sock = getConnection(server);
            if (log.isDebugEnabled()) log.debug("cache choose " + server + " for " + key);
            if (sock != null && sock.isConnected()) {
                if (aliveCheck) {
                    if (sock.isAlive()) {
                        return sock;
                    } else {
                        sock.close();
                        try {
                            sock.trueClose();
                        } catch (IOException ioe) {
                            log.error("failed to close dead socket");
                        }
                        sock = null;
                    }
                } else {
                    return sock;
                }
            } else {
                if (sock != null) {
                    deadPool.put(sock, ZERO);
                    sock = null;
                }
            }

            // if we do not want to failover, then bail here
            if (!failover) return null;

            // log that we tried
            tryServers.remove(server);

            if (tryServers.isEmpty()) break;
           //注意哦，下面是failover机制
        int rehashTries = 0;
            while (!tryServers.contains(server)) {
                String newKey = String.format("%s%s", rehashTries, key);
                if (log.isDebugEnabled()) log.debug("rehashing with: " + newKey);

                bucket = getBucket(newKey, null);
                server = (this.hashingAlg == CONSISTENT_HASH) ? consistentBuckets.get(bucket) : buckets.get((int) bucket);
                rehashTries++;
            }
        }
        return null;
    }

 
 

   下面这个方法是真正的从服务器获取连接

    
   

public SockIO getConnection(String host) {
        if (!this.initialized) {
            log.error("attempting to get SockIO from uninitialized pool!");
            return null;
        }

        if (host == null) return null;

        synchronized (this) {
            // if we have items in the pool
            // then we can return it
            if (availPool != null && !availPool.isEmpty()) {
                // take first connected socket
                Map<SockIO, Long> aSockets = availPool.get(host);
                if (aSockets != null && !aSockets.isEmpty()) {
                    for (Iterator<SockIO> i = aSockets.keySet().iterator(); i.hasNext();) {
                        SockIO socket = i.next();
                        if (socket.isConnected()) {
                            if (log.isDebugEnabled()) log.debug("++++ moving socket for host (" + host
                                                                + ") to busy pool ... socket: " + socket);
                            // remove from avail pool
                            i.remove();
                            // add to busy pool
                            addSocketToPool(busyPool, host, socket);
                            // return socket
                            return socket;
                        } else {
                            // add to deadpool for later reaping
                            deadPool.put(socket, ZERO);
                            // remove from avail pool
                            i.remove();
                        }
                    }
                }
            }
        }

        // create one socket -- let the maint thread take care of creating more
        SockIO socket = createSocket(host);
        if (socket != null) {
            synchronized (this) {
                addSocketToPool(busyPool, host, socket);
            }
        }
        return socket;
    }

  

 failover和failback

这两者都是发生在获取可用连接这个环节。

failover，如果为key选择的服务器不可用，则对key重新哈希选择下一个服务器，详见getSock方法的末尾。

failback，用一个hashmap存储连接失败的服务器和对应的失效持续时间，每次获取连接时，都探测是否到了重试时间。