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jvm雙親委派機(jī)制詳解

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雙親委派機(jī)制?記錄一下JVM的雙親委派機(jī)制學(xué)習(xí)記錄 。
類加載器種類?當(dāng)我們運(yùn)行某一個java類的main方法時，首先需要由java虛擬機(jī)的類加載器將我們要執(zhí)行的main方法所在的class文件加載到j(luò)vm中，這里提到的類加載器大概有4種：
引導(dǎo)類加載器：負(fù)責(zé)加載支撐JVM運(yùn)行的位于JRE的lib目錄下的核心類庫，比如rt.jar、charsets.jar等擴(kuò)展類加載器：負(fù)責(zé)加載支撐JVM運(yùn)行的位于JRE的lib目錄下的ext擴(kuò)展目錄中的JAR類包應(yīng)用程序類加載器：負(fù)責(zé)加載ClassPath路徑下的類包，主要就是加載你自己寫的那些類自定義加載器：負(fù)責(zé)加載用戶自定義路徑下的類包 。
?每個類加載器加載的包路徑都是不同的，有各自的職責(zé) 。通過一下示例，可以看出每個類加載器加載的路徑：
public class TestJDKClassLoader {public static void main(String[] args) {System.out.println(String.class.getClassLoader());System.out.println(com.sun.crypto.provider.DESKeyFactory.class.getClassLoader().getClass().getName());System.out.println(TestJDKClassLoader.class.getClassLoader().getClass().getName());System.out.println();ClassLoader appClassLoader = ClassLoader.getSystemClassLoader();ClassLoader extClassloader = appClassLoader.getParent();ClassLoader bootstrapLoader = extClassloader.getParent();System.out.println("the bootstrapLoader : " + bootstrapLoader);System.out.println("the extClassloader : " + extClassloader);System.out.println("the appClassLoader : " + appClassLoader);System.out.println();System.out.println("bootstrapLoader加載以下文件：");URL[] urls = Launcher.getBootstrapClassPath().getURLs();for (int i = 0; i < urls.length; i++) {System.out.println(urls[i]);}System.out.println();System.out.println("extClassloader加載以下文件：");System.out.println(System.getProperty("java.ext.dirs"));System.out.println();System.out.println("appClassLoader加載以下文件：");System.out.println(System.getProperty("java.class.path"));}}// 運(yùn)行結(jié)果：nullsun.misc.Launcher$ExtClassLoadersun.misc.Launcher$AppClassLoaderthe bootstrapLoader : nullthe extClassloader : sun.misc.Launcher$ExtClassLoader@330bedb4the appClassLoader : sun.misc.Launcher$AppClassLoader@14dad5dcbootstrapLoader加載以下文件：file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/resources.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/rt.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/sunrsasign.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jsse.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jce.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/charsets.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jfr.jarfile:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/classesextClassloader加載以下文件：D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext;C:\WINDOWS\Sun\Java\lib\extappClassLoader加載以下文件：D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\charsets.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\deploy.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\access-bridge-64.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\cldrdata.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\dnsns.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\jaccess.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\jfxrt.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\localedata.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\nashorn.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunec.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunjce_provider.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunmscapi.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunpkcs11.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\zipfs.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\javaws.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jce.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jfr.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jfxswt.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jsse.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\management-agent.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\plugin.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\resources.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\rt.jar;D:\Files\learn\tuling\jvm-demo\target\classes;D:\ProgramFiles\ideaIU-2021.3.3.win\lib\idea_rt.jarProcess finished with exit code 0

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