写在前面的话：为什么要研究类加载的过程？为什么要研究双亲委派机制？
研究类加载的过程就是要知道类加载的时候使用了双亲委派机制。但仅仅知道双亲委派机制不是目的，目的是要了解为什么要使用双亲委派机制，他的原理是什么？知道双亲委派机制的逻辑思想，然后这个思想是否可以被我们借鉴，为我所用。这才是学习知识的目的。
比如：双亲委派机制，避免了类的重复加载，避免了核心类库被修改。那么，我们在做框架设计的时候，框架底层的东西是不是应该是不容被串改的，或者不可以被黑客进攻的，那么我们就可以借鉴双亲委派机制了。
再比如：双亲委派机制的实现使用了责任链设计模式，我们借此可以研究一下责任链设计模式，这样就理解了委派的原理。那么哪些场景我们可以使用责任链设计模式呢？多思考，才是学习的目的和精髓所在。学到的东西，能用在工作中，才是王道。
一、什么是双亲委派机制我们先来看一个案例: 打印引导类加载器, 扩展类加载器, 应用程序类加载器加载的目录
package com.lxl.jvm; import sun.misc.Launcher; import java.net.URL; public class TestJDKClassLoader { public static void main(String[] args) { System.out.println(); System.out.println("bootstrap Loader加载以下文件:"); 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加载以下文件"); System.out.println(System.getProperty("java.ext.dirs")); System.out.println(); System.out.println("appClassLoader加载以下文件"); System.out.println(System.getProperty("java.class.path")); } }我们来看一下:
引导类加载器加载的文件是:Launcher.getBootstrapClassPath().getURLs()下的文件
扩展类加载器加载的文件是: java.ext.dirs , java扩展类目录
应用程序类加载器, 加载的是: java.class.path , java home路径下的所有类
我们来看一下打印结果
bootstrap Loader加载以下文件:
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/sunrsasign.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/classes
extClassLoader加载以下文件
/Users/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java
appClassLoader加载以下文件
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/charsets.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/deploy.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/dnsns.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jaccess.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/localedata.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/nashorn.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunec.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/ext/zipfs.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/javaws.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jce.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfr.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jfxswt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/jsse.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/management-agent.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/plugin.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/ant-javafx.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/dt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/javafx-mx.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/jconsole.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/packager.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/sa-jdi.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/lib/tools.jar:
/Users/Downloads/workspace/project-all/target/classes:
/Users/responsitory/org/springframework/boot/spring-boot-starter/2.2.8.RELEASE/spring-boot-starter-2.2.8.RELEASE.jar:
/Users/responsitory/org/springframework/boot/spring-boot/2.2.8.RELEASE/spring-boot-2.2.8.RELEASE.jar:
/Users/responsitory/org/springframework/spring-context/5.2.7.RELEASE/spring-context-5.2.7.RELEASE.jar:
/Users/responsitory/org/springframework/spring-aop/5.2.7.RELEASE/spring-aop-5.2.7.RELEASE.jar:
/Users/responsitory/org/springframework/spring-beans/5.2.7.RELEASE/spring-beans-5.2.7.RELEASE.jar:
/Users/responsitory/org/springframework/spring-expression/5.2.7.RELEASE/spring-expression-5.2.7.RELEASE.jar:
/Users/responsitory/org/springframework/boot/spring-boot-autoconfigure/2.2.8.RELEASE/spring-boot-autoconfigure-2.2.8.RELEASE.jar:
/Users/responsitory/org/springframework/boot/spring-boot-starter-logging/2.2.8.RELEASE/spring-boot-starter-logging-2.2.8.RELEASE.jar:
/Users/responsitory/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:
/Users/responsitory/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:
/Users/responsitory/org/apache/logging/log4j/log4j-to-slf4j/2.12.1/log4j-to-slf4j-2.12.1.jar:
/Users/responsitory/org/apache/logging/log4j/log4j-api/2.12.1/log4j-api-2.12.1.jar:
/Users/responsitory/org/slf4j/jul-to-slf4j/1.7.30/jul-to-slf4j-1.7.30.jar:
/Users/responsitory/jakarta/annotation/jakarta.annotation-api/1.3.5/jakarta.annotation-api-1.3.5.jar:
/Users/responsitory/org/springframework/spring-core/5.2.7.RELEASE/spring-core-5.2.7.RELEASE.jar:
/Users/responsitory/org/springframework/spring-jcl/5.2.7.RELEASE/spring-jcl-5.2.7.RELEASE.jar:
/Users/responsitory/org/yaml/snakeyaml/1.25/snakeyaml-1.25.jar:
/Users/responsitory/org/slf4j/slf4j-api/1.7.30/slf4j-api-1.7.30.jar:
/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar
通过观察,我们发现
引导类加载器,确实只加载了java home下的/jre/lib目录下的类
扩展类加载器加载了java扩展目录里面的类