[技术笔记] 【杭州校区】HikariCP源码分析之FastList

本帖最后由小江哥于 2019-2-16 17:16 编辑

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

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);
    }

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));
    }

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];
   }

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;
    }

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;
   }

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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[技术笔记] 【杭州校区】HikariCP源码分析之FastList

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