Java类加载的护城河：深入探究双亲委派机制 _Java类

前言关于类加载，有两个非常重要的内容，就是类加载器和双亲委派机制，也是面试时常见考核问题。
一、类加载器还是以这个简单的代码为例：
arduino复制代码package com.jvm.test; public class Book {public static void mAIn(String[] args) {String name = "《三体》";System.out.printf("一本你不看，都不知道何为“惊艳”二字的书：" + name);} }上面的类的加载是要通过类加载器来实现的。
JAVA中有几种类加载器：

引导类加载器(BootstrapClassLoader）: 这是JVM的内置类加载器，负责加载支撑JVM运行的位于JRE的lib目录下的核心类库，比如rt.jar、charsets.jar等，即对应如java.lang包中的类等。
它是所有其他类加载器的父加载器，是类加载器层次结构的顶层，由JVM本身实现，由C++实现。需要注意的是：这里的“父”，不是我们Java中的类继承的父类，这里的“父加载器”可以理解是逻辑上的概念，更多是指加载器之间的协作机制。
扩展类加载器（ExtClassLoader）：：这个类加载器负责加载Java的扩展类库，位于<JAVA_HOME>/lib/ext目录下的JAR文件。
他的父加载器是引导类加载器，通过扩展类加载器，可以实现对JVM的扩展功能。
应用程序类加载器（AppClassLoader）：也被称为系统类加载器，它负责加载应用程序的类，也就是我们自己编写的Java代码的加载器。
它的父加载器是扩展类加载器。
自定义类加载器（CustomClassLoader）：除了上述的三种主要的类加载器，Java还提供自定义类加载器的能力，允许开发人员根据需求实现自己的类加载逻辑。
负责加载用户自定义路径下的类包。

了解了这几种不同的加载器，如我们上述实例的代码的类的加载，应该也很容易得知是由应用程序类加载器来加载。
接下来看一个类加载器的示例：

ini复制代码package com.jvm.classloader;import sun.misc.Launcher;import java.NET.URL;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 appClassLoader : " + appClassLoader);System.out.println("the extClassloader : " + extClassloader);System.out.println("the bootstrapLoader : " + bootstrapLoader);System.out.println();System.out.println("bootstrapLoader加载以下路径文件：");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加载以下路径文件：");System.out.println(System.getProperty("java.ext.dirs"));System.out.println();System.out.println("appClassLoader加载以下路径文件：");System.out.println(System.getProperty("java.class.path"));}}

可以思考一下输出结果是啥？
输出结果：

bash复制代码nullsun.misc.Launcher$ExtClassLoadersun.misc.Launcher$AppClassLoaderthe appClassLoader : sun.misc.Launcher$AppClassLoader@18b4aac2the extClassloader : sun.misc.Launcher$ExtClassLoader@6ce253f1the bootstrapLoader : nullbootstrapLoader加载以下路径文件：file:/Library/Java/JavaVirtualmachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/resources.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/rt.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/sunrsasign.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jsse.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jce.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/charsets.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jfr.jarfile:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/classesextClassloader加载以下路径文件：/Users/lan/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/javaappClassLoader加载以下路径文件：/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/DNSns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_191.jdk/Contents/Home/lib/tools.jar:/Users/lan/lihy/study/tuling/jvm/jvm-full-gc/target/classes:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-starter-web/2.1.2.RELEASE/spring-boot-starter-web-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-starter/2.1.2.RELEASE/spring-boot-starter-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot/2.1.2.RELEASE/spring-boot-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/2.1.2.RELEASE/spring-boot-autoconfigure-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-starter-logging/2.1.2.RELEASE/spring-boot-starter-logging-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/Users/lan/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/Users/lan/.m2/repository/org/Apache/logging/log4j/log4j-to-slf4j/2.11.1/log4j-to-slf4j-2.11.1.jar:/Users/lan/.m2/repository/org/apache/logging/log4j/log4j-api/2.11.1/log4j-api-2.11.1.jar:/Users/lan/.m2/repository/org/slf4j/jul-to-slf4j/1.7.25/jul-to-slf4j-1.7.25.jar:/Users/lan/.m2/repository/javax/annotation/javax.annotation-api/1.3.2/javax.annotation-api-1.3.2.jar:/Users/lan/.m2/repository/org/yaml/snakeyaml/1.23/snakeyaml-1.23.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-starter-json/2.1.2.RELEASE/spring-boot-starter-json-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.9.8/jackson-databind-2.9.8.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.9.0/jackson-annotations-2.9.0.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.9.8/jackson-core-2.9.8.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jdk8/2.9.8/jackson-datatype-jdk8-2.9.8.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jsr310/2.9.8/jackson-datatype-jsr310-2.9.8.jar:/Users/lan/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.9.8/jackson-module-parameter-names-2.9.8.jar:/Users/lan/.m2/repository/org/springframework/boot/spring-boot-starter-Tomcat/2.1.2.RELEASE/spring-boot-starter-tomcat-2.1.2.RELEASE.jar:/Users/lan/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/9.0.14/tomcat-embed-core-9.0.14.jar:/Users/lan/.m2/repository/org/apache/tomcat/embed/tomcat-embed-el/9.0.14/tomcat-embed-el-9.0.14.jar:/Users/lan/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/9.0.14/tomcat-embed-websocket-9.0.14.jar:/Users/lan/.m2/repository/org/hibernate/validator/hibernate-validator/6.0.14.Final/hibernate-validator-6.0.14.Final.jar:/Users/lan/.m2/repository/javax/validation/validation-api/2.0.1.Final/validation-api-2.0.1.Final.jar:/Users/lan/.m2/repository/org/jboss/logging/jboss-logging/3.3.2.Final/jboss-logging-3.3.2.Final.jar:/Users/lan/.m2/repository/com/fasterxml/classmate/1.4.0/classmate-1.4.0.jar:/Users/lan/.m2/repository/org/springframework/spring-web/5.1.4.RELEASE/spring-web-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-beans/5.1.4.RELEASE/spring-beans-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-webmvc/5.1.4.RELEASE/spring-webmvc-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-aop/5.1.4.RELEASE/spring-aop-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-context/5.1.4.RELEASE/spring-context-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-expression/5.1.4.RELEASE/spring-expression-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/Users/lan/.m2/repository/org/springframework/spring-core/5.1.4.RELEASE/spring-core-5.1.4.RELEASE.jar:/Users/lan/.m2/repository/org/springframework/spring-jcl/5.1.4.RELEASE/spring-jcl-5.1.4.RELEASE.jar:/Applications/IntelliJ 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