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/*
* Copyright (C) 2005-2015 Schlichtherle IT Services.
* All rights reserved. Use is subject to license terms.
*/
package net.java.truecommons.shed;
import java.lang.ref.WeakReference;
import java.util.NoSuchElementException;
import javax.annotation.CheckForNull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.ThreadSafe;
/**
* An inheritable thread local stack of elements.
* This class works pretty much like a {@link java.util.Stack}, except that
* each call is forwarded to a private {@link InheritableThreadLocal} object.
*
* Whenever a child thread gets started, it inherits the top level element
* of the parent's thread local stack unless the parent's thread local stack
* is empty.
* However, it's not possible to {@link #pop} this inherited top level element
* off the stack - any attempt to do so will result in a
* {@link NoSuchElementException}.
*
* Disclaimer: Although this class internally uses an
* {@link InheritableThreadLocal}, it does not leak memory in multi class
* loader environments when used appropriately.
*
* @param The type of the elements in the inheritable thread local stack.
* @author Christian Schlichtherle
*/
@ThreadSafe
public final class InheritableThreadLocalStack {
private final InheritableThreadLocal> nodes
= new InheritableThreadLocal<>();
/**
* Returns {@code true} if this stack is empty.
*
* @return {@code true} if this stack is empty.
*/
public boolean isEmpty() {
return null == nodes.get();
}
/**
* Returns the top element on this stack or {@code null} if it's empty.
*
* @return The top element of this stack or {@code null} if it's empty.
*/
public @Nullable T peek() {
final Node node = nodes.get();
return null != node ? node.element : null;
}
/**
* Returns the nullable top element on this stack unless it's empty,
* in which case {@code elze} gets returned.
*
* @param elze the nullable default element.
* @return The nullable top element on this stack unless it's empty,
* in which case {@code elze} gets returned.
*/
public @Nullable T peekOrElse(final @CheckForNull T elze) {
final Node node = nodes.get();
return null != node ? node.element : elze;
}
/**
* Pushes the given element onto this stack.
*
* @param element the nullable element to push onto this stack.
* @return {@code element} - for fluent programming.
*/
public @Nullable T push(final @CheckForNull T element) {
final Node previous = nodes.get();
final Node next = new Node<>(previous, element);
nodes.set(next);
return element;
}
/**
* Removes and returns the nullable top element on this stack.
*
* @return The (then no more) nullable top element on this stack.
* @throws NoSuchElementException if this stack is empty.
*/
public @Nullable T pop() {
final Node node = nodes.get();
if (null == node) throw new NoSuchElementException();
if (!Thread.currentThread().equals(node.get()))
throw new NoSuchElementException();
nodes.set(node.previous); // may be null!
return node.element;
}
/**
* Removes and returns the nullable top element on this stack
* if it's identical to the given element.
*
* @param expected The expected top element on this stack.
* @throws IllegalStateException If the given element is not the top
* element on this stack.
*/
public void popIf(final @CheckForNull T expected) {
try {
final @CheckForNull T got = pop();
if (got != expected) {
push(got);
throw new IllegalStateException(got + " (expected " + expected + " as the top element of the inheritable thread local stack)");
}
} catch (NoSuchElementException ex) {
throw new IllegalStateException("The inheritable thread local stack is empty!", ex);
}
}
private static class Node extends WeakReference {
final @CheckForNull Node previous;
@CheckForNull T element;
Node( final @CheckForNull Node previous,
final @CheckForNull T element) {
super(Thread.currentThread());
this.previous = previous;
this.element = element;
}
} // Node
}