--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/java.base/share/classes/java/lang/ThreadLocal.java Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,720 @@
+/*
+ * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation. Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.lang;
+import java.lang.ref.*;
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+/**
+ * This class provides thread-local variables. These variables differ from
+ * their normal counterparts in that each thread that accesses one (via its
+ * {@code get} or {@code set} method) has its own, independently initialized
+ * copy of the variable. {@code ThreadLocal} instances are typically private
+ * static fields in classes that wish to associate state with a thread (e.g.,
+ * a user ID or Transaction ID).
+ *
+ * <p>For example, the class below generates unique identifiers local to each
+ * thread.
+ * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
+ * and remains unchanged on subsequent calls.
+ * <pre>
+ * import java.util.concurrent.atomic.AtomicInteger;
+ *
+ * public class ThreadId {
+ * // Atomic integer containing the next thread ID to be assigned
+ * private static final AtomicInteger nextId = new AtomicInteger(0);
+ *
+ * // Thread local variable containing each thread's ID
+ * private static final ThreadLocal<Integer> threadId =
+ * new ThreadLocal<Integer>() {
+ * @Override protected Integer initialValue() {
+ * return nextId.getAndIncrement();
+ * }
+ * };
+ *
+ * // Returns the current thread's unique ID, assigning it if necessary
+ * public static int get() {
+ * return threadId.get();
+ * }
+ * }
+ * </pre>
+ * <p>Each thread holds an implicit reference to its copy of a thread-local
+ * variable as long as the thread is alive and the {@code ThreadLocal}
+ * instance is accessible; after a thread goes away, all of its copies of
+ * thread-local instances are subject to garbage collection (unless other
+ * references to these copies exist).
+ *
+ * @author Josh Bloch and Doug Lea
+ * @since 1.2
+ */
+public class ThreadLocal<T> {
+ /**
+ * ThreadLocals rely on per-thread linear-probe hash maps attached
+ * to each thread (Thread.threadLocals and
+ * inheritableThreadLocals). The ThreadLocal objects act as keys,
+ * searched via threadLocalHashCode. This is a custom hash code
+ * (useful only within ThreadLocalMaps) that eliminates collisions
+ * in the common case where consecutively constructed ThreadLocals
+ * are used by the same threads, while remaining well-behaved in
+ * less common cases.
+ */
+ private final int threadLocalHashCode = nextHashCode();
+
+ /**
+ * The next hash code to be given out. Updated atomically. Starts at
+ * zero.
+ */
+ private static AtomicInteger nextHashCode =
+ new AtomicInteger();
+
+ /**
+ * The difference between successively generated hash codes - turns
+ * implicit sequential thread-local IDs into near-optimally spread
+ * multiplicative hash values for power-of-two-sized tables.
+ */
+ private static final int HASH_INCREMENT = 0x61c88647;
+
+ /**
+ * Returns the next hash code.
+ */
+ private static int nextHashCode() {
+ return nextHashCode.getAndAdd(HASH_INCREMENT);
+ }
+
+ /**
+ * Returns the current thread's "initial value" for this
+ * thread-local variable. This method will be invoked the first
+ * time a thread accesses the variable with the {@link #get}
+ * method, unless the thread previously invoked the {@link #set}
+ * method, in which case the {@code initialValue} method will not
+ * be invoked for the thread. Normally, this method is invoked at
+ * most once per thread, but it may be invoked again in case of
+ * subsequent invocations of {@link #remove} followed by {@link #get}.
+ *
+ * <p>This implementation simply returns {@code null}; if the
+ * programmer desires thread-local variables to have an initial
+ * value other than {@code null}, {@code ThreadLocal} must be
+ * subclassed, and this method overridden. Typically, an
+ * anonymous inner class will be used.
+ *
+ * @return the initial value for this thread-local
+ */
+ protected T initialValue() {
+ return null;
+ }
+
+ /**
+ * Creates a thread local variable. The initial value of the variable is
+ * determined by invoking the {@code get} method on the {@code Supplier}.
+ *
+ * @param <S> the type of the thread local's value
+ * @param supplier the supplier to be used to determine the initial value
+ * @return a new thread local variable
+ * @throws NullPointerException if the specified supplier is null
+ * @since 1.8
+ */
+ public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
+ return new SuppliedThreadLocal<>(supplier);
+ }
+
+ /**
+ * Creates a thread local variable.
+ * @see #withInitial(java.util.function.Supplier)
+ */
+ public ThreadLocal() {
+ }
+
+ /**
+ * Returns the value in the current thread's copy of this
+ * thread-local variable. If the variable has no value for the
+ * current thread, it is first initialized to the value returned
+ * by an invocation of the {@link #initialValue} method.
+ *
+ * @return the current thread's value of this thread-local
+ */
+ public T get() {
+ Thread t = Thread.currentThread();
+ ThreadLocalMap map = getMap(t);
+ if (map != null) {
+ ThreadLocalMap.Entry e = map.getEntry(this);
+ if (e != null) {
+ @SuppressWarnings("unchecked")
+ T result = (T)e.value;
+ return result;
+ }
+ }
+ return setInitialValue();
+ }
+
+ /**
+ * Variant of set() to establish initialValue. Used instead
+ * of set() in case user has overridden the set() method.
+ *
+ * @return the initial value
+ */
+ private T setInitialValue() {
+ T value = initialValue();
+ Thread t = Thread.currentThread();
+ ThreadLocalMap map = getMap(t);
+ if (map != null)
+ map.set(this, value);
+ else
+ createMap(t, value);
+ return value;
+ }
+
+ /**
+ * Sets the current thread's copy of this thread-local variable
+ * to the specified value. Most subclasses will have no need to
+ * override this method, relying solely on the {@link #initialValue}
+ * method to set the values of thread-locals.
+ *
+ * @param value the value to be stored in the current thread's copy of
+ * this thread-local.
+ */
+ public void set(T value) {
+ Thread t = Thread.currentThread();
+ ThreadLocalMap map = getMap(t);
+ if (map != null)
+ map.set(this, value);
+ else
+ createMap(t, value);
+ }
+
+ /**
+ * Removes the current thread's value for this thread-local
+ * variable. If this thread-local variable is subsequently
+ * {@linkplain #get read} by the current thread, its value will be
+ * reinitialized by invoking its {@link #initialValue} method,
+ * unless its value is {@linkplain #set set} by the current thread
+ * in the interim. This may result in multiple invocations of the
+ * {@code initialValue} method in the current thread.
+ *
+ * @since 1.5
+ */
+ public void remove() {
+ ThreadLocalMap m = getMap(Thread.currentThread());
+ if (m != null)
+ m.remove(this);
+ }
+
+ /**
+ * Get the map associated with a ThreadLocal. Overridden in
+ * InheritableThreadLocal.
+ *
+ * @param t the current thread
+ * @return the map
+ */
+ ThreadLocalMap getMap(Thread t) {
+ return t.threadLocals;
+ }
+
+ /**
+ * Create the map associated with a ThreadLocal. Overridden in
+ * InheritableThreadLocal.
+ *
+ * @param t the current thread
+ * @param firstValue value for the initial entry of the map
+ */
+ void createMap(Thread t, T firstValue) {
+ t.threadLocals = new ThreadLocalMap(this, firstValue);
+ }
+
+ /**
+ * Factory method to create map of inherited thread locals.
+ * Designed to be called only from Thread constructor.
+ *
+ * @param parentMap the map associated with parent thread
+ * @return a map containing the parent's inheritable bindings
+ */
+ static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
+ return new ThreadLocalMap(parentMap);
+ }
+
+ /**
+ * Method childValue is visibly defined in subclass
+ * InheritableThreadLocal, but is internally defined here for the
+ * sake of providing createInheritedMap factory method without
+ * needing to subclass the map class in InheritableThreadLocal.
+ * This technique is preferable to the alternative of embedding
+ * instanceof tests in methods.
+ */
+ T childValue(T parentValue) {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * An extension of ThreadLocal that obtains its initial value from
+ * the specified {@code Supplier}.
+ */
+ static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
+
+ private final Supplier<? extends T> supplier;
+
+ SuppliedThreadLocal(Supplier<? extends T> supplier) {
+ this.supplier = Objects.requireNonNull(supplier);
+ }
+
+ @Override
+ protected T initialValue() {
+ return supplier.get();
+ }
+ }
+
+ /**
+ * ThreadLocalMap is a customized hash map suitable only for
+ * maintaining thread local values. No operations are exported
+ * outside of the ThreadLocal class. The class is package private to
+ * allow declaration of fields in class Thread. To help deal with
+ * very large and long-lived usages, the hash table entries use
+ * WeakReferences for keys. However, since reference queues are not
+ * used, stale entries are guaranteed to be removed only when
+ * the table starts running out of space.
+ */
+ static class ThreadLocalMap {
+
+ /**
+ * The entries in this hash map extend WeakReference, using
+ * its main ref field as the key (which is always a
+ * ThreadLocal object). Note that null keys (i.e. entry.get()
+ * == null) mean that the key is no longer referenced, so the
+ * entry can be expunged from table. Such entries are referred to
+ * as "stale entries" in the code that follows.
+ */
+ static class Entry extends WeakReference<ThreadLocal<?>> {
+ /** The value associated with this ThreadLocal. */
+ Object value;
+
+ Entry(ThreadLocal<?> k, Object v) {
+ super(k);
+ value = v;
+ }
+ }
+
+ /**
+ * The initial capacity -- MUST be a power of two.
+ */
+ private static final int INITIAL_CAPACITY = 16;
+
+ /**
+ * The table, resized as necessary.
+ * table.length MUST always be a power of two.
+ */
+ private Entry[] table;
+
+ /**
+ * The number of entries in the table.
+ */
+ private int size = 0;
+
+ /**
+ * The next size value at which to resize.
+ */
+ private int threshold; // Default to 0
+
+ /**
+ * Set the resize threshold to maintain at worst a 2/3 load factor.
+ */
+ private void setThreshold(int len) {
+ threshold = len * 2 / 3;
+ }
+
+ /**
+ * Increment i modulo len.
+ */
+ private static int nextIndex(int i, int len) {
+ return ((i + 1 < len) ? i + 1 : 0);
+ }
+
+ /**
+ * Decrement i modulo len.
+ */
+ private static int prevIndex(int i, int len) {
+ return ((i - 1 >= 0) ? i - 1 : len - 1);
+ }
+
+ /**
+ * Construct a new map initially containing (firstKey, firstValue).
+ * ThreadLocalMaps are constructed lazily, so we only create
+ * one when we have at least one entry to put in it.
+ */
+ ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
+ table = new Entry[INITIAL_CAPACITY];
+ int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
+ table[i] = new Entry(firstKey, firstValue);
+ size = 1;
+ setThreshold(INITIAL_CAPACITY);
+ }
+
+ /**
+ * Construct a new map including all Inheritable ThreadLocals
+ * from given parent map. Called only by createInheritedMap.
+ *
+ * @param parentMap the map associated with parent thread.
+ */
+ private ThreadLocalMap(ThreadLocalMap parentMap) {
+ Entry[] parentTable = parentMap.table;
+ int len = parentTable.length;
+ setThreshold(len);
+ table = new Entry[len];
+
+ for (Entry e : parentTable) {
+ if (e != null) {
+ @SuppressWarnings("unchecked")
+ ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
+ if (key != null) {
+ Object value = key.childValue(e.value);
+ Entry c = new Entry(key, value);
+ int h = key.threadLocalHashCode & (len - 1);
+ while (table[h] != null)
+ h = nextIndex(h, len);
+ table[h] = c;
+ size++;
+ }
+ }
+ }
+ }
+
+ /**
+ * Get the entry associated with key. This method
+ * itself handles only the fast path: a direct hit of existing
+ * key. It otherwise relays to getEntryAfterMiss. This is
+ * designed to maximize performance for direct hits, in part
+ * by making this method readily inlinable.
+ *
+ * @param key the thread local object
+ * @return the entry associated with key, or null if no such
+ */
+ private Entry getEntry(ThreadLocal<?> key) {
+ int i = key.threadLocalHashCode & (table.length - 1);
+ Entry e = table[i];
+ if (e != null && e.get() == key)
+ return e;
+ else
+ return getEntryAfterMiss(key, i, e);
+ }
+
+ /**
+ * Version of getEntry method for use when key is not found in
+ * its direct hash slot.
+ *
+ * @param key the thread local object
+ * @param i the table index for key's hash code
+ * @param e the entry at table[i]
+ * @return the entry associated with key, or null if no such
+ */
+ private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
+ Entry[] tab = table;
+ int len = tab.length;
+
+ while (e != null) {
+ ThreadLocal<?> k = e.get();
+ if (k == key)
+ return e;
+ if (k == null)
+ expungeStaleEntry(i);
+ else
+ i = nextIndex(i, len);
+ e = tab[i];
+ }
+ return null;
+ }
+
+ /**
+ * Set the value associated with key.
+ *
+ * @param key the thread local object
+ * @param value the value to be set
+ */
+ private void set(ThreadLocal<?> key, Object value) {
+
+ // We don't use a fast path as with get() because it is at
+ // least as common to use set() to create new entries as
+ // it is to replace existing ones, in which case, a fast
+ // path would fail more often than not.
+
+ Entry[] tab = table;
+ int len = tab.length;
+ int i = key.threadLocalHashCode & (len-1);
+
+ for (Entry e = tab[i];
+ e != null;
+ e = tab[i = nextIndex(i, len)]) {
+ ThreadLocal<?> k = e.get();
+
+ if (k == key) {
+ e.value = value;
+ return;
+ }
+
+ if (k == null) {
+ replaceStaleEntry(key, value, i);
+ return;
+ }
+ }
+
+ tab[i] = new Entry(key, value);
+ int sz = ++size;
+ if (!cleanSomeSlots(i, sz) && sz >= threshold)
+ rehash();
+ }
+
+ /**
+ * Remove the entry for key.
+ */
+ private void remove(ThreadLocal<?> key) {
+ Entry[] tab = table;
+ int len = tab.length;
+ int i = key.threadLocalHashCode & (len-1);
+ for (Entry e = tab[i];
+ e != null;
+ e = tab[i = nextIndex(i, len)]) {
+ if (e.get() == key) {
+ e.clear();
+ expungeStaleEntry(i);
+ return;
+ }
+ }
+ }
+
+ /**
+ * Replace a stale entry encountered during a set operation
+ * with an entry for the specified key. The value passed in
+ * the value parameter is stored in the entry, whether or not
+ * an entry already exists for the specified key.
+ *
+ * As a side effect, this method expunges all stale entries in the
+ * "run" containing the stale entry. (A run is a sequence of entries
+ * between two null slots.)
+ *
+ * @param key the key
+ * @param value the value to be associated with key
+ * @param staleSlot index of the first stale entry encountered while
+ * searching for key.
+ */
+ private void replaceStaleEntry(ThreadLocal<?> key, Object value,
+ int staleSlot) {
+ Entry[] tab = table;
+ int len = tab.length;
+ Entry e;
+
+ // Back up to check for prior stale entry in current run.
+ // We clean out whole runs at a time to avoid continual
+ // incremental rehashing due to garbage collector freeing
+ // up refs in bunches (i.e., whenever the collector runs).
+ int slotToExpunge = staleSlot;
+ for (int i = prevIndex(staleSlot, len);
+ (e = tab[i]) != null;
+ i = prevIndex(i, len))
+ if (e.get() == null)
+ slotToExpunge = i;
+
+ // Find either the key or trailing null slot of run, whichever
+ // occurs first
+ for (int i = nextIndex(staleSlot, len);
+ (e = tab[i]) != null;
+ i = nextIndex(i, len)) {
+ ThreadLocal<?> k = e.get();
+
+ // If we find key, then we need to swap it
+ // with the stale entry to maintain hash table order.
+ // The newly stale slot, or any other stale slot
+ // encountered above it, can then be sent to expungeStaleEntry
+ // to remove or rehash all of the other entries in run.
+ if (k == key) {
+ e.value = value;
+
+ tab[i] = tab[staleSlot];
+ tab[staleSlot] = e;
+
+ // Start expunge at preceding stale entry if it exists
+ if (slotToExpunge == staleSlot)
+ slotToExpunge = i;
+ cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
+ return;
+ }
+
+ // If we didn't find stale entry on backward scan, the
+ // first stale entry seen while scanning for key is the
+ // first still present in the run.
+ if (k == null && slotToExpunge == staleSlot)
+ slotToExpunge = i;
+ }
+
+ // If key not found, put new entry in stale slot
+ tab[staleSlot].value = null;
+ tab[staleSlot] = new Entry(key, value);
+
+ // If there are any other stale entries in run, expunge them
+ if (slotToExpunge != staleSlot)
+ cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
+ }
+
+ /**
+ * Expunge a stale entry by rehashing any possibly colliding entries
+ * lying between staleSlot and the next null slot. This also expunges
+ * any other stale entries encountered before the trailing null. See
+ * Knuth, Section 6.4
+ *
+ * @param staleSlot index of slot known to have null key
+ * @return the index of the next null slot after staleSlot
+ * (all between staleSlot and this slot will have been checked
+ * for expunging).
+ */
+ private int expungeStaleEntry(int staleSlot) {
+ Entry[] tab = table;
+ int len = tab.length;
+
+ // expunge entry at staleSlot
+ tab[staleSlot].value = null;
+ tab[staleSlot] = null;
+ size--;
+
+ // Rehash until we encounter null
+ Entry e;
+ int i;
+ for (i = nextIndex(staleSlot, len);
+ (e = tab[i]) != null;
+ i = nextIndex(i, len)) {
+ ThreadLocal<?> k = e.get();
+ if (k == null) {
+ e.value = null;
+ tab[i] = null;
+ size--;
+ } else {
+ int h = k.threadLocalHashCode & (len - 1);
+ if (h != i) {
+ tab[i] = null;
+
+ // Unlike Knuth 6.4 Algorithm R, we must scan until
+ // null because multiple entries could have been stale.
+ while (tab[h] != null)
+ h = nextIndex(h, len);
+ tab[h] = e;
+ }
+ }
+ }
+ return i;
+ }
+
+ /**
+ * Heuristically scan some cells looking for stale entries.
+ * This is invoked when either a new element is added, or
+ * another stale one has been expunged. It performs a
+ * logarithmic number of scans, as a balance between no
+ * scanning (fast but retains garbage) and a number of scans
+ * proportional to number of elements, that would find all
+ * garbage but would cause some insertions to take O(n) time.
+ *
+ * @param i a position known NOT to hold a stale entry. The
+ * scan starts at the element after i.
+ *
+ * @param n scan control: {@code log2(n)} cells are scanned,
+ * unless a stale entry is found, in which case
+ * {@code log2(table.length)-1} additional cells are scanned.
+ * When called from insertions, this parameter is the number
+ * of elements, but when from replaceStaleEntry, it is the
+ * table length. (Note: all this could be changed to be either
+ * more or less aggressive by weighting n instead of just
+ * using straight log n. But this version is simple, fast, and
+ * seems to work well.)
+ *
+ * @return true if any stale entries have been removed.
+ */
+ private boolean cleanSomeSlots(int i, int n) {
+ boolean removed = false;
+ Entry[] tab = table;
+ int len = tab.length;
+ do {
+ i = nextIndex(i, len);
+ Entry e = tab[i];
+ if (e != null && e.get() == null) {
+ n = len;
+ removed = true;
+ i = expungeStaleEntry(i);
+ }
+ } while ( (n >>>= 1) != 0);
+ return removed;
+ }
+
+ /**
+ * Re-pack and/or re-size the table. First scan the entire
+ * table removing stale entries. If this doesn't sufficiently
+ * shrink the size of the table, double the table size.
+ */
+ private void rehash() {
+ expungeStaleEntries();
+
+ // Use lower threshold for doubling to avoid hysteresis
+ if (size >= threshold - threshold / 4)
+ resize();
+ }
+
+ /**
+ * Double the capacity of the table.
+ */
+ private void resize() {
+ Entry[] oldTab = table;
+ int oldLen = oldTab.length;
+ int newLen = oldLen * 2;
+ Entry[] newTab = new Entry[newLen];
+ int count = 0;
+
+ for (Entry e : oldTab) {
+ if (e != null) {
+ ThreadLocal<?> k = e.get();
+ if (k == null) {
+ e.value = null; // Help the GC
+ } else {
+ int h = k.threadLocalHashCode & (newLen - 1);
+ while (newTab[h] != null)
+ h = nextIndex(h, newLen);
+ newTab[h] = e;
+ count++;
+ }
+ }
+ }
+
+ setThreshold(newLen);
+ size = count;
+ table = newTab;
+ }
+
+ /**
+ * Expunge all stale entries in the table.
+ */
+ private void expungeStaleEntries() {
+ Entry[] tab = table;
+ int len = tab.length;
+ for (int j = 0; j < len; j++) {
+ Entry e = tab[j];
+ if (e != null && e.get() == null)
+ expungeStaleEntry(j);
+ }
+ }
+ }
+}