调用 intern() 方法后，内存中的 new String() 对象何时被清除

In this case, one of the best approaches is to use a local map, not being subject to thread synchronization, dropping it after the process, to avoid keeping references longer than necessary, without having to use special interaction with the garbage collector. This implies that occurrences of the same strings within different data sources are not canonicalized (but still being subject to the JVM’s String Deduplication), but it’s a reasonable trade-off. By using an ordinary resizable HashMap, we also do not have the issues of the fixed intern table.

例如

static List<String> parse(CharSequence input) {
    List<String> result = new ArrayList<>();

    Matcher m = TOKEN_PATTERN.matcher(input);
    CharBuffer cb = CharBuffer.wrap(input);
    HashMap<CharSequence,String> cache = new HashMap<>();
    while(m.find()) {
        result.add(
            cache.computeIfAbsent(cb.subSequence(m.start(), m.end()), Object::toString));
    }
    return result;
}

注意这里 CharBuffer 的使用:它包装输入序列并且它的 subSequence 方法返回另一个具有不同开始和结束索引的包装器，为我们的 HashMap 实现正确的 equals 和 hashCode 方法，而 computeIfAbsent 只会调用 toString 方法，如果键之前不存在于地图中.因此，与使用 intern() 不同的是，不会为已经遇到的字符串创建 String 实例，从而节省了其中最昂贵的方面，即字符数组的复制.

static List<String> parse(CharSequence input) {
    List<String> result = new ArrayList<>();

    Matcher m = TOKEN_PATTERN.matcher(input);
    CharBuffer cb = CharBuffer.wrap(input);
    HashMap<CharSequence,String> cache = new HashMap<>();
    while(m.find()) {
        cb.limit(m.end()).position(m.start());
        String s = cache.get(cb);
        if(s == null) {
            s = cb.toString();
            cache.put(CharBuffer.wrap(s), s);
        }
        result.add(s);
    }
    return result;
}