类加载机制，双亲委派的那些事

　　首先我们来看张图，自己就不画了，网上找了张图。

 　　（内存模型就先不看了，1。8之后已经不是这个模型了）

　　我们java的程序在加载到jvm之前，经历过的事情上面在类加载器之前我就不解释了，在.class文件到类加载器之间，还有一些过程。看下图

 　　加载：io读取磁盘上面的字节码文件，在对象被使用到的时候加载到内存

　　验证：验证内存中读取字节码文件内容是否正确（里面是很多0-9和字母组成，class文件不要随意去更改）

　　准备：这个阶段给静态成员分配地址，给默认值（基本类型都有默认值，如果成员中有对象的话，此时为null，即使是你代码中new了）

　　解析：符号（类名，方法名，包名）这个静态链接过程怎么去解释呢，（这个理解了往后补充）

　　初始化：把静态变量赋值为我们给的值

　　我们再看类加载器：

　　　　1：启动类加载器：加载jre的lib下的核心库类（rt.jar，charsets.jar等）

　　　　2：拓展类加载器：加载jre的lib下的ext拓展文件中的jar包类

　　　　3：应用程序类加载器：加载classpath下的类（自己的类）

　　　　4：自定义类加载器：加载用户指定的类

　　先来看下自定义加载器，我们如何去定义一个类加载器：

public class Test {

    public void helle () {
        System.out.println("helle_classloader");
    }

}

定义一个Test类

package com.ghsy.user.test;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;

public class MyClassLoader extends ClassLoader {

    private String classPath;


    public MyClassLoader(String classPath) {
        this.classPath = classPath;
    }

    /**
     * 重写findClass方法
     * @param name 是我们这个类的全路径
     * @return
     * @throws ClassNotFoundException
     */
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class log = null;
        // 获取该class文件字节码数组
        byte[] classData = getData();

        if (classData != null) {
            // 将class的字节码数组转换成Class类的实例
            log = defineClass(name, classData, 0, classData.length);
        }
        return log;
    }

    /**
     * 将class文件转化为字节码数组
     * @return
     */
    private byte[] getData() {

        File file = new File(classPath);
        if (file.exists()){
            FileInputStream in = null;
            ByteArrayOutputStream out = null;
            try {
                in = new FileInputStream(file);
                out = new ByteArrayOutputStream();

                byte[] buffer = new byte[1024];
                int size = 0;
                while ((size = in.read(buffer)) != -1) {
                    out.write(buffer, 0, size);
                }

            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    in.close();
                } catch (IOException e) {

                    e.printStackTrace();
                }
            }
            return out.toByteArray();
        }else{
            return null;
        }


    }


    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException,
            InstantiationException, NoSuchMethodException, InvocationTargetException {
        MyClassLoader classLoader = new MyClassLoader("C:/test/Test.class");
        Class clazz = classLoader.findClass("com.ghsy.user.test.Test");
        Object object = clazz.newInstance();
        clazz.getMethod("helle",null).invoke(object, null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }

}

自定义一个类加载器MyClassLoader。

我们把target下classes里面的Test.class拷贝到C盘test目录中

控制台输出：

D:workjavajdk1.8.0_60injava.exe "-javaagent:D:workideaIntelliJ IDEA 2018.2.5libidea_rt.jar=53557:D:workideaIntelliJ IDEA 2018.2.5in" -Dfile.encoding=UTF-8 -classpath D:workjavajdk1.8.0_60jrelibcharsets.jar;D:workjavajdk1.8.0_60jrelibdeploy.jar;D:workjavajdk1.8.0_60jrelibextaccess-bridge-64.jar;D:workjavajdk1.8.0_60jrelibextcldrdata.jar;D:workjavajdk1.8.0_60jrelibextdnsns.jar;D:workjavajdk1.8.0_60jrelibextjaccess.jar;D:workjavajdk1.8.0_60jrelibextjfxrt.jar;D:workjavajdk1.8.0_60jrelibextlocaledata.jar;D:workjavajdk1.8.0_60jrelibext
ashorn.jar;D:workjavajdk1.8.0_60jrelibextsunec.jar;D:workjavajdk1.8.0_60jrelibextsunjce_provider.jar;D:workjavajdk1.8.0_60jrelibextsunmscapi.jar;D:workjavajdk1.8.0_60jrelibextsunpkcs11.jar;D:workjavajdk1.8.0_60jrelibextzipfs.jar;D:workjavajdk1.8.0_60jrelibjavaws.jar;D:workjavajdk1.8.0_60jrelibjce.jar;D:workjavajdk1.8.0_60jrelibjfr.jar;D:workjavajdk1.8.0_60jrelibjfxswt.jar;D:workjavajdk1.8.0_60jrelibjsse.jar;D:workjavajdk1.8.0_60jrelibmanagement-agent.jar;D:workjavajdk1.8.0_60jrelibplugin.jar;D:workjavajdk1.8.0_60jrelib
esources.jar;D:workjavajdk1.8.0_60jrelib
t.jar;D:workmycodecloud-parentapp-user	argetclasses;D:workjar-resourceorgspringframeworkootspring-boot-starter2.2.1.RELEASEspring-boot-starter-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot2.2.1.RELEASEspring-boot-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-context5.2.1.RELEASEspring-context-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-autoconfigure2.2.1.RELEASEspring-boot-autoconfigure-2.2.1.RELEASE.jar;D:workjar-resourcejakartaannotationjakarta.annotation-api1.3.5jakarta.annotation-api-1.3.5.jar;D:workjar-resourceorgspringframeworkspring-core5.2.1.RELEASEspring-core-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-jcl5.2.1.RELEASEspring-jcl-5.2.1.RELEASE.jar;D:workjar-resourceorgyamlsnakeyaml1.25snakeyaml-1.25.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-netflix-eureka-client2.2.3.RELEASEspring-cloud-starter-netflix-eureka-client-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter2.2.3.RELEASEspring-cloud-starter-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-context2.2.3.RELEASEspring-cloud-context-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworksecurityspring-security-crypto5.2.1.RELEASEspring-security-crypto-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-commons2.2.3.RELEASEspring-cloud-commons-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworksecurityspring-security-rsa1.0.9.RELEASEspring-security-rsa-1.0.9.RELEASE.jar;D:workjar-resourceorgouncycastlecpkix-jdk15on1.64cpkix-jdk15on-1.64.jar;D:workjar-resourceorgouncycastlecprov-jdk15on1.64cprov-jdk15on-1.64.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-netflix-hystrix2.2.3.RELEASEspring-cloud-netflix-hystrix-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-aop2.2.1.RELEASEspring-boot-starter-aop-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-netflix-eureka-client2.2.3.RELEASEspring-cloud-netflix-eureka-client-2.2.3.RELEASE.jar;D:workjar-resourcecom
etflixeurekaeureka-client1.9.21eureka-client-1.9.21.jar;D:workjar-resourceorgcodehausjettisonjettison1.3.7jettison-1.3.7.jar;D:workjar-resourcestaxstax-api1.0.1stax-api-1.0.1.jar;D:workjar-resourcecom
etflix
etflix-commons
etflix-eventbus.3.0
etflix-eventbus-0.3.0.jar;D:workjar-resourcecom
etflix
etflix-commons
etflix-infix.3.0
etflix-infix-0.3.0.jar;D:workjar-resourcecommons-jxpathcommons-jxpath1.3commons-jxpath-1.3.jar;D:workjar-resourcejoda-timejoda-time2.10.5joda-time-2.10.5.jar;D:workjar-resourceorgantlrantlr-runtime3.4antlr-runtime-3.4.jar;D:workjar-resourceorgantlrstringtemplate3.2.1stringtemplate-3.2.1.jar;D:workjar-resourceantlrantlr2.7.7antlr-2.7.7.jar;D:workjar-resourcecomgooglecodegsongson2.8.6gson-2.8.6.jar;D:workjar-resourceorgapachecommonscommons-math2.2commons-math-2.2.jar;D:workjar-resourcecom
etflixarchaiusarchaius-core.7.6archaius-core-0.7.6.jar;D:workjar-resourcejavaxws
sjsr311-api1.1.1jsr311-api-1.1.1.jar;D:workjar-resourcecom
etflixservoservo-core.12.21servo-core-0.12.21.jar;D:workjar-resourcecomsunjerseyjersey-core1.19.1jersey-core-1.19.1.jar;D:workjar-resourcecomsunjerseyjersey-client1.19.1jersey-client-1.19.1.jar;D:workjar-resourcecomsunjerseycontribsjersey-apache-client41.19.1jersey-apache-client4-1.19.1.jar;D:workjar-resourceorgapachehttpcomponentshttpclient4.5.10httpclient-4.5.10.jar;D:workjar-resourceorgapachehttpcomponentshttpcore4.4.12httpcore-4.4.12.jar;D:workjar-resourcecommons-codeccommons-codec1.13commons-codec-1.13.jar;D:workjar-resourcecommons-configurationcommons-configuration1.10commons-configuration-1.10.jar;D:workjar-resourcecommons-langcommons-lang2.6commons-lang-2.6.jar;D:workjar-resourcecomgoogleinjectguice4.1.0guice-4.1.0.jar;D:workjar-resourcejavaxinjectjavax.inject1javax.inject-1.jar;D:workjar-resourceaopallianceaopalliance1.0aopalliance-1.0.jar;D:workjar-resourcecomfasterxmljacksoncorejackson-annotations2.10.0jackson-annotations-2.10.0.jar;D:workjar-resourcecomfasterxmljacksoncorejackson-core2.10.0jackson-core-2.10.0.jar;D:workjar-resourcecomfasterxmljacksoncorejackson-databind2.10.0jackson-databind-2.10.0.jar;D:workjar-resourcecomfasterxmlwoodstoxwoodstox-core5.3.0woodstox-core-5.3.0.jar;D:workjar-resourceorgcodehauswoodstoxstax2-api4.2stax2-api-4.2.jar;D:workjar-resourcecom
etflixeurekaeureka-core1.9.21eureka-core-1.9.21.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-netflix-archaius2.2.3.RELEASEspring-cloud-starter-netflix-archaius-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-netflix-archaius2.2.3.RELEASEspring-cloud-netflix-archaius-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-netflix-ribbon2.2.3.RELEASEspring-cloud-starter-netflix-ribbon-2.2.3.RELEASE.jar;D:workjar-resourcecom
etflix
ibbon
ibbon2.3.0
ibbon-2.3.0.jar;D:workjar-resourcecom
etflix
ibbon
ibbon-transport2.3.0
ibbon-transport-2.3.0.jar;D:workjar-resourceio
eactivex
xnetty-contexts.4.9
xnetty-contexts-0.4.9.jar;D:workjar-resourceio
eactivex
xnetty-servo.4.9
xnetty-servo-0.4.9.jar;D:workjar-resourceio
eactivex
xnetty.4.9
xnetty-0.4.9.jar;D:workjar-resourcecom
etflix
ibbon
ibbon-core2.3.0
ibbon-core-2.3.0.jar;D:workjar-resourcecom
etflix
ibbon
ibbon-httpclient2.3.0
ibbon-httpclient-2.3.0.jar;D:workjar-resourcecommons-collectionscommons-collections3.2.2commons-collections-3.2.2.jar;D:workjar-resourcecom
etflix
etflix-commons
etflix-commons-util.3.0
etflix-commons-util-0.3.0.jar;D:workjar-resourcecom
etflix
ibbon
ibbon-loadbalancer2.3.0
ibbon-loadbalancer-2.3.0.jar;D:workjar-resourcecom
etflix
etflix-commons
etflix-statistics.1.1
etflix-statistics-0.1.1.jar;D:workjar-resourceio
eactivex
xjava1.3.8
xjava-1.3.8.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-loadbalancer2.2.3.RELEASEspring-cloud-starter-loadbalancer-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-loadbalancer2.2.3.RELEASEspring-cloud-loadbalancer-2.2.3.RELEASE.jar;D:workjar-resourceioprojectreactor
eactor-core3.3.0.RELEASE
eactor-core-3.3.0.RELEASE.jar;D:workjar-resourceioprojectreactoraddons
eactor-extra3.3.0.RELEASE
eactor-extra-3.3.0.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-cache2.2.1.RELEASEspring-boot-starter-cache-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-context-support5.2.1.RELEASEspring-context-support-5.2.1.RELEASE.jar;D:workjar-resourcecomstoyanrevictor1.0.0evictor-1.0.0.jar;D:workjar-resourcecom
etflix
ibbon
ibbon-eureka2.3.0
ibbon-eureka-2.3.0.jar;D:workjar-resourceorgslf4jslf4j-api1.7.29slf4j-api-1.7.29.jar;D:workjar-resourcecom	houghtworksxstreamxstream1.4.11.1xstream-1.4.11.1.jar;D:workjar-resourcexmlpullxmlpull1.1.3.1xmlpull-1.1.3.1.jar;D:workjar-resourcexpp3xpp3_min1.1.4cxpp3_min-1.1.4c.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-actuator2.2.1.RELEASEspring-boot-starter-actuator-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-actuator-autoconfigure2.2.1.RELEASEspring-boot-actuator-autoconfigure-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-actuator2.2.1.RELEASEspring-boot-actuator-2.2.1.RELEASE.jar;D:workjar-resourcecomfasterxmljacksondatatypejackson-datatype-jsr3102.10.0jackson-datatype-jsr310-2.10.0.jar;D:workjar-resourceiomicrometermicrometer-core1.3.1micrometer-core-1.3.1.jar;D:workjar-resourceorghdrhistogramHdrHistogram2.1.11HdrHistogram-2.1.11.jar;D:workjar-resourceorglatencyutilsLatencyUtils2.0.3LatencyUtils-2.0.3.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-netflix-hystrix2.2.3.RELEASEspring-cloud-starter-netflix-hystrix-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-netflix-ribbon2.2.3.RELEASEspring-cloud-netflix-ribbon-2.2.3.RELEASE.jar;D:workjar-resourcecom
etflixhystrixhystrix-core1.5.18hystrix-core-1.5.18.jar;D:workjar-resourcecom
etflixhystrixhystrix-serialization1.5.18hystrix-serialization-1.5.18.jar;D:workjar-resourcecomfasterxmljacksonmodulejackson-module-afterburner2.10.0jackson-module-afterburner-2.10.0.jar;D:workjar-resourcecom
etflixhystrixhystrix-metrics-event-stream1.5.18hystrix-metrics-event-stream-1.5.18.jar;D:workjar-resourcecom
etflixhystrixhystrix-javanica1.5.18hystrix-javanica-1.5.18.jar;D:workjar-resourceorgapachecommonscommons-lang33.9commons-lang3-3.9.jar;D:workjar-resourceorgow2asmasm5.0.4asm-5.0.4.jar;D:workjar-resourceorgaspectjaspectjweaver1.9.4aspectjweaver-1.9.4.jar;D:workjar-resourcecomgoogleguavaguava29.0-androidguava-29.0-android.jar;D:workjar-resourcecomgoogleguavafailureaccess1.0.1failureaccess-1.0.1.jar;D:workjar-resourcecomgoogleguavalistenablefuture9999.0-empty-to-avoid-conflict-with-guavalistenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar;D:workjar-resourceorgcheckerframeworkchecker-compat-qual2.5.5checker-compat-qual-2.5.5.jar;D:workjar-resourcecomgoogleerrorproneerror_prone_annotations2.3.4error_prone_annotations-2.3.4.jar;D:workjar-resourcecomgooglej2objcj2objc-annotations1.3j2objc-annotations-1.3.jar;D:workjar-resourceio
eactivex
xjava-reactive-streams1.2.1
xjava-reactive-streams-1.2.1.jar;D:workjar-resourceorg
eactivestreams
eactive-streams1.0.3
eactive-streams-1.0.3.jar;D:workjar-resourceorgprojectlomboklombok1.18.10lombok-1.18.10.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-log4j22.2.1.RELEASEspring-boot-starter-log4j2-2.2.1.RELEASE.jar;D:workjar-resourceorgapachelogginglog4jlog4j-slf4j-impl2.12.1log4j-slf4j-impl-2.12.1.jar;D:workjar-resourceorgapachelogginglog4jlog4j-api2.12.1log4j-api-2.12.1.jar;D:workjar-resourceorgapachelogginglog4jlog4j-core2.12.1log4j-core-2.12.1.jar;D:workjar-resourceorgapachelogginglog4jlog4j-jul2.12.1log4j-jul-2.12.1.jar;D:workjar-resourceorgslf4jjul-to-slf4j1.7.29jul-to-slf4j-1.7.29.jar;D:workmycodecloud-parentapp-user-domain	argetclasses;D:workjar-resourceorgmybatisspringootmybatis-spring-boot-starter1.3.2mybatis-spring-boot-starter-1.3.2.jar;D:workjar-resourceorgmybatisspringootmybatis-spring-boot-autoconfigure1.3.2mybatis-spring-boot-autoconfigure-1.3.2.jar;D:workjar-resourceorgmybatismybatis3.4.6mybatis-3.4.6.jar;D:workjar-resourceorgmybatismybatis-spring1.3.2mybatis-spring-1.3.2.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-jdbc2.2.1.RELEASEspring-boot-starter-jdbc-2.2.1.RELEASE.jar;D:workjar-resourcecomzaxxerHikariCP3.4.1HikariCP-3.4.1.jar;D:workjar-resourceorgspringframeworkspring-jdbc5.2.1.RELEASEspring-jdbc-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-tx5.2.1.RELEASEspring-tx-5.2.1.RELEASE.jar;D:workjar-resourceorgmybatisgeneratormybatis-generator-core1.3.7mybatis-generator-core-1.3.7.jar;D:workjar-resourcecomalibabadruid-spring-boot-starter1.1.10druid-spring-boot-starter-1.1.10.jar;D:workjar-resourcecomalibabadruid1.1.10druid-1.1.10.jar;D:workjar-resourcemysqlmysql-connector-java8.0.18mysql-connector-java-8.0.18.jar;D:workmycodecloud-parentcloud-fegin	argetclasses;D:workjar-resourceorgspringframeworkcloudspring-cloud-starter-openfeign2.2.3.RELEASEspring-cloud-starter-openfeign-2.2.3.RELEASE.jar;D:workjar-resourceorgspringframeworkcloudspring-cloud-openfeign-core2.2.3.RELEASEspring-cloud-openfeign-core-2.2.3.RELEASE.jar;D:workjar-resourceiogithubopenfeignformfeign-form-spring3.8.0feign-form-spring-3.8.0.jar;D:workjar-resourceiogithubopenfeignformfeign-form3.8.0feign-form-3.8.0.jar;D:workjar-resourcecommons-fileuploadcommons-fileupload1.4commons-fileupload-1.4.jar;D:workjar-resourcecommons-iocommons-io2.2commons-io-2.2.jar;D:workjar-resourceiogithubopenfeignfeign-core10.10.1feign-core-10.10.1.jar;D:workjar-resourceiogithubopenfeignfeign-slf4j10.10.1feign-slf4j-10.10.1.jar;D:workjar-resourceiogithubopenfeignfeign-hystrix10.10.1feign-hystrix-10.10.1.jar;D:workjar-resourceiogithubopenfeignfeign-okhttp10.10.1feign-okhttp-10.10.1.jar;D:workjar-resourcecomsquareupokhttp3okhttp3.14.4okhttp-3.14.4.jar;D:workjar-resourcecomsquareupokiookio1.17.2okio-1.17.2.jar;D:workmycodecloud-parentapp-common	argetclasses;D:workjar-resourceorgspringframeworkootspring-boot-starter-web2.2.1.RELEASEspring-boot-starter-web-2.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-json2.2.1.RELEASEspring-boot-starter-json-2.2.1.RELEASE.jar;D:workjar-resourcecomfasterxmljacksondatatypejackson-datatype-jdk82.10.0jackson-datatype-jdk8-2.10.0.jar;D:workjar-resourcecomfasterxmljacksonmodulejackson-module-parameter-names2.10.0jackson-module-parameter-names-2.10.0.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-tomcat2.2.1.RELEASEspring-boot-starter-tomcat-2.2.1.RELEASE.jar;D:workjar-resourceorgapache	omcatembed	omcat-embed-core9.0.27	omcat-embed-core-9.0.27.jar;D:workjar-resourceorgapache	omcatembed	omcat-embed-el9.0.27	omcat-embed-el-9.0.27.jar;D:workjar-resourceorgapache	omcatembed	omcat-embed-websocket9.0.27	omcat-embed-websocket-9.0.27.jar;D:workjar-resourceorgspringframeworkootspring-boot-starter-validation2.2.1.RELEASEspring-boot-starter-validation-2.2.1.RELEASE.jar;D:workjar-resourcejakartavalidationjakarta.validation-api2.0.1jakarta.validation-api-2.0.1.jar;D:workjar-resourceorghibernatevalidatorhibernate-validator6.0.18.Finalhibernate-validator-6.0.18.Final.jar;D:workjar-resourceorgjbossloggingjboss-logging3.4.1.Finaljboss-logging-3.4.1.Final.jar;D:workjar-resourcecomfasterxmlclassmate1.5.1classmate-1.5.1.jar;D:workjar-resourceorgspringframeworkspring-web5.2.1.RELEASEspring-web-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-beans5.2.1.RELEASEspring-beans-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-webmvc5.2.1.RELEASEspring-webmvc-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-aop5.2.1.RELEASEspring-aop-5.2.1.RELEASE.jar;D:workjar-resourceorgspringframeworkspring-expression5.2.1.RELEASEspring-expression-5.2.1.RELEASE.jar com.ghsy.user.test.MyClassLoader
helle_classloader
com.ghsy.user.test.MyClassLoader

Process finished with exit code 0

这个时候ClassLoader是我们自己定义的ClassLoader，不再是由AppClassLoader去加载了。

　　问题1：如果自定义一个类加载器去加载？

　　　　继承ClassLoader，重写里面的findClass方法。

　　在看下这些类加载器之间的关系：

　　

 　　自己画的一个草图，上面的null，网上由其他资料说是BootstrapClassLoader，我在jdk中并未找到明确的类（无法作证），这个启动类加载器为底层c语言实现（navtice），不需要去特意关注。

　　这里提一个经常提到的概念：类加载器的双亲委派机制，下面我们来看源码（ClassLoader中的loadclass方法）

我们先来求证一件事情，上图所对应的类加载器关系是不是这样呢？我们看AppClassLoader的源码：

//代码【1】

static class AppClassLoader extends URLClassLoader {
        final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
　　　　　　

　　　　　　//这个方法返回的是一个AppClassLoader
        public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException {　　　　//传入的为ClassLoader实现类(这里为ExtClassLoader，下面会解释)
            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);
                    return new Launcher.AppClassLoader(var1x, var0);
                }
            });
        }

        AppClassLoader(URL[] var1, ClassLoader var2) {
            super(var1, var2, Launcher.factory);　　　　//调用父类的构造函数（看第代码【3】）
            this.ucp.initLookupCache(this);
        }

        public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
            int var3 = var1.lastIndexOf(46);
            if (var3 != -1) {
                SecurityManager var4 = System.getSecurityManager();
                if (var4 != null) {
                    var4.checkPackageAccess(var1.substring(0, var3));
                }
            }

            if (this.ucp.knownToNotExist(var1)) {
                Class var5 = this.findLoadedClass(var1);
                if (var5 != null) {
                    if (var2) {
                        this.resolveClass(var5);
                    }

                    return var5;
                } else {
                    throw new ClassNotFoundException(var1);
                }
            } else {
                return super.loadClass(var1, var2);
            }
        }

        protected PermissionCollection getPermissions(CodeSource var1) {
            PermissionCollection var2 = super.getPermissions(var1);
            var2.add(new RuntimePermission("exitVM"));
            return var2;
        }

        private void appendToClassPathForInstrumentation(String var1) {
            assert Thread.holdsLock(this);

            super.addURL(Launcher.getFileURL(new File(var1)));
        }

        private static AccessControlContext getContext(File[] var0) throws MalformedURLException {
            PathPermissions var1 = new PathPermissions(var0);
            ProtectionDomain var2 = new ProtectionDomain(new CodeSource(var1.getCodeBase(), (Certificate[])null), var1);
            AccessControlContext var3 = new AccessControlContext(new ProtectionDomain[]{var2});
            return var3;
        }

        static {
            ClassLoader.registerAsParallelCapable();
        }
    }



//这里为LaunCh的构造方法源码(AppClassLoader和ExtClassLoader都为LaunCh类的内部类)
//代码【2】

public Launcher() {
    Launcher.ExtClassLoader var1;
    try {
        var1 = Launcher.ExtClassLoader.getExtClassLoader();　　　　//获取了一个ExtClassLoader
    } catch (IOException var10) {
        throw new InternalError("Could not create extension class loader", var10);
    }

    try {
        this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);　　　　//传入的是ExtClassLoader（由此证明上面AppClassLoader的getAppClassLoader方法传入的是ExtClassLoader）

    } 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);
    }

}

//URLClassLoader（AppClassLoader和ExtClassLoader的父类）的构造方法　　在点进源码中去看，最终发现URLClassLoader是ClassLoader子类
//代码【3】

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();
    }
    ucp = new URLClassPath(urls, factory);
    acc = AccessController.getContext();
}

//SecureClassLoader（URLClassLoader父类）中的构造方法，继续调用父类的构造方法，我们再去ClassLoader中看
//代码【4】

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;
}

//ClassLoader
//代码【5】

private ClassLoader(Void unused, ClassLoader parent) {　　　　//这里可以看到，AppClassLoader的Parent其实已经是个ExtClssLoader的实例，
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//同理，ExtClassLoader都为URLClassLoader子类，这里就做过多赘述

    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;
    }
}

//再来看下ExtClassLoader的构造函数和getExtDirs方法，由此可证(ExtClassLoader加载的是ext扩展包中的类)

 //代码【6】

public ExtClassLoader(File[] var1) throws IOException {
    super(getExtURLs(var1), (ClassLoader)null, Launcher.factory);　　　　　　//构造函数传入的ClassLoader是null，说明一开始执行的main函数输出String的类加载器为null是正确的，
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//网上其他BootStrapClassLoader人云亦云
    SharedSecrets.getJavaNetAccess().getURLClassPath(this).initLookupCache(this);
}

private static File[] getExtDirs() {
    String var0 = System.getProperty("java.ext.dirs");
    File[] var1;
    if (var0 != null) {
        StringTokenizer var2 = new StringTokenizer(var0, File.pathSeparator);
        int var3 = var2.countTokens();
        var1 = new File[var3];

        for(int var4 = 0; var4 < var3; ++var4) {
            var1[var4] = new File(var2.nextToken());
        }
    } else {
        var1 = new File[0];
    }

    return var1;
}

//最后我们来看下loadclass方发，什么情况下去调用的启动类加载器(null)
代码【7】

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();
            try {
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                    c = findBootstrapClassOrNull(name);　　　　　//这个位置是ExtClassLoader加载器所进入到的判断（具体为啥往上翻----代码【6】，因为ExtClassLoader构造函数里面parent传入的就是null）
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                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;
    }
}

//再来看findBootstrapClassOrNull方法
//代码【8】

/**
 * Returns a class loaded by the bootstrap class loader;　　　　　　//返回由启动类加载器在载的Class，如果启动类无法加载，则返回null
 * or return null if not found.
 */
private Class<?> findBootstrapClassOrNull(String name)
{
    if (!checkName(name)) return null;

    return findBootstrapClass(name);
}

//findBootstrapClass方法为native方法（由c语言实现，不可见）次方法为加载jdk核心jar类的方法

// return null if not found
private native Class<?> findBootstrapClass(String name);

　　问题1：自定义类加载器，为什么默认的parent会是AppClassLoader呢？我们来看ClassLoader的默认构造函数：

//默认构造函数
protected ClassLoader() {
        this(checkCreateClassLoader(), getSystemClassLoader());
    }

//带有参数的构造函数，上面默认的构造函数调用了可以看到，这时候已经传入了parent，我们再来看getSystemClassLoader方法
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;
        }
    }

//getSystemClassLoader方法，调用了initSystemClassLoader方法，并且返回了scl(内部变量ClassLoader)，接着来看initSystemClassLoader方法

public static ClassLoader getSystemClassLoader() {
    initSystemClassLoader();
    if (scl == null) {
        return null;
    }
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        checkClassLoaderPermission(scl, Reflection.getCallerClass());
    }
    return scl;
}

//initSystemClassLoader方法

private static synchronized void initSystemClassLoader() {
    if (!sclSet) {
        if (scl != null)
            throw new IllegalStateException("recursive invocation");
        sun.misc.Launcher l = sun.misc.Launcher.getLauncher();　　　　　　//返回了Launcher对象(包含了内部类AppClassLoader和ExtClassLoader)，这个方法不过多解释，看一下就知道返回了一个LaunCher内部变量LaunCher
        if (l != null) {
            Throwable oops = null;
            scl = l.getClassLoader();　　　　　　//我们再看这个getClassLoader方法，返回了内部成员CLassLoader，这个是AppClassLoader（具体为什么请看上面的Launcher类的构造函数，源码第49行）
            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;
    }
}

至此，默认的parent赋值为AppClassLoader，上面画的个个类加载器关系全部串联起来！！！！！！

　　答案在上面代码中，因为默认的构造函数给parent初始化为AppClassLoader了。

　　问题2：类加载器的双亲委派是怎么样实现的？

　　　　我们还是结合源码来说：下面看ClassLoader的源码（为什么只看ClassLoader源码不在赘述，因为AppClassLoader和ExtClassLoader都执行的是ClassLoader中的laodClass方法）

protected Class<?> loadClass(String name, boolean resolve)　　　　//这是一个套娃过程，等会来张图解释
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        Class<?> c = findLoadedClass(name);　　　　　　　　//校验传入的类名在该类加载器中是否已经加载过了，如果加载过了，直接返回。（如果是父级加载器，也是需要判断是否加载过传入的类）
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {　　　　　　　　　　　　//判断父级类加载器是否为空，不为空则由父级去加载此类（往上委托-----关键来了，父级也是调用次方法，再由父级去执行次方法，到ExtClassLoader这层）
                    c = parent.loadClass(name, false);　　　//这里开始套娃，一个个去调用父加载器，直到启动类加载器（这是个类似于递归调用）
                } else {
                    c = findBootstrapClassOrNull(name);　　//会到这里说明已经是启动类加载器了，这个时候调用本地native方法去尝试在jre核心库区加载，如果加载不到，将返回null，在打回到ExtClassLoader，一层一层往下回，直到最底层类加载器找到类为止
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//这个位置是父级加载器没有找到类，尝试自己去加载类，这就是为什么我们自定义类加载器需要去重写这个findCLass方法
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                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;
    }
}

 

 　　这就是双亲委派机制

　　问题3：为什么要使用双亲委派这个机制

　　　　1：防止类重复加载（保证相同类只被加载一次），上面源码中已经说明。

　　　　2：防止jdk核心jar包类被篡改，增加程序安全（这个看问题4）

　　问题4：老生常谈的，我自定义一个java.lang.String会不会被加载到jvm里面去

　　　　下面有代码来证明吧

 

 　　抛出异常了，String找不到main方法，明明不是有main方法吗？

　　类加载机制是一个双亲委派过程，最开始是由启动类加载器去加载，启动类加载器在rt.jar中找到了java.lang.String，所以就加载了rt.jar下的String，我们自定义的String并没有被加载到jvm中。

　　那么问题来了，我们自定义一个加载器能行吗？

　　把上面的MyClassLoader改造一下，我们去rt.jar中找到String.class，放入到我们的test中看看呢？

 　　直接就抛出安全异常了，jvm不允许我们去加载以java开头包名下面的类！！！

　　问题5：如何打破双亲委派机制

　　　　这个问题我就不放代码和源码了，回头往上看讲双亲委派的那个地方，用的是loadclass方法，我们只需要自定义类加载器去继承ClassLoader类，重写里面的loadclass方法，

　　　　不要去调用parent中的laodclass即可

　　

　　问题6：什么情况下去打破双亲委派机制？

　　　　举个例子说明，我们的tomcat容器，里面由两个应用，一个是spring4版本，一个是spring5版本。如果这个时候还是双亲委派机制，那么加载了其中一个应用的spring版本，

　　　　另一个应用spring将不会再去加载（为啥?双亲委派机制会先去校验是否已经被加载过），那么可想而知的后果是非常严重的。后面博客会讲述tomcat相关的类加载

　　生而为人，我很抱歉