让我来详细讲解“Java分布式锁的三种实现方案”的完整攻略。
什么是分布式锁?
分布式锁是控制分布式系统之间同步访问共享资源的一种方式。在分布式系统中,多个节点会竞争同一个锁,这个锁可以是基于数据库或者基于缓存等其他方式实现的。
Java分布式锁的三种实现方案
基于数据库的分布式锁
这种锁的实现方式比较简单,通过数据库的行锁来实现分布式锁,通过insert或者update语句,对数据库表的某一行进行加锁,其他节点通过获取该行的锁状态来判断是否可以获取锁。
下面是一个基于数据库的分布式锁的实现示例:
public class DistributedLock {
private Connection connection;
private String tableName;
public DistributedLock(Connection connection, String tableName) {
this.connection = connection;
this.tableName = tableName;
}
public boolean acquire(String lockName) {
try {
PreparedStatement ps = connection.prepareStatement(
"INSERT INTO " + tableName + " (lock_name) VALUES (?)");
ps.setString(1, lockName);
ps.executeUpdate();
ps.close();
return true;
} catch (SQLException e) {
return false;
}
}
public boolean release(String lockName) {
try {
PreparedStatement ps = connection.prepareStatement(
"DELETE FROM " + tableName + " WHERE lock_name = ?");
ps.setString(1, lockName);
ps.executeUpdate();
ps.close();
return true;
} catch (SQLException e) {
return false;
}
}
}
基于缓存的分布式锁
基于缓存的分布式锁也比较常用,它的原理是通过利用缓存的原子性来实现分布式锁。以Redis为例,我们通过使用Redis的setnx(set if not exists)命令来实现分布式锁。
setnx命令的具体用法是:当指定的键不存在时,将设置键的值为指定的值;如果键已经存在,则不执行任何操作。
下面是一个基于Redis实现的分布式锁的示例:
public class DistributedLock {
private RedisTemplate redisTemplate;
public DistributedLock(RedisTemplate redisTemplate) {
this.redisTemplate = redisTemplate;
}
public boolean acquire(String lockName, long timeout, TimeUnit unit) {
boolean acquired = false;
String key = "lock:" + lockName;
long expireTime = unit.toMillis(timeout);
long start = System.currentTimeMillis();
while (!acquired && (System.currentTimeMillis() - start) < expireTime) {
acquired = redisTemplate.opsForValue().setIfAbsent(key, System.currentTimeMillis() + "");
}
if (acquired) {
redisTemplate.expire(key, timeout, unit);
}
return acquired;
}
public boolean release(String lockName) {
String key = "lock:" + lockName;
if (redisTemplate.hasKey(key)) {
redisTemplate.delete(key);
return true;
}
return false;
}
}
基于Zookeeper的分布式锁
基于Zookeeper实现分布式锁的原理是利用ZooKeeper节点的唯一性,通过创建一个唯一的节点代表锁,其他节点通过判断该节点是否存在来实现分布式锁。
下面是一个基于Zookeeper实现的分布式锁的示例:
public class DistributedLock implements Watcher {
private ZooKeeper zooKeeper;
private String lockName;
private String lockPath;
public DistributedLock(ZooKeeper zooKeeper, String lockName) {
this.zooKeeper = zooKeeper;
this.lockName = lockName;
}
public boolean acquire(long timeout, TimeUnit unit) {
try {
lockPath = zooKeeper.create("/locks/" + lockName + "-", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
List<String> locks = zooKeeper.getChildren("/locks", false);
Collections.sort(locks);
if (lockPath.endsWith(locks.get(0))) {
return true;
} else {
String lowerLockPath = null;
for (String path : locks) {
if (lockPath.endsWith(path)) {
lowerLockPath = path;
break;
}
}
if (lowerLockPath != null) {
Stat stat = zooKeeper.exists("/locks/" + lowerLockPath, this);
if (stat == null) {
return acquire(timeout, unit);
} else {
synchronized (this) {
this.wait(unit.toMillis(timeout));
}
return false;
}
}
}
} catch (Exception e) {
return false;
}
return false;
}
public boolean release() {
try {
zooKeeper.delete(lockPath, -1);
return true;
} catch (Exception e) {
return false;
}
}
@Override
public void process(WatchedEvent watchedEvent) {
synchronized (this) {
this.notifyAll();
}
}
}
以上就是Java分布式锁的三种实现方案的详细分析。
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