源码来源地址:https://github.com/netty/netty/tree/4.0
不恰当的数据结构导致内存在占用过大。这个问题,就完全可以使用 Netty 的IntObjectHashMap 数据结构来解决,只需要换个类,就可以节省非常多的资源。
相同点:
IntObjectHashMap和HashMap都是以键值对的形式,储存对象。
不同点:
IntObjectHashMap的键只能是int类型,值是Object类型。
HashMap的键和值都是Object类型。
目录
一、IntObjectMap.java
package com; import java.util.Collection; /** * Interface for a primitive map that uses {@code int}s as keys. * * @param <V> the value type stored in the map. */ public interface IntObjectMap<V> { /** * An Entry in the map. * * @param <V> the value type stored in the map. */ interface Entry<V> { /** * Gets the key for this entry. */ int key(); /** * Gets the value for this entry. */ V value(); /** * Sets the value for this entry. */ void setValue(V value); } /** * Gets the value in the map with the specified key. * * @param key the key whose associated value is to be returned. * @return the value or {@code null} if the key was not found in the map. */ V get(int key); /** * Puts the given entry into the map. * * @param key the key of the entry. * @param value the value of the entry. * @return the previous value for this key or {@code null} if there was no * previous mapping. */ V put(int key, V value); /** * Puts all of the entries from the given map into this map. */ void putAll(IntObjectMap<V> sourceMap); /** * Removes the entry with the specified key. * * @param key the key for the entry to be removed from this map. * @return the previous value for the key, or {@code null} if there was no * mapping. */ V remove(int key); /** * Returns the number of entries contained in this map. */ int size(); /** * Indicates whether or not this map is empty (i.e {@link #size()} == * {@code 0]). */ boolean isEmpty(); /** * Clears all entries from this map. */ void clear(); /** * Indicates whether or not this map contains a value for the specified key. */ boolean containsKey(int key); /** * Indicates whether or not the map contains the specified value. */ boolean containsValue(V value); /** * Gets an iterable collection of the entries contained in this map. */ Iterable<Entry<V>> entries(); /** * Gets the keys contained in this map. */ int[] keys(); /** * Gets the values contained in this map. */ V[] values(Class<V> clazz); /** * Gets the values contatins in this map as a {@link Collection}. */ Collection<V> values(); }
二、IntObjectHashMap.java
package com; /* * Copyright 2014 The Netty Project * * The Netty Project licenses this file to you 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. */ import java.lang.reflect.Array; import java.util.AbstractCollection; import java.util.Arrays; import java.util.Collection; import java.util.Iterator; import java.util.NoSuchElementException; /** * A hash map implementation of {@link IntObjectMap} that uses open addressing * for keys. To minimize the memory footprint, this class uses open addressing * rather than chaining. Collisions are resolved using linear probing. Deletions * implement compaction, so cost of remove can approach O(N) for full maps, * which makes a small loadFactor recommended. * * @param <V> The value type stored in the map. */ public class IntObjectHashMap<V> implements IntObjectMap<V>, Iterable<IntObjectMap.Entry<V>> { /** Default initial capacity. Used if not specified in the constructor */ private static final int DEFAULT_CAPACITY = 11; /** Default load factor. Used if not specified in the constructor */ private static final float DEFAULT_LOAD_FACTOR = 0.5f; /** * Placeholder for null values, so we can use the actual null to mean available. * (Better than using a placeholder for available: less references for GC * processing.) */ private static final Object NULL_VALUE = new Object(); /** The maximum number of elements allowed without allocating more space. */ private int maxSize; /** The load factor for the map. Used to calculate {@link #maxSize}. */ private final float loadFactor; private int[] keys; private V[] values; private Collection<V> valueCollection; private int size; public IntObjectHashMap() { this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR); } public IntObjectHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } public IntObjectHashMap(int initialCapacity, float loadFactor) { if (initialCapacity < 1) { throw new IllegalArgumentException("initialCapacity must be >= 1"); } if (loadFactor <= 0.0f || loadFactor > 1.0f) { // Cannot exceed 1 because we can never store more than capacity elements; // using a bigger loadFactor would trigger rehashing before the desired load is // reached. throw new IllegalArgumentException("loadFactor must be > 0 and <= 1"); } this.loadFactor = loadFactor; // Adjust the initial capacity if necessary. int capacity = adjustCapacity(initialCapacity); // Allocate the arrays. keys = new int[capacity]; @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" }) V[] temp = (V[]) new Object[capacity]; values = temp; // Initialize the maximum size value. maxSize = calcMaxSize(capacity); } private static <T> T toExternal(T value) { return value == NULL_VALUE ? null : value; } @SuppressWarnings("unchecked") private static <T> T toInternal(T value) { return value == null ? (T) NULL_VALUE : value; } @Override public V get(int key) { int index = indexOf(key); return index == -1 ? null : toExternal(values[index]); } @Override public V put(int key, V value) { int startIndex = hashIndex(key); int index = startIndex; for (;;) { if (values[index] == null) { // Found empty slot, use it. keys[index] = key; values[index] = toInternal(value); growSize(); return null; } if (keys[index] == key) { // Found existing entry with this key, just replace the value. V previousValue = values[index]; values[index] = toInternal(value); return toExternal(previousValue); } // Conflict, keep probing ... if ((index = probeNext(index)) == startIndex) { // Can only happen if the map was full at MAX_ARRAY_SIZE and couldn't grow. throw new IllegalStateException("Unable to insert"); } } } private int probeNext(int index) { return index == values.length - 1 ? 0 : index + 1; } @Override public void putAll(IntObjectMap<V> sourceMap) { if (sourceMap instanceof IntObjectHashMap) { // Optimization - iterate through the arrays. IntObjectHashMap<V> source = (IntObjectHashMap<V>) sourceMap; for (int i = 0; i < source.values.length; ++i) { V sourceValue = source.values[i]; if (sourceValue != null) { put(source.keys[i], sourceValue); } } return; } // Otherwise, just add each entry. for (Entry<V> entry : sourceMap.entries()) { put(entry.key(), entry.value()); } } @Override public V remove(int key) { int index = indexOf(key); if (index == -1) { return null; } V prev = values[index]; removeAt(index); return toExternal(prev); } @Override public int size() { return size; } @Override public boolean isEmpty() { return size == 0; } @Override public void clear() { Arrays.fill(keys, 0); Arrays.fill(values, null); size = 0; } @Override public boolean containsKey(int key) { return indexOf(key) >= 0; } @Override public boolean containsValue(V value) { V v1 = toInternal(value); for (V v2 : values) { // The map supports null values; this will be matched as // NULL_VALUE.equals(NULL_VALUE). if (v2 != null && v2.equals(v1)) { return true; } } return false; } @Override public Iterable<Entry<V>> entries() { return this; } @Override public Iterator<Entry<V>> iterator() { return new IteratorImpl(); } @Override public int[] keys() { int[] outKeys = new int[size()]; int targetIx = 0; for (int i = 0; i < values.length; ++i) { if (values[i] != null) { outKeys[targetIx++] = keys[i]; } } return outKeys; } @Override public V[] values(Class<V> clazz) { @SuppressWarnings("unchecked") V[] outValues = (V[]) Array.newInstance(clazz, size()); int targetIx = 0; for (V value : values) { if (value != null) { outValues[targetIx++] = value; } } return outValues; } @Override public Collection<V> values() { Collection<V> valueCollection = this.valueCollection; if (valueCollection == null) { this.valueCollection = valueCollection = new AbstractCollection<V>() { @Override public Iterator<V> iterator() { return new Iterator<V>() { final Iterator<Entry<V>> iter = IntObjectHashMap.this.iterator(); @Override public boolean hasNext() { return iter.hasNext(); } @Override public V next() { return iter.next().value(); } @Override public void remove() { throw new UnsupportedOperationException(); } }; } @Override public int size() { return size; } }; } return valueCollection; } @Override public int hashCode() { // Hashcode is based on all non-zero, valid keys. We have to scan the whole keys // array, which may have different lengths for two maps of same size(), so the // capacity cannot be used as input for hashing but the size can. int hash = size; for (int key : keys) { // 0 can be a valid key or unused slot, but won't impact the hashcode in either // case. // This way we can use a cheap loop without conditionals, or hard-to-unroll // operations, // or the devastatingly bad memory locality of visiting value objects. // Also, it's important to use a hash function that does not depend on the // ordering // of terms, only their values; since the map is an unordered collection and // entries can end up in different positions in different maps that have the // same // elements, but with different history of puts/removes, due to conflicts. hash ^= key; } return hash; } @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (!(obj instanceof IntObjectMap)) { return false; } @SuppressWarnings("rawtypes") IntObjectMap other = (IntObjectMap) obj; if (size != other.size()) { return false; } for (int i = 0; i < values.length; ++i) { V value = values[i]; if (value != null) { int key = keys[i]; Object otherValue = other.get(key); if (value == NULL_VALUE) { if (otherValue != null) { return false; } } else if (!value.equals(otherValue)) { return false; } } } return true; } /** * Locates the index for the given key. This method probes using double hashing. * * @param key the key for an entry in the map. * @return the index where the key was found, or {@code -1} if no entry is found * for that key. */ private int indexOf(int key) { int startIndex = hashIndex(key); int index = startIndex; for (;;) { if (values[index] == null) { // It's available, so no chance that this value exists anywhere in the map. return -1; } if (key == keys[index]) { return index; } // Conflict, keep probing ... if ((index = probeNext(index)) == startIndex) { return -1; } } } /** * Returns the hashed index for the given key. */ private int hashIndex(int key) { // Allowing for negative keys by adding the length after the first mod // operation. return (key % keys.length + keys.length) % keys.length; } /** * Grows the map size after an insertion. If necessary, performs a rehash of the * map. */ private void growSize() { size++; if (size > maxSize) { // Need to grow the arrays. We take care to detect integer overflow, // also limit array size to ArrayList.MAX_ARRAY_SIZE. rehash(adjustCapacity((int) Math.min(keys.length * 2.0, Integer.MAX_VALUE - 8))); } else if (size == keys.length) { // Open addressing requires that we have at least 1 slot available. Need to // refresh // the arrays to clear any removed elements. rehash(keys.length); } } /** * Adjusts the given capacity value to ensure that it's odd. Even capacities can * break probing. */ private static int adjustCapacity(int capacity) { return capacity | 1; } /** * Removes entry at the given index position. Also performs opportunistic, * incremental rehashing if necessary to not break conflict chains. * * @param index the index position of the element to remove. */ private void removeAt(int index) { --size; // Clearing the key is not strictly necessary (for GC like in a regular // collection), // but recommended for security. The memory location is still fresh in the cache // anyway. keys[index] = 0; values[index] = null; // In the interval from index to the next available entry, the arrays may have // entries // that are displaced from their base position due to prior conflicts. Iterate // these // entries and move them back if possible, optimizing future lookups. // Knuth Section 6.4 Algorithm R, also used by the JDK's IdentityHashMap. int nextFree = index; for (int i = probeNext(index); values[i] != null; i = probeNext(i)) { int bucket = hashIndex(keys[i]); if (i < bucket && (bucket <= nextFree || nextFree <= i) || bucket <= nextFree && nextFree <= i) { // Move the displaced entry "back" to the first available position. keys[nextFree] = keys[i]; values[nextFree] = values[i]; // Put the first entry after the displaced entry keys[i] = 0; values[i] = null; nextFree = i; } } } /** * Calculates the maximum size allowed before rehashing. */ private int calcMaxSize(int capacity) { // Clip the upper bound so that there will always be at least one available // slot. int upperBound = capacity - 1; return Math.min(upperBound, (int) (capacity * loadFactor)); } /** * Rehashes the map for the given capacity. * * @param newCapacity the new capacity for the map. */ private void rehash(int newCapacity) { int[] oldKeys = keys; V[] oldVals = values; keys = new int[newCapacity]; @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" }) V[] temp = (V[]) new Object[newCapacity]; values = temp; maxSize = calcMaxSize(newCapacity); // Insert to the new arrays. for (int i = 0; i < oldVals.length; ++i) { V oldVal = oldVals[i]; if (oldVal != null) { // Inlined put(), but much simpler: we don't need to worry about // duplicated keys, growing/rehashing, or failing to insert. int oldKey = oldKeys[i]; int index = hashIndex(oldKey); for (;;) { if (values[index] == null) { keys[index] = oldKey; values[index] = toInternal(oldVal); break; } // Conflict, keep probing. Can wrap around, but never reaches startIndex again. index = probeNext(index); } } } } /** * Iterator for traversing the entries in this map. */ private final class IteratorImpl implements Iterator<Entry<V>>, Entry<V> { private int prevIndex = -1; private int nextIndex = -1; private int entryIndex = -1; private void scanNext() { for (;;) { if (++nextIndex == values.length || values[nextIndex] != null) { break; } } } @Override public boolean hasNext() { if (nextIndex == -1) { scanNext(); } return nextIndex < keys.length; } @Override public Entry<V> next() { if (!hasNext()) { throw new NoSuchElementException(); } prevIndex = nextIndex; scanNext(); // Always return the same Entry object, just change its index each time. entryIndex = prevIndex; return this; } @Override public void remove() { if (prevIndex < 0) { throw new IllegalStateException("next must be called before each remove."); } removeAt(prevIndex); prevIndex = -1; } // Entry implementation. Since this implementation uses a single Entry, we // coalesce that // into the Iterator object (potentially making loop optimization much easier). @Override public int key() { return keys[entryIndex]; } @Override public V value() { return toExternal(values[entryIndex]); } @Override public void setValue(V value) { values[entryIndex] = toInternal(value); } } @Override public String toString() { if (size == 0) { return "{}"; } StringBuilder sb = new StringBuilder(4 * size); for (int i = 0; i < values.length; ++i) { V value = values[i]; if (value != null) { sb.append(sb.length() == 0 ? "{" : ", "); sb.append(keyToString(keys[i])).append('=').append(value == this ? "(this Map)" : value); } } return sb.append('}').toString(); } /** * Helper method called by {@link #toString()} in order to convert a single map * key into a string. */ protected String keyToString(int key) { return Integer.toString(key); } }