基于zookeeper分布式锁

手写代码模拟zookeeper分布式锁

• 1.zookeeper服务器
• 2.代码模拟分布式锁

第一部如何安装Zookeeper这里就不再累述了，网上有很多教程，自行百度一波，具体看代码 如何实现

2.1 maven工程

新建maven工程，导入zookeeper 和zkClent依赖，这里使用ZkClient客户进行操作，代码量更加精简

   com.101tec
   zkclient
   0.11


   org.apache.zookeeper
   zookeeper
   3.4.13
   pom

2.2 编写分布式锁实现类

• 定义自己的序列化协议

• package com.xp.zookeeper;
  
  import java.io.UnsupportedEncodingException;
  
  import org.I0Itec.zkclient.exception.ZkMarshallingError;
  import org.I0Itec.zkclient.serialize.ZkSerializer;
  /**
   * 自定义序列化
   * @author liuj
   * @date 2019-12-01
   */
  public class CustomeZkSerializer implements ZkSerializer{
  
      String charset = "UTF-8";
  
      public Object deserialize(byte[] bytes) throws ZkMarshallingError {
          try {
              return new String(bytes, charset);
          } catch (UnsupportedEncodingException e) {
              throw new ZkMarshallingError(e);
          }
      }
  
      public byte[] serialize(Object obj) throws ZkMarshallingError {
          try {
              return String.valueOf(obj).getBytes(charset);
          } catch (UnsupportedEncodingException e) {
              throw new ZkMarshallingError(e);
          }
      }
  
  }

这里只是简单的实现了自己的序列化协议，也可自己后期扩展，重点看分布式锁的加锁和解锁类的实现

package com.xp.zookeeper;
/**
 * 分布式锁实现
 * @author liuj
 * @date 2019-12-01
 */

import java.util.Collections;
import java.util.List;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
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;

public class DistributeLock implements Lock{

    private String lockPath;

    private ZkClient zkClient;

    private ThreadLocal currentPath = new ThreadLocal<>();

    private ThreadLocal beforePath = new ThreadLocal<>();

    private ThreadLocal reentCount = new ThreadLocal<>();

    public DistributeLock(String lockPath){
        super();
        this.lockPath = lockPath;
        zkClient = new ZkClient("localhost:2181");
        zkClient.setZkSerializer(new CustomeZkSerializer());
        if(!this.zkClient.exists(lockPath)){
            try{

                this.zkClient.createPersistent(lockPath);
            }catch(Exception e){

            }
        }

    }

    @Override
    public void lock() {
        if (!tryLock()) {
            waitForLock();
            lock();
        }
    }

    private void waitForLock(){
        CountDownLatch latch = new CountDownLatch(1);
        IZkDataListener listener = new IZkDataListener() {
            @Override
            public void handleDataDeleted(String arg0) throws Exception {
                System.out.println("监听节点被删除");
                latch.countDown();
            }
            @Override
            public void handleDataChange(String arg0, Object arg1) throws Exception {

            }
        };
        //注册当前节点上一个节点事件
        this.zkClient.subscribeDataChanges(this.beforePath.get(), listener);

        //当前一个节点还存在的情况阻塞自己
        if(this.zkClient.exists(this.beforePath.get())){
            try {
                latch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        //取消前一个节点的注册监听事件
        this.zkClient.unsubscribeDataChanges(this.beforePath.get(), listener);
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {

    }

    @Override
    public boolean tryLock() {
        //可重入锁逻辑实现
        if(this.reentCount.get() != null){
            //当前线程不是第一获取锁
            int reentCount = this.reentCount.get();
            if(reentCount > 0){
                this.reentCount.set(++reentCount);
                return true;
            }
        }
        if(this.currentPath.get() == null){
            currentPath.set(this.zkClient.createPersistentSequential(lockPath + "/","bbb"));
        }
        //获取当前节点下面所有的子节点
        List childList = zkClient.getChildren(lockPath);
        Collections.sort(childList);
        //判断当前节点是否最小的节点
        if(currentPath.get().equals(lockPath + "/" + childList.get(0))){
            return true;
        }else {
            int currentIndex = childList.indexOf(currentPath.get().substring(lockPath.length() + 1));
            beforePath.set(lockPath + "/" + childList.get(currentIndex - 1));
        }
        return false;
    }

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

    @Override
    public void unlock() {
        if(this.reentCount.get() != null){
            int reentCount = this.reentCount.get();
            if(reentCount > 1){
                this.reentCount.set(--reentCount);
            }else {
                this.reentCount.set(null);
            }
        }
        this.zkClient.delete(this.currentPath.get());

    }

    @Override
    public Condition newCondition() {
        return null;
    }

    public static void main(String[] args) {
        // 并发数
        int currency = 50;

        // 循环屏障
        CyclicBarrier cb = new CyclicBarrier(currency);

        // 多线程模拟高并发
        for (int i = 0; i < currency; i++) {
            new Thread(new Runnable() {
                public void run() {
                    System.out.println(Thread.currentThread().getName() + "---------所有线程准备好---------------");
                    // 等待一起出发
                    try {
                        cb.await();
                    } catch (InterruptedException | BrokenBarrierException e) {
                        e.printStackTrace();
                    }
                    DistributeLock lock = new DistributeLock("/distLock");

                    try {
                        lock.lock();
                        System.out.println(Thread.currentThread().getName() + " 获得锁！");
                    } finally {
                        lock.unlock();
                    }
                }
            }).start();

        }
    }
}

大体的思路：首先通过DistributeLock类构造方法初始化一个持久化的根节点，lock方法首先调用tryLock方法尝试在zookeeper上面创建临时节点，如果创建成功的化，就代表加锁成功，否则的化，进行调用waitForLock方法阻塞等待，在这个方法里面利用了zookeeper的wathch机制，监听上一个节点的删除时间，当当前节点的上一个节点释放锁，也就是删除了节点事件，因为这个方法首先创建一个CountDownLatch的发令枪，并且当当前节点有上一个节点的时候就会一直阻塞在这里，当监听到上一个节点删除的时候，发令枪计数器就会减一，这个时候当前节点就会被唤醒，从而lock里面就会执行当前节点的加锁逻辑，这里的思路是只注册上一个节点的的删除事件，而不是监听根节点下面所有的节点的删除事件，如果有成千上万的节点竞争锁的化，就会出现惊群效应，这里就巧妙的避免了这个问题