完全 模仿ReentrantLock， 通过继承 java.util.concurrent.locks.Lock , 内置 AbstractQueuedSynchronizer 实现类，限制可以重入次数的锁。
这个其实不难， 只要稍微注意一点就好了
ReenterLimitedLock 关键实现：
/**
* l.k
*
* 限制可以重入次数的锁，默认是2次
*
*/
public class ReenterLimitedLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
private final Sync sync; /**
* Creates an instance of {@code ReenterLimitedLock}.
* This is equivalent to using {@code ReenterLimitedLock(false)}.
*/
public ReenterLimitedLock() {
sync = new NonfairSync(2);
} public ReenterLimitedLock(int reenterLimit) {
sync = new NonfairSync(reenterLimit);
} public ReenterLimitedLock(boolean fair, int reenterLimit) {
sync = fair ? new FairSync(reenterLimit) : new NonfairSync(reenterLimit);
} public ReenterLimitedLock(boolean fair) {
sync = fair ? new FairSync(2) : new NonfairSync(2);
} abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
private boolean exceeded;
private int reenterLimit; public Sync(int reenterLimit) {
this.reenterLimit = reenterLimit;
} abstract void lock(); final boolean nonfairTryAcquire(int acquires) throws MaximumLockCountExceededException {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
// if (nextc < 0) // overflow
if (nextc > reenterLimit) { // 关键
exceeded = true;
// return false;
// throw new IllegalStateException("Maximum lock count exceeded" + 2);
throw new MaximumLockCountExceededException(reenterLimit);
}
setState(nextc);
return true;
}
return false;
} protected final boolean tryRelease(int releases) {
if (isExceeded()) {
return false;
}
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
} protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
} final ConditionObject newCondition() {
return new ConditionObject();
} // Methods relayed from outer class final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
} final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
} final boolean isLocked() {
return getState() != 0;
} /**
* Reconstitutes the instance from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
} public boolean isExceeded() {
return exceeded;
} public void setExceeded(boolean exceeded) {
this.exceeded = exceeded;
} public int getReenterLimit() {
return reenterLimit;
} public void setReenterLimit(int reenterLimit) {
this.reenterLimit = reenterLimit;
} } /**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L; public NonfairSync(int reenterLimit) {
super(reenterLimit);
} /**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
} protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
} /**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L; public FairSync(int reenterLimit) {
super(reenterLimit);
} final void lock() {
acquire(1);
} /**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
// if (nextc < 0)
if (nextc > getReenterLimit()) // 这里是关键
throw new MaximumLockCountExceededException(getReenterLimit());
// throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
} public void lock() {
sync.lock();
} public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
} public boolean tryLock() {
return sync.nonfairTryAcquire(1);
} public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
} public void unlock() {
sync.release(1);
} public Condition newCondition() {
return sync.newCondition();
} public int getHoldCount() {
return sync.getHoldCount();
} public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
} public boolean isLocked() {
return sync.isLocked();
} public final boolean isFair() {
return sync instanceof FairSync;
} protected Thread getOwner() {
return sync.getOwner();
} public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
} public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
} public final int getQueueLength() {
return sync.getQueueLength();
} protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
} public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
} public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
} protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
} public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
}
MaximumLockCountExceededException 异常：
public class MaximumLockCountExceededException extends IllegalStateException {
//public class MaximumLockCountExceededException extends Exception {
public MaximumLockCountExceededException(String s) {
super(s);
}
public MaximumLockCountExceededException(int n) {
super("Maximum lock count exceeded: " + n);
}
}测试：
/**
* Created by L.k on 2018/11/29.
*/
public class Testaa { ReenterLimitedLock twiceLock = new ReenterLimitedLock(); public static void main(String[] args) {
ReenterLimitedLock twiceLock = new ReenterLimitedLock();
Testaa testaa = new Testaa(twiceLock);
testaa.aa("asf");
} public Testaa(ReenterLimitedLock twiceLock) {
this.twiceLock = twiceLock;
} public void aa(String aa) {
try {
twiceLock.lock();
Thread.sleep(1000);
System.out.println("aa = [" + aa + "]");
bb(aa);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
twiceLock.unlock();
}
} public void bb(String bb) {
try {
twiceLock.lock();
Thread.sleep(1000); System.out.println("bb = [" + bb + "]"); cc(bb); System.out.println("Testaa.bb"); } catch (InterruptedException e) {
e.printStackTrace();
} finally {
twiceLock.unlock();
}
} public void cc(String cc) {
try {
twiceLock.lock();
Thread.sleep(1000); System.out.println("cc = [" + cc + "]"); } catch (InterruptedException e) {
e.printStackTrace();
// } catch (InterruptedException | IllegalStateException e) {
} catch (MaximumLockCountExceededException e) { // 这里必须要 捕获 MaximumLockCountExceededException， 否则 bb 方法也会异常退出
// e.printStackTrace();
System.out.println("xxxx IllegalStateException = " + e);
} finally {
twiceLock.unlock();
}
}
}