JVM知识点(二) classLoader源码分析

待我称王封你为后i 2022-12-06 12:48 139阅读 0赞

### 目录 ###

*  JVM知识点划分
 *  定义
 *  类加载器分类
 *   *  系统级别
     *  用户级别
     *  层级结构
 *  代码实现
 *   *  启动类加载器
     *  扩展类加载器
     *  应用类加载器
 *  双亲委派原则
 *   *  代码实现
 *  源码探索
 *   *  什么时候出发类加载
     *  重要方法
     *   *  loadClass()
 *  加载流程
 *  手写自定义加载器
 *   *  新建需要加载的类
     *  自定义CustomerClassLoader
     *  测试类

# JVM知识点划分 #

*  class文件结构
 *  classLoader
 *  jvm运行时数据区
 *  垃圾回收器及垃圾回收算法
 *  jit

# 定义 #

ClassLoader 顾名思义就是类加载器,**ClassLoader 作用**：

*  负责将 Class 加载到 JVM 中
 *  审查每个类由谁加载（父优先的等级加载机制）
 *  将 Class 字节码重新解析成 JVM 统一要求的对象格式

# 类加载器分类 #

## 系统级别 ##

*  启动类加载器(C实现)
 *  扩展类加载器(java实现)
 *  系统类加载器(APP类加载器)

## 用户级别 ##

自定义类加载器,集成自ClassLoader

## 层级结构 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppbmlhbjIwMTY_size_16_color_FFFFFF_t_70_pic_center]

# 代码实现 #

## 启动类加载器 ##

public static void BootStrapClassLoader() { 
    
            //获取启动类的加载器的加载目录
            String bootStrapClassLoaderPath = System.getProperty("sun.boot.class.path");
            //把加载后的目录转化为集合
            List<String> bootStrapClassLoaderPathList = Arrays.asList(bootStrapClassLoaderPath.split(";"));
    
            for (String path : bootStrapClassLoaderPathList) { 
                System.out.println("[启动类加载器-加载目录]" + path);
            }
        }

输出:

[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\resources.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\rt.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\sunrsasign.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jsse.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jce.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\charsets.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfr.jar
    [启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\classes

## 扩展类加载器 ##

public static void ExtClassLoader() { 
    
            //获取扩展类的加载器的加载目录
            String extClassLoaderPath = System.getProperty("java.ext.dirs");
            //把加载后的目录转化为集合
            List<String> extClassLoaderPathList = Arrays.asList(extClassLoaderPath.split(";"));
    
            for (String path : extClassLoaderPathList) { 
                System.out.println("[扩展类加载器-加载目录]" + path);
            }
        }

输出:

[扩展类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext
    [扩展类加载器-加载目录]C:\WINDOWS\Sun\Java\lib\ext

## 应用类加载器 ##

public static void APPClassLoader() { 
    
            //应用类的加载器的加载目录
            String appClassLoaderPath = System.getProperty("java.class.path");
            //把加载后的目录转化为集合
            List<String> appClassLoaderPathList = Arrays.asList(appClassLoaderPath.split(";"));
    
            for (String path : appClassLoaderPathList) { 
                System.out.println("[应用类加载器-加载目录]" + path);
            }
        }

输出

[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\charsets.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\deploy.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\access-bridge-64.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\cldrdata.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\dnsns.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\jaccess.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\jfxrt.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\localedata.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\nashorn.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\ojdbc6-11.2.0.3.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunec.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunjce_provider.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunmscapi.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunpkcs11.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\zipfs.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\javaws.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jce.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfr.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfxswt.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jsse.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\management-agent.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\plugin.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\resources.jar
    [应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\rt.jar
    [应用类加载器-加载目录]C:\work\demo\jxs-parent\jxs-user\target\classes
    [应用类加载器-加载目录]C:\work\demo\jxs-parent\jxs-common\jxs-common-base\target\classes
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-web\2.2.5.RELEASE\spring-boot-starter-web-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter\2.2.5.RELEASE\spring-boot-starter-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot\2.2.5.RELEASE\spring-boot-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-logging\2.2.5.RELEASE\spring-boot-starter-logging-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\logging\log4j\log4j-to-slf4j\2.12.1\log4j-to-slf4j-2.12.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\logging\log4j\log4j-api\2.12.1\log4j-api-2.12.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\slf4j\jul-to-slf4j\1.7.30\jul-to-slf4j-1.7.30.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\annotation\jakarta.annotation-api\1.3.5\jakarta.annotation-api-1.3.5.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-core\5.2.4.RELEASE\spring-core-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-jcl\5.2.4.RELEASE\spring-jcl-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\yaml\snakeyaml\1.25\snakeyaml-1.25.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-json\2.2.5.RELEASE\spring-boot-starter-json-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-databind\2.10.2\jackson-databind-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-core\2.10.2\jackson-core-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\datatype\jackson-datatype-jdk8\2.10.2\jackson-datatype-jdk8-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\datatype\jackson-datatype-jsr310\2.10.2\jackson-datatype-jsr310-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\module\jackson-module-parameter-names\2.10.2\jackson-module-parameter-names-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-tomcat\2.2.5.RELEASE\spring-boot-starter-tomcat-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-core\9.0.31\tomcat-embed-core-9.0.31.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-el\9.0.31\tomcat-embed-el-9.0.31.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-websocket\9.0.31\tomcat-embed-websocket-9.0.31.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-validation\2.2.5.RELEASE\spring-boot-starter-validation-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\validation\jakarta.validation-api\2.0.2\jakarta.validation-api-2.0.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\validator\hibernate-validator\6.0.18.Final\hibernate-validator-6.0.18.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-web\5.2.4.RELEASE\spring-web-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-beans\5.2.4.RELEASE\spring-beans-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-webmvc\5.2.4.RELEASE\spring-webmvc-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-aop\5.2.4.RELEASE\spring-aop-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-context\5.2.4.RELEASE\spring-context-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-expression\5.2.4.RELEASE\spring-expression-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-actuator\2.2.5.RELEASE\spring-boot-starter-actuator-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-actuator-autoconfigure\2.2.5.RELEASE\spring-boot-actuator-autoconfigure-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-actuator\2.2.5.RELEASE\spring-boot-actuator-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\micrometer\micrometer-core\1.3.5\micrometer-core-1.3.5.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\hdrhistogram\HdrHistogram\2.1.11\HdrHistogram-2.1.11.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\latencyutils\LatencyUtils\2.0.3\LatencyUtils-2.0.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-data-jpa\2.2.5.RELEASE\spring-boot-starter-data-jpa-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-aop\2.2.5.RELEASE\spring-boot-starter-aop-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\aspectj\aspectjweaver\1.9.5\aspectjweaver-1.9.5.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-jdbc\2.2.5.RELEASE\spring-boot-starter-jdbc-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\zaxxer\HikariCP\3.4.2\HikariCP-3.4.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-jdbc\5.2.4.RELEASE\spring-jdbc-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\activation\jakarta.activation-api\1.2.2\jakarta.activation-api-1.2.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\persistence\jakarta.persistence-api\2.2.3\jakarta.persistence-api-2.2.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\transaction\jakarta.transaction-api\1.3.3\jakarta.transaction-api-1.3.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\hibernate-core\5.4.12.Final\hibernate-core-5.4.12.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\jboss\logging\jboss-logging\3.4.1.Final\jboss-logging-3.4.1.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\javassist\javassist\3.24.0-GA\javassist-3.24.0-GA.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\net\bytebuddy\byte-buddy\1.10.8\byte-buddy-1.10.8.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\antlr\antlr\2.7.7\antlr-2.7.7.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\jboss\jandex\2.1.1.Final\jandex-2.1.1.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\dom4j\dom4j\2.1.1\dom4j-2.1.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\common\hibernate-commons-annotations\5.1.0.Final\hibernate-commons-annotations-5.1.0.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\glassfish\jaxb\jaxb-runtime\2.3.2\jaxb-runtime-2.3.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\xml\bind\jakarta.xml.bind-api\2.3.2\jakarta.xml.bind-api-2.3.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\glassfish\jaxb\txw2\2.3.2\txw2-2.3.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\sun\istack\istack-commons-runtime\3.0.8\istack-commons-runtime-3.0.8.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\jvnet\staxex\stax-ex\1.8.1\stax-ex-1.8.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\sun\xml\fastinfoset\FastInfoset\1.2.16\FastInfoset-1.2.16.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\data\spring-data-jpa\2.2.5.RELEASE\spring-data-jpa-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\data\spring-data-commons\2.2.5.RELEASE\spring-data-commons-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-orm\5.2.4.RELEASE\spring-orm-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-tx\5.2.4.RELEASE\spring-tx-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-aspects\5.2.4.RELEASE\spring-aspects-5.2.4.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\mysql\mysql-connector-java\8.0.19\mysql-connector-java-8.0.19.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\alibaba\druid-spring-boot-starter\1.1.20\druid-spring-boot-starter-1.1.20.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\alibaba\druid\1.1.20\druid-1.1.20.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\slf4j\slf4j-api\1.7.30\slf4j-api-1.7.30.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-autoconfigure\2.2.5.RELEASE\spring-boot-autoconfigure-2.2.5.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger2\2.9.2\springfox-swagger2-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\swagger\swagger-annotations\1.5.20\swagger-annotations-1.5.20.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\swagger\swagger-models\1.5.20\swagger-models-1.5.20.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-annotations\2.10.2\jackson-annotations-2.10.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-spi\2.9.2\springfox-spi-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-core\2.9.2\springfox-core-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-schema\2.9.2\springfox-schema-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger-common\2.9.2\springfox-swagger-common-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-spring-web\2.9.2\springfox-spring-web-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\guava\28.2-android\guava-28.2-android.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\failureaccess\1.0.1\failureaccess-1.0.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\listenablefuture\9999.0-empty-to-avoid-conflict-with-guava\listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\code\findbugs\jsr305\3.0.2\jsr305-3.0.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\checkerframework\checker-compat-qual\2.5.5\checker-compat-qual-2.5.5.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\errorprone\error_prone_annotations\2.3.4\error_prone_annotations-2.3.4.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\google\j2objc\j2objc-annotations\1.3\j2objc-annotations-1.3.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\classmate\1.5.1\classmate-1.5.1.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\plugin\spring-plugin-core\1.2.0.RELEASE\spring-plugin-core-1.2.0.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\plugin\spring-plugin-metadata\1.2.0.RELEASE\spring-plugin-metadata-1.2.0.RELEASE.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\mapstruct\mapstruct\1.2.0.Final\mapstruct-1.2.0.Final.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger-ui\2.9.2\springfox-swagger-ui-2.9.2.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-boot-starter\3.1.0\mybatis-plus-boot-starter-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus\3.1.0\mybatis-plus-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-generator\3.1.0\mybatis-plus-generator-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-extension\3.1.0\mybatis-plus-extension-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-core\3.1.0\mybatis-plus-core-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-annotation\3.1.0\mybatis-plus-annotation-3.1.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\mybatis\mybatis\3.5.0\mybatis-3.5.0.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\com\github\jsqlparser\jsqlparser\1.4\jsqlparser-1.4.jar
    [应用类加载器-加载目录]C:\work\maven\Repositories\org\mybatis\mybatis-spring\2.0.0\mybatis-spring-2.0.0.jar
    [应用类加载器-加载目录]C:\Program Files\JetBrains\IntelliJ IDEA 2019.2\lib\idea_rt.jar

# 双亲委派原则 #

## 代码实现 ##

System.out.println("当前加载器:"+DiffClassLoader.class.getClassLoader());
     System.out.println("当前加载器-父类:"+DiffClassLoader.class.getClassLoader().getParent());
     System.out.println("当前加载器-父类-父类:"+DiffClassLoader.class.getClassLoader().getParent().getParent());

输出:

当前加载器:sun.misc.Launcher$AppClassLoader@18b4aac2
    当前加载器-父类:sun.misc.Launcher$ExtClassLoader@37f8bb67
    当前加载器-父类-父类:null

# 源码探索 #

## 什么时候出发类加载 ##

*  访问类的静态变量 (除常量【 被final修辞的静态变量】 原因:常量一种特殊的变量，因为编译器把他们当作值(value)而不是域(field)来对待。如果你的代码中用到了常变量(constant variable)，编译器并不会生成字节码来从对象中载入域的值，而是直接把这个值插入到字节码中。这是一种很有用的优化，但是如果你需要改变final域的值那么每一块用到那个域的代码都需要重新编译。
 *  访问类的静态方法
 *  虚拟机启动时，定义了main()方法的那个类先初始化
 *  创建类的实例 new指令
 *  反射 如( Class.forName(“my.xyz.Test”) )
 *  子类初始化,一定会出发父类加载

> 初始化一定会类加载  
> 类加载不一定会触发初始化

## 重要方法 ##

### loadClass() ###

/** * 加载指定的限定名,默认实现以下方法: * 1. 调用 findLoadedClass(String)方法来校验class是否被加载 * 2. 调用loadClass(String) 方法,在父类加载器上,如果父类加载器是null,则调用虚拟机 提供的加载器,即为BootStrapClassLoader * 3. 调用findClass(String) 方法, 来寻找class * * 参数: * name:全限定名 * resolve 如果是true 解决这个类 * * * * 如果在上述步骤中找到类, 且 "resolve" 属性是true ,将会调用resolveClass(Class) * 在最后返回class时 * * 子类加载器鼓励去实现 findClass(String)方法 * */
       protected Class<?> loadClass(String name, boolean resolve)
           throws ClassNotFoundException
       { 
           synchronized (getClassLoadingLock(name)) { 
               // 1. 校验类是否被加载
               Class<?> c = findLoadedClass(name);
               if (c == null) { 
                   long t0 = System.nanoTime();
                   try { 
                       if (parent != null) { 
                       //父加载器不为空则调用父加载器的loadClass
                           c = parent.loadClass(name, false);
                       } else { 
                       //如果为空调用启动类加载器
                           c = findBootstrapClassOrNull(name);
                       }
                   } catch (ClassNotFoundException e) { 
                       // ClassNotFoundException thrown if class not found
                       // from the non-null parent class loader
                   }
    
                   if (c == null) { 
                       //如果分类加载器没有找到,则调用findClass 来寻找类
                       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) { 
               调用resolveClass()
                   resolveClass(c);
               }
               return c;
           }
       }

# 加载流程 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppbmlhbjIwMTY_size_16_color_FFFFFF_t_70_pic_center 1]

# 手写自定义加载器 #

## 新建需要加载的类 ##

package com.jxs.user.classLoader;
    
    public class Person { 
    
        public void say() { 
            System.out.println("Say Hello");
        }
    }

## 自定义CustomerClassLoader ##

package com.jxs.user.classLoader;
    
    import java.io.*;
    
    public class CustomerClassLoader extends ClassLoader { 
    
        private String mLibPath;
    
    
        public CustomerClassLoader(String path) { 
            // TODO Auto-generated constructor stub
            mLibPath = path;
        }
    
    
        /** * Finds the class with the specified <a href="#name">binary name</a>. * This method should be overridden by class loader implementations that * follow the delegation model for loading classes, and will be invoked by * the {@link #loadClass <tt>loadClass</tt>} method after checking the * parent class loader for the requested class. The default implementation * throws a <tt>ClassNotFoundException</tt>. * * @param name The <a href="#name">binary name</a> of the class * @return The resulting <tt>Class</tt> object * @throws ClassNotFoundException If the class could not be found * @since 1.2 */
        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException { 
    
            String fileName = getFileName(name);
            //加载文件
            File file = new File(mLibPath, fileName);
    
            try { 
                FileInputStream is = new FileInputStream(file);
    
                //输出流
                ByteArrayOutputStream bos = new ByteArrayOutputStream();
    
                int len = 0;
                if ((len = is.read()) != -1) { 
                    bos.write(len);
                }
                byte[] data = bos.toByteArray();
    
                is.close();
                bos.close();
    
                return defineClass(name, data, 0, data.length);
            } catch (FileNotFoundException e) { 
                e.printStackTrace();
            } catch (IOException e) { 
                e.printStackTrace();
            }
            return super.findClass(name);
        }
    
        /** * @param name 获取要加载 的class文件名 * @return */
        private String getFileName(String name) { 
            // TODO Auto-generated method stub
            int index = name.lastIndexOf('.');
            if (index == -1) { 
                return name + ".class";
            } else { 
                return name.substring(index + 1) + ".class";
            }
        }
    }

## 测试类 ##

package com.jxs.user.classLoader;
    
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    
    public class ClassLoaderTest { 
    
        public static void main(String[] args) { 
            // TODO Auto-generated method stub
    
            //创建自定义classloader对象。
            CustomerClassLoader diskLoader = new CustomerClassLoader("C:\\work\\demo\\jxs-parent\\jxs-user\\target\\classes");
            try { 
                //加载class文件
                Class c = diskLoader.loadClass("com.jxs.user.classLoader.Person");
    
                if(c != null){ 
                    try { 
                        Object obj = c.newInstance();
                        Method method = c.getDeclaredMethod("say",null);
                        //通过反射调用Test类的say方法
                        method.invoke(obj, null);
                    } catch (InstantiationException | IllegalAccessException
                            | NoSuchMethodException
                            | SecurityException |
                            IllegalArgumentException |
                            InvocationTargetException e) { 
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            } catch (ClassNotFoundException e) { 
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
    
        }
    
    }

输出:

Say Hello

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppbmlhbjIwMTY_size_16_color_FFFFFF_t_70_pic_center]: /images/20221123/9bac332ee2e64577a47fb9ea41b5c78a.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppbmlhbjIwMTY_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221123/c812ecc1204e4ff5ab11c8bc27cf8fcb.png