{"id":240,"date":"2017-03-22T01:32:33","date_gmt":"2017-03-22T00:32:33","guid":{"rendered":"http:\/\/antek.atthost24.pl\/?p=240"},"modified":"2017-03-22T22:57:41","modified_gmt":"2017-03-22T21:57:41","slug":"ile-on-wazy","status":"publish","type":"post","link":"https:\/\/blog.lantkowiak.pl\/index.php\/2017\/03\/22\/ile-on-wazy\/","title":{"rendered":"Ile on wa\u017cy?!"},"content":{"rendered":"

Czy zastanawiali\u015bcie si\u0119 kiedy\u015b jak du\u017co pami\u0119ci zajmuj\u0105 obiekty w Javie? Du\u017co os\u00f3b pewnie nigdy si\u0119 nad tym nie g\u0142owi\u0142o, a przynajmniej nie przed pierwszym OutOfMemoryError \ud83d\ude09<\/p>\n

W tym wpisie postaram si\u0119 pokaza\u0107 w jaki spos\u00f3b oszacowa\u0107 wielko\u015b\u0107 obiekt\u00f3w, kt\u00f3re tworzymy w Javie.<\/p>\n

Typy prymitywne<\/h3>\n

Niew\u0105tpliwie naj\u0142atwiej jest okre\u015bli\u0107 ilo\u015b\u0107 pami\u0119ci zajmowanej przez typy prymitywne. Poni\u017cej przedstawiam tabelk\u0119, kt\u00f3ra pokazuje ile ka\u017cdy z typ\u00f3w potrzebuj\u0119 bajt\u00f3w w pami\u0119ci.<\/p>\n\n\n\n\n\t\n\n\t\n\t\n\t\n\t\n\t\n\t\n\t
Typ<\/th>bajty<\/th>\n<\/tr>\n<\/thead>\n
boolean<\/td>1<\/td>\n<\/tr>\n
byte<\/td>1<\/td>\n<\/tr>\n
char<\/td>2<\/td>\n<\/tr>\n
int<\/td>4<\/td>\n<\/tr>\n
float<\/td>4<\/td>\n<\/tr>\n
long<\/td>8<\/td>\n<\/tr>\n
double<\/td>8<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n

Ilo\u015b\u0107 pami\u0119ci potrzebnej do przechowywania booleana mo\u017ce troch\u0119 dziwi\u0107. W ko\u0144cu mo\u017ce on przyj\u0105\u0107 tylko dwa warto\u015bci i w teorii powinien zajmowa\u0107 1 bit. Specyfikacja JVM nie definiuje ile pami\u0119ci jest potrzebne na przechowanie booleana i zale\u017cy to od danej implementacji. Dla uproszczenia mo\u017cemy \u015bmia\u0142o przyj\u0105\u0107, \u017ce jest potrzebny 1 bajt.<\/p>\n

Reszta warto\u015bci wydaj\u0119 si\u0119 by\u0107 w miar\u0119 oczywista \ud83d\ude42<\/p>\n

Tablice<\/h3>\n

Ka\u017cda tablica\u00a0zajmuje wielokrotno\u015b\u0107 danego typu (co wydaj\u0119 si\u0119 oczywiste ;)) oraz dodatkowe 24 bajty, kt\u00f3re s\u0105 narzutem tablicy. Poni\u017cej przedstawiam tabelk\u0119 z przyk\u0142adami, kt\u00f3re powinny wszystko wyja\u015bni\u0107.<\/p>\n\n\n\n\n\t\n\n\t\n\t\n\t\n\t
Typ<\/th>bajty<\/th>\n<\/tr>\n<\/thead>\n
byte[]<\/td>N + 24<\/td>\n<\/tr>\n
char[]<\/td>2N+24<\/td>\n<\/tr>\n
int[]<\/td>4N+24<\/td>\n<\/tr>\n
double[]<\/td>8N+24<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n

Obiekty<\/h3>\n

I teraz przechodzimy do mi\u0119sa tematu \u00a0– czyli jak oszacowa\u0107 wielko\u015b\u0107 obiektu naszej klasy.<\/p>\n

Aby to zrobi\u0107 potrzebujemy kilku dodatkowych informacji.<\/p>\n

Narzut obiektu<\/h4>\n

Ka\u017cdy obiekt, podobnie jak tablica, powoduje\u00a0dodatkowy narzut pami\u0119ci. W tym przypadku jest to 16 bajt\u00f3w.<\/p>\n

Referencja<\/h4>\n

Ka\u017cda przetrzymywana referencja w naszym obiekcie to dodatkowe 8 bajt\u00f3w.<\/p>\n

Padding<\/h4>\n

Wielko\u015b\u0107 ka\u017cdego obiektu jest uzupe\u0142niana do wielokrotno\u015bci 8 bajt\u00f3w.<\/p>\n

Praktyka \ud83d\ude42<\/h4>\n

Jako przyk\u0142ad we\u017amy klas\u0119 String. Poni\u017cej przedstawiam wycinek implementacji tej klasy, kt\u00f3ry jest potrzebny do oszacowania wielko\u015bci obiektu.<\/p>\n

public final class String\r\n\timplements java.io.Serializable, Comparable<String>, CharSequence {\r\n    \r\n\tprivate final char value[];\r\n\tprivate final int offset;\r\n\tprivate final int count;\r\n\tprivate int hash;\r\n\r\n\t...\r\n}<\/pre>\n
Do obliczenia potrzebujemy zsumowa\u0107 kilka rzeczy:<\/p>\n
    \n
  1. Narzut obiektu – 16 bajt\u00f3w<\/li>\n
  2. Referencja do tablicy – 8 bajt\u00f3w<\/li>\n
  3. Tablica – 2N + narzut tablicy 24 bajty<\/li>\n
  4. Pole offset – 4 bajty<\/li>\n
  5. Pole count – 4 bajty<\/li>\n
  6. Pole hash – 4 bajty<\/li>\n<\/ol>\n
    Zsumowuj\u0105c wszystko wychodzi nam 2N + 60 bajt\u00f3w. W takim przypadku musimy jeszcze pami\u0119ta\u0107 o paddingu, czyli dodajemy 4 bajty, aby ca\u0142kowita wielko\u015b\u0107 obiektu by\u0142a wielokrotno\u015bci\u0105 8.<\/p>\n
    Podsumowanie<\/h3>\n
    Powy\u017cej przedstawi\u0142em og\u00f3lne zasady, kt\u00f3rych mo\u017cemy u\u017cy\u0107, aby oszacowa\u0107 wielko\u015b\u0107 naszych obiekt\u00f3w w Javie. Dzi\u0119ki temu mo\u017cemy lepiej\u00a0zrozumie\u0107 co jest alokowane w pami\u0119ci i jaki rozmiar zajmuje.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Czy zastanawiali\u015bcie si\u0119 kiedy\u015b jak du\u017co pami\u0119ci zajmuj\u0105 obiekty w Javie? Du\u017co os\u00f3b pewnie nigdy si\u0119 nad tym nie g\u0142owi\u0142o, a przynajmniej nie przed pierwszym OutOfMemoryError \ud83d\ude09 W…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,7,16],"tags":[8,17,18],"class_list":["post-240","post","type-post","status-publish","format-standard","hentry","category-dsp2017","category-java","category-pamiec-w-jvm","tag-java","tag-jvm","tag-pamiec"],"yoast_head":"\nIle on wa\u017cy?! - Lukasz Antkowiak's blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.lantkowiak.pl\/index.php\/2017\/03\/22\/ile-on-wazy\/\" \/>\n<meta property=\"og:locale\" content=\"pl_PL\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Ile on wa\u017cy?! - Lukasz Antkowiak's blog\" \/>\n<meta property=\"og:description\" content=\"Czy zastanawiali\u015bcie si\u0119 kiedy\u015b jak du\u017co pami\u0119ci zajmuj\u0105 obiekty w Javie? 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