我们在做分布式(多JVM实例)/集群应用设计时，通常会尽量避免分布式锁，但在某些需求场景中，还是会存在对它的需求，并且使用分布式锁需要考虑的问题更多，例如怎样保证某个进程在持有锁时意外终止后，其它进程也能够正常地获得锁等。
Java中的关键字synchronized，Singleton单例以及concurrent locks并发锁等并不能满足。我们一般会引入第三者实现分布式锁服务，如zookeeper, redis, memcached等。
这里使用简单轻量的JGroups(可靠性消息传输的工具包)实现一个分布式锁，从JGroups主页可以了解到更多详细信息。
首先，定义Lock接口，最基本的两个方法：
package com.dunkcoder;
public interface Lock {
void lock(String name);
void release(String name);
}接着，是JGroups Protocol Stacks协议栈，默认UDP：
<config xmlns="urn:org:jgroups" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:org:jgroups http://www.jgroups.org/schema/JGroups-3.3.xsd">
<UDP mcast_port="${jgroups.udp.mcast_port:12345}" receive_on_all_interfaces="true" />
<PING />
<MERGE2 />
<FD_SOCK />
<FD_ALL />
<VERIFY_SUSPECT />
<BARRIER />
<pbcast.NAKACK2 />
<UNICAST2 />
<pbcast.STABLE />
<pbcast.GMS />
<UFC />
<MFC />
<FRAG2 />
<RSVP />
<pbcast.STATE_TRANSFER />
<CENTRAL_LOCK />
</config>基于此协议栈，实现Lock接口：
package com.dunkcoder;
import org.jgroups.JChannel;
import org.jgroups.blocks.locking.LockService;
public class JgroupsLock implements Lock {
private JChannel channel;
private LockService lockService;
public JgroupsLock() {
try {
channel = new JChannel("udp-lock-stack.xml");
lockService = new LockService(channel);
channel.connect("LockCluster");
} catch (Exception e) {
throw new IllegalStateException(e);
}
}
public void close() {
lockService.unlockAll();
channel.close();
}
public void lock(String name) {
lockService.getLock(name).lock();
}
public void release(String name) {
lockService.getLock(name).unlock();
}
}我们起一个Swing界面来测试一下：
package com.dunkcoder;
import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.event.ActionEvent;
@SuppressWarnings("serial")
public class LockWindow extends JFrame {
private final static Icon LOCKED_ICON;
private final static Icon UNLOCKED_ICON;
static {
try {
ClassLoader cl = LockWindow.class.getClassLoader();
LOCKED_ICON = new ImageIcon(ImageIO.read(cl.getResource("locked.png")));
UNLOCKED_ICON = new ImageIcon(ImageIO.read(cl.getResource("unlocked.png")));
} catch (Exception e) {
throw new IllegalStateException(e);
}
}
private final Lock lock;
private final String lockName;
public LockWindow(Lock lock, String lockName) {
super("Click the icon to see distributed lock");
this.lock = lock;
this.lockName = lockName;
Action toggleAction = new ToggleLockAction();
JButton toggleButton = new JButton(toggleAction);
add(toggleButton);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
pack();
setVisible(true);
}
private class ToggleLockAction extends AbstractAction {
private boolean hasLock = false;
private ToggleLockAction() {
putValue(Action.LARGE_ICON_KEY, UNLOCKED_ICON);
}
public void actionPerformed(ActionEvent e) {
if (hasLock) {
lock.release(lockName);
hasLock = false;
putValue(Action.LARGE_ICON_KEY, UNLOCKED_ICON);
} else {
lock.lock(lockName);
hasLock = true;
putValue(Action.LARGE_ICON_KEY, LOCKED_ICON);
}
}
}
}
package com.dunkcoder;
import javax.swing.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
public class Main {
public static void main(String[] args) {
new Main();
}
private final JgroupsLock lockManager;
private LockWindow lockWindow;
public Main() {
lockManager = new JgroupsLock();
try {
SwingUtilities.invokeAndWait(new Runnable() {
public void run() {
lockWindow = new LockWindow(lockManager, "LockName");
lockWindow.addWindowListener(new WindowAdapter() {
@Override
public void windowClosed(WindowEvent e) {
lockManager.close();
}
});
}
});
} catch (Exception e) {
throw new IllegalStateException(e);
}
}
}jgroupsLock项目代码在这里，切换到目录执行命令mvn clean package打包成jgroupsLock-1.0.one-jar.jar后，启动两个以上实例，点击Icon测试分布式锁。
java -jar -Djava.net.preferIPv4Stack=true jgroupsLock-1.0.one-jar.jar
其它zookeeper方式可以参考下面几篇文章：
Distributed Lock
JGroups 入门实践
zookeeper分布式锁的实现
就酱，嗯。
您有任何问题或建议，请给我写邮件。