详解Java的 StringTable

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oop StringTable::do_intern(Handle string_or_null_h, const jchar* name,
                           int len, uintx hash, TRAPS) {
  HandleMark hm(THREAD);  // cleanup strings created
  Handle string_h;

  if (!string_or_null_h.is_null()) {
    string_h = string_or_null_h;
  } else {
    string_h = java_lang_String::create_from_unicode(name, len, CHECK_NULL);
  }

  assert(java_lang_String::equals(string_h(), name, len),
         "string must be properly initialized");
  assert(len == java_lang_String::length(string_h()), "Must be same length");

  // Notify deduplication support that the string is being interned.  A string
  // must never be deduplicated after it has been interned.  Doing so interferes
  // with compiler optimizations done on e.g. interned string literals.
  if (StringDedup::is_enabled()) {
    StringDedup::notify_intern(string_h());
  }

  StringTableLookupOop lookup(THREAD, hash, string_h);
  StringTableGet stg(THREAD);

  bool rehash_warning;
  do {
    // Callers have already looked up the String using the jchar* name, so just go to add.
    WeakHandle wh(_oop_storage, string_h);
    // The hash table takes ownership of the WeakHandle, even if it's not inserted.
    if (_local_table->insert(THREAD, lookup, wh, &rehash_warning)) {
      update_needs_rehash(rehash_warning);
      return wh.resolve();
    }
    // In case another thread did a concurrent add, return value already in the table.
    // This could fail if the String got gc'ed concurrently, so loop back until success.
    if (_local_table->get(THREAD, lookup, stg, &rehash_warning)) {
      update_needs_rehash(rehash_warning);
      return stg.get_res_oop();
    }
  } while(true);
}

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// Rehash
bool StringTable::do_rehash() {
  if (!_local_table->is_safepoint_safe()) {
    return false;
  }

  // We use current size, not max size.
  size_t new_size = _local_table->get_size_log2(Thread::current());
  StringTableHash* new_table = new StringTableHash(new_size, END_SIZE, REHASH_LEN, true);
  // Use alt hash from now on
  _alt_hash = true;
  if (!_local_table->try_move_nodes_to(Thread::current(), new_table)) {
    _alt_hash = false;
    delete new_table;
    return false;
  }

  // free old table
  delete _local_table;
  _local_table = new_table;

  return true;
}

void StringTable::rehash_table() {
  static bool rehashed = false;
  log_debug(stringtable)("Table imbalanced, rehashing called.");

  // Grow instead of rehash.
  if (get_load_factor() > PREF_AVG_LIST_LEN &&
      !_local_table->is_max_size_reached()) {
    log_debug(stringtable)("Choosing growing over rehashing.");
    trigger_concurrent_work();
    _needs_rehashing = false;
    return;
  }
  // Already rehashed.
  if (rehashed) {
    log_warning(stringtable)("Rehashing already done, still long lists.");
    trigger_concurrent_work();
    _needs_rehashing = false;
    return;
  }

  _alt_hash_seed = AltHashing::compute_seed();
  {
    if (do_rehash()) {
      rehashed = true;
    } else {
      log_info(stringtable)("Resizes in progress rehashing skipped.");
    }
  }
  _needs_rehashing = false;
}