jdk-sandbox: comparison src/java.base/share/classes/java/lang/ThreadLocal.java

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+/*
+* 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&lt;Integer&gt; threadId =
+*         new ThreadLocal&lt;Integer&gt;() {
+*             &#64;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);
+}
+}
+}
+}

changeset 47216	71c04702a3d5
parent 25859	3317bb8137f4
child 50719	106dc156ce6b