Java中的幾種鎖:synchronized,ReentrantLock,ReentrantReadWriteLock基本上可以滿足程式需求,但其粒度都太大,同一時刻只有一個執行緒能進入同步區塊,這對於某些高並發的場景並不適用。
下面來提供幾個更細的粒度鎖:
1. 分段鎖
借鑒concurrentHashMap的分段思想,先生成一定數量的鎖,具體使用的時候再根據key來返回對應的lock。這是幾個實作裡最簡單,效能最高,也是最終被採用的鎖策略,程式碼如下:
/** * 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. 哈希鎖
上述分段鎖的基礎上發展起來的第二種鎖策略,目的是實現真正意義上的細粒度鎖。每個哈希值不同的物件都能獲得自己獨立的鎖。在測試中,在被鎖定的程式碼執行速度快速的情況下,效率比分段鎖定慢 30% 左右。如果有長耗時操作,感覺表現應該會更好。程式碼如下:
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. 弱引用鎖
哈希鎖因為引入的分段鎖來保證鎖創建和銷毀的同步,總感覺有點瑕疵,所以寫了第三個鎖來尋求更好的性能和更細粒度的鎖。這個鎖的想法是藉助java的弱引用來創建鎖,把鎖的銷毀交給jvm的垃圾回收,來避免額外的消耗。
有點遺憾的是因為使用了ConcurrentHashMap作為鎖的容器,所以沒能真正意義上的擺脫分段鎖。這個鎖的效能比 HashLock 快10% 左右。鎖碼:
/** * 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.基於KEY(主鍵)的互斥鎖
KeyLock是對所需處理的資料的KEY(主鍵)進行加鎖,只要是對不同key操作,其就可以並行處理,大幅提升了執行緒的平行度
KeyLock有以下幾個特性:
1、細粒度,高並行性
比ReentrantLock大,適用於處理耗時長、key範圍大的場景
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使用範例: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);
}
}
測試程式碼如下:
//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); } } } }
測試結果:
次): 類型:NonLockRunner(未加鎖)耗時:2482ms
耗時:2482ms (無法保證原子性)
型態:SynchronizedRunner(加上synchronized鎖定)
耗時: 20872ms
初始總金額:1000000
終止總金額:1000000
耗時:21588ms
初始總金額:1000000
以終止者:1000000
終止總金額:1000000
耗時:2831ms
初始總金額:1000000