concurrenthashmap | 易学教程

Are there any drawbacks with ConcurrentHashMap?

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问题 I need a HashMap that is accessible from multiple threads. There are two simple options, using a normal HashMap and synchronizing on it or using a ConcurrentHashMap. Since ConcurrentHashMap does not block on read operations it seems much better suited for my needs (almost exclusively reads, almost never updates). On the other hand, I expect very low concurrency anyway, so there should be no blocking (just the cost of managing the lock). The Map will also be very small (under ten entries), if

Is Transactional possible extend to other operations more than to db in Spring?

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问题 I explain the context: I implemented the export and import records from two differents Data base in other words a refresh of seven tables. I used JdbcCursorItemReader to perform the select chunked queries because there are over 600000 records for each table and save every chunk with jdbc batch insert on the other DB. The implented idea has been the followed: REMARK on requirement : All tables must be refreshed in unique transaction, if only a chunk of any table fails rollback I have created a

Is Transactional possible extend to other operations more than to db in Spring?

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Since Java 9 HashMap.computeIfAbsent() throws ConcurrentModificationException on attempt to memoize recursive function results

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来源： https://stackoverflow.com/questions/54824656/since-java-9-hashmap-computeifabsent-throws-concurrentmodificationexception-on

Java Concurrent Hashmap initTable() Why the try/finally block?

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来源： https://stackoverflow.com/questions/63651473/java-concurrent-hashmap-inittable-why-the-try-finally-block

How to update a Value in a ConcurrentHashMap threadsafe

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问题 I need to update a value in a ConcurrentHashmap but I am not sure on how to do this thread safe. The Hashmap is a ConcurrentHashMap and I need to get the instance of the custom class, perform some operations on it and then put the updated value back in. Is there any way to combine this get-alter-put operation to something atomic? Thanks in advance 回答1: You can use ConcurrentHashMaps computeIfPresent https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html

Why is removing the 1st entry from a ConcurrentHashMap not immediately reflected in the iterator, but removing the 2nd or subsequent entries is?

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问题 I created an iterator() and then removed the 1st entry from the map before iterating it. I always get the 1st item returned from the iterator. But when I remove the 2nd or subsequent entries, the current iterator does not return that entry. Example of removing 1st entry from map: Map<Integer,Integer> m1 = new ConcurrentHashMap<>(); m1.put(4, 1); m1.put(5, 2); m1.put(6, 3); Iterator i1 = m1.entrySet().iterator(); m1.remove(4); // remove entry from map while (i1.hasNext()) System.out.println(

ConcurrentHashMap总结

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【推荐阅读】微服务还能火多久？>>> 并发编程实践中，ConcurrentHashMap是一个经常被使用的数据结构，相比于Hashtable以及Collections.synchronizedMap()，ConcurrentHashMap在线程安全的基础上提供了更好的写并发能力，但同时降低了对读一致性的要求（这点好像CAP理论啊 O(∩_∩)O）。ConcurrentHashMap的设计与实现非常精巧，大量的利用了volatile，final，CAS等lock-free技术来减少锁竞争对于性能的影响，无论对于Java并发编程的学习还是Java内存模型的理解，ConcurrentHashMap的设计以及源码都值得非常仔细的阅读与揣摩。这篇日志记录了自己对ConcurrentHashMap的一些总结，由于JDK6，7，8中实现都不同，需要分开阐述在不同版本中的ConcurrentHashMap。之前已经在 ConcurrentHashMap原理分析中解释了ConcurrentHashMap的原理，主要是从代码的角度来阐述是源码是如何写的，本文仍然从源码出发，挑选个人觉得重要的点（会用红色标注）再次进行回顾，以及阐述ConcurrentHashMap的一些注意点。 1. JDK6与JDK7中的实现 1.1 设计思路 ConcurrentHashMap采用了分段锁的设计

【ArrayList】为什么java.util.concurrent 包里没有并发的ArrayList实现？

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为什么java.util.concurrent 包里没有并发的ArrayList实现？问：JDK 5在java.util.concurrent里引入了ConcurrentHashMap，在需要支持高并发的场景，我们可以使用它代替HashMap。但是为什么没有ArrayList的并发实现呢？难道在多线程场景下我们只有Vector这一种线程安全的数组实现可以选择么？为什么在java.util.concurrent 没有一个类可以代替Vector呢？答：我认为在java.util.concurrent包中没有加入并发的ArrayList实现的主要原因是：很难去开发一个通用并且没有并发瓶颈的线程安全的List。像ConcurrentHashMap这样的类的真正价值（The real point / value of classes）并不是它们保证了线程安全。而在于它们在保证线程安全的同时不存在并发瓶颈。举个例子，ConcurrentHashMap采用了锁分段技术和弱一致性的Map迭代器去规避并发瓶颈。所以问题在于，像“Array List”这样的数据结构，你不知道如何去规避并发的瓶颈。拿contains() 这样一个操作来说，当你进行搜索的时候如何避免锁住整个list？另一方面，Queue 和Deque (基于Linked List

CopyOnWriteArrayList(写入并复制) & CountDownLatch(闭锁)

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ConcurrentHashMap： ①Java 5.0 在 java.util.concurrent 包中提供了多种并发容器类来改进同步容器的性能。 ② ConcurrentHashMap 同步容器类是Java 5 增加的一个线程安全的哈希表。对与多线程的操作，介于 HashMap 与 Hashtable 之间。内部采用“锁分段” 机制替代 Hashtable 的独占锁。进而提高性能。 ③此包还提供了设计用于多线程上下文中的 Collection 实现： ConcurrentHashMap、ConcurrentSkipListMap、ConcurrentSkipListSet、 CopyOnWriteArrayList 和 CopyOnWriteArraySet。当期望许多线程访问一个给定 collection 时，ConcurrentHashMap 通常优于同步的 HashMap， ConcurrentSkipListMap 通常优于同步的 TreeMap。当期望的读数和遍历远远大于列表的更新数时，CopyOnWriteArrayList 优于同步的 ArrayList。 TestCopyOnWriteArrayList package com.aff.juc; import java.util.Iterator; import java.util