（一）zookeeper集群模式的启动

• QuorumPeerMain
  这是zookeeper服务的启动类,它提供了一个main()方法用来启动zookeeper服务,如下：

    public static void main(String[] args) {
        QuorumPeerMain main = new QuorumPeerMain();
        try {
            main.initializeAndRun(args);
        } catch (IllegalArgumentException e) {
            LOG.error("Invalid arguments, exiting abnormally", e);
            LOG.info(USAGE);
            System.err.println(USAGE);
            System.exit(ExitCode.INVALID_INVOCATION.getValue());
        } catch (ConfigException e) {
            LOG.error("Invalid config, exiting abnormally", e);
            System.err.println("Invalid config, exiting abnormally");
            System.exit(ExitCode.INVALID_INVOCATION.getValue());
        } catch (DatadirException e) {
            LOG.error("Unable to access datadir, exiting abnormally", e);
            System.err.println("Unable to access datadir, exiting abnormally");
            System.exit(ExitCode.UNABLE_TO_ACCESS_DATADIR.getValue());
        } catch (AdminServerException e) {
            LOG.error("Unable to start AdminServer, exiting abnormally", e);
            System.err.println("Unable to start AdminServer, exiting abnormally");
            System.exit(ExitCode.ERROR_STARTING_ADMIN_SERVER.getValue());
        } catch (Exception e) {
            LOG.error("Unexpected exception, exiting abnormally", e);
            System.exit(ExitCode.UNEXPECTED_ERROR.getValue());
        }
        LOG.info("Exiting normally");
        System.exit(ExitCode.EXECUTION_FINISHED.getValue());
    }

它就做了两件事，一:初始化QuorumPeerMain，二:initializeAndRun(args)启动服务.接下来看看initializeAndRun(args)这个方法：

    protected void initializeAndRun(String[] args)
        throws ConfigException, IOException, AdminServerException
    {
        QuorumPeerConfig config = new QuorumPeerConfig();
        if (args.length == 1) {
            config.parse(args[0]);
        }

        // Start and schedule the the purge task
        DatadirCleanupManager purgeMgr = new DatadirCleanupManager(config
                .getDataDir(), config.getDataLogDir(), config
                .getSnapRetainCount(), config.getPurgeInterval());
        purgeMgr.start();

        if (args.length == 1 && config.isDistributed()) {
            runFromConfig(config);
        } else {
            LOG.warn("Either no config or no quorum defined in config, running "
                    + " in standalone mode");
            // there is only server in the quorum -- run as standalone
            ZooKeeperServerMain.main(args);
        }
    }

这里做了三件事：
1.解析zookeeper的配置文件
2.创建并启动数据文件清理管理器(包括快照文件和事务日志)
3.判断是单机还是分布式部署方式来启动zookeeper

集群模式将会调用runFromConfig(config)这个方法启动zookeeper服务

    public void runFromConfig(QuorumPeerConfig config)
            throws IOException, AdminServerException
    {
      try {
          ManagedUtil.registerLog4jMBeans();
      } catch (JMException e) {
          LOG.warn("Unable to register log4j JMX control", e);
      }

      LOG.info("Starting quorum peer");
      MetricsProvider metricsProvider;
      try {
        metricsProvider = MetricsProviderBootstrap
                      .startMetricsProvider(config.getMetricsProviderClassName(),
                                            config.getMetricsProviderConfiguration());
      } catch (MetricsProviderLifeCycleException error) {
        throw new IOException("Cannot boot MetricsProvider " + config.getMetricsProviderClassName(),
                      error);
      }
      try {
          ServerMetrics.metricsProviderInitialized(metricsProvider);
          ServerCnxnFactory cnxnFactory = null;
          ServerCnxnFactory secureCnxnFactory = null;

          if (config.getClientPortAddress() != null) {
              cnxnFactory = ServerCnxnFactory.createFactory();
              cnxnFactory.configure(config.getClientPortAddress(),
                      config.getMaxClientCnxns(),
                      config.getClientPortListenBacklog(), false);
          }

          if (config.getSecureClientPortAddress() != null) {
              secureCnxnFactory = ServerCnxnFactory.createFactory();
              secureCnxnFactory.configure(config.getSecureClientPortAddress(),
                      config.getMaxClientCnxns(),
                      config.getClientPortListenBacklog(), true);
          }

          quorumPeer = getQuorumPeer();
          quorumPeer.setTxnFactory(new FileTxnSnapLog(
                      config.getDataLogDir(),
                      config.getDataDir()));
          quorumPeer.enableLocalSessions(config.areLocalSessionsEnabled());
          quorumPeer.enableLocalSessionsUpgrading(
              config.isLocalSessionsUpgradingEnabled());
          //quorumPeer.setQuorumPeers(config.getAllMembers());
          quorumPeer.setElectionType(config.getElectionAlg());
          quorumPeer.setMyid(config.getServerId());
          quorumPeer.setTickTime(config.getTickTime());
          quorumPeer.setMinSessionTimeout(config.getMinSessionTimeout());
          quorumPeer.setMaxSessionTimeout(config.getMaxSessionTimeout());
          quorumPeer.setInitLimit(config.getInitLimit());
          quorumPeer.setSyncLimit(config.getSyncLimit());
          quorumPeer.setObserverMasterPort(config.getObserverMasterPort());
          quorumPeer.setConfigFileName(config.getConfigFilename());
          quorumPeer.setClientPortListenBacklog(config.getClientPortListenBacklog());
          quorumPeer.setZKDatabase(new ZKDatabase(quorumPeer.getTxnFactory()));
          quorumPeer.setQuorumVerifier(config.getQuorumVerifier(), false);
          if (config.getLastSeenQuorumVerifier()!=null) {
              quorumPeer.setLastSeenQuorumVerifier(config.getLastSeenQuorumVerifier(), false);
          }
          quorumPeer.initConfigInZKDatabase();
          quorumPeer.setCnxnFactory(cnxnFactory);
          quorumPeer.setSecureCnxnFactory(secureCnxnFactory);
          quorumPeer.setSslQuorum(config.isSslQuorum());
          quorumPeer.setUsePortUnification(config.shouldUsePortUnification());
          quorumPeer.setLearnerType(config.getPeerType());
          quorumPeer.setSyncEnabled(config.getSyncEnabled());
          quorumPeer.setQuorumListenOnAllIPs(config.getQuorumListenOnAllIPs());
          if (config.sslQuorumReloadCertFiles) {
              quorumPeer.getX509Util().enableCertFileReloading();
          }

          // sets quorum sasl authentication configurations
          quorumPeer.setQuorumSaslEnabled(config.quorumEnableSasl);
          if(quorumPeer.isQuorumSaslAuthEnabled()){
              quorumPeer.setQuorumServerSaslRequired(config.quorumServerRequireSasl);
              quorumPeer.setQuorumLearnerSaslRequired(config.quorumLearnerRequireSasl);
              quorumPeer.setQuorumServicePrincipal(config.quorumServicePrincipal);
              quorumPeer.setQuorumServerLoginContext(config.quorumServerLoginContext);
              quorumPeer.setQuorumLearnerLoginContext(config.quorumLearnerLoginContext);
          }
          quorumPeer.setQuorumCnxnThreadsSize(config.quorumCnxnThreadsSize);
          quorumPeer.initialize();

          if(config.jvmPauseMonitorToRun) {
              quorumPeer.setJvmPauseMonitor(new JvmPauseMonitor(config));
          }

          quorumPeer.start();
          quorumPeer.join();
      } catch (InterruptedException e) {
          // warn, but generally this is ok
          LOG.warn("Quorum Peer interrupted", e);
      } finally {
          if (metricsProvider != null) {
              try {
                  metricsProvider.stop();
              } catch (Throwable error) {
                  LOG.warn("Error while stopping metrics", error);
              }
          }
      }
    }

这个方法做了以下几件事：
1.注册log4j JMX控件
2.创建性能指标统计类MetricsProvider
3.创建服务端与客户端通信工厂类ServerCnxnFactory
4.初始化QuorumPeer并启动这个线程