| /* |
| * Copyright (c) 2010, 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.util.WeakHashMap; |
| import java.lang.ref.WeakReference; |
| import java.util.concurrent.atomic.AtomicInteger; |
| |
| import jdk.internal.misc.Unsafe; |
| |
| import static java.lang.ClassValue.ClassValueMap.probeHomeLocation; |
| import static java.lang.ClassValue.ClassValueMap.probeBackupLocations; |
| |
| /** |
| * Lazily associate a computed value with (potentially) every type. |
| * For example, if a dynamic language needs to construct a message dispatch |
| * table for each class encountered at a message send call site, |
| * it can use a {@code ClassValue} to cache information needed to |
| * perform the message send quickly, for each class encountered. |
| * @author John Rose, JSR 292 EG |
| * @since 1.7 |
| */ |
| public abstract class ClassValue<T> { |
| /** |
| * Sole constructor. (For invocation by subclass constructors, typically |
| * implicit.) |
| */ |
| protected ClassValue() { |
| } |
| |
| /** |
| * Computes the given class's derived value for this {@code ClassValue}. |
| * <p> |
| * This method will be invoked within the first thread that accesses |
| * the value with the {@link #get get} method. |
| * <p> |
| * Normally, this method is invoked at most once per class, |
| * but it may be invoked again if there has been a call to |
| * {@link #remove remove}. |
| * <p> |
| * If this method throws an exception, the corresponding call to {@code get} |
| * will terminate abnormally with that exception, and no class value will be recorded. |
| * |
| * @param type the type whose class value must be computed |
| * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface |
| * @see #get |
| * @see #remove |
| */ |
| protected abstract T computeValue(Class<?> type); |
| |
| /** |
| * Returns the value for the given class. |
| * If no value has yet been computed, it is obtained by |
| * an invocation of the {@link #computeValue computeValue} method. |
| * <p> |
| * The actual installation of the value on the class |
| * is performed atomically. |
| * At that point, if several racing threads have |
| * computed values, one is chosen, and returned to |
| * all the racing threads. |
| * <p> |
| * The {@code type} parameter is typically a class, but it may be any type, |
| * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}. |
| * <p> |
| * In the absence of {@code remove} calls, a class value has a simple |
| * state diagram: uninitialized and initialized. |
| * When {@code remove} calls are made, |
| * the rules for value observation are more complex. |
| * See the documentation for {@link #remove remove} for more information. |
| * |
| * @param type the type whose class value must be computed or retrieved |
| * @return the current value associated with this {@code ClassValue}, for the given class or interface |
| * @throws NullPointerException if the argument is null |
| * @see #remove |
| * @see #computeValue |
| */ |
| public T get(Class<?> type) { |
| // non-racing this.hashCodeForCache : final int |
| Entry<?>[] cache; |
| Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this); |
| // racing e : current value <=> stale value from current cache or from stale cache |
| // invariant: e is null or an Entry with readable Entry.version and Entry.value |
| if (match(e)) |
| // invariant: No false positive matches. False negatives are OK if rare. |
| // The key fact that makes this work: if this.version == e.version, |
| // then this thread has a right to observe (final) e.value. |
| return e.value(); |
| // The fast path can fail for any of these reasons: |
| // 1. no entry has been computed yet |
| // 2. hash code collision (before or after reduction mod cache.length) |
| // 3. an entry has been removed (either on this type or another) |
| // 4. the GC has somehow managed to delete e.version and clear the reference |
| return getFromBackup(cache, type); |
| } |
| |
| /** |
| * Removes the associated value for the given class. |
| * If this value is subsequently {@linkplain #get read} for the same class, |
| * its value will be reinitialized by invoking its {@link #computeValue computeValue} method. |
| * This may result in an additional invocation of the |
| * {@code computeValue} method for the given class. |
| * <p> |
| * In order to explain the interaction between {@code get} and {@code remove} calls, |
| * we must model the state transitions of a class value to take into account |
| * the alternation between uninitialized and initialized states. |
| * To do this, number these states sequentially from zero, and note that |
| * uninitialized (or removed) states are numbered with even numbers, |
| * while initialized (or re-initialized) states have odd numbers. |
| * <p> |
| * When a thread {@code T} removes a class value in state {@code 2N}, |
| * nothing happens, since the class value is already uninitialized. |
| * Otherwise, the state is advanced atomically to {@code 2N+1}. |
| * <p> |
| * When a thread {@code T} queries a class value in state {@code 2N}, |
| * the thread first attempts to initialize the class value to state {@code 2N+1} |
| * by invoking {@code computeValue} and installing the resulting value. |
| * <p> |
| * When {@code T} attempts to install the newly computed value, |
| * if the state is still at {@code 2N}, the class value will be initialized |
| * with the computed value, advancing it to state {@code 2N+1}. |
| * <p> |
| * Otherwise, whether the new state is even or odd, |
| * {@code T} will discard the newly computed value |
| * and retry the {@code get} operation. |
| * <p> |
| * Discarding and retrying is an important proviso, |
| * since otherwise {@code T} could potentially install |
| * a disastrously stale value. For example: |
| * <ul> |
| * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N} |
| * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it |
| * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time |
| * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N} |
| * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)} |
| * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work |
| * <li> the previous actions of {@code T2} are repeated several times |
| * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ... |
| * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em> |
| * </ul> |
| * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly |
| * observe the time-dependent states as it computes {@code V1}, etc. |
| * This does not remove the threat of a stale value, since there is a window of time |
| * between the return of {@code computeValue} in {@code T} and the installation |
| * of the new value. No user synchronization is possible during this time. |
| * |
| * @param type the type whose class value must be removed |
| * @throws NullPointerException if the argument is null |
| */ |
| public void remove(Class<?> type) { |
| ClassValueMap map = getMap(type); |
| map.removeEntry(this); |
| } |
| |
| // Possible functionality for JSR 292 MR 1 |
| /*public*/ void put(Class<?> type, T value) { |
| ClassValueMap map = getMap(type); |
| map.changeEntry(this, value); |
| } |
| |
| /// -------- |
| /// Implementation... |
| /// -------- |
| |
| /** Return the cache, if it exists, else a dummy empty cache. */ |
| private static Entry<?>[] getCacheCarefully(Class<?> type) { |
| // racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y] |
| // Android-changed: Android stores classValueMap in extData. |
| // ClassValueMap map = type.classValueMap; |
| ClassValueMap map = (ClassValueMap) type.ensureExtDataPresent().classValueMap; |
| if (map == null) return EMPTY_CACHE; |
| Entry<?>[] cache = map.getCache(); |
| return cache; |
| // invariant: returned value is safe to dereference and check for an Entry |
| } |
| |
| /** Initial, one-element, empty cache used by all Class instances. Must never be filled. */ |
| private static final Entry<?>[] EMPTY_CACHE = { null }; |
| |
| /** |
| * Slow tail of ClassValue.get to retry at nearby locations in the cache, |
| * or take a slow lock and check the hash table. |
| * Called only if the first probe was empty or a collision. |
| * This is a separate method, so compilers can process it independently. |
| */ |
| private T getFromBackup(Entry<?>[] cache, Class<?> type) { |
| Entry<T> e = probeBackupLocations(cache, this); |
| if (e != null) |
| return e.value(); |
| return getFromHashMap(type); |
| } |
| |
| // Hack to suppress warnings on the (T) cast, which is a no-op. |
| @SuppressWarnings("unchecked") |
| Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; } |
| |
| /** Called when the fast path of get fails, and cache reprobe also fails. |
| */ |
| private T getFromHashMap(Class<?> type) { |
| // The fail-safe recovery is to fall back to the underlying classValueMap. |
| ClassValueMap map = getMap(type); |
| for (;;) { |
| Entry<T> e = map.startEntry(this); |
| if (!e.isPromise()) |
| return e.value(); |
| try { |
| // Try to make a real entry for the promised version. |
| e = makeEntry(e.version(), computeValue(type)); |
| } finally { |
| // Whether computeValue throws or returns normally, |
| // be sure to remove the empty entry. |
| e = map.finishEntry(this, e); |
| } |
| if (e != null) |
| return e.value(); |
| // else try again, in case a racing thread called remove (so e == null) |
| } |
| } |
| |
| /** Check that e is non-null, matches this ClassValue, and is live. */ |
| boolean match(Entry<?> e) { |
| // racing e.version : null (blank) => unique Version token => null (GC-ed version) |
| // non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile) |
| return (e != null && e.get() == this.version); |
| // invariant: No false positives on version match. Null is OK for false negative. |
| // invariant: If version matches, then e.value is readable (final set in Entry.<init>) |
| } |
| |
| /** Internal hash code for accessing Class.classValueMap.cacheArray. */ |
| final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK; |
| |
| /** Value stream for hashCodeForCache. See similar structure in ThreadLocal. */ |
| private static final AtomicInteger nextHashCode = new AtomicInteger(); |
| |
| /** Good for power-of-two tables. See similar structure in ThreadLocal. */ |
| private static final int HASH_INCREMENT = 0x61c88647; |
| |
| /** Mask a hash code to be positive but not too large, to prevent wraparound. */ |
| static final int HASH_MASK = (-1 >>> 2); |
| |
| /** |
| * Private key for retrieval of this object from ClassValueMap. |
| */ |
| static class Identity { |
| } |
| /** |
| * This ClassValue's identity, expressed as an opaque object. |
| * The main object {@code ClassValue.this} is incorrect since |
| * subclasses may override {@code ClassValue.equals}, which |
| * could confuse keys in the ClassValueMap. |
| */ |
| final Identity identity = new Identity(); |
| |
| /** |
| * Current version for retrieving this class value from the cache. |
| * Any number of computeValue calls can be cached in association with one version. |
| * But the version changes when a remove (on any type) is executed. |
| * A version change invalidates all cache entries for the affected ClassValue, |
| * by marking them as stale. Stale cache entries do not force another call |
| * to computeValue, but they do require a synchronized visit to a backing map. |
| * <p> |
| * All user-visible state changes on the ClassValue take place under |
| * a lock inside the synchronized methods of ClassValueMap. |
| * Readers (of ClassValue.get) are notified of such state changes |
| * when this.version is bumped to a new token. |
| * This variable must be volatile so that an unsynchronized reader |
| * will receive the notification without delay. |
| * <p> |
| * If version were not volatile, one thread T1 could persistently hold onto |
| * a stale value this.value == V1, while another thread T2 advances |
| * (under a lock) to this.value == V2. This will typically be harmless, |
| * but if T1 and T2 interact causally via some other channel, such that |
| * T1's further actions are constrained (in the JMM) to happen after |
| * the V2 event, then T1's observation of V1 will be an error. |
| * <p> |
| * The practical effect of making this.version be volatile is that it cannot |
| * be hoisted out of a loop (by an optimizing JIT) or otherwise cached. |
| * Some machines may also require a barrier instruction to execute |
| * before this.version. |
| */ |
| private volatile Version<T> version = new Version<>(this); |
| Version<T> version() { return version; } |
| void bumpVersion() { version = new Version<>(this); } |
| static class Version<T> { |
| private final ClassValue<T> classValue; |
| private final Entry<T> promise = new Entry<>(this); |
| Version(ClassValue<T> classValue) { this.classValue = classValue; } |
| ClassValue<T> classValue() { return classValue; } |
| Entry<T> promise() { return promise; } |
| boolean isLive() { return classValue.version() == this; } |
| } |
| |
| /** One binding of a value to a class via a ClassValue. |
| * States are:<ul> |
| * <li> promise if value == Entry.this |
| * <li> else dead if version == null |
| * <li> else stale if version != classValue.version |
| * <li> else live </ul> |
| * Promises are never put into the cache; they only live in the |
| * backing map while a computeValue call is in flight. |
| * Once an entry goes stale, it can be reset at any time |
| * into the dead state. |
| */ |
| static class Entry<T> extends WeakReference<Version<T>> { |
| final Object value; // usually of type T, but sometimes (Entry)this |
| Entry(Version<T> version, T value) { |
| super(version); |
| this.value = value; // for a regular entry, value is of type T |
| } |
| private void assertNotPromise() { assert(!isPromise()); } |
| /** For creating a promise. */ |
| Entry(Version<T> version) { |
| super(version); |
| this.value = this; // for a promise, value is not of type T, but Entry! |
| } |
| /** Fetch the value. This entry must not be a promise. */ |
| @SuppressWarnings("unchecked") // if !isPromise, type is T |
| T value() { assertNotPromise(); return (T) value; } |
| boolean isPromise() { return value == this; } |
| Version<T> version() { return get(); } |
| ClassValue<T> classValueOrNull() { |
| Version<T> v = version(); |
| return (v == null) ? null : v.classValue(); |
| } |
| boolean isLive() { |
| Version<T> v = version(); |
| if (v == null) return false; |
| if (v.isLive()) return true; |
| clear(); |
| return false; |
| } |
| Entry<T> refreshVersion(Version<T> v2) { |
| assertNotPromise(); |
| @SuppressWarnings("unchecked") // if !isPromise, type is T |
| Entry<T> e2 = new Entry<>(v2, (T) value); |
| clear(); |
| // value = null -- caller must drop |
| return e2; |
| } |
| static final Entry<?> DEAD_ENTRY = new Entry<>(null, null); |
| } |
| |
| /** Return the backing map associated with this type. */ |
| private static ClassValueMap getMap(Class<?> type) { |
| // racing type.classValueMap : null (blank) => unique ClassValueMap |
| // if a null is observed, a map is created (lazily, synchronously, uniquely) |
| // all further access to that map is synchronized |
| // Android-changed: Android stores classValueMap in extData. |
| // ClassValueMap map = type.classValueMap; |
| ClassValueMap map = (ClassValueMap) type.ensureExtDataPresent().classValueMap; |
| if (map != null) return map; |
| return initializeMap(type); |
| } |
| |
| private static final Object CRITICAL_SECTION = new Object(); |
| private static final Unsafe UNSAFE = Unsafe.getUnsafe(); |
| private static ClassValueMap initializeMap(Class<?> type) { |
| ClassValueMap map; |
| synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks |
| // happens about once per type |
| // Android-changed: Android stores classValueMap in extData. |
| // if ((map = type.classValueMap) == null) { |
| if ((map = (ClassValueMap) type.ensureExtDataPresent().classValueMap) == null) { |
| map = new ClassValueMap(); |
| // Place a Store fence after construction and before publishing to emulate |
| // ClassValueMap containing final fields. This ensures it can be |
| // published safely in the non-volatile field Class.classValueMap, |
| // since stores to the fields of ClassValueMap will not be reordered |
| // to occur after the store to the field type.classValueMap |
| UNSAFE.storeFence(); |
| |
| // Android-changed: Android stores classValueMap in extData. |
| // type.classValueMap = map; |
| type.ensureExtDataPresent().classValueMap = map; |
| } |
| } |
| return map; |
| } |
| |
| static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) { |
| // Note that explicitVersion might be different from this.version. |
| return new Entry<>(explicitVersion, value); |
| |
| // As soon as the Entry is put into the cache, the value will be |
| // reachable via a data race (as defined by the Java Memory Model). |
| // This race is benign, assuming the value object itself can be |
| // read safely by multiple threads. This is up to the user. |
| // |
| // The entry and version fields themselves can be safely read via |
| // a race because they are either final or have controlled states. |
| // If the pointer from the entry to the version is still null, |
| // or if the version goes immediately dead and is nulled out, |
| // the reader will take the slow path and retry under a lock. |
| } |
| |
| // The following class could also be top level and non-public: |
| |
| /** A backing map for all ClassValues. |
| * Gives a fully serialized "true state" for each pair (ClassValue cv, Class type). |
| * Also manages an unserialized fast-path cache. |
| */ |
| static class ClassValueMap extends WeakHashMap<ClassValue.Identity, Entry<?>> { |
| private Entry<?>[] cacheArray; |
| private int cacheLoad, cacheLoadLimit; |
| |
| /** Number of entries initially allocated to each type when first used with any ClassValue. |
| * It would be pointless to make this much smaller than the Class and ClassValueMap objects themselves. |
| * Must be a power of 2. |
| */ |
| private static final int INITIAL_ENTRIES = 32; |
| |
| /** Build a backing map for ClassValues. |
| * Also, create an empty cache array and install it on the class. |
| */ |
| ClassValueMap() { |
| sizeCache(INITIAL_ENTRIES); |
| } |
| |
| Entry<?>[] getCache() { return cacheArray; } |
| |
| /** Initiate a query. Store a promise (placeholder) if there is no value yet. */ |
| synchronized |
| <T> Entry<T> startEntry(ClassValue<T> classValue) { |
| @SuppressWarnings("unchecked") // one map has entries for all value types <T> |
| Entry<T> e = (Entry<T>) get(classValue.identity); |
| Version<T> v = classValue.version(); |
| if (e == null) { |
| e = v.promise(); |
| // The presence of a promise means that a value is pending for v. |
| // Eventually, finishEntry will overwrite the promise. |
| put(classValue.identity, e); |
| // Note that the promise is never entered into the cache! |
| return e; |
| } else if (e.isPromise()) { |
| // Somebody else has asked the same question. |
| // Let the races begin! |
| if (e.version() != v) { |
| e = v.promise(); |
| put(classValue.identity, e); |
| } |
| return e; |
| } else { |
| // there is already a completed entry here; report it |
| if (e.version() != v) { |
| // There is a stale but valid entry here; make it fresh again. |
| // Once an entry is in the hash table, we don't care what its version is. |
| e = e.refreshVersion(v); |
| put(classValue.identity, e); |
| } |
| // Add to the cache, to enable the fast path, next time. |
| checkCacheLoad(); |
| addToCache(classValue, e); |
| return e; |
| } |
| } |
| |
| /** Finish a query. Overwrite a matching placeholder. Drop stale incoming values. */ |
| synchronized |
| <T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) { |
| @SuppressWarnings("unchecked") // one map has entries for all value types <T> |
| Entry<T> e0 = (Entry<T>) get(classValue.identity); |
| if (e == e0) { |
| // We can get here during exception processing, unwinding from computeValue. |
| assert(e.isPromise()); |
| remove(classValue.identity); |
| return null; |
| } else if (e0 != null && e0.isPromise() && e0.version() == e.version()) { |
| // If e0 matches the intended entry, there has not been a remove call |
| // between the previous startEntry and now. So now overwrite e0. |
| Version<T> v = classValue.version(); |
| if (e.version() != v) |
| e = e.refreshVersion(v); |
| put(classValue.identity, e); |
| // Add to the cache, to enable the fast path, next time. |
| checkCacheLoad(); |
| addToCache(classValue, e); |
| return e; |
| } else { |
| // Some sort of mismatch; caller must try again. |
| return null; |
| } |
| } |
| |
| /** Remove an entry. */ |
| synchronized |
| void removeEntry(ClassValue<?> classValue) { |
| Entry<?> e = remove(classValue.identity); |
| if (e == null) { |
| // Uninitialized, and no pending calls to computeValue. No change. |
| } else if (e.isPromise()) { |
| // State is uninitialized, with a pending call to finishEntry. |
| // Since remove is a no-op in such a state, keep the promise |
| // by putting it back into the map. |
| put(classValue.identity, e); |
| } else { |
| // In an initialized state. Bump forward, and de-initialize. |
| classValue.bumpVersion(); |
| // Make all cache elements for this guy go stale. |
| removeStaleEntries(classValue); |
| } |
| } |
| |
| /** Change the value for an entry. */ |
| synchronized |
| <T> void changeEntry(ClassValue<T> classValue, T value) { |
| @SuppressWarnings("unchecked") // one map has entries for all value types <T> |
| Entry<T> e0 = (Entry<T>) get(classValue.identity); |
| Version<T> version = classValue.version(); |
| if (e0 != null) { |
| if (e0.version() == version && e0.value() == value) |
| // no value change => no version change needed |
| return; |
| classValue.bumpVersion(); |
| removeStaleEntries(classValue); |
| } |
| Entry<T> e = makeEntry(version, value); |
| put(classValue.identity, e); |
| // Add to the cache, to enable the fast path, next time. |
| checkCacheLoad(); |
| addToCache(classValue, e); |
| } |
| |
| /// -------- |
| /// Cache management. |
| /// -------- |
| |
| // Statics do not need synchronization. |
| |
| /** Load the cache entry at the given (hashed) location. */ |
| static Entry<?> loadFromCache(Entry<?>[] cache, int i) { |
| // non-racing cache.length : constant |
| // racing cache[i & (mask)] : null <=> Entry |
| return cache[i & (cache.length-1)]; |
| // invariant: returned value is null or well-constructed (ready to match) |
| } |
| |
| /** Look in the cache, at the home location for the given ClassValue. */ |
| static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) { |
| return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache)); |
| } |
| |
| /** Given that first probe was a collision, retry at nearby locations. */ |
| static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) { |
| if (PROBE_LIMIT <= 0) return null; |
| // Probe the cache carefully, in a range of slots. |
| int mask = (cache.length-1); |
| int home = (classValue.hashCodeForCache & mask); |
| Entry<?> e2 = cache[home]; // victim, if we find the real guy |
| if (e2 == null) { |
| return null; // if nobody is at home, no need to search nearby |
| } |
| // assume !classValue.match(e2), but do not assert, because of races |
| int pos2 = -1; |
| for (int i = home + 1; i < home + PROBE_LIMIT; i++) { |
| Entry<?> e = cache[i & mask]; |
| if (e == null) { |
| break; // only search within non-null runs |
| } |
| if (classValue.match(e)) { |
| // relocate colliding entry e2 (from cache[home]) to first empty slot |
| cache[home] = e; |
| if (pos2 >= 0) { |
| cache[i & mask] = Entry.DEAD_ENTRY; |
| } else { |
| pos2 = i; |
| } |
| cache[pos2 & mask] = ((entryDislocation(cache, pos2, e2) < PROBE_LIMIT) |
| ? e2 // put e2 here if it fits |
| : Entry.DEAD_ENTRY); |
| return classValue.castEntry(e); |
| } |
| // Remember first empty slot, if any: |
| if (!e.isLive() && pos2 < 0) pos2 = i; |
| } |
| return null; |
| } |
| |
| /** How far out of place is e? */ |
| private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) { |
| ClassValue<?> cv = e.classValueOrNull(); |
| if (cv == null) return 0; // entry is not live! |
| int mask = (cache.length-1); |
| return (pos - cv.hashCodeForCache) & mask; |
| } |
| |
| /// -------- |
| /// Below this line all functions are private, and assume synchronized access. |
| /// -------- |
| |
| private void sizeCache(int length) { |
| assert((length & (length-1)) == 0); // must be power of 2 |
| cacheLoad = 0; |
| cacheLoadLimit = (int) ((double) length * CACHE_LOAD_LIMIT / 100); |
| cacheArray = new Entry<?>[length]; |
| } |
| |
| /** Make sure the cache load stays below its limit, if possible. */ |
| private void checkCacheLoad() { |
| if (cacheLoad >= cacheLoadLimit) { |
| reduceCacheLoad(); |
| } |
| } |
| private void reduceCacheLoad() { |
| removeStaleEntries(); |
| if (cacheLoad < cacheLoadLimit) |
| return; // win |
| Entry<?>[] oldCache = getCache(); |
| if (oldCache.length > HASH_MASK) |
| return; // lose |
| sizeCache(oldCache.length * 2); |
| for (Entry<?> e : oldCache) { |
| if (e != null && e.isLive()) { |
| addToCache(e); |
| } |
| } |
| } |
| |
| /** Remove stale entries in the given range. |
| * Should be executed under a Map lock. |
| */ |
| private void removeStaleEntries(Entry<?>[] cache, int begin, int count) { |
| if (PROBE_LIMIT <= 0) return; |
| int mask = (cache.length-1); |
| int removed = 0; |
| for (int i = begin; i < begin + count; i++) { |
| Entry<?> e = cache[i & mask]; |
| if (e == null || e.isLive()) |
| continue; // skip null and live entries |
| Entry<?> replacement = null; |
| if (PROBE_LIMIT > 1) { |
| // avoid breaking up a non-null run |
| replacement = findReplacement(cache, i); |
| } |
| cache[i & mask] = replacement; |
| if (replacement == null) removed += 1; |
| } |
| cacheLoad = Math.max(0, cacheLoad - removed); |
| } |
| |
| /** Clearing a cache slot risks disconnecting following entries |
| * from the head of a non-null run, which would allow them |
| * to be found via reprobes. Find an entry after cache[begin] |
| * to plug into the hole, or return null if none is needed. |
| */ |
| private Entry<?> findReplacement(Entry<?>[] cache, int home1) { |
| Entry<?> replacement = null; |
| int haveReplacement = -1, replacementPos = 0; |
| int mask = (cache.length-1); |
| for (int i2 = home1 + 1; i2 < home1 + PROBE_LIMIT; i2++) { |
| Entry<?> e2 = cache[i2 & mask]; |
| if (e2 == null) break; // End of non-null run. |
| if (!e2.isLive()) continue; // Doomed anyway. |
| int dis2 = entryDislocation(cache, i2, e2); |
| if (dis2 == 0) continue; // e2 already optimally placed |
| int home2 = i2 - dis2; |
| if (home2 <= home1) { |
| // e2 can replace entry at cache[home1] |
| if (home2 == home1) { |
| // Put e2 exactly where he belongs. |
| haveReplacement = 1; |
| replacementPos = i2; |
| replacement = e2; |
| } else if (haveReplacement <= 0) { |
| haveReplacement = 0; |
| replacementPos = i2; |
| replacement = e2; |
| } |
| // And keep going, so we can favor larger dislocations. |
| } |
| } |
| if (haveReplacement >= 0) { |
| if (cache[(replacementPos+1) & mask] != null) { |
| // Be conservative, to avoid breaking up a non-null run. |
| cache[replacementPos & mask] = (Entry<?>) Entry.DEAD_ENTRY; |
| } else { |
| cache[replacementPos & mask] = null; |
| cacheLoad -= 1; |
| } |
| } |
| return replacement; |
| } |
| |
| /** Remove stale entries in the range near classValue. */ |
| private void removeStaleEntries(ClassValue<?> classValue) { |
| removeStaleEntries(getCache(), classValue.hashCodeForCache, PROBE_LIMIT); |
| } |
| |
| /** Remove all stale entries, everywhere. */ |
| private void removeStaleEntries() { |
| Entry<?>[] cache = getCache(); |
| removeStaleEntries(cache, 0, cache.length + PROBE_LIMIT - 1); |
| } |
| |
| /** Add the given entry to the cache, in its home location, unless it is out of date. */ |
| private <T> void addToCache(Entry<T> e) { |
| ClassValue<T> classValue = e.classValueOrNull(); |
| if (classValue != null) |
| addToCache(classValue, e); |
| } |
| |
| /** Add the given entry to the cache, in its home location. */ |
| private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) { |
| if (PROBE_LIMIT <= 0) return; // do not fill cache |
| // Add e to the cache. |
| Entry<?>[] cache = getCache(); |
| int mask = (cache.length-1); |
| int home = classValue.hashCodeForCache & mask; |
| Entry<?> e2 = placeInCache(cache, home, e, false); |
| if (e2 == null) return; // done |
| if (PROBE_LIMIT > 1) { |
| // try to move e2 somewhere else in his probe range |
| int dis2 = entryDislocation(cache, home, e2); |
| int home2 = home - dis2; |
| for (int i2 = home2; i2 < home2 + PROBE_LIMIT; i2++) { |
| if (placeInCache(cache, i2 & mask, e2, true) == null) { |
| return; |
| } |
| } |
| } |
| // Note: At this point, e2 is just dropped from the cache. |
| } |
| |
| /** Store the given entry. Update cacheLoad, and return any live victim. |
| * 'Gently' means return self rather than dislocating a live victim. |
| */ |
| private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) { |
| Entry<?> e2 = overwrittenEntry(cache[pos]); |
| if (gently && e2 != null) { |
| // do not overwrite a live entry |
| return e; |
| } else { |
| cache[pos] = e; |
| return e2; |
| } |
| } |
| |
| /** Note an entry that is about to be overwritten. |
| * If it is not live, quietly replace it by null. |
| * If it is an actual null, increment cacheLoad, |
| * because the caller is going to store something |
| * in its place. |
| */ |
| private <T> Entry<T> overwrittenEntry(Entry<T> e2) { |
| if (e2 == null) cacheLoad += 1; |
| else if (e2.isLive()) return e2; |
| return null; |
| } |
| |
| /** Percent loading of cache before resize. */ |
| private static final int CACHE_LOAD_LIMIT = 67; // 0..100 |
| /** Maximum number of probes to attempt. */ |
| private static final int PROBE_LIMIT = 6; // 1.. |
| // N.B. Set PROBE_LIMIT=0 to disable all fast paths. |
| } |
| } |
