Zookeeper启动的main方法是org.apache.zookeeper.server.quorum.QuorumPeerMain类的main方法：
public static void main(String[] args) {
QuorumPeerMain main = new QuorumPeerMain();
try {
main.initializeAndRun(args);
} ...
...
}
进入到initializeAndRun方法：
protected void initializeAndRun(String[] args)
throws ConfigException, IOException
{
// args[0]是传入的配置文件
QuorumPeerConfig config = new QuorumPeerConfig();
if (args.length == 1) {
// 解析配置文件
config.parse(args[0]);
}
// Start and schedule the the purge task
// 启动一个定时清除日志的任务
DatadirCleanupManager purgeMgr = new DatadirCleanupManager(config
.getDataDir(), config.getDataLogDir(), config
.getSnapRetainCount(), config.getPurgeInterval());
purgeMgr.start();
// 我们是集群启动Zookeeper，必须使用到配置文件，因此接下来运行到runFromConfig
if (args.length == 1 && config.servers.size() > 0) {
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);
}
}
这里传入的配置文件就是Zookeeper根目录下的conf/zoo.cfg文件(可参考启动脚本zkServer.sh)，该配置文件可以配置哪些配置项可以参考QuorumPeerConfig的解析配置文件过程。接下来进入runFromConfig方法：
public void runFromConfig(QuorumPeerConfig config) throws IOException {
try {
ManagedUtil.registerLog4jMBeans();
} catch (JMException e) {
LOG.warn("Unable to register log4j JMX control", e);
}
LOG.info("Starting quorum peer");
try {
// cnxnFactory用于处理本节点上的一些IO操作，默认情况下是NIOServerCnxnFactory的对象
ServerCnxnFactory cnxnFactory = ServerCnxnFactory.createFactory();
// 配置本节点监听的客户端端口和连接数
cnxnFactory.configure(config.getClientPortAddress(),
config.getMaxClientCnxns());
// QuorumPeer是zk的重要类之一
quorumPeer = getQuorumPeer();
// 设置从配置文件读取到的所有配置节点(zoo.cfg中的server.1=localhost:2888:3888配置项)
quorumPeer.setQuorumPeers(config.getServers());
quorumPeer.setTxnFactory(new FileTxnSnapLog(
new File(config.getDataLogDir()),
new File(config.getDataDir())));
// 设置使用的Leader选举算法
quorumPeer.setElectionType(config.getElectionAlg());
quorumPeer.setMyid(config.getServerId());
quorumPeer.setTickTime(config.getTickTime());
quorumPeer.setInitLimit(config.getInitLimit());
quorumPeer.setSyncLimit(config.getSyncLimit());
quorumPeer.setQuorumListenOnAllIPs(config.getQuorumListenOnAllIPs());
quorumPeer.setCnxnFactory(cnxnFactory);
quorumPeer.setQuorumVerifier(config.getQuorumVerifier());
quorumPeer.setClientPortAddress(config.getClientPortAddress());
quorumPeer.setMinSessionTimeout(config.getMinSessionTimeout());
quorumPeer.setMaxSessionTimeout(config.getMaxSessionTimeout());
quorumPeer.setZKDatabase(new ZKDatabase(quorumPeer.getTxnFactory()));
quorumPeer.setLearnerType(config.getPeerType());
quorumPeer.setSyncEnabled(config.getSyncEnabled());
// 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();
quorumPeer.start();
quorumPeer.join();
} catch (InterruptedException e) {
// warn, but generally this is ok
LOG.warn("Quorum Peer interrupted", e);
}
}
runFromConfig方法主要将从zoo.cfg中读取到的配置设置到QuorumPeer的对象属性中区，QuorumPeer继承了ZooKeeperThread类，而ZooKeeperThread继承了Thread类，因此QuorumPeer是一个线程类，稍后我们将分析其run方法。进入到quorumPeer.start()方法：
@Override
public synchronized void start() {
// 从快照文件(dataDir目录下snapshot文件)中恢复会话和zk节点信息
loadDataBase();
// 处理IO事件(暂不了解这个的作用，之后的文章会对这个进行分析，现在跳过不影响我们分析Leader的选举)
cnxnFactory.start();
// 开始进行Leader的选举
startLeaderElection();
// 开始监听本节点的状态，根据不同的状态(LOOKING、OBSERVING、FOLLOWING、LEADING)执行相应操作
super.start();
}
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