前言
最近做項目,需要一邊遍歷一邊刪除list與map,主要是ArrayList與HashMap。發現list與map刪除報錯了。而筆者同時需要保留舊的list與map,並執行增刪改操作時,使用克隆的方式,然而克隆map與list,發現引用對象在map或者list是淺克隆,即克隆引用或者指針。
筆者環境:Oracle JDK8
1. 遍歷刪除
1.1 ArrayList遍歷刪除
筆者查詢發現只能通過迭代器刪除。否則報錯java.util.ConcurrentModificationException。
public class IteratorTest {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
list.add("444");
for (String str : list) {
if ("222".equals(str)) {
list.remove(str);
}
}
System.out.println(list);
}
}
筆者發現ArrayList的list.remove方法沒問題啊
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;
}
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
仔細發現modCount++; 這句代碼尤其要注意,這就是造成問題的誘因。
當筆者查看異常堆棧時
想起來foreach是執行迭代器語句,反編譯一下,果然,其實使用下標迭代是可以刪除的,ArrayList就是數組嘛,注意一下size判斷循環條件就可以了。
源碼分析,在ArrayList中,next方法,迭代器是ArrayList內部類實現的
這個check方法,很簡單,簡單粗暴拋異常。
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
這個modCount是ArrayList的全局變量,而expectedModCount是迭代器初始化時copy一份當時的modCount,當remove時
modCount++;
而迭代器的值初始化就固定了,所以值不相等,拋異常了,next進行不下去了。
解決辦法:
①不用迭代器,下標刪除,注意刪除後size改變,判定條件也要改變
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
list.add("444");
for (int i = 0; i < list.size(); i++) {
if ("222".equals(list.get(i))) {
list.remove(i);
}
}
System.out.println(list);
}
②迭代器提供的刪除方法
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
list.add("444");
Iterator var2 = list.iterator();
while(var2.hasNext()) {
String str = (String)var2.next();
if ("222".equals(str)) {
var2.remove();
}
}
System.out.println(list);
}
解析源碼,迭代器會把modCount同步過來,expectedModCount = modCount;
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
不過,頻繁增刪改的list不推薦使用ArrayList,LinkedList更爲方便,效率更高,當然這個問題仍然存在,還是要使用這2種方法處理。
1.2 HashMap同理
2. 克隆後增刪改
2.1 HashMap克隆
Person類省略
package com.feng.clone;
import java.util.HashMap;
import java.util.Map;
public class MapCloneTest {
public static void main(String[] args) {
HashMap<String, Person> map = new HashMap<>();
Person p1 = new Person();
p1.setName("tom");
p1.setAge(12);
Person p2 = new Person();
p2.setAge(15);
p2.setName("JIM");
map.put("person1", p1);
map.put("person2", p2);
HashMap<String, Person> mapClone = (HashMap<String, Person>) map.clone();
for (Map.Entry<String, Person> entry : map.entrySet()) {
mapClone.get(entry.getKey()).setName("aaaaaaaa");
mapClone.remove(entry.getKey());
Person p3 = new Person();
p3.setAge(88);
p3.setName("kkkk");
mapClone.put("person3", p3);
System.out.println(entry.getKey() + "" + entry.getValue());
}
}
}
運行發現,克隆map修改會影響本體map的bean,增加刪除不會影響本體map
Connected to the target VM, address: '127.0.0.1:51147', transport: 'socket'
person2Person(name=aaaaaaaa, age=15)
person1Person(name=aaaaaaaa, age=12)
Disconnected from the target VM, address: '127.0.0.1:51147', transport: 'socket'
原理分析
看看HashMap的clone方法
public Object clone() {
HashMap<K,V> result;
try {
//1.clone
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
//2.reinit
result.reinitialize();
//3.copy table[]
result.putMapEntries(this, false);
return result;
}
void reinitialize() {
table = null;
entrySet = null;
keySet = null;
values = null;
modCount = 0;
threshold = 0;
size = 0;
}
可以看出
①直接clone的hashmap
②重新初始化
③複製本體map的table[]數組
/**
* Implements Map.putAll and Map constructor.
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)
resize();
//循環遍歷設置,但是隻是引用,對象並未深度克隆
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
2.2 ArrayList克隆
package com.feng.clone;
import java.util.ArrayList;
public class ArrayListClone {
public static void main(String[] args) {
ArrayList<Person> list = new ArrayList<>();
Person p1 = new Person();
p1.setName("tom");
p1.setAge(12);
Person p2 = new Person();
p2.setAge(15);
p2.setName("JIM");
list.add(p1);
list.add(p2);
ArrayList<Person> listClone = (ArrayList<Person>) list.clone();
for (Person p : list) {
listClone.get(0).setName("aaaaaaaa");
listClone.remove(p);
Person p3 = new Person();
p3.setAge(88);
p3.setName("kkkk");
listClone.add(p3);
System.out.println(p);
}
}
}
運行示例,可以看出跟map相同的現象,僅是arraylist克隆,對象只克隆引用
Person(name=aaaaaaaa, age=12)
Person(name=aaaaaaaa, age=15)
Process finished with exit code 0
ArrayList的clone原理分析
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
很簡單,直接克隆arraylist,然後複製數組,裏面的元素僅複製引用
總結
筆者在使用HashMap和ArrayList遍歷刪除元素的時候,也想到了copOnWriteList,但是Map沒法實現,並且筆者的需求有新舊map和list只能修改新的map或者list,所以想到了克隆,但是HashMap和ArrayList的元素bean未克隆,筆者必須取出bean,然後對bean克隆然後設置到新的Map或者list中,HashMap和ArrayList是淺克隆,深克隆推薦序列化。