深入理解java虚拟机（五）

1.各个文件里存放有不同编译好的class

2.

import java.io.ByteArrayOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.InputStream;

public class MyClassLoader extends ClassLoader {
     private String name ;//类加载器名字
     private String path="d:\\";//加载类的路径
     
     public String getPath() {
return path;
}


public void setPath(String path) {
this.path = path;
}


private final String fileType=".class";//class文件的扩展名
     
     public MyClassLoader(String name){
     super();//让系统类加载器成为该类加载器的父加载器
     this.name=name;
     }
     
     public MyClassLoader(ClassLoader parent ,String name){
     super(parent);//显示的指定该类加载器的父加载器
     this.name=name;
     }


@Override
public String toString() {
return "MyClassLoader [name=" + name + "]";
}

@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
byte[] data=this.loadClassData(name);
return this.defineClass(name, data, 0,data.length);

}
     //通过类的名字从硬盘上找到相应文件，得到class的二进制的数组
private byte[] loadClassData(String name){
//传进name是带.的形式com.ygl-->要转换为com\ygl
InputStream in=null;
byte[] data=null;
ByteArrayOutputStream  byteArrayOutputStream=null;
try {
this.name=this.name.replace(".", "\\");
in=new FileInputStream(new File(path+name+fileType));
byteArrayOutputStream=new ByteArrayOutputStream();

int ch=0;
while((ch=in.read())!=-1){
byteArrayOutputStream.write(ch);
}

data=byteArrayOutputStream.toByteArray();

} catch (Exception e) {
e.printStackTrace();
}finally{
try {
in.close();
byteArrayOutputStream.close();
} catch (Exception e2) {
e2.printStackTrace();
}
}
return data;
}
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
MyClassLoader  loader1=new MyClassLoader("loader1");//父类加载器是系统类加载器
loader1.setPath("d:\\myapp\\serverlib\\");
 
MyClassLoader loader2=new MyClassLoader(loader1,"loader2");//父类加载器是loader1
loader2.setPath("d:\\myapp\\clientlib\\");
 
MyClassLoader loader3=new MyClassLoader(null,"loader3");//父类加载器是根类加载器
loader3.setPath("d:\\myapp\\otherlib\\");
 
test(loader2); //loader2有父类加载器loader1，而loader1的父类是系统类加载器，拓展类加载器以及根类加载器是不加载自定义的class的，
//但系统类加载器是在当前目录(.)下查找，也即在D:\myapp\syslib下查找，并没有Simple.class,从而只有loader1在"d:\\myapp\\serverlib\\"下查找
test(loader3);
}
public static void test(ClassLoader loader) throws ClassNotFoundException, InstantiationException, IllegalAccessException{
Class clazz=loader.loadClass("Simple");//loadClass会调用findClass
Object  object= clazz.newInstance();
 
}
 }      

//***************************

//*************************
验证父子加载器层级
public class Test9 {
 public static void main(String[] args) {
ClassLoader classLoader =ClassLoader.getSystemClassLoader();
System.out.println(classLoader);
while(null!=classLoader){
classLoader=classLoader.getParent();
System.out.println(classLoader);
}
 }
 }//*****************
sun.misc.Launcher$AppClassLoader@19821f
 sun.misc.Launcher$ExtClassLoader@addbf1
 null//************************