/*
* Copyright 2010 JBoss Inc
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.drools.core.util;
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
import java.io.Serializable;
import java.util.concurrent.locks.ReentrantLock;
import org.drools.common.InternalFactHandle;
import org.drools.core.util.AbstractHashTable.DoubleCompositeIndex;
import org.drools.core.util.AbstractHashTable.FieldIndex;
import org.drools.core.util.AbstractHashTable.Index;
import org.drools.core.util.AbstractHashTable.SingleIndex;
import org.drools.core.util.AbstractHashTable.TripleCompositeIndex;
import org.drools.core.util.index.RightTupleIndexHashTable;
import org.drools.core.util.index.RightTupleList;
import org.drools.reteoo.LeftTuple;
import org.drools.reteoo.RightTuple;
public class ConcurrentHashTable {
private static final long serialVersionUID = 510l;
/*
* The basic strategy is to subdivide the table among Segments,
* each of which itself is a concurrently readable hash table.
*/
/* ---------------- Constants -------------- */
/**
* The default initial capacity for this table,
* used when not otherwise specified in a constructor.
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* The default load factor for this table, used when not
* otherwise specified in a constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The default concurrency level for this table, used when not
* otherwise specified in a constructor.
*/
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/**
* The maximum capacity, used if a higher value is implicitly
* specified by either of the constructors with arguments. MUST
* be a power of two <= 1<<30 to ensure that entries are indexable
* using ints.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The maximum number of segments to allow; used to bound
* constructor arguments.
*/
static final int MAX_SEGMENTS = 1 << 16; // slightly conservative
/**
* Number of unsynchronized retries in size and containsValue
* methods before resorting to locking. This is used to avoid
* unbounded retries if tables undergo continuous modification
* which would make it impossible to obtain an accurate result.
*/
static final int RETRIES_BEFORE_LOCK = 2;
/* ---------------- Fields -------------- */
/**
* Mask value for indexing into segments. The upper bits of a
* key's hash code are used to choose the segment.
*/
final int segmentMask;
/**
* Shift value for indexing within segments.
*/
final int segmentShift;
/**
* The segments, each of which is a specialized hash table
*/
final Segment[] segments;
private Index index;
private int startResult;
/* ---------------- Small Utilities -------------- */
/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions. This is critical
* because ConcurrentHashMap uses power-of-two length hash tables,
* that otherwise encounter collisions for hashCodes that do not
* differ in lower or upper bits.
*/
private static int hash(int h) {
// Spread bits to regularize both segment and index locations,
// using variant of single-word Wang/Jenkins hash.
h += (h << 15) ^ 0xffffcd7d;
h ^= (h >>> 10);
h += (h << 3);
h ^= (h >>> 6);
h += (h << 2) + (h << 14);
return h ^ (h >>> 16);
}
/**
* Returns the segment that should be used for key with given hash
* @param hash the hash code for the key
* @return the segment
*/
final Segment segmentFor(int hash) {
return segments[(hash >>> segmentShift) & segmentMask];
}
/* ---------------- Inner Classes -------------- */
/**
* Segments are specialized versions of hash tables. This
* subclasses from ReentrantLock opportunistically, just to
* simplify some locking and avoid separate construction.
*/
static final class Segment extends ReentrantLock
implements
Serializable {
/*
* Segments maintain a table of entry lists that are ALWAYS
* kept in a consistent state, so can be read without locking.
* Next fields of nodes are immutable (final). All list
* additions are performed at the front of each bin. This
* makes it easy to check changes, and also fast to traverse.
* When nodes would otherwise be changed, new nodes are
* created to replace them. This works well for hash tables
* since the bin lists tend to be short. (The average length
* is less than two for the default load factor threshold.)
*
* Read operations can thus proceed without locking, but rely
* on selected uses of volatiles to ensure that completed
* write operations performed by other threads are
* noticed. For most purposes, the "count" field, tracking the
* number of elements, serves as that volatile variable
* ensuring visibility. This is convenient because this field
* needs to be read in many read operations anyway:
*
* - All (unsynchronized) read operations must first read the
* "count" field, and should not look at table entries if
* it is 0.
*
* - All (synchronized) write operations should write to
* the "count" field after structurally changing any bin.
* The operations must not take any action that could even
* momentarily cause a concurrent read operation to see
* inconsistent data. This is made easier by the nature of
* the read operations in Map. For example, no operation
* can reveal that the table has grown but the threshold
* has not yet been updated, so there are no atomicity
* requirements for this with respect to reads.
*
* As a guide, all critical volatile reads and writes to the
* count field are marked in code comments.
*/
private static final long serialVersionUID = 510l;
/**
* The number of elements in this segment's region.
*/
transient volatile int tupleCount;
transient volatile int keyCount;
/**
* Number of updates that alter the size of the table. This is
* used during bulk-read methods to make sure they see a
* consistent snapshot: If modCounts change during a traversal
* of segments computing size or checking containsValue, then
* we might have an inconsistent view of state so (usually)
* must retry.
*/
transient int modCount;
/**
* The table is rehashed when its size exceeds this threshold.
* (The value of this field is always <tt>(int)(capacity *
* loadFactor)</tt>.)
*/
transient int threshold;
/**
* The per-segment table.
*/
transient volatile RightTupleList[] table;
/**
* The load factor for the hash table. Even though this value
* is same for all segments, it is replicated to avoid needing
* links to outer object.
* @serial
*/
final float loadFactor;
private Index index;
Segment(Index index,
int initialCapacity,
float lf) {
loadFactor = lf;
setTable( new RightTupleList[initialCapacity] );
this.index = index;
}
static final Segment[] newArray(int i) {
return new Segment[i];
}
/**
* Sets table to new HashEntry array.
* Call only while holding lock or in constructor.
*/
void setTable(RightTupleList[] newTable) {
threshold = (int) (newTable.length * loadFactor);
table = newTable;
}
/**
* Returns properly casted first entry of bin for given hash.
*/
RightTupleList getFirst(int hash) {
RightTupleList[] tab = table;
return tab[hash & (tab.length - 1)];
}
/* Specialized implementations of map methods */
// Object get(Object key, int hash) {
// if (count != 0) { // read-volatile
// RightTuple e = getFirst(hash);
// while (e != null) {
// if (e.hash == hash && key.equals(e.key)) {
// Object v = e.value;
// if (v != null)
// return v;
// return readValueUnderLock(e); // recheck
// }
// e = e.next;
// }
// }
// return null;
// }
//
// boolean containsKey(Object key, int hash) {
// if (count != 0) { // read-volatile
// HashEntry e = getFirst(hash);
// while (e != null) {
// if (e.hash == hash && key.equals(e.key))
// return true;
// e = e.next;
// }
// }
// return false;
// }
void add(final RightTuple rightTuple,
int hashCode,
Object object) {
lock();
try {
final RightTupleList entry = getOrCreate( hashCode,
object );
entry.add( rightTuple );
this.tupleCount++;
} finally {
unlock();
}
}
/**
* Remove; match on key only if value null, else match both.\
*/
void remove(RightTuple rightTuple,
int hashCode,
Object object) {
lock();
try {
int c = keyCount - 1;
RightTupleList[] tab = table;
int index = hashCode & (tab.length - 1);
RightTupleList first = tab[index];
RightTupleList e = first;
while ( e != null ) {
if ( e.matches( object,
hashCode ) ) {
break;
}
e = (RightTupleList) e.next;
}
e.remove( rightTuple );
tupleCount--;
if ( e.getFirst( ) == null ) {
// list is empty, so remove it
RightTupleList newFirst = (RightTupleList) e.getNext();
for ( RightTupleList p = first; p != e; p = (RightTupleList) p.getNext() ) {
newFirst = new RightTupleList( p.getIndex(),
hashCode,
newFirst );
}
keyCount = c; // write-volatile
}
} finally {
unlock();
}
}
RightTupleList get(final int hashCode,
final LeftTuple tuple,
final InternalFactHandle factHandle) {
//this.index.setCachedValue( tuple );
lock();
try {
RightTupleList[] tab = table;
int index = hashCode & (tab.length - 1);
RightTupleList first = tab[index];
RightTupleList entry = first;
while ( entry != null ) {
if ( entry.matches( tuple,
hashCode ,
factHandle ) ) {
return entry;
}
entry = (RightTupleList) entry.getNext();
}
return entry;
} finally {
unlock();
}
}
private RightTupleList getOrCreate(int hashCode,
final Object object) {
int c = keyCount;
RightTupleList[] tab = table;
int index = hashCode & (tab.length - 1);
RightTupleList first = tab[index];
RightTupleList e = first;
while ( e != null ) {
if ( e.matches( object,
hashCode ) ) {
return e;
}
e = (RightTupleList) e.next;
}
if ( e == null ) {
if ( c++ > threshold ) // ensure capacity
rehash();
++modCount;
e = new RightTupleList( this.index,
hashCode,
first );
tab[index] = e;
keyCount = c; // write-volatile
}
return e;
}
void rehash() {
RightTupleList[] oldTable = table;
int oldCapacity = oldTable.length;
if ( oldCapacity >= MAXIMUM_CAPACITY ) return;
/*
* Reclassify nodes in each list to new Map. Because we are
* using power-of-two expansion, the elements from each bin
* must either stay at same index, or move with a power of two
* offset. We eliminate unnecessary node creation by catching
* cases where old nodes can be reused because their next
* fields won't change. Statistically, at the default
* threshold, only about one-sixth of them need cloning when
* a table doubles. The nodes they replace will be garbage
* collectable as soon as they are no longer referenced by any
* reader thread that may be in the midst of traversing table
* right now.
*/
RightTupleList[] newTable = new RightTupleList[oldCapacity << 1];
threshold = (int) (newTable.length * loadFactor);
int sizeMask = newTable.length - 1;
for ( int i = 0; i < oldCapacity; i++ ) {
// We need to guarantee that any existing reads of old Map can
// proceed. So we cannot yet null out each bin.
RightTupleList e = oldTable[i];
if ( e != null ) {
RightTupleList next = (RightTupleList) e.getNext();
int idx = e.hashCode() & sizeMask;
// Single node on list
if ( next == null ) newTable[idx] = e;
else {
// Reuse trailing consecutive sequence at same slot
RightTupleList lastRun = e;
int lastIdx = idx;
for ( RightTupleList last = next; last != null; last = (RightTupleList) last.getNext() ) {
int k = last.hashCode() & sizeMask;
if ( k != lastIdx ) {
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun;
// Clone all remaining nodes
for ( RightTupleList p = e; p != lastRun; p = (RightTupleList) p.getNext() ) {
int k = p.hashCode() & sizeMask;
RightTupleList n = newTable[k];
newTable[k] = new RightTupleList( p, n );
}
}
}
}
table = newTable;
}
void clear() {
if ( tupleCount != 0 ) {
lock();
try {
RightTupleList[] tab = table;
for ( int i = 0; i < tab.length; i++ )
tab[i] = null;
++modCount;
tupleCount = 0; // write-volatile
keyCount = 0;
} finally {
unlock();
}
}
}
}
/* ---------------- Public operations -------------- */
/**
* Creates a new, empty map with the specified initial
* capacity, load factor and concurrency level.
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
* @param concurrencyLevel the estimated number of concurrently
* updating threads. The implementation performs internal sizing
* to try to accommodate this many threads.
* @throws IllegalArgumentException if the initial capacity is
* negative or the load factor or concurrencyLevel are
* nonpositive.
*/
public ConcurrentHashTable(final FieldIndex[] index,
int initialCapacity,
float loadFactor,
int concurrencyLevel) {
this.startResult = RightTupleIndexHashTable.PRIME;
for ( int i = 0, length = index.length; i < length; i++ ) {
this.startResult = RightTupleIndexHashTable.PRIME * this.startResult + index[i].getExtractor().getIndex();
}
switch ( index.length ) {
case 0 :
throw new IllegalArgumentException( "FieldIndexHashTable cannot use an index[] of length 0" );
case 1 :
this.index = new SingleIndex( index,
this.startResult );
break;
case 2 :
this.index = new DoubleCompositeIndex( index,
this.startResult );
break;
case 3 :
this.index = new TripleCompositeIndex( index,
this.startResult );
break;
default :
throw new IllegalArgumentException( "FieldIndexHashTable cannot use an index[] of length great than 3" );
}
if ( !(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0 ) throw new IllegalArgumentException();
if ( concurrencyLevel > MAX_SEGMENTS ) concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;
while ( ssize < concurrencyLevel ) {
++sshift;
ssize <<= 1;
}
segmentShift = 32 - sshift;
segmentMask = ssize - 1;
this.segments = Segment.newArray( ssize );
if ( initialCapacity > MAXIMUM_CAPACITY ) initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if ( c * ssize < initialCapacity ) ++c;
int cap = 1;
while ( cap < c )
cap <<= 1;
for ( int i = 0; i < this.segments.length; ++i )
this.segments[i] = new Segment( this.index,
cap,
loadFactor );
}
/**
* Creates a new, empty map with the specified initial capacity
* and load factor and with the default concurrencyLevel (16).
*
* @param initialCapacity The implementation performs internal
* sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
* @throws IllegalArgumentException if the initial capacity of
* elements is negative or the load factor is nonpositive
*
* @since 1.6
*/
public ConcurrentHashTable(final FieldIndex[] index,
int initialCapacity,
float loadFactor) {
this( index,
initialCapacity,
loadFactor,
DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Creates a new, empty map with the specified initial capacity,
* and with default load factor (0.75) and concurrencyLevel (16).
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @throws IllegalArgumentException if the initial capacity of
* elements is negative.
*/
public ConcurrentHashTable(final FieldIndex[] index,
int initialCapacity) {
this( index,
initialCapacity,
DEFAULT_LOAD_FACTOR,
DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Creates a new, empty map with a default initial capacity (16),
* load factor (0.75) and concurrencyLevel (16).
*/
public ConcurrentHashTable(final FieldIndex[] index) {
this( index,
DEFAULT_INITIAL_CAPACITY,
DEFAULT_LOAD_FACTOR,
DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
public boolean isEmpty() {
final Segment[] segments = this.segments;
/*
* We keep track of per-segment modCounts to avoid ABA
* problems in which an element in one segment was added and
* in another removed during traversal, in which case the
* table was never actually empty at any point. Note the
* similar use of modCounts in the size() and containsValue()
* methods, which are the only other methods also susceptible
* to ABA problems.
*/
int[] mc = new int[segments.length];
int mcsum = 0;
for ( int i = 0; i < segments.length; ++i ) {
if ( segments[i].tupleCount != 0 ) return false;
else mcsum += mc[i] = segments[i].modCount;
}
// If mcsum happens to be zero, then we know we got a snapshot
// before any modifications at all were made. This is
// probably common enough to bother tracking.
if ( mcsum != 0 ) {
for ( int i = 0; i < segments.length; ++i ) {
if ( segments[i].tupleCount != 0 || mc[i] != segments[i].modCount ) return false;
}
}
return true;
}
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of key-value mappings in this map
*/
public int size() {
final Segment[] segments = this.segments;
long sum = 0;
long check = 0;
int[] mc = new int[segments.length];
// Try a few times to get accurate count. On failure due to
// continuous async changes in table, resort to locking.
for ( int k = 0; k < RETRIES_BEFORE_LOCK; ++k ) {
check = 0;
sum = 0;
int mcsum = 0;
for ( int i = 0; i < segments.length; ++i ) {
sum += segments[i].tupleCount;
mcsum += mc[i] = segments[i].modCount;
}
if ( mcsum != 0 ) {
for ( int i = 0; i < segments.length; ++i ) {
check += segments[i].tupleCount;
if ( mc[i] != segments[i].modCount ) {
check = -1; // force retry
break;
}
}
}
if ( check == sum ) break;
}
if ( check != sum ) { // Resort to locking all segments
sum = 0;
for ( int i = 0; i < segments.length; ++i )
segments[i].lock();
for ( int i = 0; i < segments.length; ++i )
sum += segments[i].tupleCount;
for ( int i = 0; i < segments.length; ++i )
segments[i].unlock();
}
if ( sum > Integer.MAX_VALUE ) return Integer.MAX_VALUE;
else return (int) sum;
}
public void add(final RightTuple rightTuple) {
Object object = rightTuple.getFactHandle().getObject();
final int hashCode = this.index.hashCodeOf( object );
segmentFor( hashCode ).add( rightTuple,
hashCode,
object );
}
/**
* Removes the key (and its corresponding value) from this map.
* This method does nothing if the key is not in the map.
*
* @param key the key that needs to be removed
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>
* @throws NullPointerException if the specified key is null
*/
public void remove(final RightTuple rightTuple) {
Object object = rightTuple.getFactHandle().getObject();
final int hashCode = this.index.hashCodeOf( object );
segmentFor( hashCode ).remove( rightTuple,
hashCode,
object );
}
public RightTupleList get(final LeftTuple tuple, InternalFactHandle factHandle) {
final int hashCode = this.index.hashCodeOf( tuple );
return segmentFor( hashCode ).get( hashCode,
tuple,
factHandle);
}
/**
* Removes all of the mappings from this map.
*/
public void clear() {
for ( int i = 0; i < segments.length; ++i )
segments[i].clear();
}
}