原文地址：http://www.concretepage.com/java/example_concurrenthashmap_java

On this page we will provide example of ConcurrentHashMap in java. ConcurrentHashMap is thread safe but does not use locking on complete map. It is fast and has better performance in comparison to Hashtable in concurrent environment. Find some methods of?ConcurrentHashMap.?

get()?: Pass the key as an argument and it will return associated value.?
put(): Pass key and value and it will map.?
putIfAbsent(): Pass key and value and it will map only if key is not already present.?
remove(): Removes the entry for the given key.

Contents

• Java ConcurrentHashMap Internal Working
• ConcurrentHashMap Example
• ConcurrentHashMap vs Hashtable
• ConcurrentHashMap vs HashMap

Java ConcurrentHashMap Internal Working

1.?Concurrency for retrieval: Retrieval of elements from?ConcurrentHashMap?does not use locking. It may overlap with update operation. We get the elements of last successfully completed update operation. In case of aggregate operations such as?putAll?and?clear(), concurrent retrieval may show insertion or removal of only some elements.?

2.?Iteration of ConcurrentHashMap: Iterators and Enumerations also return the elements which have been concurrently added while iterating.?ConcurrentHashMap?does not throw?ConcurrentModificationException.?

3.?Concurrency for updates: Concurrent updates are thread safe.?ConcurrentHashMap?constructor has an optional concurrency level argument. The default value is 16. This is the estimated number of concurrently updating threads. It is used in internal sizing to accommodate concurrently updating threads. Hash table is internally partitioned into the concurrency level number so that it can avoid updating concurrent thread contention.

ConcurrentHashMap Example

Find the example.?
ConcurrentHashMapDemo.java

package com.concretepage; import java.util.Iterator; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ConcurrentHashMapDemo { private final ConcurrentHashMap<Integer,String> conHashMap = new ConcurrentHashMap<Integer,String>(); public static void main(String[] args) { ExecutorService service = Executors.newFixedThreadPool(3); ConcurrentHashMapDemo ob = new ConcurrentHashMapDemo(); service.execute(ob.new WriteThreasOne()); service.execute(ob.new WriteThreasTwo()); service.execute(ob.new ReadThread()); service.shutdownNow(); } class WriteThreasOne implements Runnable { @Override public void run() { for(int i= 1; i<=10; i++) { conHashMap.putIfAbsent(i, "A"+ i); } } } class WriteThreasTwo implements Runnable { @Override public void run() { for(int i= 1; i<=5; i++) { conHashMap.put(i, "B"+ i); } }

轉載于:https://www.cnblogs.com/davidwang456/p/6018203.html

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