首页 > 编程语言 > 浅谈Java类的加载,链接及初始化
2021
09-13

浅谈Java类的加载,链接及初始化

一 类生命周期

Loading

Linking（Verification、Preparation、Resolution）

Initializing

二 类加载器

1 图解

2 代码

package jvm;
 
public class T002_ClassLoadLevel {
    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader());
        System.out.println(sun.awt.HKSCS.class.getClassLoader());
        System.out.println(sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader());
        System.out.println(T002_ClassLoadLevel.class.getClassLoader());
        System.out.println(sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader().getClass().getClassLoader());
        System.out.println(T002_ClassLoadLevel.class.getClassLoader().getClass().getClassLoader());
    }
}

3 测试 

null

null

sun.misc.Launcher$ExtClassLoader@5b2133b1

sun.misc.Launcher$AppClassLoader@18b4aac2

null

null

三 反射执行过程

四 双亲委派和父加载器

1 父加载器

父加载器不是类加载器的加载器。也不是类加载器的父类加载器。

2 双亲委派是一个孩子向父亲方向检查，然后父亲向孩子方向加载的双亲委派过程。

3 为什么要搞双亲委派

主要是出于安全原因，如果没有双亲委派，开发人员可以开发出自己的 String，用它来替换核心类库中的 String，在自定义的 String 类中收集用户信息，导致系统不安全。

4 代码

package jvm;
 
public class T004_ParentAndChild {
    public static void main(String[] args) {
        System.out.println(T004_ParentAndChild.class.getClassLoader());
        System.out.println(T004_ParentAndChild.class.getClassLoader().getClass().getClassLoader());
        System.out.println(T004_ParentAndChild.class.getClassLoader().getParent());
        System.out.println(T004_ParentAndChild.class.getClassLoader().getParent().getParent());
    }
}

5 测试

sun.misc.Launcher$AppClassLoader@18b4aac2

null

sun.misc.Launcher$ExtClassLoader@1be6f5c3

null

6 双亲委派过程

五 类加载器范围

1 代码

package jvm;
 
public class T003_ClassLoaderScope {
    public static void main(String[] args) {
        System.out.println("----------------------------------------------------------");
        String pathBoot = System.getProperty("sun.boot.class.path");
        System.out.println(pathBoot.replaceAll(";",System.lineSeparator()));
        System.out.println("----------------------------------------------------------");
        String pathExt = System.getProperty("java.ext.dirs");
        System.out.println(pathExt.replaceAll(";",System.lineSeparator()));
        System.out.println("----------------------------------------------------------");
        String pathApp = System.getProperty("java.class.path");
        System.out.println(pathApp.replaceAll(";",System.lineSeparator()));
    }
}

2 测试

----------------------------------------------------------
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\resources.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\rt.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\sunrsasign.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jsse.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jce.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\charsets.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jfr.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\classes
----------------------------------------------------------
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext
C:\WINDOWS\Sun\Java\lib\ext
----------------------------------------------------------
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\charsets.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\deploy.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\access-bridge-64.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\cldrdata.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\dnsns.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\jaccess.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\jfxrt.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\localedata.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\nashorn.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\sunec.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\sunjce_provider.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\sunmscapi.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\sunpkcs11.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\ext\zipfs.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\javaws.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jce.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jfr.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jfxswt.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\jsse.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\management-agent.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\plugin.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\resources.jar
D:\ProgramFiles\Java\jdk1.8.0_251\jre\lib\rt.jar
E:\javatest1\target\classes
D:\.m2\repos\org\springframework\boot\spring-boot-starter\2.2.6.RELEASE\spring-boot-starter-2.2.6.RELEASE.jar
D:\.m2\repos\org\springframework\boot\spring-boot\2.2.6.RELEASE\spring-boot-2.2.6.RELEASE.jar
D:\.m2\repos\org\springframework\spring-context\5.2.5.RELEASE\spring-context-5.2.5.RELEASE.jar
D:\.m2\repos\org\springframework\spring-aop\5.2.5.RELEASE\spring-aop-5.2.5.RELEASE.jar
D:\.m2\repos\org\springframework\spring-beans\5.2.5.RELEASE\spring-beans-5.2.5.RELEASE.jar
D:\.m2\repos\org\springframework\spring-expression\5.2.5.RELEASE\spring-expression-5.2.5.RELEASE.jar
D:\.m2\repos\org\springframework\boot\spring-boot-autoconfigure\2.2.6.RELEASE\spring-boot-autoconfigure-2.2.6.RELEASE.jar
D:\.m2\repos\org\springframework\boot\spring-boot-starter-logging\2.2.6.RELEASE\spring-boot-starter-logging-2.2.6.RELEASE.jar
D:\.m2\repos\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar
D:\.m2\repos\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar
D:\.m2\repos\org\slf4j\slf4j-api\1.7.30\slf4j-api-1.7.30.jar
D:\.m2\repos\org\apache\logging\log4j\log4j-to-slf4j\2.12.1\log4j-to-slf4j-2.12.1.jar
D:\.m2\repos\org\apache\logging\log4j\log4j-api\2.12.1\log4j-api-2.12.1.jar
D:\.m2\repos\org\slf4j\jul-to-slf4j\1.7.30\jul-to-slf4j-1.7.30.jar
D:\.m2\repos\jakarta\annotation\jakarta.annotation-api\1.3.5\jakarta.annotation-api-1.3.5.jar
D:\.m2\repos\org\springframework\spring-core\5.2.5.RELEASE\spring-core-5.2.5.RELEASE.jar
D:\.m2\repos\org\springframework\spring-jcl\5.2.5.RELEASE\spring-jcl-5.2.5.RELEASE.jar
D:\.m2\repos\org\yaml\snakeyaml\1.25\snakeyaml-1.25.jar
D:\.m2\repos\commons-beanutils\commons-beanutils\1.9.4\commons-beanutils-1.9.4.jar
D:\.m2\repos\commons-logging\commons-logging\1.2\commons-logging-1.2.jar
D:\.m2\repos\commons-collections\commons-collections\3.2.2\commons-collections-3.2.2.jar
D:\.m2\repos\commons-codec\commons-codec\1.14\commons-codec-1.14.jar
D:\.m2\repos\com\google\code\gson\gson\2.8.6\gson-2.8.6.jar
D:\.m2\repos\org\apache\commons\commons-lang3\3.9\commons-lang3-3.9.jar
D:\.m2\repos\cglib\cglib\2.2.2\cglib-2.2.2.jar
D:\.m2\repos\asm\asm\3.3.1\asm-3.3.1.jar
D:\.m2\repos\commons-lang\commons-lang\2.6\commons-lang-2.6.jar
D:\.m2\repos\com\mchange\c3p0\0.9.5.2\c3p0-0.9.5.2.jar
D:\.m2\repos\com\mchange\mchange-commons-java\0.2.11\mchange-commons-java-0.2.11.jar
D:\.m2\repos\org\projectlombok\lombok\1.18.12\lombok-1.18.12.jar
D:\.m2\repos\com\google\guava\guava\20.0\guava-20.0.jar
D:\.m2\repos\commons-io\commons-io\2.6\commons-io-2.6.jar
D:\.m2\repos\com\alibaba\fastjson\1.2.31\fastjson-1.2.31.jar
D:\.m2\repos\org\apache\poi\poi\3.9\poi-3.9.jar
D:\.m2\repos\org\apache\poi\poi-ooxml\3.9\poi-ooxml-3.9.jar
D:\.m2\repos\org\apache\poi\poi-ooxml-schemas\3.9\poi-ooxml-schemas-3.9.jar
D:\.m2\repos\org\apache\xmlbeans\xmlbeans\2.3.0\xmlbeans-2.3.0.jar
D:\.m2\repos\stax\stax-api\1.0.1\stax-api-1.0.1.jar
D:\.m2\repos\dom4j\dom4j\1.6.1\dom4j-1.6.1.jar
D:\.m2\repos\xml-apis\xml-apis\1.0.b2\xml-apis-1.0.b2.jar
D:\ProgramFiles\JetBrains\IDEA\lib\idea_rt.jar

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