类加载器及双亲委派机制

一. 类的加载

java class是由class loader 类加载器加载到JVM内存中的。
看下下面的demo，java中有三种类加载器。

• 首先，我们自己定义的这个classLoaderDemo类，是由AppClassLoader（应用类加载器）加载到内存的。AppClassLoader主要负责加载应用程序的主函数类。
• AppClassLoader的父classLoader是ExtClassLoader（扩展类加载器）。ExtClassLoader负责加载java的扩展类库。
• ExtClassLoader的父加载器在这个demo中看不粗来，其实它的父加载器是BootstrapClassLoader。BootstrapClassLoader也就是启动类加载器，它不是一个java class，是特定于平台的机器指令，负责开启整个加载过程。主要负责加载核心的类库(java.lang.*等)，构造ExtClassLoader和APPClassLoader。

public class ClassLoaderDemo {

    public static void main(String[] args) {
        ClassLoader cl = ClassLoaderDemo.class.getClassLoader();
        System.out.println("class loader for ClassLoaderDemo is " + cl);
        System.out.println("parent class loader is " + cl.getParent());
        System.out.println("parent for parent class loader is " + cl.getParent().getParent());
        
        ClassLoader strCl = String.class.getClassLoader();
        System.out.println("class loader for String is " + strCl);
        
        try {
            ClassLoader listCl = cl.getParent().loadClass("java.util.List").getClass().getClassLoader();
            System.out.println("class loader for List is " + listCl);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        
        System.out.println("BootStrap ClassLoader path " + System.getProperty("sun.boot.class.path"));
        System.out.println("Extention ClassLoader path " + System.getProperty("java.ext.dirs"));
        System.out.println("Application ClassLoader path  " + System.getProperty("java.class.path"));
    }

}

输出：

class loader for ClassLoaderDemo is sun.misc.Launcher$AppClassLoader@4e25154f
parent class loader is sun.misc.Launcher$ExtClassLoader@33909752
parent for parent class loader is null
class loader for String is null
class loader for List is null
BootStrap ClassLoader path /Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/sunrsasign.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/classes
Extention ClassLoader path /Users/username/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java
Application ClassLoader path  /Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_201.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/System/Library/Java/Extensions/MRJToolkit.jar:/Users/username/Documents/java/myredis/target/classes:/Users/username/.m2/repository/redis/clients/jedis/3.2.0/jedis-3.2.0.jar:/Users/username/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/Users/username/.m2/repository/org/apache/commons/commons-pool2/2.6.2/commons-pool2-2.6.2.jar:/Users/username/.m2/repository/com/alibaba/fastjson/1.2.62/fastjson-1.2.62.jar

二. 双亲委派机制

1. 工作机制

这三种类加载器，是按照双亲委派机制来工作的。

当需要加载一个类时，AppClassLoader会检查是否已加载此类，如果已加载则不再处理，如果没有加载，则会询问ExtClassLoader是否已加载此类。如果ExtClassLoader没有加载此类，会再询问BootstrapClassLoader是否已加载此类。这样一层层往上询问。

如果连BootstrapClassLoader都没有加载过此类，那么BootstrapClassLoader会检查它是否可以加载此类，如果可以，它会记载这个类。如果不能，那么它会往下询问ExtClassLoader是否可以加载，如果能则加载，不能则继续往下询问AppClassLoader。

双亲委派机制

2. 为什么采用双亲委派机制

这种机制能保证系统级别的类不会被用户覆盖，防止危险代码的植入。