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本篇內容介紹了“zookeeper分布式鎖實現的方法是什么”的有關知識，在實際案例的操作過程中，不少人都會遇到這樣的困境，接下來就讓小編帶領大家學習一下如何處理這些情況吧！希望大家仔細閱讀，能夠學有所成！
一。為何使用分布式鎖?
當應用服務器數量超過1臺，對相同數據的訪問可能造成訪問沖突（特別是寫沖突）。單純使用關系數據庫比如MYSQL的應用可以借助于事務來實現鎖，也可以使用版本號等實現樂觀鎖，最大的缺陷就是可用性降低（性能差）。對于GLEASY這種滿足大規模并發訪問請求的應用來說，使用數據庫事務來實現數據庫就有些捉襟見肘了。另外對于一些不依賴數據庫的應用，比如分布式文件系統，為了保證同一文件在大量讀寫操作情況下的正確性，必須引入分布式鎖來約束對同一文件的并發操作。
二。對分布式鎖的要求
1.高性能（分布式鎖不能成為系統的性能瓶頸）
2.避免死鎖（拿到鎖的結點掛掉不會導致其它結點永遠無法繼續）
3.支持鎖重入
三。方案1，基于zookeeper的分布式鎖
/**
* DistributedLockUtil.java
* 分布式鎖工廠類，所有分布式請求都由該工廠類負責
**/
public class DistributedLockUtil {
private static Object schemeLock = new Object();
private static Object mutexLock = new Object();
private static Map<String,Object> mutexLockMap = new ConcurrentHashMap();
private String schema;
private Map<String,DistributedReentrantLock> cache = new ConcurrentHashMap<String,DistributedReentrantLock>();
private static Map<String,DistributedLockUtil> instances = new ConcurrentHashMap();
public static DistributedLockUtil getInstance(String schema){
DistributedLockUtil u = instances.get(schema);
if(u==null){
synchronized(schemeLock){
u = instances.get(schema);
if(u == null){
u = new DistributedLockUtil(schema);
instances.put(schema, u);
}
}
}
return u;
}
private DistributedLockUtil(String schema){
this.schema = schema;
}
private Object getMutex(String key){
Object mx = mutexLockMap.get(key);
if(mx == null){
synchronized(mutexLock){
mx = mutexLockMap.get(key);
if(mx==null){
mx = new Object();
mutexLockMap.put(key,mx);
}
}
}
return mx;
}
private DistributedReentrantLock getLock(String key){
DistributedReentrantLock lock = cache.get(key);
if(lock == null){
synchronized(getMutex(key)){
lock = cache.get(key);
if(lock == null){
lock = new DistributedReentrantLock(key,schema);
cache.put(key, lock);
}
}
}
return lock;
}
public void reset(){
for(String s : cache.keySet()){
getLock(s).unlock();
}
}
/**
* 嘗試加鎖
* 如果當前線程已經擁有該鎖的話,直接返回false,表示不用再次加鎖,此時不應該再調用unlock進行解鎖
*
* @param key
* @return
* @throws InterruptedException
* @throws KeeperException
*/
public LockStat lock(String key) throws InterruptedException, KeeperException{
if(getLock(key).isOwner()){
return LockStat.NONEED;
}
getLock(key).lock();
return LockStat.SUCCESS;
}
public void clearLock(String key) throws InterruptedException, KeeperException{
synchronized(getMutex(key)){
DistributedReentrantLock l = cache.get(key);
l.clear();
cache.remove(key);
}
}
public void unlock(String key,LockStat stat) throws InterruptedException, KeeperException{
unlock(key,stat,false);
}
public void unlock(String key,LockStat stat,boolean keepalive) throws InterruptedException, KeeperException{
if(stat == null) return;
if(LockStat.SUCCESS.equals(stat)){
DistributedReentrantLock lock = getLock(key);
boolean hasWaiter = lock.unlock();
if(!hasWaiter && !keepalive){
synchronized(getMutex(key)){
lock.clear();
cache.remove(key);
}
}
}
}
public static enum LockStat{
NONEED,
SUCCESS
}
}
/**
*DistributedReentrantLock.java
*本地線程之間鎖爭用，先使用虛擬機內部鎖機制，減少結點間通信開銷
*/
public class DistributedReentrantLock {
private static final Logger logger = Logger.getLogger(DistributedReentrantLock.class);
private ReentrantLock reentrantLock = new ReentrantLock();
private WriteLock writeLock;
private long timeout = 3*60*1000;
private final Object mutex = new Object();
private String dir;
private String schema;
private final ExitListener exitListener = new ExitListener(){
@Override
public void execute() {
initWriteLock();
}
};
private synchronized void initWriteLock(){
logger.debug("初始化writeLock");
writeLock = new WriteLock(dir,new LockListener(){
@Override
public void lockAcquired() {
synchronized(mutex){
mutex.notify();
}
}
@Override
public void lockReleased() {
}
},schema);
if(writeLock != null && writeLock.zk != null){
writeLock.zk.addExitListener(exitListener);
}
synchronized(mutex){
mutex.notify();
}
}
public DistributedReentrantLock(String dir,String schema) {
this.dir = dir;
this.schema = schema;
initWriteLock();
}
public void lock(long timeout) throws InterruptedException, KeeperException {
reentrantLock.lock();//多線程競爭時，先拿到第一層鎖
try{
boolean res = writeLock.trylock();
if(!res){
synchronized(mutex){
mutex.wait(timeout);
}
if(writeLock == null || !writeLock.isOwner()){
throw new InterruptedException("鎖超時");
}
}
}catch(InterruptedException e){
reentrantLock.unlock();
throw e;
}catch(KeeperException e){
reentrantLock.unlock();
throw e;
}
}
public void lock() throws InterruptedException, KeeperException {
lock(timeout);
}
public void destroy() throws KeeperException {
writeLock.unlock();
}
public boolean unlock(){
if(!isOwner()) return false;
try{
writeLock.unlock();
reentrantLock.unlock();//多線程競爭時，釋放最外層鎖
}catch(RuntimeException e){
reentrantLock.unlock();//多線程競爭時，釋放最外層鎖
throw e;
}
return reentrantLock.hasQueuedThreads();
}
public boolean isOwner() {
return reentrantLock.isHeldByCurrentThread() && writeLock.isOwner();
}
public void clear() {
writeLock.clear();
}
}
/**
*WriteLock.java
*基于zk的鎖實現
*一個最簡單的場景如下：
*1.結點A請求加鎖，在特定路徑下注冊自己（會話自增結點)，得到一個ID號1
*2.結點B請求加鎖，在特定路徑下注冊自己（會話自增結點)，得到一個ID號2
*3.結點A獲取所有結點ID，判斷出來自己是最小結點號，于是獲得鎖
*4.結點B獲取所有結點ID，判斷出來自己不是最小結點，于是監聽小于自己的最大結點（結點A）變更事件
*5.結點A拿到鎖，處理業務，處理完，釋放鎖（刪除自己）
*6.結點B收到結點A變更事件，判斷出來自己已經是最小結點號，于是獲得鎖。
*/
public class WriteLock extends ZkPrimative {
private static final Logger LOG = Logger.getLogger(WriteLock.class);
private final String dir;
private String id;
private LockNode idName;
private String ownerId;
private String lastChildId;
private byte[] data = {0x12, 0x34};
private LockListener callback;
public WriteLock(String dir,String schema) {
super(schema,true);
this.dir = dir;
}
public WriteLock(String dir,LockListener callback,String schema) {
this(dir,schema);
this.callback = callback;
}
public LockListener getLockListener() {
return this.callback;
}
public void setLockListener(LockListener callback) {
this.callback = callback;
}
public synchronized void unlock() throws RuntimeException {
if(zk == null || zk.isClosed()){
return;
}
if (id != null) {
try {
zk.delete(id, -1);
} catch (InterruptedException e) {
LOG.warn("Caught: " + e, e);
//set that we have been interrupted.
Thread.currentThread().interrupt();
} catch (KeeperException.NoNodeException e) {
// do nothing
} catch (KeeperException e) {
LOG.warn("Caught: " + e, e);
throw (RuntimeException) new RuntimeException(e.getMessage()).
initCause(e);
}finally {
if (callback != null) {
callback.lockReleased();
}
id = null;
}
}
}
private class LockWatcher implements Watcher {
public void process(WatchedEvent event) {
LOG.debug("Watcher fired on path: " + event.getPath() + " state: " +
event.getState() + " type " + event.getType());
try {
trylock();
} catch (Exception e) {
LOG.warn("Failed to acquire lock: " + e, e);
}
}
}
private void findPrefixInChildren(String prefix, ZooKeeper zookeeper, String dir)
throws KeeperException, InterruptedException {
List<String> names = zookeeper.getChildren(dir, false);
for (String name : names) {
if (name.startsWith(prefix)) {
id = dir + "/" + name;
if (LOG.isDebugEnabled()) {
LOG.debug("Found id created last time: " + id);
}
break;
}
}
if (id == null) {
id = zookeeper.create(dir + "/" + prefix, data,
acl, EPHEMERAL_SEQUENTIAL);
if (LOG.isDebugEnabled()) {
LOG.debug("Created id: " + id);
}
}
}
public void clear() {
if(zk == null || zk.isClosed()){
return;
}
try {
zk.delete(dir, -1);
} catch (Exception e) {
LOG.error("clear error: " + e,e);
}
}
public synchronized boolean trylock() throws KeeperException, InterruptedException {
if(zk == null){
LOG.info("zk 是空");
return false;
}
if (zk.isClosed()) {
LOG.info("zk 已經關閉");
return false;
}
ensurePathExists(dir);
LOG.debug("id:"+id);
do {
if (id == null) {
long sessionId = zk.getSessionId();
String prefix = "x-" + sessionId + "-";
idName = new LockNode(id);
LOG.debug("idName:"+idName);
}
if (id != null) {
List<String> names = zk.getChildren(dir, false);
if (names.isEmpty()) {
LOG.warn("No children in: " + dir + " when we've just " +
"created one! Lets recreate it...");
id = null;
} else {
SortedSet<LockNode> sortedNames = new TreeSet<LockNode>();
for (String name : names) {
sortedNames.add(new LockNode(dir + "/" + name));
}
ownerId = sortedNames.first().getName();
LOG.debug("all:"+sortedNames);
SortedSet<LockNode> lessThanMe = sortedNames.headSet(idName);
LOG.debug("less than me:"+lessThanMe);
if (!lessThanMe.isEmpty()) {
LockNode lastChildName = lessThanMe.last();
lastChildId = lastChildName.getName();
if (LOG.isDebugEnabled()) {
LOG.debug("watching less than me node: " + lastChildId);
}
Stat stat = zk.exists(lastChildId, new LockWatcher());
if (stat != null) {
return Boolean.FALSE;
} else {
LOG.warn("Could not find the" +
" stats for less than me: " + lastChildName.getName());
}
} else {
if (isOwner()) {
if (callback != null) {
callback.lockAcquired();
}
return Boolean.TRUE;
}
}
}
}
}
while (id == null);
return Boolean.FALSE;
}
public String getDir() {
return dir;
}
public boolean isOwner() {
return id != null && ownerId != null && id.equals(ownerId);
}
public String getId() {
return this.id;
}
}使用本方案實現的分布式鎖，可以很好地解決鎖重入的問題，而且使用會話結點來避免死鎖；性能方面，根據筆者自測結果，加鎖解鎖各一次算是一個操作，本方案實現的分布式鎖，TPS大概為2000-3000，性能比較一般
“zookeeper分布式鎖實現的方法是什么”的內容就介紹到這里了，感謝大家的閱讀。如果想了解更多行業相關的知識可以關注億速云網站，小編將為大家輸出更多高質量的實用文章！
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