我竟然被“雙親委派”給虐了

這是Hollis的第?326?篇原創(chuàng)分享

作者 l Hollis

來源 l Hollis（ID：hollischuang）

• Bootstrap ClassLoader 啟動(dòng)類加載器
• Extention ClassLoader 標(biāo)準(zhǔn)擴(kuò)展類加載器
• Application ClassLoader 應(yīng)用類加載器
• User ClassLoader 用戶自定義類加載器

• Bootstrap ClassLoader ，主要負(fù)責(zé)加載Java核心類庫，%JRE_HOME%\lib下的rt.jar、resources.jar、charsets.jar和class等。
• Extention ClassLoader，主要負(fù)責(zé)加載目錄%JRE_HOME%\lib\ext目錄下的jar包和class文件。
• Application ClassLoader ，主要負(fù)責(zé)加載當(dāng)前應(yīng)用的classpath下的所有類
• User ClassLoader ， 用戶自定義的類加載器,可加載指定路徑的class文件

public?abstract?class?ClassLoader?{

????????//?The?parent?class?loader?for?delegation

????????private?final?ClassLoader?parent;

????}

protected?Class?loadClass(String?name,?boolean?resolve)

????????????throws?ClassNotFoundException

????????{

????????????synchronized?(getClassLoadingLock(name))?{

????????????????//?First,?check?if?the?class?has?already?been?loaded

????????????????Class?c?=?findLoadedClass(name);

????????????????if?(c?==?null)?{

????????????????????long?t0?=?System.nanoTime();

????????????????????try?{

????????????????????????if?(parent?!=?null)?{

????????????????????????????c?=?parent.loadClass(name,?false);

????????????????????????}?else?{

????????????????????????????c?=?findBootstrapClassOrNull(name);

????????????????????????}

????????????????????}?catch?(ClassNotFoundException?e)?{

????????????????????????//?ClassNotFoundException?thrown?if?class?not?found

????????????????????????//?from?the?non-null?parent?class?loader

????????????????????}



????????????????????if?(c?==?null)?{

????????????????????????//?If?still?not?found,?then?invoke?findClass?in?order

????????????????????????//?to?find?the?class.

????????????????????????long?t1?=?System.nanoTime();

????????????????????????c?=?findClass(name);



????????????????????????//?this?is?the?defining?class?loader;?record?the?stats

????????????????????????sun.misc.PerfCounter.getParentDelegationTime().addTime(t1?-?t0);

????????????????????????sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);

????????????????????????sun.misc.PerfCounter.getFindClasses().increment();

????????????????????}

????????????????}

????????????????if?(resolve)?{

????????????????????resolveClass(c);

????????????????}

????????????????return?c;

????????????}

????????}

代碼不難理解，主要就是以下幾個(gè)步驟：

• loadClass()?就是主要進(jìn)行類加載的方法，默認(rèn)的雙親委派機(jī)制就實(shí)現(xiàn)在這個(gè)方法中。
• findClass()?根據(jù)名稱或位置加載.class字節(jié)碼
• definclass()?把字節(jié)碼轉(zhuǎn)化為Class

/**

?????*?@since??1.2

?????*/

????protected?Class?findClass(String?name)?throws?ClassNotFoundException?{

????????throw?new?ClassNotFoundException(name);

????}

這個(gè)方法只拋出了一個(gè)異常，沒有默認(rèn)實(shí)現(xiàn)。

Connection?conn?=?DriverManager.getConnection("jdbc:mysql://localhost:3306/mysql",?"root",?"1234");

在以上代碼執(zhí)行之前，DriverManager會(huì)先被類加載器加載，因?yàn)閖ava.sql.DriverManager類是位于rt.jar下面的 ，所以他會(huì)被根加載器加載。

ServiceLoader?loadedDrivers?=?ServiceLoader.load(Driver.class);

這段代碼，會(huì)嘗試加載classpath下面的所有實(shí)現(xiàn)了Driver接口的實(shí)現(xiàn)類。

public?static??ServiceLoader?load(Class?service)?{

????????ClassLoader?cl?=?Thread.currentThread().getContextClassLoader();

????????return?ServiceLoader.load(service,?cl);

????}

第一行，獲取當(dāng)前線程的線程上下?類加載器 AppClassLoader，?于加載 classpath 中的具體實(shí)現(xiàn)類。

Class?c?=?findLoadedClass(cn);

????if?(c?==?null)?{

????????//?找到當(dāng)前類屬于哪個(gè)模塊

????????LoadedModule?loadedModule?=?findLoadedModule(cn);

????????if?(loadedModule?!=?null)?{

????????????//獲取當(dāng)前模塊的類加載器

????????????BuiltinClassLoader?loader?=?loadedModule.loader();

????????????//進(jìn)行類加載

????????????c?=?findClassInModuleOrNull(loadedModule,?cn);

?????????}?else?{

??????????????//?找不到模塊信息才會(huì)進(jìn)行雙親委派

????????????????if?(parent?!=?null)?{

??????????????????c?=?parent.loadClassOrNull(cn);

????????????????}

??????????}

????}

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