# ReentrantReadWriteLock

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  
Summary: [Nested Classes](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#nestedclasses) \| [Ctors](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#pubctors) \| [Methods](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#pubmethods) \| [Protected Methods](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#promethods) \| [Inherited Methods](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#inhmethods)  

# ReentrantReadWriteLock


`
public


class
ReentrantReadWriteLock
`


`

extends `[Object](https://developer.android.com/reference/java/lang/Object)`


`

`


implements

`[ReadWriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReadWriteLock)`,

`[Serializable](https://developer.android.com/reference/java/io/Serializable)`


`

|---|---------------------------------------------------|
| [java.lang.Object](https://developer.android.com/reference/java/lang/Object) ||
| ↳ | java.util.concurrent.locks.ReentrantReadWriteLock |

<br />

*** ** * ** ***

An implementation of [ReadWriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReadWriteLock) supporting similar
semantics to [ReentrantLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantLock).

This class has the following properties:

- **Acquisition order**

  This class does not impose a reader or writer preference
  ordering for lock access. However, it does support an optional
  *fairness* policy.

  ***Non-fair mode (default)***
  :   When constructed as non-fair (the default), the order of entry
      to the read and write lock is unspecified, subject to reentrancy
      constraints. A nonfair lock that is continuously contended may
      indefinitely postpone one or more reader or writer threads, but
      will normally have higher throughput than a fair lock.

  ***Fair mode***

  :   When constructed as fair, threads contend for entry using an approximately arrival-order policy. When the currently held lock is released, either the longest-waiting single writer thread will be assigned the write lock, or if there is a group of reader threads waiting longer than all waiting writer threads, that group will be assigned the read lock. A thread that tries to acquire a fair read lock (non-reentrantly)
      will block if either the write lock is held, or there is a waiting
      writer thread. The thread will not acquire the read lock until
      after the oldest currently waiting writer thread has acquired and
      released the write lock. Of course, if a waiting writer abandons
      its wait, leaving one or more reader threads as the longest waiters
      in the queue with the write lock free, then those readers will be
      assigned the read lock.

      A thread that tries to acquire a fair write lock (non-reentrantly)
      will block unless both the read lock and write lock are free (which
      implies there are no waiting threads). (Note that the non-blocking
      [ReadLock.tryLock()](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.ReadLock#tryLock()) and [WriteLock.tryLock()](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock#tryLock()) methods
      do not honor this fair setting and will immediately acquire the lock
      if it is possible, regardless of waiting threads.)
- **Reentrancy**

  This lock allows both readers and writers to reacquire read or
  write locks in the style of a [ReentrantLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantLock). Non-reentrant
  readers are not allowed until all write locks held by the writing
  thread have been released.

  Additionally, a writer can acquire the read lock, but not
  vice-versa. Among other applications, reentrancy can be useful
  when write locks are held during calls or callbacks to methods that
  perform reads under read locks. If a reader tries to acquire the
  write lock it will never succeed.

- **Lock downgrading**

  Reentrancy also allows downgrading from the write lock to a read lock,
  by acquiring the write lock, then the read lock and then releasing the
  write lock. However, upgrading from a read lock to the write lock is
  **not** possible.

- **Interruption of lock acquisition**

  The read lock and write lock both support interruption during lock
  acquisition.

- **[Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition) support**

  The write lock provides a [Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition) implementation that
  behaves in the same way, with respect to the write lock, as the
  [Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition) implementation provided by
  [ReentrantLock.newCondition](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantLock#newCondition()) does for [ReentrantLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantLock).
  This [Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition) can, of course, only be used with the write lock.

  The read lock does not support a [Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition) and
  `readLock().newCondition()` throws
  `UnsupportedOperationException`.

- **Instrumentation**

  This class supports methods to determine whether locks
  are held or contended. These methods are designed for monitoring
  system state, not for synchronization control.

Serialization of this class behaves in the same way as built-in
locks: a deserialized lock is in the unlocked state, regardless of
its state when serialized.

**Sample usages.** Here is a code sketch showing how to perform
lock downgrading after updating a cache (exception handling is
particularly tricky when handling multiple locks in a non-nested
fashion):

     class CachedData {
       Object data;
       boolean cacheValid;
       final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

       void processCachedData() {
         rwl.readLock().lock();
         if (!cacheValid) {
           // Must release read lock before acquiring write lock
           rwl.readLock().unlock();
           rwl.writeLock().lock();
           try {
             // Recheck state because another thread might have
             // acquired write lock and changed state before we did.
             if (!cacheValid) {
               data = ...;
               cacheValid = true;
             }
             // Downgrade by acquiring read lock before releasing write lock
             rwl.readLock().lock();
           } finally {
             rwl.writeLock().unlock(); // Unlock write, still hold read
           }
         }

         try {
           use(data);
         } finally {
           rwl.readLock().unlock();
         }
       }
     }

ReentrantReadWriteLocks can be used to improve concurrency in some uses of some kinds of Collections. This is typically worthwhile only when the collections are expected to be large, accessed by more reader threads than writer threads, and entail operations with overhead that outweighs synchronization overhead. For example, here is a class using a TreeMap that is expected to be large and concurrently accessed.  

     class RWDictionary {
       private final Map<String, Data> m = new TreeMap<>();
       private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
       private final Lock r = rwl.readLock();
       private final Lock w = rwl.writeLock();

       public Data get(String key) {
         r.lock();
         try { return m.get(key); }
         finally { r.unlock(); }
       }
       public List<String> allKeys() {
         r.lock();
         try { return new ArrayList<>(m.keySet()); }
         finally { r.unlock(); }
       }
       public Data put(String key, Data value) {
         w.lock();
         try { return m.put(key, value); }
         finally { w.unlock(); }
       }
       public void clear() {
         w.lock();
         try { m.clear(); }
         finally { w.unlock(); }
       }
     }

## Implementation Notes

This lock supports a maximum of 65535 recursive write locks
and 65535 read locks. Attempts to exceed these limits result in
[Error](https://developer.android.com/reference/java/lang/Error) throws from locking methods.

## Summary

|                                                                                                                                                    ### Nested classes                                                                                                                                                    ||
|----------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ` class` | [ReentrantReadWriteLock.ReadLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.ReadLock) The lock returned by method [ReentrantReadWriteLock.readLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#readLock()).     |
| ` class` | [ReentrantReadWriteLock.WriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock) The lock returned by method [ReentrantReadWriteLock.writeLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#writeLock()). |

|                                                                                                              ### Public constructors                                                                                                              ||
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---|
| ` `[ReentrantReadWriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#ReentrantReadWriteLock())`() ` Creates a new `ReentrantReadWriteLock` with default (nonfair) ordering properties.        |
| ` `[ReentrantReadWriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#ReentrantReadWriteLock(boolean))`(boolean fair) ` Creates a new `ReentrantReadWriteLock` with the given fairness policy. |

|                                                                                                                                                                                                                                                              ### Public methods                                                                                                                                                                                                                                                               ||
|--------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ` final int`                                                                                                                               | ` `[getQueueLength](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getQueueLength())`() ` Returns an estimate of the number of threads waiting to acquire either the read or write lock.                                                                                                                                                               |
| ` int`                                                                                                                                     | ` `[getReadHoldCount](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getReadHoldCount())`() ` Queries the number of reentrant read holds on this lock by the current thread.                                                                                                                                                                           |
| ` int`                                                                                                                                     | ` `[getReadLockCount](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getReadLockCount())`() ` Queries the number of read locks held for this lock.                                                                                                                                                                                                     |
| ` int`                                                                                                                                     | ` `[getWaitQueueLength](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getWaitQueueLength(java.util.concurrent.locks.Condition))`(`[Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition)` condition) ` Returns an estimate of the number of threads waiting on the given condition associated with the write lock. |
| ` int`                                                                                                                                     | ` `[getWriteHoldCount](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getWriteHoldCount())`() ` Queries the number of reentrant write holds on this lock by the current thread.                                                                                                                                                                        |
| ` final boolean`                                                                                                                           | ` `[hasQueuedThread](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#hasQueuedThread(java.lang.Thread))`(`[Thread](https://developer.android.com/reference/java/lang/Thread)` thread) ` Queries whether the given thread is waiting to acquire either the read or write lock.                                                                           |
| ` final boolean`                                                                                                                           | ` `[hasQueuedThreads](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#hasQueuedThreads())`() ` Queries whether any threads are waiting to acquire the read or write lock.                                                                                                                                                                               |
| ` boolean`                                                                                                                                 | ` `[hasWaiters](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#hasWaiters(java.util.concurrent.locks.Condition))`(`[Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition)` condition) ` Queries whether any threads are waiting on the given condition associated with the write lock.                              |
| ` final boolean`                                                                                                                           | ` `[isFair](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#isFair())`() ` Returns `true` if this lock has fairness set true.                                                                                                                                                                                                                           |
| ` boolean`                                                                                                                                 | ` `[isWriteLocked](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#isWriteLocked())`() ` Queries if the write lock is held by any thread.                                                                                                                                                                                                               |
| ` boolean`                                                                                                                                 | ` `[isWriteLockedByCurrentThread](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#isWriteLockedByCurrentThread())`() ` Queries if the write lock is held by the current thread.                                                                                                                                                                         |
| ` `[ReentrantReadWriteLock.ReadLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.ReadLock)   | ` `[readLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#readLock())`() ` Returns the lock used for reading.                                                                                                                                                                                                                                       |
| ` `[String](https://developer.android.com/reference/java/lang/String)                                                                      | ` `[toString](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#toString())`() ` Returns a string identifying this lock, as well as its lock state.                                                                                                                                                                                                       |
| ` `[ReentrantReadWriteLock.WriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock) | ` `[writeLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#writeLock())`() ` Returns the lock used for writing.                                                                                                                                                                                                                                     |

|                                                                                                                                                                                                                                                                        ### Protected methods                                                                                                                                                                                                                                                                        ||
|-------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ` `[Thread](https://developer.android.com/reference/java/lang/Thread)                                                                                 | ` `[getOwner](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getOwner())`() ` Returns the thread that currently owns the write lock, or `null` if not owned.                                                                                                                                                                                                      |
| ` `[Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | ` `[getQueuedReaderThreads](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getQueuedReaderThreads())`() ` Returns a collection containing threads that may be waiting to acquire the read lock.                                                                                                                                                                   |
| ` `[Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | ` `[getQueuedThreads](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getQueuedThreads())`() ` Returns a collection containing threads that may be waiting to acquire either the read or write lock.                                                                                                                                                               |
| ` `[Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | ` `[getQueuedWriterThreads](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getQueuedWriterThreads())`() ` Returns a collection containing threads that may be waiting to acquire the write lock.                                                                                                                                                                  |
| ` `[Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | ` `[getWaitingThreads](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock#getWaitingThreads(java.util.concurrent.locks.Condition))`(`[Condition](https://developer.android.com/reference/java/util/concurrent/locks/Condition)` condition) ` Returns a collection containing those threads that may be waiting on the given condition associated with the write lock. |

| ### Inherited methods |
|-----------------------|---|
| From class ` `[java.lang.Object](https://developer.android.com/reference/java/lang/Object)` ` |--------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` `[Object](https://developer.android.com/reference/java/lang/Object)          | ` `[clone](https://developer.android.com/reference/java/lang/Object#clone())`() ` Creates and returns a copy of this object.                                                                                                                                                        | | ` boolean`                                                                     | ` `[equals](https://developer.android.com/reference/java/lang/Object#equals(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` obj) ` Indicates whether some other object is "equal to" this one.                                             | | ` void`                                                                        | ` `[finalize](https://developer.android.com/reference/java/lang/Object#finalize())`() ` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.                                                            | | ` final `[Class](https://developer.android.com/reference/java/lang/Class)`<?>` | ` `[getClass](https://developer.android.com/reference/java/lang/Object#getClass())`() ` Returns the runtime class of this `Object`.                                                                                                                                                 | | ` int`                                                                         | ` `[hashCode](https://developer.android.com/reference/java/lang/Object#hashCode())`() ` Returns a hash code value for the object.                                                                                                                                                   | | ` final void`                                                                  | ` `[notify](https://developer.android.com/reference/java/lang/Object#notify())`() ` Wakes up a single thread that is waiting on this object's monitor.                                                                                                                              | | ` final void`                                                                  | ` `[notifyAll](https://developer.android.com/reference/java/lang/Object#notifyAll())`() ` Wakes up all threads that are waiting on this object's monitor.                                                                                                                           | | ` `[String](https://developer.android.com/reference/java/lang/String)          | ` `[toString](https://developer.android.com/reference/java/lang/Object#toString())`() ` Returns a string representation of the object.                                                                                                                                              | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long,%20int))`(long timeoutMillis, int nanos) ` Causes the current thread to wait until it is awakened, typically by being *notified* or *interrupted*, or until a certain amount of real time has elapsed. | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long))`(long timeoutMillis) ` Causes the current thread to wait until it is awakened, typically by being *notified* or *interrupted*, or until a certain amount of real time has elapsed.                   | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait())`() ` Causes the current thread to wait until it is awakened, typically by being *notified* or *interrupted*.                                                                                             | ||
| From interface ` `[java.util.concurrent.locks.ReadWriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReadWriteLock)` ` |---------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------| | ` abstract `[Lock](https://developer.android.com/reference/java/util/concurrent/locks/Lock) | ` `[readLock](https://developer.android.com/reference/java/util/concurrent/locks/ReadWriteLock#readLock())`() ` Returns the lock used for reading.   | | ` abstract `[Lock](https://developer.android.com/reference/java/util/concurrent/locks/Lock) | ` `[writeLock](https://developer.android.com/reference/java/util/concurrent/locks/ReadWriteLock#writeLock())`() ` Returns the lock used for writing. | ||

## Public constructors

### ReentrantReadWriteLock

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public ReentrantReadWriteLock ()
```

Creates a new `ReentrantReadWriteLock` with
default (nonfair) ordering properties.

<br />

### ReentrantReadWriteLock

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public ReentrantReadWriteLock (boolean fair)
```

Creates a new `ReentrantReadWriteLock` with
the given fairness policy.

<br />

|                                   Parameters                                    ||
|--------|-------------------------------------------------------------------------|
| `fair` | `boolean`: `true` if this lock should use a fair ordering policy <br /> |

## Public methods

### getQueueLength

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public final int getQueueLength ()
```

Returns an estimate of the number of threads waiting to acquire
either the read or write lock. The value is only an estimate
because the number of threads may change dynamically while this
method traverses internal data structures. This method is
designed for use in monitoring system state, not for
synchronization control.

<br />

|                               Returns                               ||
|-------|--------------------------------------------------------------|
| `int` | the estimated number of threads waiting for this lock <br /> |

### getReadHoldCount

Added in [API level 9](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public int getReadHoldCount ()
```

Queries the number of reentrant read holds on this lock by the
current thread. A reader thread has a hold on a lock for
each lock action that is not matched by an unlock action.

<br />

|                                                               Returns                                                                ||
|-------|-------------------------------------------------------------------------------------------------------------------------------|
| `int` | the number of holds on the read lock by the current thread, or zero if the read lock is not held by the current thread <br /> |

### getReadLockCount

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public int getReadLockCount ()
```

Queries the number of read locks held for this lock. This
method is designed for use in monitoring system state, not for
synchronization control.

<br />

|                   Returns                   ||
|-------|--------------------------------------|
| `int` | the number of read locks held <br /> |

### getWaitQueueLength

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public int getWaitQueueLength (Condition condition)
```

Returns an estimate of the number of threads waiting on the
given condition associated with the write lock. Note that because
timeouts and interrupts may occur at any time, the estimate
serves only as an upper bound on the actual number of waiters.
This method is designed for use in monitoring of the system
state, not for synchronization control.

<br />

|                   Parameters                   ||
|-------------|-----------------------------------|
| `condition` | `Condition`: the condition <br /> |

|                        Returns                        ||
|-------|------------------------------------------------|
| `int` | the estimated number of waiting threads <br /> |

|                                                                                 Throws                                                                                  ||
|----------------------------------------------------------------------------------------------------------------|---------------------------------------------------------|
| [IllegalMonitorStateException](https://developer.android.com/reference/java/lang/IllegalMonitorStateException) | if this lock is not held                                |
| [IllegalArgumentException](https://developer.android.com/reference/java/lang/IllegalArgumentException)         | if the given condition is not associated with this lock |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                 | if the condition is null                                |

### getWriteHoldCount

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public int getWriteHoldCount ()
```

Queries the number of reentrant write holds on this lock by the
current thread. A writer thread has a hold on a lock for
each lock action that is not matched by an unlock action.

<br />

|                                                                Returns                                                                 ||
|-------|---------------------------------------------------------------------------------------------------------------------------------|
| `int` | the number of holds on the write lock by the current thread, or zero if the write lock is not held by the current thread <br /> |

### hasQueuedThread

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public final boolean hasQueuedThread (Thread thread)
```

Queries whether the given thread is waiting to acquire either
the read or write lock. Note that because cancellations may
occur at any time, a `true` return does not guarantee
that this thread will ever acquire a lock. This method is
designed primarily for use in monitoring of the system state.

<br />

|              Parameters               ||
|----------|-----------------------------|
| `thread` | `Thread`: the thread <br /> |

|                                   Returns                                    ||
|-----------|-------------------------------------------------------------------|
| `boolean` | `true` if the given thread is queued waiting for this lock <br /> |

|                                                        Throws                                                         ||
|------------------------------------------------------------------------------------------------|-----------------------|
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the thread is null |

### hasQueuedThreads

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public final boolean hasQueuedThreads ()
```

Queries whether any threads are waiting to acquire the read or
write lock. Note that because cancellations may occur at any
time, a `true` return does not guarantee that any other
thread will ever acquire a lock. This method is designed
primarily for use in monitoring of the system state.

<br />

|                                      Returns                                       ||
|-----------|-------------------------------------------------------------------------|
| `boolean` | `true` if there may be other threads waiting to acquire the lock <br /> |

### hasWaiters

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public boolean hasWaiters (Condition condition)
```

Queries whether any threads are waiting on the given condition
associated with the write lock. Note that because timeouts and
interrupts may occur at any time, a `true` return does
not guarantee that a future `signal` will awaken any
threads. This method is designed primarily for use in
monitoring of the system state.

<br />

|                   Parameters                   ||
|-------------|-----------------------------------|
| `condition` | `Condition`: the condition <br /> |

|                          Returns                          ||
|-----------|------------------------------------------------|
| `boolean` | `true` if there are any waiting threads <br /> |

|                                                                                 Throws                                                                                  ||
|----------------------------------------------------------------------------------------------------------------|---------------------------------------------------------|
| [IllegalMonitorStateException](https://developer.android.com/reference/java/lang/IllegalMonitorStateException) | if this lock is not held                                |
| [IllegalArgumentException](https://developer.android.com/reference/java/lang/IllegalArgumentException)         | if the given condition is not associated with this lock |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                 | if the condition is null                                |

### isFair

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public final boolean isFair ()
```

Returns `true` if this lock has fairness set true.

<br />

|                           Returns                           ||
|-----------|--------------------------------------------------|
| `boolean` | `true` if this lock has fairness set true <br /> |

### isWriteLocked

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public boolean isWriteLocked ()
```

Queries if the write lock is held by any thread. This method is
designed for use in monitoring system state, not for
synchronization control.

<br />

|                                      Returns                                      ||
|-----------|------------------------------------------------------------------------|
| `boolean` | `true` if any thread holds the write lock and `false` otherwise <br /> |

### isWriteLockedByCurrentThread

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public boolean isWriteLockedByCurrentThread ()
```

Queries if the write lock is held by the current thread.

<br />

|                                          Returns                                          ||
|-----------|--------------------------------------------------------------------------------|
| `boolean` | `true` if the current thread holds the write lock and `false` otherwise <br /> |

### readLock

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public ReentrantReadWriteLock.ReadLock readLock ()
```

Returns the lock used for reading.

<br />

|                                                                                 Returns                                                                                 ||
|---------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [ReentrantReadWriteLock.ReadLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.ReadLock) | the lock used for reading <br /> |

### toString

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public String toString ()
```

Returns a string identifying this lock, as well as its lock state.
The state, in brackets, includes the String `"Write locks ="`
followed by the number of reentrantly held write locks, and the
String `"Read locks ="` followed by the number of held
read locks.

<br />

|                                                               Returns                                                                ||
|--------------------------------------------------------------------|------------------------------------------------------------------|
| [String](https://developer.android.com/reference/java/lang/String) | a string identifying this lock, as well as its lock state <br /> |

### writeLock

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public ReentrantReadWriteLock.WriteLock writeLock ()
```

Returns the lock used for writing.

<br />

|                                                                                  Returns                                                                                  ||
|-----------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [ReentrantReadWriteLock.WriteLock](https://developer.android.com/reference/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock) | the lock used for writing <br /> |

## Protected methods

### getOwner

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
protected Thread getOwner ()
```

Returns the thread that currently owns the write lock, or
`null` if not owned. When this method is called by a
thread that is not the owner, the return value reflects a
best-effort approximation of current lock status. For example,
the owner may be momentarily `null` even if there are
threads trying to acquire the lock but have not yet done so.
This method is designed to facilitate construction of
subclasses that provide more extensive lock monitoring
facilities.

<br />

|                                                   Returns                                                    ||
|--------------------------------------------------------------------|------------------------------------------|
| [Thread](https://developer.android.com/reference/java/lang/Thread) | the owner, or `null` if not owned <br /> |

### getQueuedReaderThreads

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
protected Collection<Thread> getQueuedReaderThreads ()
```

Returns a collection containing threads that may be waiting to
acquire the read lock. Because the actual set of threads may
change dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order. This method is
designed to facilitate construction of subclasses that provide
more extensive lock monitoring facilities.

<br />

|                                                                                       Returns                                                                                        ||
|----------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | the collection of threads <br /> |

### getQueuedThreads

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
protected Collection<Thread> getQueuedThreads ()
```

Returns a collection containing threads that may be waiting to
acquire either the read or write lock. Because the actual set
of threads may change dynamically while constructing this
result, the returned collection is only a best-effort estimate.
The elements of the returned collection are in no particular
order. This method is designed to facilitate construction of
subclasses that provide more extensive monitoring facilities.

<br />

|                                                                                       Returns                                                                                        ||
|----------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | the collection of threads <br /> |

### getQueuedWriterThreads

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
protected Collection<Thread> getQueuedWriterThreads ()
```

Returns a collection containing threads that may be waiting to
acquire the write lock. Because the actual set of threads may
change dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order. This method is
designed to facilitate construction of subclasses that provide
more extensive lock monitoring facilities.

<br />

|                                                                                       Returns                                                                                        ||
|----------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | the collection of threads <br /> |

### getWaitingThreads

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
protected Collection<Thread> getWaitingThreads (Condition condition)
```

Returns a collection containing those threads that may be
waiting on the given condition associated with the write lock.
Because the actual set of threads may change dynamically while
constructing this result, the returned collection is only a
best-effort estimate. The elements of the returned collection
are in no particular order. This method is designed to
facilitate construction of subclasses that provide more
extensive condition monitoring facilities.

<br />

|                   Parameters                   ||
|-------------|-----------------------------------|
| `condition` | `Condition`: the condition <br /> |

|                                                                                       Returns                                                                                        ||
|----------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------|
| [Collection](https://developer.android.com/reference/java/util/Collection)`<`[Thread](https://developer.android.com/reference/java/lang/Thread)`>` | the collection of threads <br /> |

|                                                                                 Throws                                                                                  ||
|----------------------------------------------------------------------------------------------------------------|---------------------------------------------------------|
| [IllegalMonitorStateException](https://developer.android.com/reference/java/lang/IllegalMonitorStateException) | if this lock is not held                                |
| [IllegalArgumentException](https://developer.android.com/reference/java/lang/IllegalArgumentException)         | if the given condition is not associated with this lock |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                 | if the condition is null                                |