ZooKeeper是一个分布式的，开放源码的分布式应用程序协调服务。基于ZooKeeper，我们可以实现一种简单的分布式互斥锁，包括可重入与不可重入。代码如下：
import java.io.IOException;
import java.util.ArrayList;
import java.util.Random;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;
public class TestZooKeeperDistributeLock {
// 是否为可重入锁
private boolean reentrant = false;
public boolean isReentrant() {
return reentrant;
}
private ZooKeeper zk = null;
public ZooKeeper getZk() {
return zk;
}
public TestZooKeeperDistributeLock(boolean reentrant) {
this.reentrant = reentrant;
// 初始化环境：连接Zookeeper并创建根目录
init();
}
private void init() {
try {
System.out.println("...");
System.out.println("...");
System.out.println("...");
System.out.println("...");
System.out.println("开始连接ZooKeeper...");
// 创建与ZooKeeper服务器的连接zk
String address = "192.168.1.226:2181";
int sessionTimeout = 3000;
zk = new ZooKeeper(address, sessionTimeout, new Watcher() {
// 监控所有被触发的事件
public void process(WatchedEvent event) {
if (event.getType() == null || "".equals(event.getType())) {
return;
}
System.out.println("已经触发了" + event.getType() + "事件！");
}
});
System.out.println("ZooKeeper连接创建成功！");
Thread.currentThread().sleep(1000l);
System.out.println("...");
System.out.println("...");
System.out.println("...");
System.out.println("...");
// 创建根目录节点
// 路径为/tmp_root_path
// 节点内容为字符串"我是根目录/tmp_root_path"
// 创建模式为CreateMode.PERSISTENT
System.out.println("开始创建根目录节点/tmp_root_path...");
zk.create("/tmp_root_path", "我是根目录/tmp_root_path".getBytes(),
Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
System.out.println("根目录节点/tmp_root_path创建成功！");
Thread.currentThread().sleep(1000l);
System.out.println("...");
System.out.println("...");
System.out.println("...");
System.out.println("...");
} catch (Exception e) {
zk = null;
}
}
public void destroy() {
// 删除根目录节点
try {
System.out.println("开始删除根目录节点/tmp_root_path...");
zk.delete("/tmp_root_path", -1);
System.out.println("根目录节点/tmp_root_path删除成功！");
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
} catch (KeeperException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
// 关闭连接
if (zk != null) {
try {
zk.close();
System.out.println("释放ZooKeeper连接成功！");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public static void main(String[] args) {
final TestZooKeeperDistributeLock testZooKeeperDistributeLock = new TestZooKeeperDistributeLock(
true);
final Random radom = new Random();
try {
Thread.currentThread().sleep(1000l);
} catch (InterruptedException e2) {
// TODO Auto-generated catch block
e2.printStackTrace();
}
ArrayList<Thread> threadList = new ArrayList<Thread>();
for (int i = 0; i < 4; i++) {
Thread thread = new Thread() {
@Override
public void run() {
boolean locked = false;
while (true) {
try {
// 创建需要获取锁的目录节点，创建成功则说明能够获取到锁，创建不成功，则说明锁已被其他线程（哪怕是不同进程的）获取
// 路径为/tmp_root_path/lock
// 节点内容为当前线程名
// 创建模式为CreateMode.PERSISTENT
System.out.println("线程"
+ Thread.currentThread().getName()
+ "尝试获取锁...");
testZooKeeperDistributeLock.getZk()
.create("/tmp_root_path/lock",
Thread.currentThread().getName()
.getBytes(),
Ids.OPEN_ACL_UNSAFE,
CreateMode.PERSISTENT);
System.out.println("线程"
+ Thread.currentThread().getName()
+ "成功获取到锁！");
locked = true;
System.out.println("线程"
+ Thread.currentThread().getName()
+ "开始处理业务逻辑...");
Thread.currentThread().sleep(
3000 + radom.nextInt(3000));
System.out.println("线程"
+ Thread.currentThread().getName()
+ "业务逻辑处理完毕！");
} catch (Exception e) {
if (testZooKeeperDistributeLock.isReentrant()) {
try {
String lockThread = new String(
testZooKeeperDistributeLock
.getZk()
.getData(
"/tmp_root_path/lock",
false, null));
if (lockThread != null) {
// 当前线程与获取到的锁线程名一致，重入锁
if (lockThread.equals(Thread
.currentThread().getName())) {
System.out.println("线程"
+ Thread.currentThread()
.getName()
+ "成功重入锁！");
locked = true;
System.out.println("线程"
+ Thread.currentThread()
.getName()
+ "开始处理业务逻辑...");
Thread.currentThread().sleep(
3000 + radom.nextInt(3000));
System.out.println("线程"
+ Thread.currentThread()
.getName()
+ "业务逻辑处理完毕！");
} else {
System.out.println("线程"
+ Thread.currentThread()
.getName()
+ "尝试获取锁失败，锁被线程"
+ lockThread + "占用！");
}
}
} catch (KeeperException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
} else {
System.out.println("线程"
+ Thread.currentThread().getName()
+ "尝试获取锁失败！");
}
try {
Thread.currentThread().sleep(
3000 + radom.nextInt(3000));
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
} finally {
try {
if (locked) {
System.out.println("线程"
+ Thread.currentThread().getName()
+ "开始释放锁...");
testZooKeeperDistributeLock.getZk().delete(
"/tmp_root_path/lock", -1);
System.out.println("线程"
+ Thread.currentThread().getName()
+ "成功释放锁！");
Thread.currentThread().sleep(
3000 + radom.nextInt(3000));
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (KeeperException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
locked = false;
}
}
}
}
};
threadList.add(thread);
thread.start();
}
try {
Thread.currentThread().sleep(1000 * 20);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
for(int i = 0; i < threadList.size(); i++){
Thread thread = threadList.get(i);
thread.stop();
}
// 释放资源
testZooKeeperDistributeLock.destroy();
}
} 运行结果如下：
... ... ... ... 开始连接ZooKeeper... ZooKeeper连接创建成功！ 已经触发了None事件！ ... ... ... ... 开始创建根目录节点/tmp_root_path... 根目录节点/tmp_root_path创建成功！ ... ... ... ... 线程Thread-0尝试获取锁... 线程Thread-2尝试获取锁... 线程Thread-3尝试获取锁... 线程Thread-1尝试获取锁... 线程Thread-0成功获取到锁！ 线程Thread-0开始处理业务逻辑... 线程Thread-1尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-2尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-3尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-1尝试获取锁... 线程Thread-1尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-2尝试获取锁... 线程Thread-2尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-3尝试获取锁... 线程Thread-3尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-0业务逻辑处理完毕！ 线程Thread-0开始释放锁... 线程Thread-0成功释放锁！ 线程Thread-2尝试获取锁... 线程Thread-2成功获取到锁！ 线程Thread-2开始处理业务逻辑... 线程Thread-1尝试获取锁... 线程Thread-1尝试获取锁失败，锁被线程Thread-2占用！ 线程Thread-3尝试获取锁... 线程Thread-3尝试获取锁失败，锁被线程Thread-2占用！ 线程Thread-0尝试获取锁... 线程Thread-0尝试获取锁失败，锁被线程Thread-2占用！ 线程Thread-2业务逻辑处理完毕！ 线程Thread-2开始释放锁... 线程Thread-2成功释放锁！ 线程Thread-3尝试获取锁... 线程Thread-1尝试获取锁... 线程Thread-3成功获取到锁！ 线程Thread-3开始处理业务逻辑... 线程Thread-1尝试获取锁失败，锁被线程Thread-3占用！ 线程Thread-0尝试获取锁... 线程Thread-0尝试获取锁失败，锁被线程Thread-3占用！ 线程Thread-2尝试获取锁... 线程Thread-2尝试获取锁失败，锁被线程Thread-3占用！ 线程Thread-3业务逻辑处理完毕！ 线程Thread-3开始释放锁... 线程Thread-3成功释放锁！ 线程Thread-0尝试获取锁... 线程Thread-0成功获取到锁！ 线程Thread-0开始处理业务逻辑... 线程Thread-1尝试获取锁... 线程Thread-1尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-2尝试获取锁... 线程Thread-2尝试获取锁失败，锁被线程Thread-0占用！ 线程Thread-0开始释放锁... 开始删除根目录节点/tmp_root_path... 线程Thread-0成功释放锁！ 根目录节点/tmp_root_path删除成功！ 释放ZooKeeper连接成功！示例可能略显粗糙，但是大体原理就是这样。我们可以利用ZooKeeper上一个固定位置的节点有无，来判断锁是否被获取到。当某一线程来临时，如果节点不存在，则说明没有其他线程占用对应锁，调用ZooKeeper的create()方法创建节点，标识分布式锁已被当前线程占用。待业务处理完毕后，再调用ZooKeeper的delete()方法删除节点，则完成锁的释放。
同时，我们可以对应节点写入线程名等区分线程唯一性的字段来标识锁被哪个线程占用。而当再有线程要获取锁时，如果为不可重入锁，无论哪个线程，即便是持有锁的线程本身，也得等锁释放后再获取锁，如果为可重入锁，则判断当前线程唯一性字段与对应节点中的数据是否一致即可。
当然，也可以利用ZooKeeper的CreateMode中PERSISTENT_SEQUENTIAL，一种顺序自动编号的目录节点，检测某节点的子目录，来实现可以排队的优先级锁！这个留待以后我们再研究！
