Zookeeper系列四：Zookeeper实现分布式锁、Zookeeper实现配置中心

一、Zookeeper实现分布式锁

分布式锁主要用于在分布式环境中保证数据的一致性。

包括跨进程、跨机器、跨网络导致共享资源不一致的问题。

1. 分布式锁的实现思路

说明：

这种实现会有一个缺点，即当有很多进程在等待锁的时候，在释放锁的时候会有很多进程就过来争夺锁，这种现象称为 “惊群效应”

2. 分布式锁优化后的实现思路

3. Zookeeper分布式锁的代码实现

准备工作：

1）安装Zookeeper，具体参考我前面的我文章Zookeeper系列一：Zookeeper介绍、Zookeeper安装配置、ZK Shell的使用

2）新建一个maven项目ZK-Demo,然后在pom.xml里面引入相关的依赖

        <dependency>
            <groupId>com.101tec</groupId>
            <artifactId>zkclient</artifactId>
            <version>0.10</version>
        </dependency>

3.1 Zookeeper分布式锁的核心代码实现

实现逻辑参考“2. 分布式锁优化后的实现思路”中的流程图

package com.study.demo.lock;

import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

import org.I0Itec.zkclient.IZkDataListener;
import org.I0Itec.zkclient.ZkClient;
import org.I0Itec.zkclient.serialize.SerializableSerializer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
*
* @Description: Zookeeper分布式锁的核心代码实现
* @author leeSmall
* @date 2018年9月4日
*
*/
public class DistributedLock implements Lock {
    private static Logger logger = LoggerFactory.getLogger(DistributedLock.class);

    private static final String ZOOKEEPER_IP_PORT = "192.168.152.130:2181";
    private static final String LOCK_PATH = "/LOCK";

    private ZkClient client = new ZkClient(ZOOKEEPER_IP_PORT, 4000, 4000, new SerializableSerializer());

    private CountDownLatch cdl;

    private String beforePath;// 当前请求的节点前一个节点
    private String currentPath;// 当前请求的节点

    // 判断有没有LOCK目录，没有则创建
    public DistributedLock() {
        if (!this.client.exists(LOCK_PATH)) {
            this.client.createPersistent(LOCK_PATH);
        }
    }

    public void lock() {
        //尝试去获取分布式锁失败
        if (!tryLock()) {
            //对次小节点进行监听
            waitForLock();
            lock();
        }
        else {
            logger.info(Thread.currentThread().getName() + " 获得分布式锁！");
        }
    }

    public boolean tryLock() {
        // 如果currentPath为空则为第一次尝试加锁，第一次加锁赋值currentPath
        if (currentPath == null || currentPath.length() <= 0) {
            // 创建一个临时顺序节点
            currentPath = this.client.createEphemeralSequential(LOCK_PATH + '/', "lock");
            System.out.println("---------------------------->" + currentPath);
        }

        // 获取所有临时节点并排序，临时节点名称为自增长的字符串如：0000000400
        List<String> childrens = this.client.getChildren(LOCK_PATH);
        //由小到大排序所有子节点
        Collections.sort(childrens);
        //判断创建的子节点/LOCK/Node-n是否最小,即currentPath,如果当前节点等于childrens中的最小的一个就占用锁
        if (currentPath.equals(LOCK_PATH + '/' + childrens.get(0))) {
            return true;
        }
        //找出比创建的临时顺序节子节点/LOCK/Node-n次小的节点,并赋值给beforePath
        else {
            int wz = Collections.binarySearch(childrens, currentPath.substring(6));
            beforePath = LOCK_PATH + '/' + childrens.get(wz - 1);
        }

        return false;
    }

    //等待锁,对次小节点进行监听
    private void waitForLock() {
        IZkDataListener listener = new IZkDataListener() {
            public void handleDataDeleted(String dataPath) throws Exception {
                logger.info(Thread.currentThread().getName() + ":捕获到DataDelete事件！---------------------------");
                if (cdl != null) {
                    cdl.countDown();
                }
            }

            public void handleDataChange(String dataPath, Object data) throws Exception {

            }
        };

        // 对次小节点进行监听,即beforePath-给排在前面的的节点增加数据删除的watcher
        this.client.subscribeDataChanges(beforePath, listener);
        if (this.client.exists(beforePath)) {
            cdl = new CountDownLatch(1);
            try {
                cdl.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        this.client.unsubscribeDataChanges(beforePath, listener);
    }

    //完成业务逻辑以后释放锁
    public void unlock() {
        // 删除当前临时节点
        client.delete(currentPath);
    }

    // ==========================================
    public void lockInterruptibly() throws InterruptedException {

    }

    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return false;
    }

    public Condition newCondition() {
        return null;
    }
}

3.2 在业务里面使用分布式锁

package com.study.demo.lock;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.locks.Lock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
*
* @Description: 在业务里面使用分布式锁
* @author leeSmall
* @date 2018年9月4日
*
*/
public class OrderServiceImpl implements Runnable {
    private static OrderCodeGenerator ong = new OrderCodeGenerator();

    private Logger logger = LoggerFactory.getLogger(OrderServiceImpl.class);
    // 同时并发的线程数
    private static final int NUM = 10;
    // 按照线程数初始化倒计数器,倒计数器
    private static CountDownLatch cdl = new CountDownLatch(NUM);

    private Lock lock = new DistributedLock();

    // 创建订单接口
    public void createOrder() {
        String orderCode = null;

        //准备获取锁
        lock.lock();
        try {
            // 获取订单编号
            orderCode = ong.getOrderCode();
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            //完成业务逻辑以后释放锁
            lock.unlock();
        }

        // ……业务代码

        logger.info("insert into DB使用id：=======================>" + orderCode);
    }

    public void run() {
        try {
            // 等待其他线程初始化
            cdl.await();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        // 创建订单
        createOrder();
    }

    public static void main(String[] args) {
        for (int i = 1; i <= NUM; i++) {
            // 按照线程数迭代实例化线程
            new Thread(new OrderServiceImpl()).start();
            // 创建一个线程，倒计数器减1
            cdl.countDown();
        }
    }
}

工具类：

package com.study.demo.lock;

import java.text.SimpleDateFormat;
import java.util.Date;

public class OrderCodeGenerator {
    // 自增长序列
    private static int i = 0;

    // 按照“年-月-日-小时-分钟-秒-自增长序列”的规则生成订单编号
    public String getOrderCode() {
        Date now = new Date();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMddHHmmss");
        return sdf.format(now) + ++i;
    }

}

二、Zookeeper实现配置中心

1. 首先在zookeeper里面创建一个Jdbc的节点，在下面分别创建4个子节点/Jdbc/url、/Jdbc/uname、/Jdbc/password、/Jdbc/driver

create /Jdbc ''
create /Jdbc/url jdbc.mysql://192.168.152.1/dbspread
create /Jdbc/uname root
create /Jdbc/password 123456
create /Jdbc/driver com.mysql.jdbc.Driver

注意：/Jdbc/url这个节点的值是错的 

2. 新建一个zkdemo的maven的web项目

项目结构如下：

2.1 在pom.xml文件里面引入下面依赖：

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.study.demo</groupId>
    <artifactId>zkdemo</artifactId>
    <packaging>war</packaging>
    <version>0.0.1-SNAPSHOT</version>
    <name>zkdemo Maven Webapp</name>
    <url>http://maven.apache.org</url>

    <properties>
        <spring.version>4.3.8.RELEASE</spring.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.apache.zookeeper</groupId>
            <artifactId>zookeeper</artifactId>
            <version>3.4.10</version>
        </dependency>

        <dependency>
            <groupId>com.101tec</groupId>
            <artifactId>zkclient</artifactId>
            <version>0.10</version>
        </dependency>

        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-framework</artifactId>
            <version>4.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-recipes</artifactId>
            <version>4.0.0</version>
        </dependency>

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>tomcat-catalina</artifactId>
            <version>7.0.39</version>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-jdbc</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${spring.version}</version>
        </dependency>

        <dependency>
            <groupId>com.zaxxer</groupId>
            <artifactId>HikariCP</artifactId>
            <version>2.7.1</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.41</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.25</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>jcl-over-slf4j</artifactId>
            <version>1.7.25</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.9.1</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.9.1</version>
        </dependency>

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>jstl</artifactId>
            <version>1.2</version>
        </dependency>

    </dependencies>
    <build>
        <finalName>zkdemo</finalName>
    </build>
</project>

2.2 新建一个zookeeper配置中心类,从zookeeper动态获取数据库配置

package com.study.demo.config;

import java.util.List;
import java.util.Properties;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.cache.TreeCache;
import org.apache.curator.framework.recipes.cache.TreeCacheEvent;
import org.apache.curator.framework.recipes.cache.TreeCacheListener;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.springframework.web.context.ContextLoader;
import org.springframework.web.context.WebApplicationContext;

import com.zaxxer.hikari.HikariDataSource;

/**
*
* @Description: zookeeper配置中心类,从zookeeper动态获取数据库配置
* @author leeSmall
* @date 2018年9月10日
*
*/
public class ZookeeperConfigurerCentral {
    //curator客户端
    private CuratorFramework zkClient;

    //curator事件监听
    private TreeCache treeCache;

    //zookeeper的ip和端口
    private String zkServers;

    //zookeeper上的/Jdbc路径
    private String zkPath;

    //超时设置
    private int sessionTimeout;

    //读取zookeeper上的数据库配置文件放到这里
    private Properties props;

    public ZookeeperConfigurerCentral(String zkServers, String zkPath, int sessionTimeout) {
        this.zkServers = zkServers;
        this.zkPath = zkPath;
        this.sessionTimeout = sessionTimeout;
        this.props = new Properties();

        //初始化curator客户端
        initZkClient();
        //从zookeeper的Jdbc节点下获取数据库配置存入props
        getConfigData();
        //对zookeeper上的数据库配置文件所在节点进行监听，如果有改变就动态刷新props
        addZkListener();
    }

    //初始化curator客户端
    private void initZkClient() {
        zkClient = CuratorFrameworkFactory.builder().connectString(zkServers).sessionTimeoutMs(sessionTimeout)
                .retryPolicy(new ExponentialBackoffRetry(1000, 3)).build();
        zkClient.start();
    }

    //从zookeeper的Jdbc节点下获取数据库配置存入props
    private void getConfigData() {
        try {
            List<String> list = zkClient.getChildren().forPath(zkPath);
            for (String key : list) {
                String value = new String(zkClient.getData().forPath(zkPath + "/" + key));
                if (value != null && value.length() > 0) {
                    props.put(key, value);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    //对zookeeper上的数据库配置文件所在节点进行监听，如果有改变就动态刷新props
    private void addZkListener() {
        TreeCacheListener listener = new TreeCacheListener() {
            public void childEvent(CuratorFramework client, TreeCacheEvent event) throws Exception {
                if (event.getType() == TreeCacheEvent.Type.NODE_UPDATED) {
                    getConfigData();
                    WebApplicationContext ctx = ContextLoader.getCurrentWebApplicationContext();
                    HikariDataSource dataSource = (HikariDataSource) ctx.getBean("dataSource");
                    System.out.println("================"+props.getProperty("url"));
                    dataSource.setJdbcUrl(props.getProperty("url"));
                    dataSource.setUsername(props.getProperty("uname"));
                    dataSource.setPassword(props.getProperty("password "));
                    dataSource.setDriverClassName(props.getProperty("driver "));
                }
            }
        };

        treeCache = new TreeCache(zkClient, zkPath);
        try {
            treeCache.start();
            treeCache.getListenable().addListener(listener);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public Properties getProps() {
        return props;
    }

    public void setZkServers(String zkServers) {
        this.zkServers = zkServers;
    }

    public void setZkPath(String zkPath) {
        this.zkPath = zkPath;
    }

    public void setSessionTimeout(int sessionTimeout) {
        this.sessionTimeout = sessionTimeout;
    }
}

2.3 新建一个加载props里面的数据库配置的类

package com.study.demo.config;

import java.util.Properties;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.PropertyPlaceholderConfigurer;

/**
*
* @Description: 加载props里面的数据库配置,这个类等价于以前在xml文件里面的配置：
* <context:property-placeholder location="classpath:config/jdbc_conf.properties"/>
* @author leeSmall
* @date 2018年9月10日
*
*/
public class ZookeeperPlaceholderConfigurer extends PropertyPlaceholderConfigurer {
    private ZookeeperConfigurerCentral zkConfigurerCentral;

    @Override
    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props)
            throws BeansException {
        System.out.println(zkConfigurerCentral.getProps());
        super.processProperties(beanFactoryToProcess, zkConfigurerCentral.getProps());
    }

    public void setzkConfigurerCentral(ZookeeperConfigurerCentral zkConfigurerCentral) {
        this.zkConfigurerCentral = zkConfigurerCentral;
    }
}

2.4 在/zkdemo/src/main/webapp/WEB-INF/config/applicationContext.xml配置2.2和2.3新建的两个主类

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config />

    <context:component-scan base-package="com.study.demo" />

    <!--通过构造函数注入zkServers、sessionTimeout、zkPath从zookeeper动态获取数据库配置  -->
    <bean id="zkConfigurerCentral" class="com.study.demo.config.ZookeeperConfigurerCentral">
        <constructor-arg name="zkServers" value="192.168.152.130:2181" />
        <constructor-arg name="sessionTimeout" value="1000" />
        <constructor-arg name="zkPath" value="/Jdbc" />
    </bean>

    <!--这个类等价于以前在xml文件里面的配置：
    <context:property-placeholder location="classpath:config/jdbc_conf.properties"/>  加载
    props里面的数据库配置
    -->
    <bean id="zkPlaceholderConfigurer" class="com.study.demo.config.ZookeeperPlaceholderConfigurer">
        <property name="zkConfigurerCentral" ref="zkConfigurerCentral" />
        <property name="ignoreUnresolvablePlaceholders" value="true" />
        <property name="order" value="1" />
    </bean>

    <bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate">
        <property name="dataSource">
            <ref bean="dataSource" />
        </property>
    </bean>

    <bean id="dataSource" class="com.zaxxer.hikari.HikariDataSource"
        destroy-method="shutdown">
        <property name="driverClassName" value="${driver}" />
        <property name="jdbcUrl" value="${url}" />
        <property name="username" value="${uname}" />
        <property name="password" value="${password}" />

        <!-- 连接只读数据库时配置为true， 保证安全 -->
        <property name="readOnly" value="false" />
        <!-- 等待连接池分配连接的最大时长（毫秒），超过这个时长还没可用的连接则发生SQLException， 缺省:30秒 -->
        <property name="connectionTimeout" value="30000" />
        <!-- 一个连接idle状态的最大时长（毫秒），超时则被释放（retired），缺省:10分钟 -->
        <property name="idleTimeout" value="600000" />
        <!-- 一个连接的生命时长（毫秒），超时而且没被使用则被释放（retired），缺省:30分钟，建议设置比数据库超时时长少30秒，参考MySQL
            wait_timeout参数（show variables like '%timeout%';） -->
        <property name="maxLifetime" value="1800000" />
        <!-- 连接池中允许的最大连接数。缺省值：10；推荐的公式：((core_count * 2) + effective_spindle_count) -->
        <property name="maximumPoolSize" value="15" />
    </bean>
</beans>

2.5 在com.study.demo.controller新建测试类

测试类1：

package com.study.demo.controller;

import java.io.Serializable;

public class OrderModel implements Serializable {
    private static final long serialVersionUID = 1L;

    private int orderId;
    private int brandId;

    public int getOrderId() {
        return orderId;
    }

    public void setOrderId(int orderId) {
        this.orderId = orderId;
    }

    public int getBrandId() {
        return brandId;
    }

    public void setBrandId(int brandId) {
        this.brandId = brandId;
    }

}

测试类2：

package com.study.demo.controller;

import java.sql.ResultSet;
import java.sql.SQLException;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.RowMapper;
import org.springframework.stereotype.Repository;

@Repository
public class OrderDao {
    @Autowired
    private JdbcTemplate jdbcTemplate;

    public OrderModel findById() {
        String sql = "select * from tbl_order where order_id = 1";
        return jdbcTemplate.queryForObject(sql, new RowMapper<OrderModel>() {
            public OrderModel mapRow(ResultSet rs, int rowNum) throws SQLException {
                OrderModel payment = new OrderModel();
                payment.setOrderId(rs.getInt("order_id"));
                payment.setBrandId(rs.getInt("brand_id"));
                return payment;
            }
        });
    }
}

测试类3：

package com.study.demo.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class OrderService {
    @Autowired
    private OrderDao dao;

    public OrderModel getById() {
        return dao.findById();
    }
}

测试类4：

package com.study.demo.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class OrderController {
    @Autowired
    private OrderService service;

    @ResponseBody
    @RequestMapping(value = "/test", method = RequestMethod.GET)
    public String test() {
        OrderModel p = service.getById();
        return p.getBrandId() + "";
    }
}

2.6 其他附加配置和数据库脚本

/zkdemo/src/main/webapp/WEB-INF/config/log4j.properties

log4j.rootLogger=INFO,console
log4j.logger.org.apache.zookeeper=DEBUG
log4j.logger.org.apache.curator=DEBUG
log4j.logger.java.lang.Exception=INFO

log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=%d{MM-dd HH:mm:ss.SSS} [%c:%p] %m%n

/zkdemo/src/main/webapp/WEB-INF/config/spring-mvc.xml

<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
    xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">

    <mvc:default-servlet-handler />
    <mvc:annotation-driven
        content-negotiation-manager="contentNegotiationManager" />

    <context:component-scan base-package="com.study.demo">
        <context:include-filter type="annotation"
            expression="org.springframework.stereotype.Controller" />
        <context:exclude-filter type="annotation"
            expression="org.springframework.stereotype.Service" />
    </context:component-scan>

    <bean id="stringHttpMessageConverter"
        class="org.springframework.http.converter.StringHttpMessageConverter">
        <property name="supportedMediaTypes">
            <list>
                <bean class="org.springframework.http.MediaType">
                    <constructor-arg index="0" value="text" />
                    <constructor-arg index="1" value="plain" />
                    <constructor-arg index="2" value="UTF-8" />
                </bean>
            </list>
        </property>
    </bean>
    <bean id="mappingJacksonHttpMessageConverter"
        class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter" />

    <bean
        class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
        <property name="messageConverters">
            <list>
                <ref bean="stringHttpMessageConverter" />
                <ref bean="mappingJacksonHttpMessageConverter" />
            </list>
        </property>
    </bean>

    <bean id="contentNegotiationManager"
        class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean">
        <property name="mediaTypes">
            <map>
                <entry key="html" value="text/html" />
                <entry key="pdf" value="application/pdf" />
                <entry key="xsl" value="application/vnd.ms-excel" />
                <entry key="xml" value="application/xml" />
                <entry key="json" value="application/json" />
            </map>
        </property>
        <property name="defaultContentType" value="text/html" />
    </bean>

    <bean id="viewResolver"
        class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
        <property name="order" value="0" />
        <property name="contentNegotiationManager" ref="contentNegotiationManager" />

        <property name="viewResolvers">
            <list>
                <bean class="org.springframework.web.servlet.view.BeanNameViewResolver" />
                <bean
                    class="org.springframework.web.servlet.view.InternalResourceViewResolver">
                    <property name="viewClass"
                        value="org.springframework.web.servlet.view.JstlView" />
                    <property name="prefix" value="/WEB-INF/pages/" />
                    <property name="suffix" value=".jsp"></property>
                </bean>
            </list>
        </property>

        <property name="defaultViews">
            <list>
                <bean
                    class="org.springframework.web.servlet.view.json.MappingJackson2JsonView">
                    <property name="extractValueFromSingleKeyModel" value="true" />
                </bean>
            </list>
        </property>
    </bean>
</beans>

/zkdemo/src/main/webapp/WEB-INF/web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.5"
    xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

    <display-name>zkdemo</display-name>
    <description>Zookeeper Demo Application</description>

    <!--============================================================== -->
    <!-- Context parameters definition -->
    <!--============================================================== -->
    <context-param>
        <param-name>webAppRootKey</param-name>
        <param-value>zkdemo.root</param-value>
    </context-param>
    <context-param>
        <param-name>log4jConfigLocation</param-name>
        <param-value>/WEB-INF/config/log4j.properties</param-value>
    </context-param>
    <context-param>
        <param-name>log4jRefreshInterval</param-name>
        <param-value>60000</param-value>
    </context-param>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/config/applicationContext.xml</param-value>
    </context-param>

    <servlet>
        <servlet-name>spring</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/config/spring-mvc.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>spring</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

    <!--============================================================== -->
    <!-- Listener definition -->
    <!--============================================================== -->
    <listener>
        <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
    </listener>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <!--============================================================== -->
    <!-- Filter definition -->
    <!--============================================================== -->
    <filter>
        <filter-name>characterEncodingFilter</filter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
        <init-param>
            <param-name>forceEncoding</param-name>
            <param-value>true</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>characterEncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <!--============================================================== -->
    <!-- Web Session definition -->
    <!--============================================================== -->
    <session-config>
        <session-timeout>20</session-timeout>
    </session-config>

    <!--============================================================== -->
    <!-- Redirect page definition -->
    <!--============================================================== -->
    <error-page>
        <error-code>403</error-code>
        <location>/403.jsp</location>
    </error-page>
    <error-page>
        <error-code>404</error-code>
        <location>/404.jsp</location>
    </error-page>
    <error-page>
        <error-code>500</error-code>
        <location>/500.jsp</location>
    </error-page>

    <!--============================================================== -->
    <!-- First page definition -->
    <!--============================================================== -->
    <welcome-file-list>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>

</web-app>

数据库脚本：

CREATE TABLE `tbl_order` (
  `order_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '订单id',
  `brand_id` int(11) DEFAULT NULL COMMENT '品牌id',
  PRIMARY KEY (`order_id`)
) ENGINE=MyISAM AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 COMMENT='订单表';INSERT INTO tbl_order VALUES('','')

2.7 启动项目在浏览器输入地址http://localhost:8080/zkdemo/test查看效果

可以看到报错了，这是因为我们之前设置了错误的url

create /Jdbc/url jdbc.mysql://192.168.152.1/dbspread 

修改url为正确的

set /Jdbc/url jdbc:mysql://192.168.152.1:3306/dbspread

再次输入地址访问查看效果：

http://localhost:8080/zkdemo/test

可以看到在没有重启服务的情况下，可以正常访问获取到值了，这是因为zookeeper的数据库的配置动态刷新到服务了！