parent
							
								
									2d81998a6c
								
							
						
					
					
						commit
						e5bd19b275
					
				| @ -0,0 +1,158 @@ | ||||
| --- | ||||
| ArrayList 和 Vector 源码分析 | ||||
| --- | ||||
| 
 | ||||
| #### 前言 | ||||
| 
 | ||||
| 基于JDK1.10。 | ||||
| 
 | ||||
| #### ArrayList | ||||
| 
 | ||||
| ArrayList实现了List接口、RandomAccess接口,可以插入空数据以及支持随机访问。 | ||||
| 
 | ||||
| ArrayList相当于动态数组,里面有两个重要属性,elementData以及size。 | ||||
| 
 | ||||
| ```java | ||||
| transient Object[] elementData; //数据 | ||||
| private int size;	//数组大小 | ||||
| ``` | ||||
| 
 | ||||
| 首先看一下**构造方法**(只罗列其中一种): | ||||
| 
 | ||||
| ```java | ||||
|     public ArrayList(int initialCapacity) { | ||||
|         if (initialCapacity > 0) { | ||||
|             this.elementData = new Object[initialCapacity]; | ||||
|         } else if (initialCapacity == 0) { | ||||
|             this.elementData = EMPTY_ELEMENTDATA; | ||||
|         } else { | ||||
|             throw new IllegalArgumentException("Illegal Capacity: "+ | ||||
|                                                initialCapacity); | ||||
|         } | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| 也就是说我们可以指定容量大小,这样可以避免扩容导致的性能损耗。 | ||||
| 
 | ||||
| **第一种方式添加数据时:** | ||||
| 
 | ||||
| ```java | ||||
|     public boolean add(E e) { | ||||
|         modCount++; | ||||
|         add(e, elementData, size); | ||||
|         return true; | ||||
|     } | ||||
|     private void add(E e, Object[] elementData, int s) { | ||||
|         if (s == elementData.length)	//第一步:扩容效验 | ||||
|             elementData = grow();	 | ||||
|         elementData[s] = e;	//第二步:在尾部添加数据并且size+1 | ||||
|         size = s + 1; | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| - 首先是扩容效验 | ||||
| - 把数据插入到最后,并且 size + 1 | ||||
| 
 | ||||
| **第二种方式添加数据时:** | ||||
| 
 | ||||
| ```java | ||||
|     public void add(int index, E element) { | ||||
|         rangeCheckForAdd(index);	//第一步:判断插入的index是否合理 | ||||
|         modCount++; | ||||
|         final int s; | ||||
|         Object[] elementData; | ||||
|         if ((s = size) == (elementData = this.elementData).length)	//第二步:扩容效验 | ||||
|             elementData = grow(); | ||||
|         System.arraycopy(elementData, index, elementData, index + 1,s - index);	//第三步:拷贝数组,并把index后的数据往后移一位,把数据插入到index位置上,size + 1 | ||||
|         elementData[index] = element; | ||||
|         size = s + 1; | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| - 首先判断插入的index是否合理,以及依旧进行扩容效验 | ||||
| - 拷贝数组插入数据,size + 1 | ||||
| 
 | ||||
| **扩容效验:** | ||||
| 
 | ||||
| ```java | ||||
|     private Object[] grow() { | ||||
|         return grow(size + 1); | ||||
|     } | ||||
|     private Object[] grow(int minCapacity) { | ||||
|         return elementData = Arrays.copyOf(elementData, newCapacity(minCapacity));	//拷贝旧数组,容量大小为以前的1.5倍 | ||||
|     } | ||||
|     private int newCapacity(int minCapacity) { | ||||
|         // overflow-conscious code | ||||
|         int oldCapacity = elementData.length; | ||||
|         int newCapacity = oldCapacity + (oldCapacity >> 1);	//容量扩大为之前的1.5倍 | ||||
|         if (newCapacity - minCapacity <= 0) { | ||||
|             if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) | ||||
|                 return Math.max(DEFAULT_CAPACITY, minCapacity);	//private static final int DEFAULT_CAPACITY = 10; | ||||
|             if (minCapacity < 0) // overflow | ||||
|                 throw new OutOfMemoryError(); | ||||
|             return minCapacity; | ||||
|         } | ||||
|         return (newCapacity - MAX_ARRAY_SIZE <= 0)	//private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; | ||||
|             ? newCapacity | ||||
|             : hugeCapacity(minCapacity); | ||||
|     } | ||||
|     private static int hugeCapacity(int minCapacity) { | ||||
|         if (minCapacity < 0) // overflow | ||||
|             throw new OutOfMemoryError(); | ||||
|         return (minCapacity > MAX_ARRAY_SIZE) | ||||
|             ? Integer.MAX_VALUE | ||||
|             : MAX_ARRAY_SIZE; | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| 可以看出,如果是非极端的情况下(比如初始容量很小和很大),都会将容量扩大至以前的1.5倍。 | ||||
| 
 | ||||
| 所以,ArrayList的性能损耗主要在于数据的拷贝,所以可以适当情况下指定容量大小,应该尽量减少扩容操作,更要避免在指定位置插入数据的操作。 | ||||
| 
 | ||||
| **序列化:** | ||||
| 
 | ||||
| 仔细看的话肯定能看到前面的elementData是用transient修饰的,也就是拒绝数据被自动序列化。因为ArrayList并不是所有位置都有数据,所以没必要全部序列话,应该只序列化有数据的部分。 | ||||
| 
 | ||||
| ```java | ||||
|     private void writeObject(java.io.ObjectOutputStream s) | ||||
|         throws java.io.IOException { | ||||
|         // Write out element count, and any hidden stuff | ||||
|         int expectedModCount = modCount; | ||||
|         s.defaultWriteObject(); | ||||
| 
 | ||||
|         // Write out size as capacity for behavioral compatibility with clone() | ||||
|         s.writeInt(size); | ||||
| 
 | ||||
|         // Write out all elements in the proper order. | ||||
|         for (int i=0; i<size; i++) {	//只序列化有数据的部分 | ||||
|             s.writeObject(elementData[i]); | ||||
|         } | ||||
| 
 | ||||
|         if (modCount != expectedModCount) { | ||||
|             throw new ConcurrentModificationException(); | ||||
|         } | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| #### Vector | ||||
| 
 | ||||
| 和ArrayList很相似,同样实现了List、RandomAccess接口,可以插入空数据以及支持随机访问。内部通过动态数组来实现,不过add、get方法都加了synchronized同步锁。 | ||||
| 
 | ||||
| ```java | ||||
|     public synchronized boolean add(E e) { | ||||
|         modCount++; | ||||
|         add(e, elementData, elementCount); | ||||
|         return true; | ||||
|     } | ||||
|     private void add(E e, Object[] elementData, int s) { | ||||
|         if (s == elementData.length) | ||||
|             elementData = grow(); | ||||
|         elementData[s] = e; | ||||
|         elementCount = s + 1; | ||||
|     } | ||||
|     public void add(int index, E element) { | ||||
|         insertElementAt(element, index); | ||||
|     } | ||||
| ``` | ||||
| 
 | ||||
| 所以,可以把Vector理解为一个加锁的ArrayList。 | ||||
| Before Width: | Height: | Size: 16 KiB After Width: | Height: | Size: 16 KiB | 
| Before Width: | Height: | Size: 153 KiB After Width: | Height: | Size: 153 KiB | 
| Before Width: | Height: | Size: 119 KiB After Width: | Height: | Size: 119 KiB | 
					Loading…
					
					
				
		Reference in new issue