呵，十個雙親委派問題，想問倒我？

• Bootstrap ClassLoader 啟動類加載器
• Extention ClassLoader 標準擴展類加載器
• Application ClassLoader 應用類加載器
• User ClassLoader 用戶自定義類加載器

• Bootstrap ClassLoader ，主要負責加載Java核心類庫，%JRE_HOME%\lib下的rt.jar、resources.jar、charsets.jar和class等。
• Extention ClassLoader，主要負責加載目錄%JRE_HOME%\lib\ext目錄下的jar包和class文件。
• Application ClassLoader ，主要負責加載當前應用的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，主要就是以下幾個步驟：

• loadClass()?就是主要進行類加載的方法，默認的雙親委派機制就實現在這個方法中。
• findClass()?根據名稱或位置加載.class字節(jié)碼
• definclass()?把字節(jié)碼轉化為Class

/**

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

?????*/

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

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

????}

這個方法只拋出了一個異常，沒有默認實現。

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

在以上代碼執(zhí)行之前，DriverManager會先被類加載器加載，因為java.sql.DriverManager類是位于rt.jar下面的 ，所以他會被根加載器加載。

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

這段代碼，會嘗試加載classpath下面的所有實現了Driver接口的實現類。

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

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

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

????}

第一行，獲取當前線程的線程上下?類加載器 AppClassLoader，?于加載 classpath 中的具體實現類。

Class?c?=?findLoadedClass(cn);

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

????????//?找到當前類屬于哪個模塊

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

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

????????????//獲取當前模塊的類加載器

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

????????????//進行類加載

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

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

??????????????//?找不到模塊信息才會進行雙親委派

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

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

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

??????????}

????}