HikariCP源码分析之FastList
FastList是一个List接口的精简实现，只实现了接口中必要的几个方法。JDK ArrayList每次调用get()方法时都会进行rangeCheck检查索引是否越界，FastList的实现中去除了这一检查，只要保证索引合法那么rangeCheck就成为了不必要的计算开销(当然开销极小)。此外，HikariCP使用List来保存打开的Statement，当Statement关闭或Connection关闭时需要将对应的Statement从List中移除。通常情况下，同一个Connection创建了多个Statement时，后打开的Statement会先关闭。ArrayList的remove(Object)方法是从头开始遍历数组，而FastList是从数组的尾部开始遍历，因此更为高效。
简而言之就是 自定义数组类型（FastList）代替ArrayList：避免每次get()调用都要进行range check，避免调用remove()时的从头到尾的扫描。
源码分析
java.util.ArrayList
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public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable</code>
com.zaxxer.hikari.util.FastList
<code class="">public final class FastList<T> implements List<T>, RandomAccess, Serializable
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我们先看一下java.util.ArrayList的get方法
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/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
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我们可以看到每次get的时候都会进行rangeCheck
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/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
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我们对比一下com.zaxxer.hikari.util.FastList的get方法，可以看到取消了rangeCheck，一定程度下追求了极致
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/**
* Get the element at the specified index.
*
* @param index the index of the element to get
* @return the element, or ArrayIndexOutOfBounds is thrown if the index is invalid
*/
@Override
public T get(int index) {
return elementData[index];
}
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我们再来看一下ArrayList的remove(Object)方法
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/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If the list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
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再对比看一下FastList的remove(Object方法)
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/**
* This remove method is most efficient when the element being removed
* is the last element. Equality is identity based, not equals() based.
* Only the first matching element is removed.
*
* @param element the element to remove
*/
@Override
public boolean remove(Object element) {
for (int index = size - 1; index >= 0; index--) {
if (element == elementData[index]) {
final int numMoved = size - index - 1;
if (numMoved > 0) {
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
}
elementData[--size] = null;
return true;
}
}
return false;
}
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好了，今天的文章就是这么简短，最后附一下FastList的源码，内容真的是十分精简的。
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/*
* Copyright (C) 2013, 2014 Brett Wooldridge
*
* 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
*
* [url=http://www.apache.org/licenses/LICENSE-2.0]http://www.apache.org/licenses/LICENSE-2.0[/url]
*
* 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 com.zaxxer.hikari.util;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.RandomAccess;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
/**
* Fast list without range checking.
*
* @author Brett Wooldridge
*/
public final class FastList<T> implements List<T>, RandomAccess, Serializable {
private static final long serialVersionUID = -4598088075242913858L;
private final Class<?> clazz;
private T[] elementData;
private int size;
/**
* Construct a FastList with a default size of 32.
* @param clazz the Class stored in the collection
*/
@SuppressWarnings("unchecked")
public FastList(Class<?> clazz) {
this.elementData = (T[]) Array.newInstance(clazz, 32);
this.clazz = clazz;
}
/**
* Construct a FastList with a specified size.
* @param clazz the Class stored in the collection
* @param capacity the initial size of the FastList
*/
@SuppressWarnings("unchecked")
public FastList(Class<?> clazz, int capacity) {
this.elementData = (T[]) Array.newInstance(clazz, capacity);
this.clazz = clazz;
}
/**
* Add an element to the tail of the FastList.
*
* @param element the element to add
*/
@Override
public boolean add(T element) {
if (size < elementData.length) {
elementData[size++] = element;
}
else {
// overflow-conscious code
final int oldCapacity = elementData.length;
final int newCapacity = oldCapacity << 1;
@SuppressWarnings("unchecked")
final T[] newElementData = (T[]) Array.newInstance(clazz, newCapacity);
System.arraycopy(elementData, 0, newElementData, 0, oldCapacity);
newElementData[size++] = element;
elementData = newElementData;
}
return true;
}
/**
* Get the element at the specified index.
*
* @param index the index of the element to get
* @return the element, or ArrayIndexOutOfBounds is thrown if the index is invalid
*/
@Override
public T get(int index) {
return elementData[index];
}
/**
* Remove the last element from the list. No bound check is performed, so if this
* method is called on an empty list and ArrayIndexOutOfBounds exception will be
* thrown.
*
* @return the last element of the list
*/
public T removeLast() {
T element = elementData[--size];
elementData[size] = null;
return element;
}
/**
* This remove method is most efficient when the element being removed
* is the last element. Equality is identity based, not equals() based.
* Only the first matching element is removed.
*
* @param element the element to remove
*/
@Override
public boolean remove(Object element) {
for (int index = size - 1; index >= 0; index--) {
if (element == elementData[index]) {
final int numMoved = size - index - 1;
if (numMoved > 0) {
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
}
elementData[--size] = null;
return true;
}
}
return false;
}
/**
* Clear the FastList.
*/
@Override
public void clear() {
for (int i = 0; i < size; i++) {
elementData[i] = null;
}
size = 0;
}
/**
* Get the current number of elements in the FastList.
*
* @return the number of current elements
*/
@Override
public int size() {
return size;
}
/** {@inheritDoc} */
@Override
public boolean isEmpty() {
return size == 0;
}
/** {@inheritDoc} */
@Override
public T set(int index, T element) {
T old = elementData[index];
elementData[index] = element;
return old;
}
/** {@inheritDoc} */
@Override
public T remove(int index) {
if (size == 0) {
return null;
}
final T old = elementData[index];
final int numMoved = size - index - 1;
if (numMoved > 0) {
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
}
elementData[--size] = null;
return old;
}
/** {@inheritDoc} */
@Override
public boolean contains(Object o) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public Iterator<T> iterator() {
return new Iterator<T>() {
private int index;
@Override
public boolean hasNext() {
return index < size;
}
@Override
public T next() {
if (index < size) {
return elementData[index++];
}
throw new NoSuchElementException("No more elements in FastList");
}
};
}
/** {@inheritDoc} */
@Override
public Object[] toArray()
{
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public <E> E[] toArray(E[] a)
{
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean containsAll(Collection<?> c) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean addAll(Collection<? extends T> c) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean addAll(int index, Collection<? extends T> c) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean removeAll(Collection<?> c) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean retainAll(Collection<?> c) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public void add(int index, T element) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public int indexOf(Object o) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public int lastIndexOf(Object o) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public ListIterator<T> listIterator() {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public ListIterator<T> listIterator(int index) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public List<T> subList(int fromIndex, int toIndex) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public Object clone() {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public void forEach(Consumer<? super T> action) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public Spliterator<T> spliterator() {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public boolean removeIf(Predicate<? super T> filter) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public void replaceAll(UnaryOperator<T> operator) {
throw new UnsupportedOperationException();
}
/** {@inheritDoc} */
@Override
public void sort(Comparator<? super T> c) {
throw new UnsupportedOperationException();
}
}
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