Java中的几种锁：synchronized，ReentrantLock，ReentrantReadWriteLock已基本可以满足编程需求，但其粒度都太大，同一时刻只有一个线程能进入同步块，这对于某些高并发的场景并不适用。
下面来提供几个更细的粒度锁：
1. 分段锁
借鉴concurrentHashMap的分段思想，先生成一定数量的锁，具体使用的时候再根据key来返回对应的lock。这是几个实现里最简单，性能最高，也是最终被采用的锁策略，代码如下：
/**
* 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. 哈希锁
上述分段锁的基础上发展起来的第二种锁策略，目的是实现真正意义上的细粒度锁。每个哈希值不同的对象都能获得自己独立的锁。在测试中，在被锁住的代码执行速度飞快的情况下，效率比分段锁慢 30% 左右。如果有长耗时操作，感觉表现应该会更好。代码如下：
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. 弱引用锁
哈希锁因为引入的分段锁来保证锁创建和销毁的同步，总感觉有点瑕疵，所以写了第三个锁来寻求更好的性能和更细粒度的锁。这个锁的思想是借助java的弱引用来创建锁，把锁的销毁交给jvm的垃圾回收，来避免额外的消耗。
有点遗憾的是因为使用了ConcurrentHashMap作为锁的容器，所以没能真正意义上的摆脱分段锁。这个锁的性能比 HashLock 快10% 左右。锁代码：
/**
* 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.基于KEY（主键）的互斥锁
KeyLock是对所需处理的数据的KEY（主键）进行加锁，只要是对不同key操作，其就可以并行处理，大大提高了线程的并行度
KeyLock有如下几个特性：
1、细粒度，高并行性
2、可重入
3、公平锁
4、加锁开销比ReentrantLock大，适用于处理耗时长、key范围大的场景
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使用示例：
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);
}
}测试代码如下：
//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);
}
}
}
}测试结果：
（8线程对100个账户随机转账总共10000次）：
类型：NonLockRunner（不加锁）
耗时：2482ms
初始总金额：1000000
终止总金额：998906（无法保证原子性）
类型：SynchronizedRunner（加synchronized锁）
耗时：20872ms
初始总金额：1000000
终止总金额：1000000
类型：ReentrantLockRunner（加ReentrantLock锁）
耗时：21588ms
初始总金额：1000000
终止总金额：1000000
类型：KeyLockRunner（加KeyLock锁）
耗时：2831ms
初始总金额：1000000
终止总金额：1000000
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