一、概述
1、传统集合类在并发访问时的问题说明
2、传统方式下用Collections工具类提供的synchronizedCollection方法来获得同步集合，分析该方法的实现源码。
3、传统方式下的Collection在迭代集合时，不允许对集合进行修改。
> 根据AbstractList的checkForComodification方法的源码，分析产生ConcurrentModificationException异常的原因。
4、Java5中提供了如下一些同步集合类
通过看java.util.concurrent包下的介绍可以知道有哪些并发集合
> ConcurrentHashMap
> CopyOnWriteArrayList
> CopyOnWriteArraySet
二、代码描述
1、User.java
package com.lh.threadtest.t14;
/***
*
* @ClassName: User
* @Description: TODO
* @author Liu
* @date 2018年1月18日 下午8:43:50
*
*/
public class User implements Cloneable {
private String name;
private int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof User)) {
return false;
}
User user = (User) obj;
// if(this.name==user.name && this.age==user.age)
if (this.name.equals(user.name) && this.age == user.age) {
return true;
} else {
return false;
}
}
public int hashCode() {
return name.hashCode() + age;
}
public String toString() {
return "{name:'" + name + "',age:" + age + "}";
}
public Object clone() {
Object object = null;
try {
object = super.clone();
} catch (CloneNotSupportedException e) {
}
return object;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
}
2、CollectionTest.java
package com.lh.threadtest.t14;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;
/***
*
* @ClassName: CollectionTest
* @Description: TODO
* @author Liu
* @date 2018年1月18日 下午8:44:27
*
*/
public class CollectionTest {
public static void main(String[] args) {
Collections.synchronizedCollection(null);
// Collection users = new ArrayList();
Collection users = new CopyOnWriteArrayList();
users.add(new User("张三", 28));
users.add(new User("李四", 25));
users.add(new User("王五", 31));
Iterator itrUsers = users.iterator();
while (itrUsers.hasNext()) {
User user = (User) itrUsers.next();
if ("张三".equals(user.getName())) {
users.remove(user);
// itrUsers.remove();
} else {
System.out.println(user);
}
}
}
}
3、Collections.synchronizedMap(Map<Object obj, Object obj> map)源码
private static class SynchronizedMap<K,V>
implements Map<K,V>, Serializable {
private static final long serialVersionUID = 1978198479659022715L;
private final Map<K,V> m; // Backing Map
final Object mutex; // Object on which to synchronize
SynchronizedMap(Map<K,V> m) {
this.m = Objects.requireNonNull(m);
mutex = this;
}
SynchronizedMap(Map<K,V> m, Object mutex) {
this.m = m;
this.mutex = mutex;
}
public int size() {
synchronized (mutex) {return m.size();}
}
public boolean isEmpty() {
synchronized (mutex) {return m.isEmpty();}
}
public boolean containsKey(Object key) {
synchronized (mutex) {return m.containsKey(key);}
}
public boolean containsValue(Object value) {
synchronized (mutex) {return m.containsValue(value);}
}
public V get(Object key) {
synchronized (mutex) {return m.get(key);}
}
public V put(K key, V value) {
synchronized (mutex) {return m.put(key, value);}
}
public V remove(Object key) {
synchronized (mutex) {return m.remove(key);}
}
public void putAll(Map<? extends K, ? extends V> map) {
synchronized (mutex) {m.putAll(map);}
}
public void clear() {
synchronized (mutex) {m.clear();}
}
private transient Set<K> keySet;
private transient Set<Map.Entry<K,V>> entrySet;
private transient Collection<V> values;
public Set<K> keySet() {
synchronized (mutex) {
if (keySet==null)
keySet = new SynchronizedSet<>(m.keySet(), mutex);
return keySet;
}
}
public Set<Map.Entry<K,V>> entrySet() {
synchronized (mutex) {
if (entrySet==null)
entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
return entrySet;
}
}
public Collection<V> values() {
synchronized (mutex) {
if (values==null)
values = new SynchronizedCollection<>(m.values(), mutex);
return values;
}
}
public boolean equals(Object o) {
if (this == o)
return true;
synchronized (mutex) {return m.equals(o);}
}
public int hashCode() {
synchronized (mutex) {return m.hashCode();}
}
public String toString() {
synchronized (mutex) {return m.toString();}
}
// Override default methods in Map
@Override
public V getOrDefault(Object k, V defaultValue) {
synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
}
@Override
public void forEach(BiConsumer<? super K, ? super V> action) {
synchronized (mutex) {m.forEach(action);}
}
@Override
public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
synchronized (mutex) {m.replaceAll(function);}
}
@Override
public V putIfAbsent(K key, V value) {
synchronized (mutex) {return m.putIfAbsent(key, value);}
}
@Override
public boolean remove(Object key, Object value) {
synchronized (mutex) {return m.remove(key, value);}
}
@Override
public boolean replace(K key, V oldValue, V newValue) {
synchronized (mutex) {return m.replace(key, oldValue, newValue);}
}
@Override
public V replace(K key, V value) {
synchronized (mutex) {return m.replace(key, value);}
}
@Override
public V computeIfAbsent(K key,
Function<? super K, ? extends V> mappingFunction) {
synchronized (mutex) {return m.computeIfAbsent(key, mappingFunction);}
}
@Override
public V computeIfPresent(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
synchronized (mutex) {return m.computeIfPresent(key, remappingFunction);}
}
@Override
public V compute(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
synchronized (mutex) {return m.compute(key, remappingFunction);}
}
@Override
public V merge(K key, V value,
BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
synchronized (mutex) {return m.merge(key, value, remappingFunction);}
}
private void writeObject(ObjectOutputStream s) throws IOException {
synchronized (mutex) {s.defaultWriteObject();}
}
}三、关键点
1、传统的ArrayList/ArraySet/HashMap集合类线程不安全的原因？
2、ConcurrentModificationException异常发生的原因？
来源：oschina
链接：https://my.oschina.net/u/3144678/blog/1609326