深入了解java虛拟機（五）

1.各個檔案裡存放有不同編譯好的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 ;//類加載器名字
     private String path="d:\\";//加載類的路徑
     
     public String getPath() {
return path;
}


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


private final String fileType=".class";//class檔案的擴充名
     
     public MyClassLoader(String name){
     super();//讓系統類加載器成為該類加載器的父加載器
     this.name=name;
     }
     
     public MyClassLoader(ClassLoader parent ,String name){
     super(parent);//顯示的指定該類加載器的父加載器
     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);

}
     //通過類的名字從硬碟上找到相應檔案，得到class的二進制的數組
private byte[] loadClassData(String name){
//傳進name是帶.的形式com.ygl-->要轉換為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");//父類加載器是系統類加載器
loader1.setPath("d:\\myapp\\serverlib\\");
 
MyClassLoader loader2=new MyClassLoader(loader1,"loader2");//父類加載器是loader1
loader2.setPath("d:\\myapp\\clientlib\\");
 
MyClassLoader loader3=new MyClassLoader(null,"loader3");//父類加載器是根類加載器
loader3.setPath("d:\\myapp\\otherlib\\");
 
test(loader2); //loader2有父類加載器loader1，而loader1的父類是系統類加載器，拓展類加載器以及根類加載器是不加載自定義的class的，
//但系統類加載器是在目前目錄(.)下查找，也即在D:\myapp\syslib下查找，并沒有Simple.class,進而隻有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會調用findClass
Object  object= clazz.newInstance();
 
}
 }      

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

//*************************
驗證父子加載器層級
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//************************