映射条目的原子更新

ConcurrentHashMap只有部分原子更新。

JavaAPI提供了一些新方法，例如：compute方法可以提供一个键和一个计算新值的函数。

map.compute(word,(k,v)->v == null ? 1 : v+1)

注释：ConcurrentHashMap中不允许有null值。很多方法都使用null来指示映射中某个给定的键不存在。

使用并发散列映射来统计一个目录树的Java文件中的所有单词

package concurrentHashMap;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Scanner;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class CHMDemo {
    
    public static ConcurrentHashMap<String,Long> map = new ConcurrentHashMap<>();
    
    public static void process(Path file) {
        try(var in = new Scanner(file)){
            while(in.hasNext()) {
                String word = in.next();
                map.merge(word, 1L, Long::sum);
            }
        }catch (Exception e) {
            e.printStackTrace();
        }
    }
    
    public static Set<Path> descendants(Path rootDir) throws IOException{
        try(Stream<Path> entries = Files.walk(rootDir)){
            return entries.collect(Collectors.toSet());
        }
    }

    public static void main(String[] args) throws InterruptedException, IOException {
        int processors = Runtime.getRuntime().availableProcessors();
        ExecutorService executor = Executors.newFixedThreadPool(processors);
        Path pathToRoot = Path.of("C:\\Program Files\\Java\\jdk1.8.0_131\\src");
        for (Path p : descendants(pathToRoot)) {
            if(p.getFileName().toString().endsWith(".java")) {
                executor.execute(()->process(p));
            }
        }
        executor.shutdown();
        executor.awaitTermination(10, TimeUnit.MINUTES);
        map.forEach((k,v)->{
            if(v>=10)
                System.out.println(k+" occurs "+v+" times");
        });
    }

}