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/* * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.lang; import java.lang.ClassValue.ClassValueMap; import java.util.WeakHashMap; import java.lang.ref.WeakReference; import java.util.concurrent.atomic.AtomicInteger; 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 { /** * Sole constructor. (For invocation by subclass constructors, typically * implicit.) */ protected ClassValue() { } /** * Computes the given class's derived value for this {@code ClassValue}. *

* This method will be invoked within the first thread that accesses * the value with the {@link #get get} method. *

* 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}. *

* 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. *

* 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. *

* 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}. *

* 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 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. *

* 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. *

* 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}. *

* 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. *

* 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}. *

* Otherwise, whether the new state is even or odd, * {@code T} will discard the newly computed value * and retry the {@code get} operation. *

* Discarding and retrying is an important proviso, * since otherwise {@code T} could potentially install * a disastrously stale value. For example: *

* 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 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] ClassValueMap map = type.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 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 castEntry(Entry e) { return (Entry) 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 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.) } /** 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. *

* 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. *

* If version were not volatile, one thread T1 could persistently hold onto * a stale value this.value == V1, while 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. *

* 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 version = new Version<>(this); Version version() { return version; } void bumpVersion() { version = new Version<>(this); } static class Version { private final ClassValue classValue; private final Entry promise = new Entry<>(this); Version(ClassValue classValue) { this.classValue = classValue; } ClassValue classValue() { return classValue; } Entry promise() { return promise; } boolean isLive() { return classValue.version() == this; } } /** One binding of a value to a class via a ClassValue. * States are:

    *
  • promise if value == Entry.this *
  • else dead if version == null *
  • else stale if version != classValue.version *
  • else live
* 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 extends WeakReference> { final Object value; // usually of type T, but sometimes (Entry)this Entry(Version 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 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 version() { return get(); } ClassValue classValueOrNull() { Version v = version(); return (v == null) ? null : v.classValue(); } boolean isLive() { Version v = version(); if (v == null) return false; if (v.isLive()) return true; clear(); return false; } Entry refreshVersion(Version v2) { assertNotPromise(); @SuppressWarnings("unchecked") // if !isPromise, type is T Entry 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 ClassValueMap map = type.classValueMap; if (map != null) return map; return initializeMap(type); } private static final Object CRITICAL_SECTION = new Object(); private static ClassValueMap initializeMap(Class type) { ClassValueMap map; synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks // happens about once per type if ((map = type.classValueMap) == null) type.classValueMap = map = new ClassValueMap(type); } return map; } static Entry makeEntry(Version 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, relative a single given type. * 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> { private final Class type; 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, relative the given type. * Also, create an empty cache array and install it on the class. */ ClassValueMap(Class type) { this.type = type; sizeCache(INITIAL_ENTRIES); } Entry[] getCache() { return cacheArray; } /** Initiate a query. Store a promise (placeholder) if there is no value yet. */ synchronized Entry startEntry(ClassValue classValue) { @SuppressWarnings("unchecked") // one map has entries for all value types Entry e = (Entry) get(classValue.identity); Version 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 Entry finishEntry(ClassValue classValue, Entry e) { @SuppressWarnings("unchecked") // one map has entries for all value types Entry e0 = (Entry) 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 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 void changeEntry(ClassValue classValue, T value) { @SuppressWarnings("unchecked") // one map has entries for all value types Entry e0 = (Entry) get(classValue.identity); Version 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 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 Entry probeHomeLocation(Entry[] cache, ClassValue classValue) { return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache)); } /** Given that first probe was a collision, retry at nearby locations. */ static Entry probeBackupLocations(Entry[] cache, ClassValue 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 void addToCache(Entry e) { ClassValue classValue = e.classValueOrNull(); if (classValue != null) addToCache(classValue, e); } /** Add the given entry to the cache, in its home location. */ private void addToCache(ClassValue classValue, Entry 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 Entry overwrittenEntry(Entry 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. } }
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