概述
一、类加载运行的全过程
当我们用java命令运行某个类的main函数时，需要通过类加载器把主类加载到jvm中
public class User {
private String name;
private int age;
}
public class ClassLoaderTest1 {
public static final int count= 1;
public static User user = new User();
public int calculate() {
int x = 1;
int y = 2;
int z = x * y;
return z;
}
public static void main(String[] args) {
ClassLoaderTest1 loaderTest1 = new ClassLoaderTest1();
loaderTest1.calculate();
}
}
通过java命令执行代码的流程大致如下：
其中，加载类的过程如下：
加载 >> 验证 >> 准备 >> 解析 >> 初始化 >> 使用 >> 卸载
加载：在硬盘上查找并通过IO读入字节码文件，使用到类时才会加载，例如调用类的 main()方法，new对象等等，在加载阶段会在内存中生成一个代表这个类的 java.lang.Class对象，作为方法区这个类的各种数据的访问入口
验证：校验字节码文件的正确性，比如开头：cafe babe，是否java语言的规范
准备：给类的静态变量分配内存，并赋予默认值
public static final int count= 1; //静态变量赋一个默认值 int->0
public static User user = new User(); //对象->null
解析：将符号引用替换为直接引用，该阶段会把一些静态方法(符号引用，比如 main()方法)替换为指向数据所存内存的指针或句柄等(直接引用)，这是所谓的静态链接过程(类加载期间完成)，动态链接是在程序运行期间完成的将符号引用替换为直接引用javap -v ClassLoaderTest1.class
Classfile /E:/code/gitee/springboot-examples/springboot-examples-demo/jvm-demo/target/test-classes/com/zengqingfa/exercise/jvm/ClassLoaderTest1.class
Last modified 2021-10-23; size 889 bytes
MD5 checksum e05503781b6953f2b3a4729adee10dba
Compiled from "ClassLoaderTest1.java"
public class com.zengqingfa.exercise.jvm.ClassLoaderTest1
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #8.#36 // java/lang/Object."<init>":()V
#2 = Class #37 // com/zengqingfa/exercise/jvm/ClassLoaderTest1
#3 = Methodref #2.#36 // com/zengqingfa/exercise/jvm/ClassLoaderTest1."<init>":()V
#4 = Methodref #2.#38 // com/zengqingfa/exercise/jvm/ClassLoaderTest1.calculate:()I
#5 = Class #39 // com/zengqingfa/exercise/jvm/User
#6 = Methodref #5.#36 // com/zengqingfa/exercise/jvm/User."<init>":()V
#7 = Fieldref #2.#40 // com/zengqingfa/exercise/jvm/ClassLoaderTest1.user:Lcom/zengqingfa/exercise/jvm/User;
#8 = Class #41 // java/lang/Object
#9 = Utf8 count
#10 = Utf8 I
#11 = Utf8 ConstantValue
#12 = Integer 1
#13 = Utf8 user
#14 = Utf8 Lcom/zengqingfa/exercise/jvm/User;
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 LocalVariableTable
#20 = Utf8 this
#21 = Utf8 Lcom/zengqingfa/exercise/jvm/ClassLoaderTest1;
#22 = Utf8 calculate
#23 = Utf8 ()I
#24 = Utf8 x
#25 = Utf8 y
#26 = Utf8 z
#27 = Utf8 main
#28 = Utf8 ([Ljava/lang/String;)V
#29 = Utf8 args
#30 = Utf8 [Ljava/lang/String;
#31 = Utf8 loaderTest1
#32 = Utf8 MethodParameters
#33 = Utf8 <clinit>
#34 = Utf8 SourceFile
#35 = Utf8 ClassLoaderTest1.java
#36 = NameAndType #15:#16 // "<init>":()V
#37 = Utf8 com/zengqingfa/exercise/jvm/ClassLoaderTest1
#38 = NameAndType #22:#23 // calculate:()I
#39 = Utf8 com/zengqingfa/exercise/jvm/User
#40 = NameAndType #13:#14 // user:Lcom/zengqingfa/exercise/jvm/User;
#41 = Utf8 java/lang/Object
{
public static final int count;
descriptor: I
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 1
public static com.zengqingfa.exercise.jvm.User user;
descriptor: Lcom/zengqingfa/exercise/jvm/User;
flags: ACC_PUBLIC, ACC_STATIC
public com.zengqingfa.exercise.jvm.ClassLoaderTest1();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 9: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/zengqingfa/exercise/jvm/ClassLoaderTest1;
public int calculate();
descriptor: ()I
flags: ACC_PUBLIC
Code:
stack=2, locals=4, args_size=1
0: iconst_1
1: istore_1
2: iconst_2
3: istore_2
4: iload_1
5: iload_2
6: imul
7: istore_3
8: iload_3
9: ireturn
LineNumberTable:
line 15: 0
line 16: 2
line 17: 4
line 18: 8
LocalVariableTable:
Start Length Slot Name Signature
0 10 0 this Lcom/zengqingfa/exercise/jvm/ClassLoaderTest1;
2 8 1 x I
4 6 2 y I
8 2 3 z I
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #2 // class com/zengqingfa/exercise/jvm/ClassLoaderTest1
3: dup
4: invokespecial #3 // Method "<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #4 // Method calculate:()I
12: pop
13: return
LineNumberTable:
line 22: 0
line 23: 8
line 24: 13
LocalVariableTable:
Start Length Slot Name Signature
0 14 0 args [Ljava/lang/String;
8 6 1 loaderTest1 Lcom/zengqingfa/exercise/jvm/ClassLoaderTest1;
MethodParameters:
Name Flags
args
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=2, locals=0, args_size=0
0: new #5 // class com/zengqingfa/exercise/jvm/User
3: dup
4: invokespecial #6 // Method com/zengqingfa/exercise/jvm/User."<init>":()V
7: putstatic #7 // Field user:Lcom/zengqingfa/exercise/jvm/User;
10: return
LineNumberTable:
line 12: 0
}
SourceFile: "ClassLoaderTest1.java"
初始化：对类的静态变量初始化为指定的值，执行静态代码块
public static final int count= 1; //赋值1
public static User user = new User(); //赋值对象user
类被加载到方法区中后主要包含运行时常量池、类型信息、字段信息、方法信息、类加载器的 引用、对应class实例的引用等信息。
类加载器的引用：这个类到类加载器实例的引用 对应class实例的引用：类加载器在加载类信息放到方法区中后，会创建一个对应的Class 类型的 对象实例放到堆(Heap)中, 作为开发人员访问方法区中类定义的入口和切入点。
**注意，主类在运行过程中如果使用到其它类，会逐步加载这些类，为懒加载。 **
jar包或war包里的类不是一次性全部加载的，是使用到时才加载。
public class LazyLoadTest {
static {
System.out.println("LazyLoadTest 静态代码块");
}
public static void main(String[] args) {
new A();
System.out.println("============== load test==============");
B b=null;//这里不会加载B,除非new B()
}
}
class A {
static {
System.out.println("==============静态代码块加载 AAAAA==============");
}
public A() {
System.out.println("==============构造函数加载 AAAAA==============");
}
}
class B {
static {
System.out.println("==============静态代码块加载 BBBBB==============");
}
public B() {
System.out.println("==============构造函数加载 BBBBB==============");
}
}
执行结果：
LazyLoadTest 静态代码块
==============静态代码块加载 AAAAA==============
==============构造函数加载 AAAAA==============
============== load test==============
二、类加载器和双亲委派机制
2.1 类加载器
上面的类加载过程主要是通过类加载器来实现的，Java里有如下几种类加载器
引导类加载器：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
扩展类加载器：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
应用程序类加载器：负责加载ClassPath路径下的类包，主要就是加载你自己写的那些类
自定义加载器：负责加载用户自定义路径下的类包
测试类加载器
package com.zengqingfa.exercise.jvm;
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 bootstrapLoader : " + bootstrapLoader);
System.out.println("the extClassloader : " + extClassloader);
System.out.println("the appClassLoader : " + appClassLoader);
System.out.println();
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();
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"));
}
}
测试结果
null
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader
=======================
the bootstrapLoader : null
the extClassloader : sun.misc.Launcher$ExtClassLoader@1b2c6ec2
the appClassLoader : sun.misc.Launcher$AppClassLoader@18b4aac2
bootstrapLoader加载以下文件：
file:/F:/jdk1.8/jre/lib/resources.jar
file:/F:/jdk1.8/jre/lib/rt.jar
file:/F:/jdk1.8/jre/lib/sunrsasign.jar
file:/F:/jdk1.8/jre/lib/jsse.jar
file:/F:/jdk1.8/jre/lib/jce.jar
file:/F:/jdk1.8/jre/lib/charsets.jar
file:/F:/jdk1.8/jre/lib/jfr.jar
file:/F:/jdk1.8/jre/classes
extClassloader加载以下文件：
F:jdk1.8jrelibext;C:WINDOWSSunJavalibext
appClassLoader加载以下文件：
F:jdk1.8jrelibcharsets.jar;F:jdk1.8jrelibdeploy.jar;F:jdk1.8jrelibextaccess-bridge-64.jar;F:jdk1.8jrelibextcldrdata.jar;F:jdk1.8jrelibextdnsns.jar;F:jdk1.8jrelibextjaccess.jar;F:jdk1.8jrelibextjfxrt.jar;F:jdk1.8jrelibextlocaledata.jar;F:jdk1.8jrelibextnashorn.jar;F:jdk1.8jrelibextsunec.jar;F:jdk1.8jrelibextsunjce_provider.jar;F:jdk1.8jrelibextsunmscapi.jar;F:jdk1.8jrelibextsunpkcs11.jar;F:jdk1.8jrelibextzipfs.jar;F:jdk1.8jrelibjavaws.jar;F:jdk1.8jrelibjce.jar;F:jdk1.8jrelibjfr.jar;F:jdk1.8jrelibjfxswt.jar;F:jdk1.8jrelibjsse.jar;F:jdk1.8jrelibmanagement-agent.jar;F:jdk1.8jrelibplugin.jar;F:jdk1.8jrelibresources.jar;F:jdk1.8jrelibrt.jar;E:codegiteespringboot-examplesspringboot-examples-demojvm-demotargettest-classes;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-web2.2.0.RELEASEspring-boot-starter-web-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter2.2.0.RELEASEspring-boot-starter-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot2.2.0.RELEASEspring-boot-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-autoconfigure2.2.0.RELEASEspring-boot-autoconfigure-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-logging2.2.0.RELEASEspring-boot-starter-logging-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositorychqoslogbacklogback-classic1.2.3logback-classic-1.2.3.jar;E:apache-maven-3.6.0repositorychqoslogbacklogback-core1.2.3logback-core-1.2.3.jar;E:apache-maven-3.6.0repositoryorgapachelogginglog4jlog4j-to-slf4j2.12.1log4j-to-slf4j-2.12.1.jar;E:apache-maven-3.6.0repositoryorgapachelogginglog4jlog4j-api2.12.1log4j-api-2.12.1.jar;E:apache-maven-3.6.0repositoryorgslf4jjul-to-slf4j1.7.28jul-to-slf4j-1.7.28.jar;E:apache-maven-3.6.0repositoryjakartaannotationjakarta.annotation-api1.3.5jakarta.annotation-api-1.3.5.jar;E:apache-maven-3.6.0repositoryorgyamlsnakeyaml1.25snakeyaml-1.25.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-json2.2.0.RELEASEspring-boot-starter-json-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksoncorejackson-databind2.10.0jackson-databind-2.10.0.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksoncorejackson-annotations2.10.0jackson-annotations-2.10.0.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksoncorejackson-core2.10.0jackson-core-2.10.0.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksondatatypejackson-datatype-jdk82.10.0jackson-datatype-jdk8-2.10.0.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksondatatypejackson-datatype-jsr3102.10.0jackson-datatype-jsr310-2.10.0.jar;E:apache-maven-3.6.0repositorycomfasterxmljacksonmodulejackson-module-parameter-names2.10.0jackson-module-parameter-names-2.10.0.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-tomcat2.2.0.RELEASEspring-boot-starter-tomcat-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgapachetomcatembedtomcat-embed-core9.0.27tomcat-embed-core-9.0.27.jar;E:apache-maven-3.6.0repositoryorgapachetomcatembedtomcat-embed-el9.0.27tomcat-embed-el-9.0.27.jar;E:apache-maven-3.6.0repositoryorgapachetomcatembedtomcat-embed-websocket9.0.27tomcat-embed-websocket-9.0.27.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-validation2.2.0.RELEASEspring-boot-starter-validation-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryjakartavalidationjakarta.validation-api2.0.1jakarta.validation-api-2.0.1.jar;E:apache-maven-3.6.0repositoryorghibernatevalidatorhibernate-validator6.0.17.Finalhibernate-validator-6.0.17.Final.jar;E:apache-maven-3.6.0repositoryorgjbossloggingjboss-logging3.4.1.Finaljboss-logging-3.4.1.Final.jar;E:apache-maven-3.6.0repositorycomfasterxmlclassmate1.5.0classmate-1.5.0.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-web5.2.0.RELEASEspring-web-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-beans5.2.0.RELEASEspring-beans-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-webmvc5.2.0.RELEASEspring-webmvc-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-aop5.2.0.RELEASEspring-aop-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-context5.2.0.RELEASEspring-context-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-expression5.2.0.RELEASEspring-expression-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgprojectlomboklombok1.18.12lombok-1.18.12.jar;E:apache-maven-3.6.0repositorycomgoogleguavaguava29.0-jreguava-29.0-jre.jar;E:apache-maven-3.6.0repositorycomgoogleguavafailureaccess1.0.1failureaccess-1.0.1.jar;E:apache-maven-3.6.0repositorycomgoogleguavalistenablefuture9999.0-empty-to-avoid-conflict-with-guavalistenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar;E:apache-maven-3.6.0repositorycomgooglecodefindbugsjsr3053.0.2jsr305-3.0.2.jar;E:apache-maven-3.6.0repositoryorgcheckerframeworkchecker-qual2.11.1checker-qual-2.11.1.jar;E:apache-maven-3.6.0repositorycomgoogleerrorproneerror_prone_annotations2.3.4error_prone_annotations-2.3.4.jar;E:apache-maven-3.6.0repositorycomgooglej2objcj2objc-annotations1.3j2objc-annotations-1.3.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-starter-test2.2.0.RELEASEspring-boot-starter-test-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-test2.2.0.RELEASEspring-boot-test-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkbootspring-boot-test-autoconfigure2.2.0.RELEASEspring-boot-test-autoconfigure-2.2.0.RELEASE.jar;E:apache-maven-3.6.0repositorycomjaywayjsonpathjson-path2.4.0json-path-2.4.0.jar;E:apache-maven-3.6.0repositorynetminidevjson-smart2.3json-smart-2.3.jar;E:apache-maven-3.6.0repositorynetminidevaccessors-smart1.2accessors-smart-1.2.jar;E:apache-maven-3.6.0repositoryorgow2asmasm5.0.4asm-5.0.4.jar;E:apache-maven-3.6.0repositoryorgslf4jslf4j-api1.7.28slf4j-api-1.7.28.jar;E:apache-maven-3.6.0repositoryjakartaxmlbindjakarta.xml.bind-api2.3.2jakarta.xml.bind-api-2.3.2.jar;E:apache-maven-3.6.0repositoryjakartaactivationjakarta.activation-api1.2.1jakarta.activation-api-1.2.1.jar;E:apache-maven-3.6.0repositoryorgjunitjupiterjunit-jupiter5.5.2junit-jupiter-5.5.2.jar;E:apache-maven-3.6.0repositoryorgjunitjupiterjunit-jupiter-api5.5.2junit-jupiter-api-5.5.2.jar;E:apache-maven-3.6.0repositoryorgopentest4jopentest4j1.2.0opentest4j-1.2.0.jar;E:apache-maven-3.6.0repositoryorgjunitplatformjunit-platform-commons1.5.2junit-platform-commons-1.5.2.jar;E:apache-maven-3.6.0repositoryorgjunitjupiterjunit-jupiter-params5.5.2junit-jupiter-params-5.5.2.jar;E:apache-maven-3.6.0repositoryorgjunitjupiterjunit-jupiter-engine5.5.2junit-jupiter-engine-5.5.2.jar;E:apache-maven-3.6.0repositoryorgjunitvintagejunit-vintage-engine5.5.2junit-vintage-engine-5.5.2.jar;E:apache-maven-3.6.0repositoryorgapiguardianapiguardian-api1.1.0apiguardian-api-1.1.0.jar;E:apache-maven-3.6.0repositoryorgjunitplatformjunit-platform-engine1.5.2junit-platform-engine-1.5.2.jar;E:apache-maven-3.6.0repositoryjunitjunit4.12junit-4.12.jar;E:apache-maven-3.6.0repositoryorgmockitomockito-junit-jupiter3.1.0mockito-junit-jupiter-3.1.0.jar;E:apache-maven-3.6.0repositoryorgassertjassertj-core3.13.2assertj-core-3.13.2.jar;E:apache-maven-3.6.0repositoryorghamcresthamcrest2.1hamcrest-2.1.jar;E:apache-maven-3.6.0repositoryorgmockitomockito-core3.1.0mockito-core-3.1.0.jar;E:apache-maven-3.6.0repositorynetbytebuddybyte-buddy1.10.1byte-buddy-1.10.1.jar;E:apache-maven-3.6.0repositorynetbytebuddybyte-buddy-agent1.10.1byte-buddy-agent-1.10.1.jar;E:apache-maven-3.6.0repositoryorgobjenesisobjenesis2.6objenesis-2.6.jar;E:apache-maven-3.6.0repositoryorgskyscreamerjsonassert1.5.0jsonassert-1.5.0.jar;E:apache-maven-3.6.0repositorycomvaadinexternalgoogleandroid-json0.0.20131108.vaadin1android-json-0.0.20131108.vaadin1.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-core5.2.0.RELEASEspring-core-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-jcl5.2.0.RELEASEspring-jcl-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgspringframeworkspring-test5.2.0.RELEASEspring-test-5.2.0.RELEASE.jar;E:apache-maven-3.6.0repositoryorgxmlunitxmlunit-core2.6.3xmlunit-core-2.6.3.jar;E:apache-maven-3.6.0repositorycomalibabafastjson1.2.70fastjson-1.2.70.jar;E:apache-maven-3.6.0repositorycomgooglecodegsongson2.8.6gson-2.8.6.jar;E:apache-maven-3.6.0repositorycnhutoolhutool-all5.3.7hutool-all-5.3.7.jar;E:apache-maven-3.6.0repositoryorgapachecommonscommons-lang33.9commons-lang3-3.9.jar;F:IDEAideaIU-2020.3.2.winlibidea_rt.jar
可以看到，appClassLoader加载的类的目录也包含bootstrapLoader加载的目录。
类加载器初始化过程
由引导类加载器通过c++代码发起sun.misc.Launcher#getLauncher方法的调用，会创建JVM启动器实例sun.misc.Launcher，
sun.misc.Launcher初始化使用了单例模式设计，保证一个JVM虚拟机内只有一个 sun.misc.Launcher实例。 在Launcher构造方法内部，其创建了两个类加载器，分别是 sun.misc.Launcher.ExtClassLoader(扩展类加载器)和sun.misc.Launcher.AppClassLoader(应 用类加载器)。 JVM默认使用Launcher的getClassLoader()方法返回的类加载器AppClassLoader的实例加载我们 的应用程序。
public class Launcher {
private static URLStreamHandlerFactory factory = new Launcher.Factory();
//静态成员变量 只有一个实例
private static Launcher launcher = new Launcher();
private static String bootClassPath = System.getProperty("sun.boot.class.path");
private ClassLoader loader;
private static URLStreamHandler fileHandler;
public static Launcher getLauncher() {
return launcher;
}
public Launcher() {
Launcher.ExtClassLoader var1;
try {
//1、获取扩展类加载器实例,并把父类加载器设置为空
var1 = Launcher.ExtClassLoader.getExtClassLoader();
} catch (IOException var10) {
throw new InternalError("Could not create extension class loader", var10);
}
try {
//2、获取应用类加载器实例,把扩展类加载器作为参数传入
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");
...
}
public ClassLoader getClassLoader() {
return this.loader;
}
static class AppClassLoader extends URLClassLoader {
final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
//2.1 获取应用程序类扩展加载器
public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException {
final String var1 = System.getProperty("java.class.path");
final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1);
return (ClassLoader)AccessController.doPrivileged(new PrivilegedAction<Launcher.AppClassLoader>() {
public Launcher.AppClassLoader run() {
URL[] var1x = var1 == null ? new URL[0] : Launcher.pathToURLs(var2);
//2.2 调用应用程序类扩展加载器构造方法 var0为扩展类加载器
return new Launcher.AppClassLoader(var1x, var0);
}
});
}
// 2.3 应用程序类扩展加载器构造方法,var2为扩展类加载器
AppClassLoader(URL[] var1, ClassLoader var2) {
//2.4 调用父类的构造函数,var2为扩展类加载器作为参数传入
//最后调用java.lang.ClassLoader#ClassLoader(java.lang.Void, java.lang.ClassLoader)
//有个成员变量parent 进行赋值操作
super(var1, var2, Launcher.factory);
this.ucp.initLookupCache(this);
}
}
static class ExtClassLoader extends URLClassLoader {
private static volatile Launcher.ExtClassLoader instance;
//1.1 获取扩展类加载器
public static Launcher.ExtClassLoader getExtClassLoader() throws IOException {
if (instance == null) {
Class var0 = Launcher.ExtClassLoader.class;
synchronized(Launcher.ExtClassLoader.class) {
if (instance == null) {
//1.2 创建扩展类加载器方法调用
instance = createExtClassLoader();
}
}
}
return instance;
}
//1.3 创建扩展类加载器方法
private static Launcher.ExtClassLoader createExtClassLoader() throws IOException {
try {
return (Launcher.ExtClassLoader)AccessController.doPrivileged(new PrivilegedExceptionAction<Launcher.ExtClassLoader>() {
public Launcher.ExtClassLoader run() throws IOException {
File[] var1 = Launcher.ExtClassLoader.getExtDirs();
int var2 = var1.length;
for(int var3 = 0; var3 < var2; ++var3) {
MetaIndex.registerDirectory(var1[var3]);
}
//1.4 创建扩展类加载器,调用构造方法
return new Launcher.ExtClassLoader(var1);
}
});
} catch (PrivilegedActionException var1) {
throw (IOException)var1.getException();
}
}
//1.5 传入文件路径进行加载
public ExtClassLoader(File[] var1) throws IOException {
//1.6 调用父类的构造方法 ,第二个参数为parent，传入的为空
//java.net.URLClassLoader#URLClassLoader(java.net.URL[], java.lang.ClassLoader, java.net.URLStreamHandlerFactory)
super(getExtURLs(var1), (ClassLoader)null, Launcher.factory);
SharedSecrets.getJavaNetAccess().getURLClassPath(this).initLookupCache(this);
}
}
//参考类
//java.net.URLClassLoader#URLClassLoader
public URLClassLoader(URL[] urls, ClassLoader parent,
URLStreamHandlerFactory factory) {
super(parent);
// this is to make the stack depth consistent with 1.1
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
acc = AccessController.getContext();
ucp = new URLClassPath(urls, factory, acc);
}
//java.security.SecureClassLoader#SecureClassLoader(java.lang.ClassLoader)
protected SecureClassLoader(ClassLoader parent) {
super(parent);
// this is to make the stack depth consistent with 1.1
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
initialized = true;
}
//java.lang.ClassLoader#ClassLoader(java.lang.ClassLoader)
public abstract class ClassLoader {
private static native void registerNatives();
static {
registerNatives();
}
// The parent class loader for delegation
// Note: VM hardcoded the offset of this field, thus all new fields
// must be added *after* it.
private final ClassLoader parent;
protected ClassLoader(ClassLoader parent) {
this(checkCreateClassLoader(), parent);
}
private ClassLoader(Void unused, ClassLoader parent) {
this.parent = parent;
if (ParallelLoaders.isRegistered(this.getClass())) {
parallelLockMap = new ConcurrentHashMap<>();
package2certs = new ConcurrentHashMap<>();
domains =
Collections.synchronizedSet(new HashSet<ProtectionDomain>());
assertionLock = new Object();
} else {
// no finer-grained lock; lock on the classloader instance
parallelLockMap = null;
package2certs = new Hashtable<>();
domains = new HashSet<>();
assertionLock = this;
}
}
...
}
类加载器结构图
2.2 双亲委派机制
JVM类加载器是有亲子层级结构的
这里类加载其实就有一个双亲委派机制，加载某个类时会先委托父加载器寻找目标类，找不到再 委托上层父加载器加载，如果所有父加载器在自己的加载类路径下都找不到目标类，则在自己的 类加载路径中查找并载入目标类。
比如我们的ClassLoaderTest类，最先会找应用程序类加载器加载，应用程序类加载器会先委托扩展类加载 器加载，扩展类加载器再委托引导类加载器，顶层引导类加载器在自己的类加载路径里找了半天 没找到ClassLoaderTest类，则向下退回加载ClassLoaderTest类的请求，扩展类加载器收到回复就自己加载，在自己的 类加载路径里找了半天也没找到ClassLoaderTest类，又向下退回ClassLoaderTest类的加载请求给应用程序类加载器， 应用程序类加载器于是在自己的类加载路径里找Math类，结果找到了就自己加载了。。
双亲委派机制说简单点就是，先找父亲加载，不行再由儿子自己加载
源码解析：
为什么开始是从应用程序类加载器开始进行加载呢？AppClassLoader
不是从引导类加载器开始加载呢？一次循环就可以了，90%的应用程序的代码都是由应用程序类加载的
只是初始化类的时候需要2次循环，加载完毕后，会保存下来java.lang.ClassLoader#findLoadedClass，下次使用的时候直接从应用程序类判断是否加载过，如果加载过，直接返回。
public class Launcher {
...
private ClassLoader loader;
public ClassLoader getClassLoader() {
return this.loader;
}
public Launcher() {
Launcher.ExtClassLoader var1;
try {
var1 = Launcher.ExtClassLoader.getExtClassLoader();
} catch (IOException var10) {
throw new InternalError("Could not create extension class loader", var10);
}
try {
//load 赋值为应用程序类加载器
this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
} catch (IOException var9) {
throw new InternalError("Could not create application class loader", var9);
}
}
...
}
通过sun.misc.Launcher类的成员变量loader进行类的加载
通过调用其父类java.lang.ClassLoader#loadClass(java.lang.String) 方法加载类
方法逻辑如下：
1. 首先，检查一下指定名称的类是否已经加载过，如果加载过了，就不需要再加载，直接 返回。
2. 如果此类没有加载过，那么，再判断一下是否有父加载器；如果有父加载器，则由父加 载器加载（即调用parent.loadClass(name, false);）.或者是调用bootstrap类加载器来加 载。
3. 如果父加载器及bootstrap类加载器都没有找到指定的类，那么调用当前类加载器的 findClass方法来完成类加载。
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
//检查该类是否已经加载过，如果加载过，直接返回，没有加载过，返回空
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
//appClassLoader的parent属性为extClassLoader
if (parent != null) {
c = parent.loadClass(name, false);
} else {
//由引导类加载器进行加载
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
//如果加载失败，则尝试自己去加载
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
//都会调用URLClassLoader的findClass方法在加载器的类路径里查找并加载该类
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
//通过全限定类名获取已经加载过的类
protected final Class<?> findLoadedClass(String name) {
if (!checkName(name))
return null;
return findLoadedClass0(name);
}
private native final Class<?> findLoadedClass0(String name);
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
调用父类的java.net.URLClassLoader#findClass方法进行类的加载
protected Class<?> findClass(final String name)
throws ClassNotFoundException
{
final Class<?> result;
try {
result = AccessController.doPrivileged(
new PrivilegedExceptionAction<Class<?>>() {
public Class<?> run() throws ClassNotFoundException {
//com/zengqingfa/exercise/jvm/ClassLoaderTest1.class
String path = name.replace('.', '/').concat(".class");
//当前类的加载的路径进行加载，如果没有则返回空
Resource res = ucp.getResource(path, false);
if (res != null) {
try {
//类加载过程
return defineClass(name, res);
} catch (IOException e) {
throw new ClassNotFoundException(name, e);
}
} else {
return null;
}
}
}, acc);
} catch (java.security.PrivilegedActionException pae) {
throw (ClassNotFoundException) pae.getException();
}
if (result == null) {
throw new ClassNotFoundException(name);
}
return result;
}
为什么要设计双亲委派机制
**沙箱安全机制：自己写的java.lang.String.class类不会被加载，这样便可以防止核心 API库被随意篡改 **
package java.lang;
public class String {
public static void main(String[] args) {
System.out.println("My String class");
}
}
报错：
错误: 在类 java.lang.String 中找不到 main 方法, 请将 main 方法定义为:
public static void main(String[] args)
否则 JavaFX 应用程序类必须扩展javafx.application.Application
类加载过程中是返回rt.jar中jdk中的String类，是不存在的main方法
避免类的重复加载：当父亲已经加载了该类时，就没有必要子ClassLoader再加载一 次，保证被加载类的唯一性
全盘负责委托机制
“全盘负责”是指当一个ClassLoder装载一个类时，除非显示的使用另外一个ClassLoder，该类所依赖及引用的类也由这个ClassLoder载入。User类和ClassLoaderTest1类使用同一个类加载器进行加载
public class ClassLoaderTest1 {
public static User user = new User();
}
自定义类加载器示例
自定义类加载器只需要继承 java.lang.ClassLoader 类，该类有两个核心方法，一个是 loadClass(String, boolean)，实现了双亲委派机制，还有一个方法是findClass，默认实现是空 方法，所以我们自定义类加载器主要是重写findClass方法。
需要加载的类：
public class User1 {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public void print() {
System.out.println("自定义类加载器加载");
}
}
自定义类加载器
package com.zengqingfa.exercise.jvm;
import java.io.FileInputStream;
import java.lang.reflect.Method;
public class MyClassLoaderTest {
static class MyClassLoader extends ClassLoader {
private String classPath;
public MyClassLoader(String classPath) {
this.classPath = classPath;
}
private byte[] loadByte(String name) throws Exception {
name = name.replaceAll("\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte(name);
//defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节 数组。
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
}
public static void main(String args[]) throws Exception {
//初始化自定义类加载器，会先初始化父类ClassLoader，其中会把自定义类加载器的父加载
//器设置为应用程序类加载器AppClassLoader
MyClassLoader classLoader = new MyClassLoader("F:/test");
//F盘创建 F:testcomzengqingfaexercisejvm 几级目录，将User类的复制类User1.class丢入该目录
Class clazz = classLoader.loadClass("com.zengqingfa.exercise.jvm.User1");
Object obj = clazz.newInstance();
Method method = clazz.getDeclaredMethod("print", null);
method.invoke(obj, null);
//如果父类appClassLoader存在该类 sun.misc.Launcher$AppClassLoader
//如果父类appClassLoader不存在该类com.zengqingfa.exercise.jvm.MyClassLoaderTest$MyClassLoader
System.out.println(clazz.getClassLoader().getClass().getName());
}
}
自定义类载器的父类加载器为什么是应用程序类加载器？
初始化自定义类加载器的时候，先初始化父类加载器
protected ClassLoader() {
this(checkCreateClassLoader(), getSystemClassLoader());
}
protected ClassLoader(ClassLoader parent) {
this(checkCreateClassLoader(), parent);
}
private ClassLoader(Void unused, ClassLoader parent) {
this.parent = parent;
if (ParallelLoaders.isRegistered(this.getClass())) {
parallelLockMap = new ConcurrentHashMap<>();
package2certs = new ConcurrentHashMap<>();
domains =
Collections.synchronizedSet(new HashSet<ProtectionDomain>());
assertionLock = new Object();
} else {
// no finer-grained lock; lock on the classloader instance
parallelLockMap = null;
package2certs = new Hashtable<>();
domains = new HashSet<>();
assertionLock = this;
}
}
public static ClassLoader getSystemClassLoader() {
initSystemClassLoader();
if (scl == null) {
return null;
}
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkClassLoaderPermission(scl, Reflection.getCallerClass());
}
return scl;
}
private static synchronized void initSystemClassLoader() {
if (!sclSet) {
if (scl != null)
throw new IllegalStateException("recursive invocation");
//获取Launcher类
sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
if (l != null) {
Throwable oops = null;
//获取对应的classLoader sun.misc.Launcher#getClassLoader 为AppClassLoader
scl = l.getClassLoader();
try {
scl = AccessController.doPrivileged(
new SystemClassLoaderAction(scl));
} catch (PrivilegedActionException pae) {
oops = pae.getCause();
if (oops instanceof InvocationTargetException) {
oops = oops.getCause();
}
}
if (oops != null) {
if (oops instanceof Error) {
throw (Error) oops;
} else {
// wrap the exception
throw new Error(oops);
}
}
}
sclSet = true;
}
}
打破双亲委派机制
尝试打破双亲委派机制，用自定义类加载器加载我们自己实现的的类
package com.zengqingfa.exercise.jvm;
import java.io.FileInputStream;
import java.lang.reflect.Method;
public class MyClassLoaderTest2 {
static class MyClassLoader extends ClassLoader {
private String classPath;
public MyClassLoader(String classPath) {
this.classPath = classPath;
}
private byte[] loadByte(String name) throws Exception {
name = name.replaceAll("\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte(name);
//defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节 数组。
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
if (!name.startsWith("com.zengqingfa.exercise")) {
c = this.getParent().loadClass(name);
} else {
c = findClass(name);
}
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
}
public static void main(String args[]) throws Exception {
//初始化自定义类加载器，会先初始化父类ClassLoader，其中会把自定义类加载器的父加载 器设置为应用程序类加载器AppClassLoader
MyClassLoader classLoader = new MyClassLoader("F:/test");
//F盘创建 F:testcomzengqingfaexercisejvm 几级目录，将User类的复制类User1.class丢入该目录
Class clazz = classLoader.loadClass("com.zengqingfa.exercise.jvm.User1");
Object obj = clazz.newInstance();
Method method = clazz.getDeclaredMethod("print", null);
method.invoke(obj, null);
//如果父类appClassLoader存在该类 sun.misc.Launcher$AppClassLoader
//如果父类appClassLoader不存在该类com.zengqingfa.exercise.jvm.MyClassLoaderTest$MyClassLoader
System.out.println(clazz.getClassLoader().getClass().getName());
}
}
Tomcat打破双亲委派机制
以Tomcat类加载为例，Tomcat 如果使用默认的双亲委派类加载机制行不行？ 我们思考一下：Tomcat是个web容器， 那么它要解决什么问题：
1. 一个web容器可能需要部署两个应用程序，不同的应用程序可能会依赖同一个第三方类库的 不同版本，不能要求同一个类库在同一个服务器只有一份，因此要保证每个应用程序的类库都是 独立的，保证相互隔离。
2. 部署在同一个web容器中相同的类库相同的版本可以共享。否则，如果服务器有10个应用程 序，那么要有10份相同的类库加载进虚拟机。
3. web容器也有自己依赖的类库，不能与应用程序的类库混淆。基于安全考虑，应该让容器的 类库和程序的类库隔离开来。
4. web容器要支持jsp的修改，我们知道，jsp 文件最终也是要编译成class文件才能在虚拟机中 运行，但程序运行后修改jsp已经是司空见惯的事情， web容器需要支持 jsp 修改后不用重启。
再看看我们的问题：Tomcat 如果使用默认的双亲委派类加载机制行不行？ 答案是不行的。为什么？
第一个问题，如果使用默认的类加载器机制，那么是无法加载两个相同类库的不同版本的，默认 的类加器是不管你是什么版本的，只在乎你的全限定类名，并且只有一份。
第二个问题，默认的类加载器是能够实现的，因为他的职责就是保证唯一性。
第三个问题和第一个问题一样。
我们再看第四个问题，我们想我们要怎么实现jsp文件的热加载，jsp 文件其实也就是class文 件，那么如果修改了，但类名还是一样，类加载器会直接取方法区中已经存在的，修改后的jsp 是不会重新加载的。那么怎么办呢？我们可以直接卸载掉这jsp文件的类加载器，所以你应该想 到了，每个jsp文件对应一个唯一的类加载器，当一个jsp文件修改了，就直接卸载这个jsp类加载 器。重新创建类加载器，重新加载jsp文件。
Tomcat自定义加载器详解
tomcat的几个主要类加载器：
commonLoader：Tomcat最基本的类加载器，加载路径中的class可以被Tomcat容 器本身以及各个Webapp访问； catalinaLoader：Tomcat容器私有的类加载器，加载路径中的class对于Webapp不 可见；
sharedLoader：各个Webapp共享的类加载器，加载路径中的class对于所有 Webapp可见，但是对于Tomcat容器不可见；
WebappClassLoader：各个Webapp私有的类加载器，加载路径中的class只对当前 Webapp可见，比如加载war包里相关的类，每个war包应用都有自己的WebappClassLoader，实现相互隔离，比如不同war包应用引入了不同的spring版本， 这样实现就能加载各自的spring版本；
从图中的委派关系中可以看出： CommonClassLoader能加载的类都可以被CatalinaClassLoader和SharedClassLoader使用， 从而实现了公有类库的共用，而CatalinaClassLoader和SharedClassLoader自己能加载的类则 与对方相互隔离。 WebAppClassLoader可以使用SharedClassLoader加载到的类，但各个WebAppClassLoader 实例之间相互隔离。 而JasperLoader的加载范围仅仅是这个JSP文件所编译出来的那一个.Class文件，它出现的目的 就是为了被丢弃：当Web容器检测到JSP文件被修改时，会替换掉目前的JasperLoader的实例， 并通过再建立一个新的Jsp类加载器来实现JSP文件的热加载功能。
模拟tomcat打破双亲委派
com.zengqingfa.exercise.jvm.User1版本一：
public class User1 {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public void print() {
System.out.println("版本一：自定义类加载器加载");
}
}
com.zengqingfa.exercise.jvm.User1版本二：
public class User1 {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public void print() {
System.out.println("版本二：自定义类加载器加载");
}
}
分别在F盘创建不同的文件夹但是相同的类路径和类名
自定义类加载器
package com.zengqingfa.exercise.jvm;
import java.io.FileInputStream;
import java.lang.reflect.Method;
public class MyClassLoaderTest3 {
static class MyClassLoader extends ClassLoader {
private String classPath;
public MyClassLoader(String classPath) {
this.classPath = classPath;
}
private byte[] loadByte(String name) throws Exception {
name = name.replaceAll("\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte(name);
//defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节 数组。
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
if (!name.startsWith("com.zengqingfa.exercise")) {
c = this.getParent().loadClass(name);
} else {
c = findClass(name);
}
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
}
public static void main(String args[]) throws Exception {
MyClassLoader classLoader1 = new MyClassLoader("F:/test1");
Class clazz = classLoader1.loadClass("com.zengqingfa.exercise.jvm.User1");
Object obj1 = clazz.newInstance();
Method method1 = clazz.getDeclaredMethod("print", null);
method1.invoke(obj1, null);
System.out.println(clazz.getClassLoader().getClass().getName());
System.out.println("========================");
MyClassLoader classLoader2 = new MyClassLoader("F:/test2");
Class clazz2 = classLoader2.loadClass("com.zengqingfa.exercise.jvm.User1");
Object obj2 = clazz2.newInstance();
Method method2 = clazz2.getDeclaredMethod("print", null);
method2.invoke(obj2, null);
System.out.println(clazz.getClassLoader().getClass().getName());
}
}
打印结果：
版本一：自定义类加载器加载
com.zengqingfa.exercise.jvm.MyClassLoaderTest3$MyClassLoader
========================
版本二：自定义类加载器加载
com.zengqingfa.exercise.jvm.MyClassLoaderTest3$MyClassLoader
最后
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