B站搜索“乐哥聊编程“有本篇文章配套视频 https://www.bilibili.com/video/BV1cP4y117Qt
负责加载支撑JVM运行的位于JRE的lib目录下的核心类库,这个加载器是由C++写的,所以我们在java源码里面是找不到它的实现,如果尝试对它进行打印,输出将为空值。
负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录中的JAR 类包,也可以使用java.ext.dirs来指定路径,一般保持默认就好。
负责加载类路径下的jar包和class文件,我们自己写的代码都是通过这个类加载加载的
我们自己去实现的类加载器,其父类加载器为应用程序类加载器。
package com.lglbc.day1;
import sun.misc.Launcher;
import java.net.URL;
/**
* @Description TODO
* @Author 乐哥聊编程
* @Date 2022/11/13 20:57
*/
public class TestClassLoader {
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("===============手动分割================");
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("===============手动分割================");
System.out.println("bootstrapLoader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("===============手动分割================");
System.out.println("extClassloader加载以下文件:");
System.out.println(System.getProperty("java.ext.dirs"));
System.out.println("===============手动分割================");
System.out.println("appClassLoader加载以下文件:");
System.out.println(System.getProperty("java.class.path"));
}
}
public Launcher() {
Launcher.ExtClassLoader var1;
try {
var1 = Launcher.ExtClassLoader.getExtClassLoader();
} catch (IOException var10) {
throw new InternalError("Could not create extension class loader", var10);
}
try {
this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
} catch (IOException var9) {
throw new InternalError("Could not create application class loader", var9);
}
Thread.currentThread().setContextClassLoader(this.loader);
String var2 = System.getProperty("java.security.manager");
if (var2 != null) {
SecurityManager var3 = null;
if (!"".equals(var2) && !"default".equals(var2)) {
try {
var3 = (SecurityManager)this.loader.loadClass(var2).newInstance();
} catch (IllegalAccessException var5) {
} catch (InstantiationException var6) {
} catch (ClassNotFoundException var7) {
} catch (ClassCastException var8) {
}
} else {
var3 = new SecurityManager();
}
if (var3 == null) {
throw new InternalError("Could not create SecurityManager: " + var2);
}
System.setSecurityManager(var3);
}
}