org.apache.commons.lang3.concurrent.locks

## Class LockingVisitors

- java.lang.Object
- - org.apache.commons.lang3.concurrent.locks.LockingVisitors

-     public class LockingVisitors
      extends Object

  Combines the monitor and visitor pattern to work with `locked objects`. Locked objects are an alternative to synchronization. This, on Wikipedia, is known as the Visitor pattern (https://en.wikipedia.org/wiki/Visitor_pattern), and from the "Gang of Four" "Design Patterns" book's Visitor pattern [Gamma, E., Helm, R., & Johnson, R. (1998). Visitor. In Design patterns elements of reusable object oriented software (pp. 331-344). Reading: Addison Wesley.].

  Locking is preferable, if there is a distinction between read access (multiple threads may have read access concurrently), and write access (only one thread may have write access at any given time). In comparison, synchronization doesn't support read access, because synchronized access is exclusive.

  Using this class is fairly straightforward:

  1. While still in single thread mode, create an instance of `LockingVisitors.StampedLockVisitor` by calling `stampedLockVisitor(Object)`, passing the object which needs to be locked. Discard all references to the locked object. Instead, use references to the lock.
  2. If you want to access the locked object, create a `FailableConsumer`. The consumer will receive the locked object as a parameter. For convenience, the consumer may be implemented as a Lambda. Then invoke `LockingVisitors.LockVisitor.acceptReadLocked(FailableConsumer)`, or `LockingVisitors.LockVisitor.acceptWriteLocked(FailableConsumer)`, passing the consumer.
  3. As an alternative, if you need to produce a result object, you may use a `FailableFunction`. This function may also be implemented as a Lambda. To have the function executed, invoke `LockingVisitors.LockVisitor.applyReadLocked(FailableFunction)`, or `LockingVisitors.LockVisitor.applyWriteLocked(FailableFunction)`.

  Example: A thread safe logger class.

         public class SimpleLogger {

           private final StampedLockVisitor<PrintStream> lock;

           public SimpleLogger(OutputStream out) {
               lock = LockingVisitors.stampedLockVisitor(new PrintStream(out));
           }

           public void log(String message) {
               lock.acceptWriteLocked((ps) -> ps.println(message));
           }

           public void log(byte[] buffer) {
               lock.acceptWriteLocked((ps) -> { ps.write(buffer); ps.println(); });
           }

  Since:
    3.11

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `LockingVisitors.LockVisitor<O,L>` Wraps a domain object and a lock for access by lambdas.
    `static class` | `LockingVisitors.ReadWriteLockVisitor<O>` This class implements a wrapper for a locked (hidden) object, and provides the means to access it.
    `static class` | `LockingVisitors.StampedLockVisitor<O>` This class implements a wrapper for a locked (hidden) object, and provides the means to access it.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `LockingVisitors()`

  - ### Method Summary

    All Methods Static Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `static <O> LockingVisitors.ReadWriteLockVisitor<O>` | `reentrantReadWriteLockVisitor(O object)` Creates a new instance of `LockingVisitors.ReadWriteLockVisitor` with the given (hidden) object.
    `static <O> LockingVisitors.StampedLockVisitor<O>` | `stampedLockVisitor(O object)` Creates a new instance of `LockingVisitors.StampedLockVisitor` with the given (hidden) object.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### LockingVisitors

          public LockingVisitors()

  - ### Method Detail

    - #### reentrantReadWriteLockVisitor

          public static <O> LockingVisitors.ReadWriteLockVisitor<O> reentrantReadWriteLockVisitor(O object)

      Creates a new instance of `LockingVisitors.ReadWriteLockVisitor` with the given (hidden) object.
      Type Parameters:
        `O` - The locked objects type.
      Parameters:
        `object` - The locked (hidden) object.
      Returns:
        The created instance, a `lock` for the given object.

    - #### stampedLockVisitor

          public static <O> LockingVisitors.StampedLockVisitor<O> stampedLockVisitor(O object)

      Creates a new instance of `LockingVisitors.StampedLockVisitor` with the given (hidden) object.
      Type Parameters:
        `O` - The locked objects type.
      Parameters:
        `object` - The locked (hidden) object.
      Returns:
        The created instance, a `lock` for the given object.

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