Several locks in Java: synchronized, ReentrantLock, ReentrantReadWriteLock can basically meet programming needs, but their granularity is too large. Only one thread can enter the synchronized block at the same time, which is not suitable for some high-concurrency scenarios. .
The following provides several more fine-grained locks:
1. Segmented locks
Draw on the segmentation idea of concurrentHashMap, first generate a certain number of locks, and use them specifically Then return the corresponding lock based on the key. This is the simplest and highest performing among several implementations, and it is also the lock strategy that was finally adopted. The code is as follows:
/**
* 分段锁，系统提供一定数量的原始锁，根据传入对象的哈希值获取对应的锁并加锁
* 注意：要锁的对象的哈希值如果发生改变，有可能导致锁无法成功释放!!!
*/
public class SegmentLock<T> {
private Integer segments = 16;//默认分段数量
private final HashMap<Integer, ReentrantLock> lockMap = new HashMap<>();
public SegmentLock() {
init(null, false);
}
public SegmentLock(Integer counts, boolean fair) {
init(counts, fair);
}
private void init(Integer counts, boolean fair) {
if (counts != null) {
segments = counts;
}
for (int i = 0; i < segments; i++) {
lockMap.put(i, new ReentrantLock(fair));
}
}
public void lock(T key) {
ReentrantLock lock = lockMap.get(key.hashCode() % segments);
lock.lock();
}
public void unlock(T key) {
ReentrantLock lock = lockMap.get(key.hashCode() % segments);
lock.unlock();
}
}2. Hash lock
was developed on the basis of the above segmented lock. The second lock strategy aims to achieve true fine-grained locking. Each object with a different hash value can obtain its own independent lock. In tests, when the locked code executed very quickly, the efficiency was about 30% slower than segmented locking. If there are long-term operations, it feels like the performance should be better. The code is as follows:
public class HashLock<T> {
private boolean isFair = false;
private final SegmentLock<T> segmentLock = new SegmentLock<>();//分段锁
private final ConcurrentHashMap<T, LockInfo> lockMap = new ConcurrentHashMap<>();
public HashLock() {
}
public HashLock(boolean fair) {
isFair = fair;
}
public void lock(T key) {
LockInfo lockInfo;
segmentLock.lock(key);
try {
lockInfo = lockMap.get(key);
if (lockInfo == null) {
lockInfo = new LockInfo(isFair);
lockMap.put(key, lockInfo);
} else {
lockInfo.count.incrementAndGet();
}
} finally {
segmentLock.unlock(key);
}
lockInfo.lock.lock();
}
public void unlock(T key) {
LockInfo lockInfo = lockMap.get(key);
if (lockInfo.count.get() == 1) {
segmentLock.lock(key);
try {
if (lockInfo.count.get() == 1) {
lockMap.remove(key);
}
} finally {
segmentLock.unlock(key);
}
}
lockInfo.count.decrementAndGet();
lockInfo.unlock();
}
private static class LockInfo {
public ReentrantLock lock;
public AtomicInteger count = new AtomicInteger(1);
private LockInfo(boolean fair) {
this.lock = new ReentrantLock(fair);
}
public void lock() {
this.lock.lock();
}
public void unlock() {
this.lock.unlock();
}
}
}3. Weak reference lock
The hash lock always feels a bit flawed because of the segmented lock introduced to ensure the synchronization of lock creation and destruction, so I wrote the third one. locks to seek better performance and more fine-grained locks. The idea of this lock is to use Java's weak reference to create the lock, and hand over the destruction of the lock to the jvm's garbage collection to avoid additional consumption.
It is a pity that because ConcurrentHashMap is used as the lock container, it cannot really get rid of the segmentation lock. The performance of this lock is about 10% faster than HashLock. Lock code:
/**
* 弱引用锁，为每个独立的哈希值提供独立的锁功能
*/
public class WeakHashLock<T> {
private ConcurrentHashMap<T, WeakLockRef<T, ReentrantLock>> lockMap = new ConcurrentHashMap<>();
private ReferenceQueue<ReentrantLock> queue = new ReferenceQueue<>();
public ReentrantLock get(T key) {
if (lockMap.size() > 1000) {
clearEmptyRef();
}
WeakReference<ReentrantLock> lockRef = lockMap.get(key);
ReentrantLock lock = (lockRef == null ? null : lockRef.get());
while (lock == null) {
lockMap.putIfAbsent(key, new WeakLockRef<>(new ReentrantLock(), queue, key));
lockRef = lockMap.get(key);
lock = (lockRef == null ? null : lockRef.get());
if (lock != null) {
return lock;
}
clearEmptyRef();
}
return lock;
}
@SuppressWarnings("unchecked")
private void clearEmptyRef() {
Reference<? extends ReentrantLock> ref;
while ((ref = queue.poll()) != null) {
WeakLockRef<T, ? extends ReentrantLock> weakLockRef = (WeakLockRef<T, ? extends ReentrantLock>) ref;
lockMap.remove(weakLockRef.key);
}
}
private static final class WeakLockRef<T, K> extends WeakReference<K> {
final T key;
private WeakLockRef(K referent, ReferenceQueue<? super K> q, T key) {
super(referent, q);
this.key = key;
}
}
}4. Mutex lock based on KEY (primary key)
KeyLock is performed on the KEY (primary key) of the data to be processed Locking, as long as it operates on different keys, can be processed in parallel, greatly improving the parallelism of threads
KeyLock has the following characteristics ：
1. Fine-grained, high parallelism
2. Reentrant
3. Fair lock
4. The locking overhead is larger than ReentrantLock, suitable for long processing time and key range Large scene
public class KeyLock<K> {
// 保存所有锁定的KEY及其信号量
private final ConcurrentMap<K, Semaphore> map = new ConcurrentHashMap<K, Semaphore>();
// 保存每个线程锁定的KEY及其锁定计数
private final ThreadLocal<Map<K, LockInfo>> local = new ThreadLocal<Map<K, LockInfo>>() {
@Override
protected Map<K, LockInfo> initialValue() {
return new HashMap<K, LockInfo>();
}
};
/**
* 锁定key，其他等待此key的线程将进入等待，直到调用{@link #unlock(K)}
* 使用hashcode和equals来判断key是否相同，因此key必须实现{@link #hashCode()}和
* {@link #equals(Object)}方法
*
* @param key
*/
public void lock(K key) {
if (key == null)
return;
LockInfo info = local.get().get(key);
if (info == null) {
Semaphore current = new Semaphore(1);
current.acquireUninterruptibly();
Semaphore previous = map.put(key, current);
if (previous != null)
previous.acquireUninterruptibly();
local.get().put(key, new LockInfo(current));
} else {
info.lockCount++;
}
}
/**
* 释放key，唤醒其他等待此key的线程
* @param key
*/
public void unlock(K key) {
if (key == null)
return;
LockInfo info = local.get().get(key);
if (info != null && --info.lockCount == 0) {
info.current.release();
map.remove(key, info.current);
local.get().remove(key);
}
}
/**
* 锁定多个key
* 建议在调用此方法前先对keys进行排序，使用相同的锁定顺序，防止死锁发生
* @param keys
*/
public void lock(K[] keys) {
if (keys == null)
return;
for (K key : keys) {
lock(key);
}
}
/**
* 释放多个key
* @param keys
*/
public void unlock(K[] keys) {
if (keys == null)
return;
for (K key : keys) {
unlock(key);
}
}
private static class LockInfo {
private final Semaphore current;
private int lockCount;
private LockInfo(Semaphore current) {
this.current = current;
this.lockCount = 1;
}
}
}KeyLock usage example:
private int[] accounts;
private KeyLock<Integer> lock = new KeyLock<Integer>();
public boolean transfer(int from, int to, int money) {
Integer[] keys = new Integer[] {from, to};
Arrays.sort(keys); //对多个key进行排序，保证锁定顺序防止死锁
lock.lock(keys);
try {
//处理不同的from和to的线程都可进入此同步块
if (accounts[from] < money)
return false;
accounts[from] -= money;
accounts[to] += money;
return true;
} finally {
lock.unlock(keys);
}
}The test code is as follows:
//场景：多线程并发转账
public class Test {
private final int[] account; // 账户数组，其索引为账户ID，内容为金额
public Test(int count, int money) {
account = new int[count];
Arrays.fill(account, money);
}
boolean transfer(int from, int to, int money) {
if (account[from] < money)
return false;
account[from] -= money;
try {
Thread.sleep(2);
} catch (Exception e) {
}
account[to] += money;
return true;
}
int getAmount() {
int result = 0;
for (int m : account)
result += m;
return result;
}
public static void main(String[] args) throws Exception {
int count = 100; //账户个数
int money = 10000; //账户初始金额
int threadNum = 8; //转账线程数
int number = 10000; //转账次数
int maxMoney = 1000; //随机转账最大金额
Test test = new Test(count, money);
//不加锁
// Runner runner = test.new NonLockRunner(maxMoney, number);
//加synchronized锁
// Runner runner = test.new SynchronizedRunner(maxMoney, number);
//加ReentrantLock锁
// Runner runner = test.new ReentrantLockRunner(maxMoney, number);
//加KeyLock锁
Runner runner = test.new KeyLockRunner(maxMoney, number);
Thread[] threads = new Thread[threadNum];
for (int i = 0; i < threadNum; i++)
threads[i] = new Thread(runner, "thread-" + i);
long begin = System.currentTimeMillis();
for (Thread t : threads)
t.start();
for (Thread t : threads)
t.join();
long time = System.currentTimeMillis() - begin;
System.out.println("类型：" + runner.getClass().getSimpleName());
System.out.printf("耗时：%dms\n", time);
System.out.printf("初始总金额：%d\n", count * money);
System.out.printf("终止总金额：%d\n", test.getAmount());
}
// 转账任务
abstract class Runner implements Runnable {
final int maxMoney;
final int number;
private final Random random = new Random();
private final AtomicInteger count = new AtomicInteger();
Runner(int maxMoney, int number) {
this.maxMoney = maxMoney;
this.number = number;
}
@Override
public void run() {
while(count.getAndIncrement() < number) {
int from = random.nextInt(account.length);
int to;
while ((to = random.nextInt(account.length)) == from)
;
int money = random.nextInt(maxMoney);
doTransfer(from, to, money);
}
}
abstract void doTransfer(int from, int to, int money);
}
// 不加锁的转账
class NonLockRunner extends Runner {
NonLockRunner(int maxMoney, int number) {
super(maxMoney, number);
}
@Override
void doTransfer(int from, int to, int money) {
transfer(from, to, money);
}
}
// synchronized的转账
class SynchronizedRunner extends Runner {
SynchronizedRunner(int maxMoney, int number) {
super(maxMoney, number);
}
@Override
synchronized void doTransfer(int from, int to, int money) {
transfer(from, to, money);
}
}
// ReentrantLock的转账
class ReentrantLockRunner extends Runner {
private final ReentrantLock lock = new ReentrantLock();
ReentrantLockRunner(int maxMoney, int number) {
super(maxMoney, number);
}
@Override
void doTransfer(int from, int to, int money) {
lock.lock();
try {
transfer(from, to, money);
} finally {
lock.unlock();
}
}
}
// KeyLock的转账
class KeyLockRunner extends Runner {
private final KeyLock<Integer> lock = new KeyLock<Integer>();
KeyLockRunner(int maxMoney, int number) {
super(maxMoney, number);
}
@Override
void doTransfer(int from, int to, int money) {
Integer[] keys = new Integer[] {from, to};
Arrays.sort(keys);
lock.lock(keys);
try {
transfer(from, to, money);
} finally {
lock.unlock(keys);
}
}
}
}Test result:
(8 threads A total of 10,000 random transfers to 100 accounts):
Type: NonLockRunner (unlocked)
Time consuming: 2482ms
Initial total amount: 1000000
Total termination amount: 998906 (unable to Guaranteed atomicity)
Type: SynchronizedRunner (plus synchronized lock)
Time consuming: 20872ms
Initial total amount: 1000000
Total termination amount: 1000000
Type: ReentrantLockRunner (Add ReentrantLock lock)
Time consumption: 21588ms
Initial total amount: 1000000
Total termination amount: 1000000
Type: KeyLockRunner (Add KeyLock lock)
Time consumption: 2831ms
Initial total amount: 1000000
Total termination amount: 1000000
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