服務器動態上下線程序的工作機制

服務器代碼：
補充：volatile關鍵字：java中一切都是對象，當多個線程操作同一個對象時候，該對象會放在堆內存中，而多個線程相當于在多個棧中，當A線程想要去除對象中的數據并修改，往往不是直接拿對象的值直接改變其內容，而是先把中的對象賦值一份到A線程棧中，然后再對賦值的對象進行修改，最后把賦值對象與堆中的對象進行比較，不同則修改堆中對象，這樣當多個線程訪問對象時會存在，當A線程修改了堆中賦值對象的值，但還沒來得及修改堆中的對象，而B線程此時拿到的仍然是原對象值并沒有發生更改，為了避免這種問題。因此同volatile關鍵字讓線程直接獲取對象并修改內容無需賦值一份再去修改。

package cn.itcast.bigdata.zkdist;import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooDefs.Ids; import org.apache.zookeeper.ZooKeeper;public class DistributedServer {private static final String connectString = "mini1:2181,mini2:2181,mini3:2181";private static final int sessionTimeout = 2000;private static final String parentNode = "/servers";private ZooKeeper zk = null;/*** 創建到zk的客戶端連接* * @throws Exception*/public void getConnect() throws Exception {zk = new ZooKeeper(connectString, sessionTimeout, new Watcher() {@Overridepublic void process(WatchedEvent event) {// 收到事件通知后的回調函數（應該是我們自己的事件處理邏輯）System.out.println(event.getType() + "---" + event.getPath());try {zk.getChildren("/", true);} catch (Exception e) {}}});}/*** 向zk集群注冊服務器信息* * @param hostname* @throws Exception*/public void registerServer(String hostname) throws Exception {String create = zk.create(parentNode + "/server", hostname.getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);//當服務斷掉時ZooKeeper將此臨時節點刪除，這樣client就不會得到服務的信息了System.out.println(hostname + "is online.." + create);}/*** 業務功能* * @throws InterruptedException*/public void handleBussiness(String hostname) throws InterruptedException {System.out.println(hostname + "start working.....");Thread.sleep(Long.MAX_VALUE);}public static void main(String[] args) throws Exception {// 獲取zk連接DistributedServer server = new DistributedServer();server.getConnect();// 利用zk連接注冊服務器信息server.registerServer(args[0]);// 啟動業務功能server.handleBussiness(args[0]);}}

輸出：當args[0]等于weijie1時候，輸出如下

log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper). log4j:WARN Please initialize the log4j system properly. 45null weijie1is online/servers/server0000000006 weijie1 is starting

客戶端代碼：

package com.itcast.zookpeer.zk;import java.io.IOException; import java.util.ArrayList; import java.util.List;import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooKeeper;public class DistributedClient {private static final String connectString = "weijie1:2181,weijie2:2181,weijie3:2181";private static final int sessionTimeout = 2000;private static final String parentNode = "/servers";private static volatile List<String> serverList; ZooKeeper zk = null;//創建客戶端連接private void getConnection() throws Exception {zk = new ZooKeeper(connectString, sessionTimeout, new Watcher(){@Overridepublic void process(WatchedEvent event) {System.out.println(event.getType()+"--"+event.getPath());try{getServerList();}catch(Exception e){}}});}//獲取服務器信息列表private void getServerList() throws Exception {//獲取服務器子節點的信息，并且對父節點進行監聽List<String> server = zk.getChildren(parentNode, true);//先創建一個list來存儲服務器信息List<String> servers = new ArrayList<String>();for (String string : server) {byte[] data = zk.getData(parentNode+"/"+string, false, null);servers.add(new String(data));//new String對data進行解析轉成字符串類型}serverList = servers;System.out.println(serverList);}private void BundleBusiness() throws Exception {System.out.println("is starting");Thread.sleep(Long.MAX_VALUE);}public static void main(String[] args) throws Exception {DistributedClient distribute = new DistributedClient();distribute.getConnection();distribute.getServerList();distribute.BundleBusiness();}}

輸出：

當啟動一次服務器時候此時輸出結果
log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
log4j:WARN Please initialize the log4j system properly.
None--null
[weijie1]
is starting
[weijie1]

再啟動一次DistributedServer時候輸出
log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
log4j:WARN Please initialize the log4j system properly.
None--null
[weijie1]
is starting
[weijie1]
NodeChildrenChanged--/servers
[weijie1, weijie1]
分析：當啟動兩次服務器此時在servers目錄下又創建了一個臨時帶序號的節點，此時總共有兩個臨時節點，因此輸出主機名兩次，因為在List<String> server = zk.getChildren(parentNode, true);中多父節點”/servers”進行了監聽。

總結

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