Added FastThreadLocal

Dorkbox-Util/src/dorkbox/util/FastThreadLocal.java

@@ -0,0 +1,117 @@
+/*
+ * Copyright © 2012-2014 Lightweight Java Game Library Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are
+ * permitted provided that the following conditions are met:
+ *
+ *   - Redistributions of source code must retain the above copyright notice, this list
+ *     of conditions and the following disclaimer.
+ *   - Redistributions in binary form must reproduce the above copyright notice, this
+ *     list of conditions and the following disclaimer in the documentation and/or
+ *     other materials provided with the distribution.
+ *   - Neither the name of 'Light Weight Java Game Library' nor the names of its
+ *     contributors may be used to endorse or promote products derived from this
+ *     software without specific prior written permission.
+ *
+ *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *   AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *   ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ *   LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *   CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *   SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *   CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+ *   IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
+ *   OF SUCH DAMAGE.
+ */
+package dorkbox.util;
+
+import java.util.Arrays;
+
+/**
+ * Fast {@code ThreadLocal} implementation, adapted from the
+ * <a href="https://github.com/riven8192/LibStruct/blob/master/src/net/indiespot/struct/runtime/FastThreadLocal.java">LibStruct</a> library.
+ *
+ * <p>This implementation replaces the {@code ThreadLocalMap} lookup in {@link ThreadLocal} with a simple array access. The big advantage of this method is
+ * that thread-local accesses are identified as invariant by the JVM, which enables significant code-motion optimizations.</p>
+ *
+ * <p>The underlying array contains a slot for each thread that uses the {@link FastThreadLocal} instance. The slot is indexed by {@link Thread#getId()}. The
+ * array grows if necessary when the {@link #set} method is called.</p>
+ *
+ * <p>It is assumed that usages of this class will be read heavy, so any contention/false-sharing issues caused by the {@link #set} method are ignored.</p>
+ *
+ * @param <T> the thread-local value type
+ *
+ * @author Riven
+ * @see java.lang.ThreadLocal
+ */
+public class FastThreadLocal<T> {
+
+	@SuppressWarnings("unchecked")
+    private T[] threadIDMap = (T[])new Object[1];
+
+	/** Creates a thread local variable. */
+	public FastThreadLocal() {
+	}
+
+	/**
+	 * Returns the current thread's "initial value" for this thread-local variable.
+	 *
+	 * @see ThreadLocal#initialValue()
+	 */
+	public T initialValue() {
+		return null;
+	}
+
+	/**
+	 * Sets the current thread's copy of this thread-local variable to the specified value.
+	 *
+	 * @param value the value to be stored in the current thread's copy of this thread-local.
+	 *
+	 * @see ThreadLocal#set(T)
+	 */
+	public void set(T value) {
+		int id = (int)Thread.currentThread().getId();
+
+		synchronized ( this ) {
+			int len = threadIDMap.length;
+            if (len <= id) {
+                threadIDMap = Arrays.copyOf(threadIDMap, id + 1);
+            }
+
+			threadIDMap[id] = value;
+		}
+	}
+
+	/**
+	 * Returns the value in the current thread's copy of this thread-local variable.
+	 *
+	 * @see ThreadLocal#get()
+	 */
+	public final T get() {
+		int id = (int)Thread.currentThread().getId();
+
+		T[] threadIDMap = this.threadIDMap; // It's OK if the array is resized after this access, will just use the old array.
+
+		T value = threadIDMap.length <= id ? null : threadIDMap[id];
+
+		if ( value == null ) {
+			value = initialValue();
+			set(value);
+		}
+
+		return value;
+	}
+
+	/**
+	 * Removes the current thread's value for this thread-local variable.
+	 *
+	 * @see ThreadLocal#remove()
+	 */
+	public void remove() {
+		set(null);
+	}
+
+}