ConcurrentMap线程安全的正确用法

素颜马尾好姑娘i 2022-06-05 12:56 189阅读 0赞

import java.util.HashMap;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    import java.util.concurrent.ConcurrentMap;
    import java.util.concurrent.ExecutionException;
    
    public class ConcurrentMapWithMap {  
        
        private static Map<String, Long> mapWordCounts = new HashMap<>();  
        private static ConcurrentMap<String, Long> concurrentMapWordCounts = new ConcurrentHashMap<>();  
        public static int count=0;  
          
        public static long mapIncrease(String word) {  
            Long oldValue = mapWordCounts.get(word);  
            Long newValue = (oldValue == null) ? 1L : oldValue + 1;  
            mapWordCounts.put(word, newValue);  
            return newValue;  
        }  
          
           
        public static long ConcurrentMapIncrease(String word) {  
            Long oldValue, newValue;  
            while (true) {  
                oldValue = concurrentMapWordCounts.get(word);  
                if (oldValue == null) {  
                    newValue = 1L;  
                    //已经有key了就返回放入的值，否则返回空
                    if (concurrentMapWordCounts.putIfAbsent(word, newValue) == null) {  
                        break;  
                    }  
                } else {  
                    newValue = oldValue + 1;  
                    //值替换，每次替换时都会比较上面拿到oldValue是否就是当前map里面的值，是才替换，否则继续获取
                    if (concurrentMapWordCounts.replace(word, oldValue, newValue)) {  
                        break;  
                    }  
                }  
            }  
            return newValue;
        }  
          
        public static void mapWordCount() throws InterruptedException, ExecutionException {  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000) 
                        ConcurrentMapWithMap.mapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   ;
                        ConcurrentMapWithMap.mapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                       ConcurrentMapWithMap.mapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                        ConcurrentMapWithMap.mapIncrease("work");  
                }  
            }).start();  
        }                 
              
        public static void concurrentWordCount() throws InterruptedException, ExecutionException {  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                        ConcurrentMapWithMap.ConcurrentMapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                        ConcurrentMapWithMap.ConcurrentMapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                        ConcurrentMapWithMap.ConcurrentMapIncrease("work");  
                }  
            }).start();  
            new Thread(new Runnable(){    
                public void run() {  
                    int count=0;  
                    while(count++<10000)   
                        ConcurrentMapWithMap.ConcurrentMapIncrease("work");  
                }  
            }).start();  
        }    
      
        public static void main(String[] args) throws InterruptedException, ExecutionException {  
            ConcurrentMapWithMap.mapWordCount();  
            Thread.sleep(10000);  
            System.out.println("final count map"+ConcurrentMapWithMap.mapWordCounts.get("work"));//多线程累加，每次都少于40000，故线程不安全  
            ConcurrentMapWithMap.concurrentWordCount();  
            Thread.sleep(10000);  
            System.out.println("final count concurrentMap"+ConcurrentMapWithMap.concurrentMapWordCounts.get("work"));//多线程累加，每次都是40000  
        }  
      
    }