Java ekosisteminin temel ta\u015flar\u0131ndan biri olan ClassLoader mekanizmas\u0131, Java Sanal Makinesi’nin (JVM) \u00e7al\u0131\u015fma zaman\u0131nda s\u0131n\u0131flar\u0131 nas\u0131l y\u00fckledi\u011fini, ba\u011flad\u0131\u011f\u0131n\u0131 ve ba\u015flatt\u0131\u011f\u0131n\u0131 y\u00f6neten kritik bir bile\u015fendir. Bir Java uygulamas\u0131n\u0131n dinamik do\u011fas\u0131n\u0131n merkezinde yer alan ClassLoader’lar, kodun mod\u00fcler, g\u00fcvenli ve esnek bir \u015fekilde \u00e7al\u0131\u015fmas\u0131n\u0131 sa\u011flar. Bu makalede, Java ClassLoader’\u0131n ne oldu\u011funu, nas\u0131l \u00e7al\u0131\u015ft\u0131\u011f\u0131n\u0131, JVM’in yerle\u015fik ClassLoader’lar\u0131n\u0131, \u00f6zel ClassLoader’lar\u0131n nas\u0131l olu\u015fturulaca\u011f\u0131n\u0131 ve bu mekanizman\u0131n ileri d\u00fczey konular\u0131n\u0131 ve potansiyel tuzaklar\u0131n\u0131 ayr\u0131nt\u0131l\u0131 bir \u015fekilde inceleyece\u011fiz.<\/p>\n
Bir Java program\u0131 \u00e7al\u0131\u015ft\u0131\u011f\u0131nda, t\u00fcm s\u0131n\u0131flar ba\u015flang\u0131\u00e7ta bellekte bulunmaz. Bunun yerine, JVM, bir s\u0131n\u0131fa ilk kez ihtiya\u00e7 duyuldu\u011funda onu dinamik olarak y\u00fckler. Bu s\u00fcre\u00e7, \u00fc\u00e7 ana a\u015famadan olu\u015fur: Y\u00fckleme (Loading), Ba\u011flama (Linking) ve Ba\u015flatma (Initialization).<\/p>\n
Y\u00fckleme a\u015famas\u0131, bir s\u0131n\u0131f\u0131n ikili temsilini (genellikle bir Ba\u011flama, y\u00fcklenen s\u0131n\u0131f\u0131n do\u011frulu\u011funu ve tutarl\u0131l\u0131\u011f\u0131n\u0131 kontrol eden, bellekte gerekli yap\u0131lar\u0131 haz\u0131rlayan ve sembolik referanslar\u0131 do\u011frudan referanslara d\u00f6n\u00fc\u015ft\u00fcren bir s\u00fcre\u00e7tir. \u00dc\u00e7 alt a\u015famadan olu\u015fur:<\/p>\n Do\u011frulama, y\u00fcklenen s\u0131n\u0131f\u0131n bytecode’unun JVM spesifikasyonlar\u0131na uygun olup olmad\u0131\u011f\u0131n\u0131 kontrol eder. Bu a\u015fama, k\u00f6t\u00fc niyetli veya hatal\u0131 bytecode’un JVM’i \u00e7\u00f6kertmesini veya g\u00fcvenlik a\u00e7\u0131klar\u0131na yol a\u00e7mas\u0131n\u0131 engeller. \u00d6rne\u011fin, bytecode’un tip g\u00fcvenli\u011fini sa\u011flamas\u0131, y\u0131\u011f\u0131n ta\u015fmalar\u0131n\u0131 \u00f6nlemesi ve eri\u015fim izinlerini ihlal etmemesi gibi kontroller yap\u0131l\u0131r. Do\u011frulama ba\u015far\u0131s\u0131z olursa, bir Haz\u0131rlama a\u015famas\u0131nda, s\u0131n\u0131f\u0131n statik alanlar\u0131 i\u00e7in bellek ayr\u0131l\u0131r ve bu alanlara varsay\u0131lan ba\u015flang\u0131\u00e7 de\u011ferleri atan\u0131r. \u00d6rne\u011fin, bir \u00c7\u00f6z\u00fcmleme, s\u0131n\u0131f\u0131n sembolik referanslar\u0131n\u0131 (\u00f6rne\u011fin, di\u011fer s\u0131n\u0131flara, metotlara veya alanlara yap\u0131lan referanslar) do\u011frudan referanslara d\u00f6n\u00fc\u015ft\u00fcrme i\u015flemidir. Java bytecode’unda, di\u011fer s\u0131n\u0131flara yap\u0131lan referanslar sembolik olarak saklan\u0131r (yani, sadece adlar\u0131yla). \u00c7\u00f6z\u00fcmleme, bu sembolik adlar\u0131 kullanarak ilgili s\u0131n\u0131flar\u0131 bulur ve bellekteki ger\u00e7ek adreslerine veya do\u011frudan i\u015faret\u00e7ilerine d\u00f6n\u00fc\u015ft\u00fcr\u00fcr. Bu s\u00fcre\u00e7, tembel (lazy) bir \u015fekilde ger\u00e7ekle\u015ftirilebilir; yani, bir referans\u0131n ilk kullan\u0131ld\u0131\u011f\u0131 anda \u00e7\u00f6z\u00fcmlenmesi m\u00fcmk\u00fcnd\u00fcr. \u00c7\u00f6z\u00fcmleme ba\u015far\u0131s\u0131z olursa, bir Ba\u015flatma, bir s\u0131n\u0131f\u0131n kullan\u0131m i\u00e7in haz\u0131r hale getirildi\u011fi son a\u015famad\u0131r. Bu a\u015famada, s\u0131n\u0131f\u0131n statik ba\u015flat\u0131c\u0131 bloklar\u0131 ( Ba\u015flatma, her s\u0131n\u0131f i\u00e7in yaln\u0131zca bir kez ger\u00e7ekle\u015fir ve i\u015f par\u00e7ac\u0131\u011f\u0131 g\u00fcvenli bir \u015fekilde yap\u0131l\u0131r; yani birden fazla i\u015f par\u00e7ac\u0131\u011f\u0131 ayn\u0131 anda bir s\u0131n\u0131f\u0131 ba\u015flatmaya \u00e7al\u0131\u015f\u0131rsa, JVM bu i\u015flemi senkronize eder.<\/p>\n Java ClassLoader’lar\u0131, “parent-first” (\u00f6nce ebeveyn) olarak bilinen bir delegasyon modeli kullanarak \u00e7al\u0131\u015f\u0131r. Bu model, bir s\u0131n\u0131f\u0131n y\u00fcklenmesi istendi\u011finde, iste\u011fi do\u011frudan y\u00fcklemek yerine \u00f6nce ebeveyn ClassLoader’a devretmesini gerektirir.<\/p>\n Bir ClassLoader’a bir s\u0131n\u0131f y\u00fckleme iste\u011fi geldi\u011finde ( Bu hiyerar\u015fik yap\u0131, ClassLoader’lar\u0131n bir a\u011fa\u00e7 yap\u0131s\u0131 olu\u015fturmas\u0131n\u0131 sa\u011flar ve her ClassLoader’\u0131n bir ebeveyni (Bootstrap ClassLoader hari\u00e7) bulunur.<\/p>\n * Tutarl\u0131l\u0131k ve G\u00fcvenlik:<\/strong> JVM, varsay\u0131lan olarak \u00fc\u00e7 ana ClassLoader ile gelir ve bunlar belirli g\u00f6revleri yerine getirmek \u00fczere tasarlanm\u0131\u015ft\u0131r:<\/p>\n * G\u00f6revi:<\/strong> Bu, JVM’in en \u00fcst d\u00fczey ClassLoader’\u0131d\u0131r ve hi\u00e7bir ebeveyni yoktur. \u00c7ekirdek Java API s\u0131n\u0131flar\u0131n\u0131 y\u00fcklemekten sorumludur. Bu s\u0131n\u0131flar genellikle * G\u00f6revi:<\/strong> Bootstrap ClassLoader’\u0131n alt ClassLoader’\u0131d\u0131r. Java uzant\u0131lar\u0131 dizininden (genellikle * G\u00f6revi:<\/strong> Extension ClassLoader’\u0131n alt ClassLoader’\u0131d\u0131r. Genellikle kullan\u0131c\u0131n\u0131n yazd\u0131\u011f\u0131 uygulama s\u0131n\u0131flar\u0131n\u0131 ve uygulaman\u0131n ba\u011f\u0131ml\u0131 oldu\u011fu \u00fc\u00e7\u00fcnc\u00fc taraf k\u00fct\u00fcphanelerini y\u00fcklemekten sorumludur. Bu s\u0131n\u0131flar\u0131, Java’n\u0131n yerle\u015fik ClassLoader’lar\u0131 \u00e7o\u011fu durumda yeterli olsa da, baz\u0131 geli\u015fmi\u015f senaryolarda kendi \u00f6zel ClassLoader’\u0131n\u0131z\u0131 olu\u015fturman\u0131z gerekebilir.<\/p>\n * Standart Olmayan Kaynaklardan S\u0131n\u0131f Y\u00fckleme:<\/strong> S\u0131n\u0131flar\u0131 dosya sisteminden veya JAR’dan de\u011fil, bir veritaban\u0131ndan, a\u011f \u00fczerinden (\u00f6rne\u011fin, RMI, OSGi), \u015fifrelenmi\u015f bir dosyadan veya dinamik olarak olu\u015fturulmu\u015f bytecode’dan y\u00fcklemek istedi\u011finizde. Kendi ClassLoader’\u0131n\u0131z\u0131 olu\u015fturmak i\u00e7in 1. En \u00f6nemli nokta, \u00d6rnek (Kavramsal): Basit Bir Dosya Sistemi ClassLoader’\u0131<\/strong><\/p>\n Bu \u00f6rnekte Java ClassLoader’lar\u0131, her ClassLoader’\u0131n kendi “namespace”ine sahip olmas\u0131 prensibine dayan\u0131r. Bu, ayn\u0131 tam nitelikli ada sahip iki s\u0131n\u0131f\u0131n, farkl\u0131 ClassLoader’lar taraf\u0131ndan y\u00fcklendi\u011finde, JVM i\u00e7in tamamen farkl\u0131 tipler olarak kabul edildi\u011fi anlam\u0131na gelir.<\/p>\n * Farkl\u0131 Namespace’ler:<\/strong> Bir ClassLoader, kendisi taraf\u0131ndan y\u00fcklenen t\u00fcm s\u0131n\u0131flar\u0131 ve ebeveynleri taraf\u0131ndan y\u00fcklenen t\u00fcm s\u0131n\u0131flar\u0131 g\u00f6r\u00fcr. Ancak, karde\u015f veya alt ClassLoader’lar taraf\u0131ndan y\u00fcklenen s\u0131n\u0131flar\u0131 do\u011frudan g\u00f6remez. Bu durum, * ClassLoader mekanizmas\u0131 g\u00fc\u00e7l\u00fc olsa da, yanl\u0131\u015f kullan\u0131ld\u0131\u011f\u0131nda veya anla\u015f\u0131lmad\u0131\u011f\u0131nda karma\u015f\u0131k sorunlara yol a\u00e7abilir.<\/p>\n \u00d6zel ClassLoader’lar, \u00f6zellikle uygulama sunucular\u0131 gibi uzun \u00f6m\u00fcrl\u00fc ortamlarda bellek s\u0131z\u0131nt\u0131lar\u0131na yol a\u00e7abilecek yayg\u0131n bir kaynakt\u0131r. Bir uygulama yeniden da\u011f\u0131t\u0131ld\u0131\u011f\u0131nda, genellikle eski uygulama ClassLoader’\u0131 ve onun y\u00fckledi\u011fi t\u00fcm s\u0131n\u0131flar bellekten temizlenmelidir. Ancak, a\u015fa\u011f\u0131daki durumlarda bu ger\u00e7ekle\u015fmeyebilir:<\/p>\n * Statik Referanslar:<\/strong> Eski ClassLoader taraf\u0131ndan y\u00fcklenen bir s\u0131n\u0131f\u0131n statik bir alan\u0131, yeni ClassLoader taraf\u0131ndan y\u00fcklenen bir s\u0131n\u0131fa veya JVM’in kendisi taraf\u0131ndan y\u00fcklenen bir s\u0131n\u0131fa (\u00f6rne\u011fin, Java’n\u0131n g\u00fcvenlik modeli, ClassLoader’lar\u0131 \u00f6nemli bir bile\u015fen olarak kullan\u0131r. Bir s\u0131n\u0131f ClassLoader taraf\u0131ndan y\u00fcklendi\u011finde, o s\u0131n\u0131f\u0131n nereden geldi\u011fi bilgisi (code source) de kaydedilir. Java ClassLoader mekanizmas\u0131, Java platformunun dinamik, esnek ve g\u00fcvenli do\u011fas\u0131n\u0131n temelini olu\u015fturur. S\u0131n\u0131flar\u0131n ya\u015fam d\u00f6ng\u00fcs\u00fcn\u00fc (y\u00fckleme, ba\u011flama, ba\u015flatma) y\u00f6neterek, uygulamalar\u0131n mod\u00fcler bir \u015fekilde \u00e7al\u0131\u015fmas\u0131n\u0131 sa\u011flar. Yerle\u015fik Bootstrap, Extension ve Application ClassLoader’lar \u00e7o\u011fu ihtiyac\u0131 kar\u015f\u0131larken, \u00f6zel ClassLoader’lar; dinamik kod y\u00fckleme, izolasyon ve \u00e7al\u0131\u015fma zaman\u0131 kod manip\u00fclasyonu gibi geli\u015fmi\u015f senaryolar i\u00e7in g\u00fc\u00e7l\u00fc bir ara\u00e7 sunar.<\/p>\n Delegasyon modelini anlamak, ClassLoader hiyerar\u015fisini kavramak ve .class<\/code> dosyas\u0131ndan veya ba\u015fka bir kaynaktan) al\u0131p JVM belle\u011fine getirme s\u00fcrecidir. Bu a\u015famada ClassLoader, belirli bir s\u0131n\u0131f\u0131n tam nitelikli ad\u0131n\u0131 (Fully Qualified Name – FQN) kullanarak bu s\u0131n\u0131f\u0131n bytecode’unu bulur ve okur. Bytecode okunduktan sonra, JVM bu veriyi kullanarak bellekte bir java.lang.Class<\/code> nesnesi olu\u015fturur. Bu Class<\/code> nesnesi, s\u0131n\u0131f\u0131n \u00e7al\u0131\u015fma zaman\u0131 temsilidir ve s\u0131n\u0131f\u0131n ad\u0131n\u0131, s\u00fcper s\u0131n\u0131f\u0131n\u0131, uygulad\u0131\u011f\u0131 aray\u00fczleri, alanlar\u0131n\u0131 ve metotlar\u0131n\u0131 i\u00e7erir. Her y\u00fcklenen s\u0131n\u0131f i\u00e7in bellekte yaln\u0131zca bir adet Class<\/code> nesnesi bulunur.<\/p>\nBa\u011flama (Linking)<\/h3>\n
Do\u011frulama (Verification)<\/h4>\n
VerifyError<\/code> f\u0131rlat\u0131l\u0131r.<\/p>\nHaz\u0131rlama (Preparation)<\/h4>\n
int<\/code> statik alan\u0131 0<\/code> ile, bir boolean<\/code> false<\/code> ile ve bir nesne referans\u0131 null<\/code> ile ba\u015flat\u0131l\u0131r. Bu a\u015famada, alanlara programc\u0131 taraf\u0131ndan belirlenen ger\u00e7ek de\u011ferler atanmaz; bu i\u015flem Ba\u015flatma a\u015famas\u0131nda ger\u00e7ekle\u015fir.<\/p>\n\u00c7\u00f6z\u00fcmleme (Resolution)<\/h4>\n
NoClassDefFoundError<\/code> veya IllegalAccessError<\/code> gibi hatalar f\u0131rlat\u0131labilir.<\/p>\nBa\u015flatma (Initialization)<\/h3>\n
static { ... }<\/code>) y\u00fcr\u00fct\u00fcl\u00fcr ve statik alanlara programc\u0131 taraf\u0131ndan belirlenen ger\u00e7ek de\u011ferler atan\u0131r. Bir s\u0131n\u0131f, ancak a\u015fa\u011f\u0131daki durumlardan biri ger\u00e7ekle\u015fti\u011finde ba\u015flat\u0131l\u0131r:
\n* Bir s\u0131n\u0131f\u0131n \u00f6rne\u011fi olu\u015fturuldu\u011funda (new<\/code> anahtar kelimesi ile).
\n* Bir s\u0131n\u0131f\u0131n statik bir metodu \u00e7a\u011fr\u0131ld\u0131\u011f\u0131nda.
\n* Bir s\u0131n\u0131f\u0131n statik bir alan\u0131na eri\u015fildi\u011finde (final olmayan veya derleme zaman\u0131nda bilinen bir sabit olmayan).
\n* Yans\u0131ma (reflection) API’leri kullan\u0131larak bir s\u0131n\u0131f ba\u015flat\u0131ld\u0131\u011f\u0131nda.
\n* Bir alt s\u0131n\u0131f ba\u015flat\u0131ld\u0131\u011f\u0131nda, \u00fcst s\u0131n\u0131f\u0131 da ba\u015flat\u0131l\u0131r.
\n* JVM ba\u015flang\u0131\u00e7 s\u0131n\u0131f\u0131n\u0131 i\u00e7eren bir metodu \u00e7a\u011fr\u0131ld\u0131\u011f\u0131nda.<\/p>\nDelegasyon Modeli<\/h2>\n
Parent-First Delegasyonun \u0130\u015fleyi\u015fi<\/h3>\n
loadClass()<\/code> metodu \u00e7a\u011fr\u0131ld\u0131\u011f\u0131nda), a\u015fa\u011f\u0131daki ad\u0131mlar izlenir:
\n1. \u00d6nbelle\u011fi Kontrol Et:<\/strong> ClassLoader, bu s\u0131n\u0131f\u0131n daha \u00f6nce kendisi veya ebeveynleri taraf\u0131ndan y\u00fcklenip y\u00fcklenmedi\u011fini kendi \u00f6nbelle\u011finde kontrol eder. Y\u00fcklendiyse, mevcut Class<\/code> nesnesini d\u00f6nd\u00fcr\u00fcr.
\n2. Ebeveyne Devret:<\/strong> S\u0131n\u0131f \u00f6nbellekte bulunamazsa, ClassLoader iste\u011fi ebeveyn ClassLoader’\u0131na devrederek s\u0131n\u0131f\u0131 y\u00fcklemesini ister.
\n3. Kendisi Y\u00fckle:<\/strong> E\u011fer ebeveyn ClassLoader s\u0131n\u0131f\u0131 bulamaz ve y\u00fckleyemezse, orijinal ClassLoader s\u0131n\u0131f\u0131 kendi kaynaklar\u0131ndan (\u00f6rne\u011fin, kendi belirledi\u011fi dosya sisteminden, a\u011fdan vb.) bulmaya ve y\u00fcklemeye \u00e7al\u0131\u015f\u0131r.
\n4. Hata:<\/strong> E\u011fer ClassLoader da s\u0131n\u0131f\u0131 bulamazsa, bir ClassNotFoundException<\/code> f\u0131rlat\u0131l\u0131r.<\/p>\nDelegasyon Modelinin Faydalar\u0131<\/h3>\n
java.lang.Object<\/code> gibi temel Java API s\u0131n\u0131flar\u0131n\u0131n her zaman ayn\u0131 ClassLoader (Bootstrap ClassLoader) taraf\u0131ndan y\u00fcklenmesini ve dolay\u0131s\u0131yla her zaman ayn\u0131 s\u00fcr\u00fcm\u00fcn\u00fcn kullan\u0131lmas\u0131n\u0131 sa\u011flar. Bu, temel Java API’lerinin k\u00f6t\u00fc niyetli kodlar taraf\u0131ndan de\u011fi\u015ftirilmesini veya farkl\u0131 s\u00fcr\u00fcmlerinin y\u00fcklenmesini engeller.
\n* Tekrarlamay\u0131 \u00d6nleme:<\/strong> Ayn\u0131 s\u0131n\u0131f\u0131n birden fazla kez y\u00fcklenmesini \u00f6nler. E\u011fer bir ebeveyn ClassLoader zaten bir s\u0131n\u0131f\u0131 y\u00fcklediyse, alt ClassLoader’lar bu s\u0131n\u0131f\u0131 tekrar y\u00fcklemeye \u00e7al\u0131\u015fmaz, bellekte gereksiz kopyalar\u0131n olu\u015fmas\u0131n\u0131 engeller.
\n* Hiyerar\u015fik Yap\u0131:<\/strong> Uygulama ve k\u00fct\u00fcphane ba\u011f\u0131ml\u0131l\u0131klar\u0131n\u0131n daha d\u00fczenli y\u00f6netilmesine olanak tan\u0131r.<\/p>\nYerle\u015fik ClassLoader’lar<\/h2>\n
Bootstrap ClassLoader (\u00d6ny\u00fckleme S\u0131n\u0131f Y\u00fckleyici)<\/h3>\n
rt.jar<\/code> (runtime library) veya daha yeni Java s\u00fcr\u00fcmlerinde mod\u00fcl yolu (jmods<\/code>) \u00fczerinde bulunur.
\n* Uygulama:<\/strong> Bootstrap ClassLoader, Java koduyla de\u011fil, JVM’in yerel (native) koduyla uygulan\u0131r. Bu nedenle, java.lang.ClassLoader.getSystemClassLoader().getParent()<\/code> metodu \u00e7a\u011fr\u0131ld\u0131\u011f\u0131nda null<\/code> d\u00f6nd\u00fcr\u00fcr.
\n* Yolu:<\/strong> sun.boot.class.path<\/code> sistem \u00f6zelli\u011fi ile belirtilen dizinlerden s\u0131n\u0131flar\u0131 y\u00fckler.<\/p>\nExtension ClassLoader (Uzant\u0131 S\u0131n\u0131f Y\u00fckleyici)<\/h3>\n
jre\/lib\/ext<\/code> veya java.ext.dirs<\/code> sistem \u00f6zelli\u011fi ile belirtilen dizinler) s\u0131n\u0131flar\u0131 y\u00fckler. Bu dizinler, JDK’ya eklenen veya \u00fc\u00e7\u00fcnc\u00fc taraf k\u00fct\u00fcphanelerin yerle\u015ftirilebilece\u011fi yerlerdir.
\n* Uygulama:<\/strong> sun.misc.Launcher$ExtClassLoader<\/code> s\u0131n\u0131f\u0131 taraf\u0131ndan uygulan\u0131r.
\n* Ebeveyn:<\/strong> Bootstrap ClassLoader’d\u0131r.
\n* \u00c7ocuk:<\/strong> Application ClassLoader’d\u0131r.<\/p>\nApplication ClassLoader (Uygulama S\u0131n\u0131f Y\u00fckleyici) \/ System ClassLoader<\/h3>\n
CLASSPATH<\/code> ortam de\u011fi\u015fkeni, -classpath<\/code> veya -cp<\/code> komut sat\u0131r\u0131 arg\u00fcman\u0131 ile belirtilen yollardan y\u00fckler.
\n* Uygulama:<\/strong> sun.misc.Launcher$AppClassLoader<\/code> s\u0131n\u0131f\u0131 taraf\u0131ndan uygulan\u0131r.
\n* Ebeveyn:<\/strong> Extension ClassLoader’d\u0131r.
\n* getSystemClassLoader()<\/code>:<\/strong> ClassLoader.getSystemClassLoader()<\/code> metodu \u00e7a\u011fr\u0131ld\u0131\u011f\u0131nda d\u00f6nd\u00fcr\u00fclen ClassLoader budur. \u00c7o\u011fu zaman, bir Java uygulamas\u0131ndaki varsay\u0131lan ClassLoader’d\u0131r.<\/p>\n\u00d6zel ClassLoader’lar<\/h2>\n
Ne Zaman \u00d6zel ClassLoader Kullan\u0131lmal\u0131?<\/h3>\n
\n* S\u0131n\u0131f Yeniden Y\u00fckleme (Hot Swapping):<\/strong> Bir uygulaman\u0131n \u00e7al\u0131\u015fmas\u0131n\u0131 durdurmadan veya JVM’i yeniden ba\u015flatmadan s\u0131n\u0131flar\u0131 g\u00fcncellemek (\u00f6rne\u011fin, geli\u015ftirme ortamlar\u0131nda veya uygulama sunucular\u0131nda). Her yeni ClassLoader, ayn\u0131 s\u0131n\u0131f\u0131n yeni bir s\u00fcr\u00fcm\u00fcn\u00fc y\u00fckleyebilir.
\n* Kod \u00dcretimi ve De\u011fi\u015fimi:<\/strong> \u00c7al\u0131\u015fma zaman\u0131nda bytecode \u00fcreten veya mevcut bytecode’u de\u011fi\u015ftiren ara\u00e7lar (\u00f6rne\u011fin, AOP \u00e7er\u00e7eveleri, ORM ara\u00e7lar\u0131) genellikle \u00f6zel ClassLoader’lar kullan\u0131r.
\n* \u0130zolasyon ve G\u00fcvenlik (Sandboxing):<\/strong> Farkl\u0131 uygulamalar\u0131 veya eklentileri birbirinden izole etmek. Her uygulama veya eklenti kendi ClassLoader’\u0131 ile y\u00fcklenebilir, b\u00f6ylece birbirlerinin s\u0131n\u0131flar\u0131na do\u011frudan eri\u015femezler veya birbirlerinin ba\u011f\u0131ml\u0131l\u0131klar\u0131n\u0131 etkilemezler. \u00d6rne\u011fin, servlet konteynerleri (Tomcat, Jetty) her web uygulamas\u0131n\u0131 kendi ClassLoader’\u0131 ile y\u00fckleyerek izolasyon sa\u011flar.
\n* \u00c7oklu S\u00fcr\u00fcm Deste\u011fi:<\/strong> Ayn\u0131 k\u00fct\u00fcphanenin farkl\u0131 s\u00fcr\u00fcmlerini ayn\u0131 JVM i\u00e7inde \u00e7al\u0131\u015ft\u0131rmak.<\/p>\n\u00d6zel ClassLoader Nas\u0131l Olu\u015fturulur?<\/h3>\n
java.lang.ClassLoader<\/code> soyut s\u0131n\u0131f\u0131n\u0131 geni\u015fletmeniz gerekir. Genellikle iki ana metodu ge\u00e7ersiz k\u0131lman\u0131z yeterlidir:<\/p>\nfindClass(String name)<\/code>:<\/strong> Bu metot, ClassLoader’\u0131n s\u0131n\u0131f\u0131 kendi kaynaklar\u0131ndan (ebeveynine devrettikten sonra) bulmas\u0131ndan ve y\u00fcklemesinden sorumludur. Bu metot, s\u0131n\u0131f\u0131n bytecode’unu bir byte[]<\/code> dizisi olarak d\u00f6nd\u00fcrmelidir.
\n2. defineClass(String name, byte[] b, int off, int len)<\/code>:<\/strong> Bu metot, findClass<\/code> taraf\u0131ndan bulunan bytecode dizisini alarak JVM’de bir Class<\/code> nesnesine d\u00f6n\u00fc\u015ft\u00fcr\u00fcr. Genellikle bu metodu do\u011frudan \u00e7a\u011f\u0131rmak yerine, ClassLoader<\/code> s\u0131n\u0131f\u0131n\u0131n varsay\u0131lan defineClass<\/code> implementasyonunu kullan\u0131rs\u0131n\u0131z.<\/p>\nloadClass(String name)<\/code> metodunu ge\u00e7ersiz k\u0131lmaktan ka\u00e7\u0131nmakt\u0131r. loadClass<\/code> metodu, delegasyon modelini uygulayan ana metottur. E\u011fer bu metodu ge\u00e7ersiz k\u0131larsan\u0131z, parent-first delegasyon modelini bozabilirsiniz. \u00d6zel bir ClassLoader olu\u015ftururken, genellikle findClass<\/code> metodunu override ederek kendi s\u0131n\u0131f bulma mant\u0131\u011f\u0131n\u0131z\u0131 eklersiniz. loadClass<\/code> metodu, \u00f6nce ebeveynini kontrol eder, ebeveyn bulamazsa findClass<\/code> metodunuzu \u00e7a\u011f\u0131r\u0131r.<\/p>\nimport java.io.ByteArrayOutputStream;\nimport java.io.File;\nimport java.io.IOException;\nimport java.io.InputStream;\nimport java.nio.file.Files;\nimport java.nio.file.Path;\nimport java.nio.file.Paths;\n\npublic class CustomFileSystemClassLoader extends ClassLoader {\n private String classPath;\n\n public CustomFileSystemClassLoader(String classPath, ClassLoader parent) {\n super(parent);\n this.classPath = classPath;\n }\n\n @Override\n protected Class findClass(String name) throws ClassNotFoundException {\n \/\/ S\u0131n\u0131f\u0131n dosya yolunu olu\u015ftur\n String filePath = classPath + File.separator + name.replace('.', File.separatorChar) + \".class\";\n Path path = Paths.get(filePath);\n\n try {\n \/\/ Bytecode'u dosyadan oku\n byte[] classBytes = Files.readAllBytes(path);\n \/\/ Bytecode'u bir Class nesnesine d\u00f6n\u00fc\u015ft\u00fcr\n return defineClass(name, classBytes, 0, classBytes.length);\n } catch (IOException e) {\n \/\/ Dosya bulunamazsa veya okunamazsa\n throw new ClassNotFoundException(\"S\u0131n\u0131f bulunamad\u0131: \" + name + \" (Dosya: \" + filePath + \")\", e);\n }\n }\n\n \/\/ Basit bir test metodu\n public static void main(String[] args) throws Exception {\n \/\/ \u00d6rnek: 'myclasses' dizininden MyClass.class dosyas\u0131n\u0131 y\u00fckle\n \/\/ myclasses\/com\/example\/MyClass.class\n \/\/ MyClass.java i\u00e7eri\u011fi:\n \/\/ package com.example;\n \/\/ public class MyClass {\n \/\/ public void sayHello() {\n \/\/ System.out.println(\"Hello from MyClass loaded by custom ClassLoader!\");\n \/\/ }\n \/\/ }\n\n String customClassPath = \".\/myclasses\"; \/\/ S\u0131n\u0131flar\u0131n bulundu\u011fu dizin\n CustomFileSystemClassLoader customClassLoader = new CustomFileSystemClassLoader(customClassPath, ClassLoader.getSystemClassLoader());\n\n \/\/ com.example.MyClass s\u0131n\u0131f\u0131n\u0131 y\u00fckle\n Class myClass = customClassLoader.loadClass(\"com.example.MyClass\");\n Object instance = myClass.getDeclaredConstructor().newInstance();\n myClass.getMethod(\"sayHello\").invoke(instance);\n\n \/\/ MyClass'\u0131n hangi ClassLoader taraf\u0131ndan y\u00fcklendi\u011fini g\u00f6ster\n System.out.println(\"MyClass y\u00fcklendi\u011fi ClassLoader: \" + myClass.getClassLoader());\n System.out.println(\"Bu main metodunun ClassLoader'\u0131: \" + CustomFileSystemClassLoader.class.getClassLoader());\n }\n}<\/code><\/pre>\nCustomFileSystemClassLoader<\/code>, belirtilen bir dizinden .class<\/code> dosyalar\u0131n\u0131 okuyarak s\u0131n\u0131flar\u0131 y\u00fckler. main<\/code> metodu, bu ClassLoader’\u0131 kullanarak com.example.MyClass<\/code> adl\u0131 bir s\u0131n\u0131f\u0131 dinamik olarak y\u00fckler ve bir metodunu \u00e7a\u011f\u0131r\u0131r.<\/p>\nClassLoader Hiyerar\u015fisi ve \u0130zolasyon<\/h2>\n
\n* instanceof<\/code> ve Tip D\u00f6n\u00fc\u015f\u00fcm\u00fc Sorunlar\u0131:<\/strong> Bu durumun \u00f6nemli sonu\u00e7lar\u0131 vard\u0131r. E\u011fer A<\/code> s\u0131n\u0131f\u0131 ClassLoaderX<\/code> taraf\u0131ndan, B<\/code> s\u0131n\u0131f\u0131 da ClassLoaderY<\/code> taraf\u0131ndan y\u00fcklenmi\u015fse ve A<\/code> ile B<\/code> ayn\u0131 tam nitelikli ada sahip olsalar bile, A<\/code> bir B<\/code> \u00f6rne\u011fi olarak kabul edilmez. \u00d6rne\u011fin:<\/p>\nObject obj = classLoadedByClassLoaderX.newInstance();\n \/\/ E\u011fer classLoadedByClassLoaderY taraf\u0131ndan y\u00fcklenen bir MyInterface varsa\n \/\/ obj instanceof MyInterfaceClassLoadedByClassLoaderY -> false olabilir\n \/\/ (MyInterface'in her iki ClassLoader taraf\u0131ndan da y\u00fcklendi\u011fi varsay\u0131ld\u0131\u011f\u0131nda)<\/code><\/pre>\nClassCastException<\/code>‘lara yol a\u00e7abilir. \u00d6zellikle, bir k\u00fct\u00fcphane veya \u00e7er\u00e7eve, kendi ClassLoader’\u0131 ile bir aray\u00fcz\u00fc y\u00fcklerken, uygulaman\u0131z bu aray\u00fcz\u00fcn kendi ClassLoader’\u0131 ile y\u00fckledi\u011fi bir implementasyonunu kullanmaya \u00e7al\u0131\u015ft\u0131\u011f\u0131nda bu t\u00fcr hatalar ortaya \u00e7\u0131kabilir. \u00c7\u00f6z\u00fcm genellikle, aray\u00fczlerin veya temel tiplerin ortak bir ClassLoader (genellikle Application ClassLoader veya Extension ClassLoader) taraf\u0131ndan y\u00fcklenmesini sa\u011flamakt\u0131r.<\/p>\nNoClassDefFoundError<\/code> ve ClassNotFoundException<\/code>:<\/strong>
\n * ClassNotFoundException<\/code>: Bir ClassLoader, loadClass()<\/code> veya findClass()<\/code> metotlar\u0131 arac\u0131l\u0131\u011f\u0131yla bir s\u0131n\u0131f\u0131 bulamad\u0131\u011f\u0131nda f\u0131rlat\u0131l\u0131r. Bu genellikle s\u0131n\u0131f\u0131n classpath’te olmamas\u0131 veya ClassLoader’\u0131n arama yolunda bulunmamas\u0131 durumunda meydana gelir.
\n * NoClassDefFoundError<\/code>: Bir s\u0131n\u0131f\u0131n ba\u015far\u0131l\u0131 bir \u015fekilde y\u00fcklendi\u011fi (yani bytecode’u JVM’e getirildi\u011fi) ancak ba\u015flatma a\u015famas\u0131nda bir hatayla kar\u015f\u0131la\u015f\u0131ld\u0131\u011f\u0131nda f\u0131rlat\u0131l\u0131r. \u00d6rne\u011fin, s\u0131n\u0131f\u0131n statik ba\u015flat\u0131c\u0131 blo\u011fu bir istisna f\u0131rlatt\u0131\u011f\u0131nda veya s\u0131n\u0131f\u0131n ba\u011f\u0131ml\u0131 oldu\u011fu ba\u015fka bir s\u0131n\u0131f bulunamad\u0131\u011f\u0131nda bu hata olu\u015fabilir. Bu, genellikle bir derleme zaman\u0131 ba\u011f\u0131ml\u0131l\u0131\u011f\u0131n\u0131n \u00e7al\u0131\u015fma zaman\u0131nda eksik oldu\u011fu anlam\u0131na gelir.<\/p>\n\u0130leri D\u00fczey Konular ve Potansiyel Tuzaklar<\/h2>\n
Context ClassLoader (Ba\u011flam S\u0131n\u0131f Y\u00fckleyici)<\/h3>\n
Thread.currentThread().getContextClassLoader()<\/code> metodu ile eri\u015filen Context ClassLoader, delegasyon modelinin “parent-first” kural\u0131n\u0131n tersine \u00e7evrildi\u011fi durumlarda kullan\u0131l\u0131r.
\n* \u0130htiya\u00e7 Duyulma Sebebi:<\/strong> Baz\u0131 \u00e7er\u00e7eveler (\u00f6rne\u011fin, JDBC s\u00fcr\u00fcc\u00fcleri, JAXB, Log4j) veya servis sa\u011flay\u0131c\u0131 aray\u00fczleri (SPI’lar), \u00e7ekirdek Java API’sinin bir par\u00e7as\u0131 olarak veya Extension ClassLoader taraf\u0131ndan y\u00fcklenir. Bu \u00e7er\u00e7eveler, uygulaman\u0131n kendi ClassLoader’\u0131 taraf\u0131ndan y\u00fcklenen kullan\u0131c\u0131 tan\u0131ml\u0131 s\u0131n\u0131flar\u0131 (\u00f6rne\u011fin, bir JDBC s\u00fcr\u00fcc\u00fcs\u00fc implementasyonu veya bir log appender) bulmak ve y\u00fcklemek isteyebilir. Standart delegasyon modeliyle, \u00fcst ClassLoader’lar alt ClassLoader’lar\u0131n s\u0131n\u0131flar\u0131n\u0131 g\u00f6remez.
\n* \u00c7\u00f6z\u00fcm:<\/strong> Context ClassLoader, bu “ters” delegasyon i\u00e7in bir mekanizma sa\u011flar. Bir i\u015f par\u00e7ac\u0131\u011f\u0131 olu\u015fturuldu\u011funda, genellikle Application ClassLoader, o i\u015f par\u00e7ac\u0131\u011f\u0131n\u0131n Context ClassLoader’\u0131 olarak ayarlan\u0131r. \u00c7er\u00e7eveler, kendi kodlar\u0131 daha y\u00fcksek bir ClassLoader taraf\u0131ndan y\u00fcklenmi\u015f olsa bile, kullan\u0131c\u0131 tan\u0131ml\u0131 s\u0131n\u0131flar\u0131 bulmak i\u00e7in Thread.currentThread().getContextClassLoader()<\/code>‘\u0131 kullan\u0131r. Bu, bir “child-first” (\u00f6nce \u00e7ocuk) veya “sibling-first” (\u00f6nce karde\u015f) y\u00fckleme davran\u0131\u015f\u0131n\u0131 etkinle\u015ftirmenin bir yoludur.
\n* Kullan\u0131m:<\/strong> Kendi \u00f6zel ClassLoader’\u0131n\u0131z\u0131 olu\u015fturdu\u011funuzda veya bir uygulama sunucusu ortam\u0131nda \u00e7al\u0131\u015f\u0131rken, Context ClassLoader’\u0131 do\u011fru bir \u015fekilde ayarlamak veya kullanmak \u00f6nemlidir.<\/p>\nBellek S\u0131z\u0131nt\u0131lar\u0131<\/h3>\n
java.util.logging.LogManager<\/code>) referans veriyorsa, eski ClassLoader ve t\u00fcm s\u0131n\u0131flar\u0131 \u00e7\u00f6p toplay\u0131c\u0131 taraf\u0131ndan toplanamaz.
\n* Thread Lokal Referanslar\u0131:<\/strong> ThreadLocal<\/code> de\u011fi\u015fkenleri, ClassLoader’lara veya onlar\u0131n y\u00fckledi\u011fi s\u0131n\u0131flara referans tutuyorsa, bu da s\u0131z\u0131nt\u0131ya yol a\u00e7abilir.
\n* JVM Kay\u0131tlar\u0131:<\/strong> Eski ClassLoader taraf\u0131ndan y\u00fcklenen s\u0131n\u0131flar\u0131n, JVM’in kendi i\u00e7 mekanizmalar\u0131na (\u00f6rne\u011fin, JDBC s\u00fcr\u00fcc\u00fcleri DriverManager<\/code>‘a kaydoldu\u011funda) kaydolmas\u0131 ve bu kay\u0131tlar\u0131n uygulaman\u0131n durdurulmas\u0131 s\u0131ras\u0131nda d\u00fczg\u00fcn bir \u015fekilde kald\u0131r\u0131lmamas\u0131.
\n* \u00c7\u00f6z\u00fcm:<\/strong> Uygulama durdurulurken t\u00fcm kaynaklar\u0131n (i\u015f par\u00e7ac\u0131klar\u0131, statik alanlar, JVM kay\u0131tlar\u0131) temizlendi\u011finden emin olmak kritik \u00f6neme sahiptir. \u00d6zellikle statik alanlar ve ThreadLocal<\/code>‘lar ClassLoader’\u0131n \u00f6mr\u00fcn\u00fc uzatabilecek potansiyel tehlikelerdir.<\/p>\nG\u00fcvenlik Y\u00f6neticisi Etkile\u015fimi<\/h3>\n
java.lang.SecurityManager<\/code>, belirli bir s\u0131n\u0131f\u0131n belirli bir eylemi ger\u00e7ekle\u015ftirme iznine sahip olup olmad\u0131\u011f\u0131n\u0131 belirlerken bu bilgiyi kullan\u0131r. \u00d6rne\u011fin, bir dosya sistemine eri\u015fim veya a\u011f ba\u011flant\u0131s\u0131 kurma gibi i\u015flemler, s\u0131n\u0131f\u0131 y\u00fckleyen ClassLoader’\u0131n kaynak konumuna g\u00f6re k\u0131s\u0131tlanabilir. \u00d6zel ClassLoader’lar, g\u00fcvenli bir “sandbox” ortam\u0131 olu\u015fturmak i\u00e7in bu mekanizma ile birlikte kullan\u0131labilir.<\/p>\nSonu\u00e7<\/h2>\n
Context ClassLoader<\/code> gibi ileri d\u00fczey konular\u0131n fark\u0131nda olmak, ClassNotFoundException<\/code>, NoClassDefFoundError<\/code> ve ClassCastException<\/code> gibi yayg\u0131n sorunlar\u0131 gidermek ve bellek s\u0131z\u0131nt\u0131lar\u0131n\u0131 \u00f6nlemek i\u00e7in hayati \u00f6neme sahiptir. Java ClassLoader’lar\u0131na hakim olmak, \u00f6zellikle b\u00fcy\u00fck \u00f6l\u00e7ekli uygulamalar, uygulama sunucular\u0131 veya eklenti mimarileri geli\u015ftiren her Java geli\u015ftiricisi i\u00e7in vazge\u00e7ilmez bir beceridir. Bu derinlemesine bilgi, Java uygulamalar\u0131n\u0131z\u0131n performans\u0131n\u0131, kararl\u0131l\u0131\u011f\u0131n\u0131 ve g\u00fcvenli\u011fini \u00f6nemli \u00f6l\u00e7\u00fcde art\u0131rman\u0131za yard\u0131mc\u0131 olacakt\u0131r.<\/body><\/p>\n","protected":false},"excerpt":{"rendered":"Java ClassLoader: Derinlemesine Bir Bak\u0131\u015f\nJava ekosisteminin temel ta\u015flar\u0131ndan biri olan ClassLoader mekanizmas\u0131, Java Sanal Makinesi’nin (JVM)","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"csco_page_header_type":"","csco_page_load_nextpost":"","csco_page_subscribe_form":"","csco_page_contact_form":"","footnotes":""},"categories":[874],"tags":[],"class_list":{"0":"post-40273","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-server","7":"cs-entry","8":"cs-video-wrap"},"yoast_head":"\n