【JUC】ConcurrentHashMap之computeIfAbsent

ConcurrentHashMap之computeIfAbsent

ConcurrentHashMap的锁粒度更细

public class TGestWordCount {public static void main(String[] args) {demo(() -> new ConcurrentHashMap<String, LongAdder>(),(map, words) -> {for(String word:words){// 如果缺少这个key， 就根据后面的函数接口返回一个value 并放入map中，// 如果有，则返回这个valueLongAdder value = map.computeIfAbsent(word, (key) -> new LongAdder());// 第一次时 value时new出来的， 第二次以后， 是返回map中那个存在的LongAdder对象。// 累加器进行累加value.increment();// map.put(word, map.getOrDefault(word, 0) + 1);}});}private static <V> void demo(Supplier<Map<String, V>> supplier, BiConsumer<Map<String, V>, List<String>> consumer) {Map<String, V> counterMap = supplier.get();List<Thread> ts = new ArrayList<>();for (int i = 1; i <= 26; i++) {int idx = i;Thread thread = new Thread(() -> {List<String> words = readFromFile(idx);consumer.accept(counterMap, words);});ts.add(thread);}ts.forEach(Thread::start);ts.forEach(t -> {try {t.join();} catch (InterruptedException e) {e.printStackTrace();}});System.out.println(counterMap);}private static List<String> readFromFile(int idx) {ArrayList<String> words = new ArrayList<>();try (BufferedReader in = new BufferedReader(new InputStreamReader(new FileInputStream("tmp/" + idx + ".txt")))) {while (true) {String word = in.readLine();if (word == null) {break;}words.add(word);}return words;} catch (IOException e) {throw new RuntimeException(e);}}}

点击查看测试案例生成代码

public class TestSampleForCountAlpha {static final String ALPHA = "abcedfghijklmnopqrstuvwxyz";public static void main(String[] args) {int length = ALPHA.length();int count = 200;ArrayList<String> list = new ArrayList<>(length * count);for (int i = 0; i < length; i++) {char ch = ALPHA.charAt(i);for (int j = 0; j < count; j++) {list.add(String.valueOf(ch));}}Collections.shuffle(list);for (int i = 0; i < 26; i++) {try (PrintWriter out = new PrintWriter(new OutputStreamWriter(new FileOutputStream("tmp/" + (i+1) + ".txt")))) {String collect = list.subList(i * count, (i+1) * count).stream().collect(Collectors.joining("\n"));out.print(collect);} catch (FileNotFoundException e) {throw new RuntimeException(e);}}}
}