一. 双亲委派机制

1.1 什么是双亲委派机制

我们先来看一个案例:

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加载一下文件:");
        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"));
    }
}

这是打印引导类加载器, 扩展类加载器, 应用程序类加载器加载的目录.

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

应用程学类加载器, 加载的是: java.class.path , java home路径下的所有类

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我们来看一下打印结果

bootstrap Loader加载一下文件:
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目录下面类

扩展类加载器加载了java扩展目录里面的类

但是, 应用程序类加载器, 加载的类包含了java home下/jre/lib目录, java home扩展目录下的类, 还有responsitory仓库下的类, 还有idea的类, 还有就是我们的类路径下target的类.

问题来了, 为什么AppClassLoader加载器加载了引导类加载器和扩展类加载器要加载的类呢? 这样加载不是重复了么?

其实, 不会重复加载, appClassLoader主要加载的类就是target目录下的类, 其他目录下的类基本上不会加载. 为什么呢? 这就是下面要说的双亲委派机制.

上面这个图就是双亲委派机制的图. 什么意思呢?

比如: 我现在有一个自定义的java.lxl.jvm.Math类. 首先是由应用程序类加载器去加载java.lxl.jvm.Math类, 他要去看他已经加载的类中是否有这个类, 如果有, 就直接返回回来, 如果没有, 就委托扩展类加载器去加载. 扩展类加载器去查看已经加载的类是否有java.lxl.jvm.Math, 如果有就返回,如果没有就继续委托它的父类引导类加载器去加载. 这时候, 我们都知道, Math类是我自己定义的, 引导类加载器中不可能有, 所以, 他就会让扩展类加载器去加载, 扩展类加载器中有没有呢? 当然也没有, 于是委托应用程序类加载器, ok,应用程序类加载器是有的, 于是就可以加载, 然后返回了.

那么, 这里有一个问题, 那就是, 由应用程序类加载器首先加载, 然后最后又回到了应用程序类加载器. 绕了一圈又回来了, 这样是不是有些多此一举呢, 循环了两次? 为什么一定要从应用程序类加载器加载呢? 直接从引导类加载器加载不好么?只循环一次啊....

其实, 对于我们的项目来说, 95%的类都是我们自己写的, 因此, 而我们自己写的类是有应用程序类加载器加载. 其实,应用程序类加载器只有在第一次的时候, 才会加载两次. 以后, 当再次使用到这个类的时候, 直接去问应用程序类加载器, 有这个类么? 已经有了, 就直接返回了.

1.2 源码分析双亲委派机制

我们来看一下类加载器.类加载器主要调用的是classLoader.loadClass("com.lxl.Math") 这个方法来实现双亲委派机制的. 根据上面的分析, 我们知道, 在Launcher类初始化的时候, loadClass是AppClassLoader, 那么也就是说, 双亲委派机制的起点是AppClassLoader.

下面我们来看一下源码, 我们采用断点的方式来分析

首先, 我们在Launcher的AppClassLoader的loadClass(String var1, boolean var2) 这个方法添加一个断点, 并将其赋值为我们的com.lxl.jvm.Math类

然后运行Math的main方法,我们来看一下这个类到底是如何被加载的

启动debug调试模式, 首先进入了Launch.AppClassLoader.loadClass(....)方法

我们来具体看看这个方法的实现

上面都是在做权限校验, 我们看重点代码. 重点代码是调用了super.loadClass(var1,var2), 而这个super是谁呢? 我们来看看AppClassLoader的集成关系

在mac上按option+command+u查看集成关系图

我们看到AppClassLoader继承自URLClassLoader, 而URLClassLoader又继承了上面四个类,最终有继承一个叫做ClassLoader的类, 所有的类加载器, 最终都要继承这个ClassLoader类.

而这里调用的是super.loadClass(),我们来看看URLClassLoader中是否有loadClass()类, 看过之后发现,他没有, 最终这个super.loadClass()是继承了ClassLoader类的loadClass(....)方法

正式这个类实现了双亲委派机制

1.2 双亲委派机制及其原理

1. 类加载的过程

2. jvm核心类加载器

一. 双亲委派机制

1.1 什么是双亲委派机制

1.2 源码分析双亲委派机制

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