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ZooKeeperServer，为所有服务器的父类。
QuorumZooKeeperServer，其是所有参与选举的服务器的父类，是抽象类，其继承了ZooKeeperServer类。
LeaderZooKeeperServer，Leader服务器，继承了QuorumZooKeeperServer类，也会继承ZooKeeperServer中的很多方法。
LearnerZooKeeper，其是Learner服务器的父类，为抽象类，也继承了QuorumZooKeeperServer类。
FollowerZooKeeperServer，Follower服务器，继承了LearnerZooKeeper。
ObserverZooKeeperServer，Observer服务器，继承了LearnerZooKeeper。
ReadOnlyZooKeeperServer，只读服务器，不提供写服务，继承QuorumZooKeeperServer。
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public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {}ZooKeeperServer是ZooKeeper中所有服务器的父类，其实现了Session.Expirer和ServerStats.Provider接口，SessionExpirer中定义了expire方法（表示会话过期）和getServerId方法（表示获取服务器ID），而Provider则主要定义了获取服务器某些数据的方法。
protected static final Logger LOG;
static {
LOG = LoggerFactory.getLogger(ZooKeeperServer.class);
Environment.logEnv("Server environment:", LOG);
}
//jmx服务
protected ZooKeeperServerBean jmxServerBean;
protected DataTreeBean jmxDataTreeBean;
// 默认心跳频率
public static final int DEFAULT_TICK_TIME = 3000;
protected int tickTime = DEFAULT_TICK_TIME;
// 最小会话过期时间
/** value of -1 indicates unset, use default */
protected int minSessionTimeout = -1;
// 最大会话过期时间
/** value of -1 indicates unset, use default */
protected int maxSessionTimeout = -1;
protected SessionTracker sessionTracker;
// 事务日志快照
private FileTxnSnapLog txnLogFactory = null;
// Zookeeper内存数据库
private ZKDatabase zkDb;
private final AtomicLong hzxid = new AtomicLong(0);
public final static Exception ok = new Exception("No prob");
// 请求处理器
protected RequestProcessor firstProcessor;
protected volatile State state = State.INITIAL;
protected enum State {
INITIAL, RUNNING, SHUTDOWN, ERROR
}
/**
* This is the secret that we use to generate passwords. For the moment,
* it's more of a checksum that's used in reconnection, which carries no
* security weight, and is treated internally as if it carries no
* security weight.
*/
static final private long superSecret = 0XB3415C00L;
private final AtomicInteger requestsInProcess = new AtomicInteger(0);
// 未处理的ChangeRecord
final Deque outstandingChanges = new ArrayDeque<>();
// this data structure must be accessed under the outstandingChanges lock
// 记录path对应的ChangeRecord
final HashMap outstandingChangesForPath =
new HashMap();
protected ServerCnxnFactory serverCnxnFactory;
protected ServerCnxnFactory secureServerCnxnFactory;
// 服务器统计数据
private final ServerStats serverStats;
private final ZooKeeperServerListener listener;
private ZooKeeperServerShutdownHandler zkShutdownHandler;
private volatile int createSessionTrackerServerId = 1; 该函数用于加载数据，其首先会判断内存库是否已经加载设置zxid，之后会调用killSession函数删除过期的会话
if(zkDb.isInitialized()){ // 内存数据库已被初始化
// 设置为最后处理的Zxid
setZxid(zkDb.getDataTreeLastProcessedZxid());
}
else {
// 未被初始化，则加载数据库
setZxid(zkDb.loadDataBase());
}
// Clean up dead sessions
LinkedList deadSessions = new LinkedList();
for (Long session : zkDb.getSessions()) {// 遍历所有的会话
if (zkDb.getSessionWithTimeOuts().get(session) == null) {
deadSessions.add(session);
}
}
for (long session : deadSessions) { // 删除过期的会话
// XXX: Is lastProcessedZxid really the best thing to use?
killSession(session, zkDb.getDataTreeLastProcessedZxid());
}
// Make a clean snapshot
//初始化一个快照
takeSnapshot(); 提交请求，处理器进行处理
public void submitRequest(Request si) {
if (firstProcessor == null) {// 第一个处理器为空
synchronized (this) {
try {
// Since all requests are passed to the request
// processor it should wait for setting up the request
// processor chain. The state will be updated to RUNNING
// after the setup.
//服务器调用链还未初始化完成
while (state == State.INITIAL) {
wait(1000);
}
} catch (InterruptedException e) {
LOG.warn("Unexpected interruption", e);
}
if (firstProcessor == null || state != State.RUNNING) {
throw new RuntimeException("Not started");
}
}
}
try {
touch(si.cnxn);
// 是否为合法的请求
boolean validpacket = Request.isValid(si.type);
if (validpacket) {
//调用链第一处理器开始处理
firstProcessor.proce***equest(si);
if (si.cnxn != null) {
incInProcess();
}
} else {
LOG.warn("Received packet at server of unknown type " + si.type);
new UnimplementedRequestProcessor().proce***equest(si);
}
} catch (MissingSessionException e) {
if (LOG.isDebugEnabled()) {
LOG.debug("Dropping request: " + e.getMessage());
}
} catch (RequestProcessorException e) {
LOG.error("Unable to process request:" + e.getMessage(), e);
}
}// 提交请求处理器
CommitProcessor commitProcessor;
//处理链请求第一个处理处理器
PrepRequestProcessor prepRequestProcessor;LeaderZooKeeperServer(FileTxnSnapLog logFactory, QuorumPeer self, ZKDatabase zkDb) throws IOException {
super(logFactory, self.tickTime, self.minSessionTimeout, self.maxSessionTimeout, zkDb, self);
}直接调用父类QuorumZooKeeperServer的构造函数，然后再调用ZooKeeperServer的构造函数，逐级构造。
@Override
protected void setupRequestProcessors() {
//创建FinalRequestProcessor,处理链最后一个处理器
RequestProcessor finalProcessor = new FinalRequestProcessor(this);
//创建ToBeAppliedRequestProcessor
RequestProcessor toBeAppliedProcessor = new Leader.ToBeAppliedRequestProcessor(finalProcessor, getLeader());
// 创建CommitProcessor,提交处理器
commitProcessor = new CommitProcessor(toBeAppliedProcessor,
Long.toString(getServerId()), false,
getZooKeeperServerListener());
// 启动CommitProcessor
commitProcessor.start();
// 创建ProposalRequestProcessor
ProposalRequestProcessor proposalProcessor = new ProposalRequestProcessor(this,
commitProcessor);
// 初始化ProposalProcessor
proposalProcessor.initialize();
//创建PrepRequestProcessor，作为以第一个处理链处理器
prepRequestProcessor = new PrepRequestProcessor(this, proposalProcessor);
prepRequestProcessor.start();
// firstProcessor为PrepRequestProcessor
firstProcessor = new LeaderRequestProcessor(this, prepRequestProcessor);
setupContainerManager();
}setupRequestProcessors函数表示创建处理链，可以看到其处理链的顺序为PrepRequestProcessor -> ProposalRequestProcessor -> CommitProcessor -> Leader.ToBeAppliedRequestProcessor -> FinalRequestProcessor。