public void start()

in zookeeper-server/src/main/java/org/apache/zookeeper/server/embedded/ZooKeeperServerEmbeddedImpl.java [87:191]
99 lines of code
12 McCabe index (conditional complexity)

    public void start(long startupTimeout) throws Exception {
        switch (exitHandler) {
            case EXIT:
                ServiceUtils.setSystemExitProcedure(ServiceUtils.SYSTEM_EXIT);
                break;
            case LOG_ONLY:
                ServiceUtils.setSystemExitProcedure(ServiceUtils.LOG_ONLY);
                break;
            default:
                ServiceUtils.setSystemExitProcedure(ServiceUtils.SYSTEM_EXIT);
                break;
        }
        final CompletableFuture<String> started = new CompletableFuture<>();

        if (config.getServers().size() > 1 || config.isDistributed()) {
            LOG.info("Running ZK Server in single Quorum MODE");

            maincluster = new QuorumPeerMain() {
                protected QuorumPeer getQuorumPeer() throws SaslException {
                    return new QuorumPeer() {
                        @Override
                        public void start() {
                            super.start();
                            boundClientPort = getClientPort();
                            boundSecureClientPort = getSecureClientPort();
                            LOG.info("ZK Server {} started", this);
                            started.complete(null);
                        }
                    };
                }
            };

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

            thread = new Thread("zkservermainrunner") {
                @Override
                public void run() {
                    try {
                        maincluster.runFromConfig(config);
                        maincluster.close();
                        LOG.info("ZK server died. Requesting stop on JVM");
                        if (!stopping) {
                            ServiceUtils.requestSystemExit(ExitCode.EXECUTION_FINISHED.getValue());
                        }
                    } catch (Throwable t) {
                        LOG.error("error during server lifecycle", t);
                        maincluster.close();
                        if (!stopping) {
                            ServiceUtils.requestSystemExit(ExitCode.INVALID_INVOCATION.getValue());
                        }
                    }
                }
            };
            thread.start();
        } else {
            LOG.info("Running ZK Server in single STANDALONE MODE");
            mainsingle = new ZooKeeperServerMain() {
                @Override
                public void serverStarted() {
                    LOG.info("ZK Server started");
                    boundClientPort = getClientPort();
                    boundSecureClientPort = getSecureClientPort();
                    started.complete(null);
                }
            };
            purgeMgr = new DatadirCleanupManager(config
                    .getDataDir(), config.getDataLogDir(), config
                    .getSnapRetainCount(), config.getPurgeIntervalInMs());
            purgeMgr.start();
            thread = new Thread("zkservermainrunner") {
                @Override
                public void run() {
                    try {
                        ServerConfig cc = new ServerConfig();
                        cc.readFrom(config);
                        LOG.info("ZK server starting");
                        mainsingle.runFromConfig(cc);
                        LOG.info("ZK server died. Requesting stop on JVM");
                        if (!stopping) {
                            ServiceUtils.requestSystemExit(ExitCode.EXECUTION_FINISHED.getValue());
                        }
                    } catch (Throwable t) {
                        LOG.error("error during server lifecycle", t);
                        mainsingle.close();
                        if (!stopping) {
                            ServiceUtils.requestSystemExit(ExitCode.INVALID_INVOCATION.getValue());
                        }
                    }
                }
            };
            thread.start();
        }

        try {
            started.get(startupTimeout, TimeUnit.MILLISECONDS);
        } catch (TimeoutException err) {
            LOG.info("Startup timed out, trying to close");
            close();
            throw err;
        }
    }