JDK8/Java8源码在线阅读
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*
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* accompanied this code).
*
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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<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 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<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 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
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 <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, 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<ClassValue.Identity, Entry<?>> {
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
<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) {
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