fail safe java_java中fail-fast 和 fail-safe的区别

首先要弄明白同步修改的概念，指的是一个或者多个线程正在遍历一个集合，此时另一个线程修改了Collection的数据结构(添加，删除或者修改)；

1.fail-fast机制

fail-fast机制在集合被遍历时，如果集合元素被修改，直接抛出Concurrent Modification Exception，有两种情况；

1>单线程环境下

集合被创建后，在遍历它的过程中修改了结构。注意 remove()方法会让expectModcount和modcount 相等，所以是不会抛出这个异常。

2>多线程环境下

一个线程在遍历元素时，另一个线程对元素进行了修改

fail-fast机制校验的原理是内部维护了一个标识“mode”,当集合数据结构被修改时，修改mode；在每次遍历的next和hasNext方法会去检验“mode”是否被修改，若修改则抛出Concurrent Modification Exception

例如ArrayList迭代部分的源码

private class Itr implements Iterator {

int cursor; // index of next element to return

int lastRet = -1; // index of last element returned; -1 if no such

int expectedModCount = modCount;

public boolean hasNext() {

return cursor != size;

}

@SuppressWarnings("unchecked")

public E next() {

checkForComodification();

int i = cursor;

if (i >= size)

throw new NoSuchElementException();

Object[] elementData = ArrayList.this.elementData;

if (i >= elementData.length)

throw new ConcurrentModificationException();

cursor = i + 1;

return (E) elementData[lastRet = i];

}

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();

}

}

2.fail-safe机制

fail-safe机制在每次对集合进行修改时会复制一个新的对象，因此不会抛出ConcurrentModificationException，

fail-safe机制有两个问题

(1)需要复制集合，产生大量的无效对象，开销大

(2)无法保证读取的数据是目前原始数据结构中的数据。

3.fail-fast和fail-safe机制例子

package com.example.springbootDemo.service;

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

public class FailFastExample {

public static void main(String[] args) {

Map premiumPhone = new HashMap();

premiumPhone.put("Apple", "iPhone");

premiumPhone.put("HTC", "HTC one");

premiumPhone.put("Samsung","S5");

Iterator iterator = premiumPhone.keySet().iterator();

while (iterator.hasNext())

{

System.out.println(premiumPhone.get(iterator.next()));

premiumPhone.put("Sony", "Xperia Z");

}

}

}

package com.example.springbootDemo.service;

import java.util.Iterator;

import java.util.concurrent.ConcurrentHashMap;

public class FailSafeExample {

public static void main(String[] args) {

ConcurrentHashMap premiumPhone =

new ConcurrentHashMap();

premiumPhone.put("Apple", "iPhone");

premiumPhone.put("HTC", "HTC one");

premiumPhone.put("Samsung","S5");

Iterator iterator = premiumPhone.keySet().iterator();

while (iterator.hasNext())

{

System.out.println(premiumPhone.get(iterator.next()));

premiumPhone.put("Sony", "Xperia Z");

}

System.out.printf("premiumPhone:"+premiumPhone.toString());

}

}