zk基于顺序节点实现分布式锁demo

叁歲伎倆 2022-10-17 01:42 152阅读 0赞

笔记来源: Java工程师面试突击专栏  
[https://apppukyptrl1086.pc.xiaoe-tech.com/detail/p\_5d3114935b4d7\_CEcL8yMS/6][https_apppukyptrl1086.pc.xiaoe-tech.com_detail_p_5d3114935b4d7_CEcL8yMS_6]

public class ZooKeeperDistributedLock implements Watcher{ 
    	
        private ZooKeeper zk;
        private String locksRoot= "/locks";
        private String productId;
        private String waitNode;
        private String lockNode;
        private CountDownLatch latch;
        private CountDownLatch connectedLatch = new CountDownLatch(1);
    private int sessionTimeout = 30000; 
    
        public ZooKeeperDistributedLock(String productId){ 
            this.productId = productId;
             try { 
    	   String address = "192.168.31.187:2181,192.168.31.19:2181,192.168.31.227:2181";
                zk = new ZooKeeper(address, sessionTimeout, this);
                connectedLatch.await();
            } catch (IOException e) { 
                throw new LockException(e);
            } catch (KeeperException e) { 
                throw new LockException(e);
            } catch (InterruptedException e) { 
                throw new LockException(e);
            }
        }
    	
    	// 锁释放后, 回调此方法
        public void process(WatchedEvent event) { 
            if(event.getState()==KeeperState.SyncConnected){ 
            // 执行countDown , 让线程进入可运行的状态
                connectedLatch.countDown();
                return;
            }
    
            if(this.latch != null) {   
                this.latch.countDown(); 
            }
        }
    
        public void acquireDistributedLock() {    
            try { 
                if(this.tryLock()){ 
                    return;
                }
                else{ 
                    waitForLock(waitNode, sessionTimeout);
                }
            } catch (KeeperException e) { 
                throw new LockException(e);
            } catch (InterruptedException e) { 
                throw new LockException(e);
            } 
    }
    
        public boolean tryLock() { 
            try { 
     		// 传入进去的locksRoot + “/” + productId
    		// 假设productId代表了一个商品id，比如说1
    		// locksRoot = locks
    		// /locks/10000000000，/locks/10000000001，/locks/10000000002
                lockNode = zk.create(locksRoot + "/" + productId, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL); // 尝试去创建临时顺序节点
       
                // 看看刚创建的节点是不是最小的节点
    	 	// locks：10000000000，10000000001，10000000002
                List<String> locks = zk.getChildren(locksRoot, false);
                // 排序 从小到大 
                Collections.sort(locks);
    			
    			// 判断是不是第一个节点
                if(lockNode.equals(locksRoot+"/"+ locks.get(0))){ 
                    //如果是最小的节点,则表示取得锁
                    return true;
                }
    	
                //如果不是最小的节点，找到比自己小1的节点
    	  int previousLockIndex = -1;
                for(int i = 0; i < locks.size(); i++) { 
    		if(lockNode.equals(locksRoot + “/” + locks.get(i))) { 
    	         	    previousLockIndex = i - 1;
    		    break;
    		}
    	   }
    	   
    	   this.waitNode = locks.get(previousLockIndex);
            } catch (KeeperException e) { 
                throw new LockException(e);
            } catch (InterruptedException e) { 
                throw new LockException(e);
            }
            return false;
        }
         
        private boolean waitForLock(String waitNode, long waitTime) throws InterruptedException, KeeperException { 
        // 判断上一个节点是否存在 , 存在代表上一个系统还没有释放锁, 不存在则说明已经释放锁. 
            Stat stat = zk.exists(locksRoot + "/" + waitNode, true);
            if(stat != null){ 
            // 锁还没有释放, 使用CountDownLatch去等待 
                this.latch = new CountDownLatch(1);
                this.latch.await(waitTime, TimeUnit.MILLISECONDS);            	  
                 this.latch = null;
            }
            return true;
    }
     // 释放锁 
        public void unlock() { 
            try { 
    		// 删除/locks/10000000000节点
    		// 删除/locks/10000000001节点
                System.out.println("unlock " + lockNode);
                // 删除自身的节点 
                zk.delete(lockNode,-1);
                lockNode = null;
                zk.close();
            } catch (InterruptedException e) { 
                e.printStackTrace();
            } catch (KeeperException e) { 
                e.printStackTrace();
            }
    }
    
        public class LockException extends RuntimeException { 
            private static final long serialVersionUID = 1L;
            public LockException(String e){ 
                super(e);
            }
            public LockException(Exception e){ 
                super(e);
            }
    }

如果有一把锁，被多个人给竞争，此时多个人会排队，第一个拿到锁的人会执行，然后释放锁，后面的每个人都会去监听排在自己前面的那个人创建的node上，一旦某个人释放了锁，排在自己后面的人就会被zookeeper给通知，一旦被通知了之后，就ok了，自己就获取到了锁，就可以执行代码了

[https_apppukyptrl1086.pc.xiaoe-tech.com_detail_p_5d3114935b4d7_CEcL8yMS_6]: https://apppukyptrl1086.pc.xiaoe-tech.com/detail/p_5d3114935b4d7_CEcL8yMS/6