Java类加载器层次结构原理解析

public class Demo02 {
  public static void main(String[] args) {
    System.out.println(ClassLoader.getSystemClassLoader());
    System.out.println(ClassLoader.getSystemClassLoader().getParent());;
    System.out.println(ClassLoader.getSystemClassLoader().getParent().getParent());;
  }
}

package com.test;

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * 自定义文件系统加载器
 * @author We.lxk
 *
 */
public class FileSystemClassLoader extends ClassLoader{
  private String rootDir;
  
  public FileSystemClassLoader(String rootDir) {
    this.rootDir = rootDir;
  }
  
  private byte[] getClassData(String classname){    //com.test.User -> rootDir/com/test/User
    String path = rootDir +"/"+classname.replace(".", "/")+".class";
    //IOUtils 可以使用它将读取的流数据转换为字节数组
    InputStream is = null;
    ByteArrayOutputStream baos = new ByteArrayOutputStream(); 
    try{
      is = new FileInputStream(path);
      
      byte[] buffer = new byte[1024];
      int temp = 0;
      while((temp=is.read(buffer))!=-1){
        baos.write(buffer, 0, temp);
      }
      return baos.toByteArray();
    }catch(Exception e){
      e.printStackTrace();
      return null;
    }finally{
        try {
          if(is!=null)
          is.close();
        } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
        try {
          if(baos!=null)
          baos.close();
        } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
    }
  }
  
  @Override
  public Class<?> loadClass(String name) throws ClassNotFoundException {
    Class<?> c = findLoadedClass(name);
    
    //应该先查询有没有加载过这个类。已经加载，则直接返回加载好的类。
    if(c!=null){
      return c;
    }else{
      ClassLoader parent = this.getParent();
      try{
        //System.out.println("hello");
        c = parent.loadClass(name);          //委派给父类加载
      }catch(Exception e){
        //e.printStackTrace();
      }
      if(c!=null){
        return c;
      }else{
        byte[] classData = getClassData(name);
        if(classData==null){
          throw new ClassNotFoundException();
        }else{
          c = defineClass(name, classData, 0, classData.length);
        }
      }
    }
    return c;
  }
}

package com.test;

/**
 *  测试自定义的FileSystemClassLoader 
 * @author We.lxk
 *
 */
public class Demo03 {
  public static void main(String[] args) throws Exception {
    FileSystemClassLoader loader = new FileSystemClassLoader("D:/myJava");
    FileSystemClassLoader loader2 = new FileSystemClassLoader("D:/myJava");
    
    Class<?> c = loader.loadClass("com.test.Demos");
    Class<?> c2 = loader.loadClass("com.test.Demos");
    Class<?> c3 = loader2.loadClass("com.test.Demos");
    
    Class<?> c4 = loader2.loadClass("java.lang.String");
    Class<?> c5 = loader.loadClass("com.test.Demo");
    
    
    System.out.println(c.hashCode()+" "+c.getClassLoader());
    System.out.println(c2.hashCode()+" "+c2.getClassLoader());
    System.out.println(c3.hashCode()+" "+c3.getClassLoader());
    System.out.println(c4.hashCode()+" "+c4.getClassLoader());
    System.out.println(c5.hashCode()+" "+c5.getClassLoader());
    //System.out.println(.getClassLoader());
  }
}