concurrenthashmap

ConcurrentHashMap read while resizing

由 一世执手 提交于 2019-12-10 22:58:05
问题 I would like to know the possible scenarios that can occur when we are trying to read a ConcurrentHashMap while it is resizing. I know that during read, first attempt is always unsynchronized. In second attempt it will try to acquire lock and retry. But if it happens during resizing how would it work? Thanks 回答1: By looking at the source: A ConcurrentHashMap contains one or more segments, depending on the concurrency level. During a rehash, a segment is locked and the new table is built next

Implementation independent way to see if a map contains null key

由 断了今生、忘了曾经 提交于 2019-12-10 21:04:59
问题 I have an API that receives a parameter which implements the Map interface. I need to check to see if this Map contains any null key. The problem is there are certain implementations of Map , such as ConcurrentHashMap which throw NPE if you call contains(null) on them. What is a good, implementation independent way to see if an object that adheres to the Map interface contains a null key or not? Note that I can't use keySet and check to see if that contains a null, because ConcurrentHashMap

Java Summarize

由 ε祈祈猫儿з 提交于 2019-12-10 17:41:01
Ssm的独立搭建: 1,依赖 1.1.servlet 1.2.spring ---------spring- context -core-beans 1.3.springmvc 1.4.mybatis 1.5.mybatis-spring 1.6.mysql-connector 1.7.dbcp 1.8.spring-jdbc 2.配置web.xml 2.1.spring的核心容器的启动 2.2.springmvc核心控制器 3.配置applicationContext.xml 3.1.数据库连接池 3.2.sqlSessionFactory 3.3.mapperscanner 3.4声明式事务---------->简单说一下事务有声明式和编程式 4.配置springMvc.xml 4.1.扫描控制层 4.2.扫描注解驱动 ★★★为什么HashMap最大容量是2的30次方? 因为int的最大值是2的31-1,而hashMap最大容量不可以超过int的最大值,所有是2的30次方, ★★★为什么HashMap是2的N次方?HashTable是什么? 2的N次方有一个特点,对一个数取余的时候,可以通过移位运算来进行,HashMap保证性能,所以是2的N次方, 而HashTable则是通过取余,他是任意容量, ★★★ Map我想保证效率,还想保证安全,怎么办?

Regarding internal working of concurrent hashmap

由 点点圈 提交于 2019-12-10 15:34:30
问题 I was going through the ConcurrentHashMap and this related tutorial, and had some questions. In the article, it was mention that ConcurrentHashMap allows multiple readers to read concurrently without any blocking. This is achieved by partitioning the Map into different parts based on concurrency level and locking only a portion of the Map during updates. Default concurrency level is 16, and accordingly the Map is divided into 16 part and each part is governed with a different lock. This means

Java: How to take static snapshot of ConcurrentHashMap?

由 99封情书 提交于 2019-12-10 12:54:06
问题 Java doc says that return values of method values() and entrySet() are backed by the map. So changes to the map are reflected in the set and vice versa. I don't want this to happen to my static copy. Essentially, I want lots of concurrent operations to be done on my DS. But for some cases I want to iterate over its static snapshot. I want to iterate over static snapshot, as I am assuming iterating over static snapshot will be faster as compared to a version which is being updated concurrently

Laziness of eviction in Guava's maps

由 只谈情不闲聊 提交于 2019-12-10 07:46:57
问题 Current eviction algorithm for maps is quite lazy. It looks like expired objects are evicted only when the data structure is accessed. For example, the map from addresses to indexers defined as: ConcurrentMap<Address, Indexer> indexers = new MapMaker() .expireAfterAccess( EXPIRATION, TimeUnit.SECONDS) .evictionListener( new IndexEvicted()) .makeMap(); leads to quite surprising pattern: while containsKey() for the given address returns false, immediately after that indexer for that address is

JAVA-JDK1.7-ConCurrentHashMap-测试和验证

由 左心房为你撑大大i 提交于 2019-12-10 05:30:55
概述 上次记录了关于ConCurrentHashMap的原理以及源码，现在我对其进行有关功能的测试，下面就讲解一下我测试的内容和代码。这些测试主要针对JDK1.7版本。 GET安全测试 上一篇写过get方法是没有加锁的，因为HashEntry的value和next属性是volatile的，volatile直接保证了可见性，所以读的时候可以不加锁，现在写一个程序，启动20个线程，只有一个key="count",每个线程都要执行 map.put(key, 1) 或者 map.put(key, value + 1) ， 所以理论上说，20个线程会得到值（"count",20）,所有的源码如下： 1 package test; 2 3 import java.io.Serializable; 4 import java.util.concurrent.ConcurrentHashMap; 5 import java.util.concurrent.ExecutorService; 6 import java.util.concurrent.Executors; 7 import java.util.concurrent.locks.ReentrantLock; 8 9 /** 10 * Created by Administrator on 2019/12/4. 11 */ 12

Java ConcurrentHashMap not thread safe.. wth?

由 风流意气都作罢 提交于 2019-12-10 03:19:17
问题 I was using HashMap before like public Map<SocketChannel, UserProfile> clients = new HashMap<SocketChannel, UserProfile>(); now I've switched to ConcurrentHashMap to avoid synchronized blocks and now i'm experiencing problems my server is heavily loaded with 200-400 concurrent clients every second which is expected to grow over time. which now looks like this public ConcurrentHashMap<SocketChannel, UserProfile> clients = new ConcurrentHashMap<SocketChannel, UserProfile>(); My server design

List、set、Map的底层实现原理

由 和自甴很熟 提交于 2019-12-10 00:16:58
版权声明：本文为博主原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接和本声明。 本文链接：https://blog.csdn.net/xzp_12345/article/details/79251174 本文主要是参考了网上一些对java集合讲解的比较详细博客进行归纳总结，下面对 java集合中几个比较常用的类归纳分析。 ArrayList实现原理要点概括 参考文献： http://zhangshixi.iteye.com/blog/674856l https://www.cnblogs.com/leesf456/p/5308358.html ArrayList是List接口的可变数组非同步实现，并允许包括null在内的所有元素。底层使用数组实现该集合是可变长度数组，数组扩容时，会将老数组中的元素重新拷贝一份到新的数组中，每次数组容量增长大约是其容量的1.5倍，这种操作的代价很高。采用了Fail-Fast机制，面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险remove方法会让下标到数组末尾的元素向前移动一个单位，并把最后一位的值置空，方便GC LinkedList实现原理要点概括 参考文献： 1.http://www.cnblogs.com/ITtangtang/p/3948610.htmll 2

concurrent hashMap putIfAbsent method functionality

由 风格不统一 提交于 2019-12-09 07:17:50
问题 I am a new bie to the world of java and exploring the concurrent hash map, while exploring the concurrent hashmap API , I discover the putifAbsent() method public V putIfAbsent(K paramK, V paramV) { if (paramV == null) throw new NullPointerException(); int i = hash(paramK.hashCode()); return segmentFor(i).put(paramK, i, paramV, true); } Now please advise what is it functionality and when do we practically require it , if possible please explain with a small simple example. 回答1: A
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