Package org.apache.commons.lang3.concurrent.locks

Class LockingVisitors

  • public class LockingVisitors
extends java.lang.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

    • Constructor Detail

      • LockingVisitors

        public LockingVisitors()

    • Method Detail

      • create

        public static <O> LockingVisitors.ReadWriteLockVisitor<O> create​(O object,
                                                                 java.util.concurrent.locks.ReadWriteLock readWriteLock)
        Creates a new instance of LockingVisitors.ReadWriteLockVisitor with the given (hidden) object and lock.
        Type Parameters:
        O - The locked objects type.
        Parameters:
        object - The locked (hidden) object.
        readWriteLock - The lock to use.
        Returns:
        The created instance, a lock for the given object.
        Since:
        3.13.0

      • 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.