类加载链接初始化

朴灿烈づ我的快乐病毒、 2021-07-24 23:40 395阅读 0赞

# 一 类生命周期 #

Linking（Verification、Preparation、Resolution）

Initializing

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

# 二 类加载器 #

## 1 图解 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

## 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 #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 2][]

# 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**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 3][]

# 五 类加载器范围 #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 4][]

## 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 ##

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