自定义类加载器

流程

1. 继承ClassLoader类；
2. 检查请求的类型是否已经被这个类加载器装载到命名空间了，如果已经装载直接返回；
3. 委派类加载请求给父类加载器，如果父类加载器能够完成，则返回父类加载器加载的Class实例；
4. 调用本类加载器的findClass()方法，获取对象字节码，获取到调用defineClass()导入类型到方法区；获取不到或其他原因导致失败，返回异常给loadClass()，再转抛异常，终止加载。
   说明：两个类加载器加载相同的类，JVM认为是不同的类。

代码实现
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.InputStream;

/**

• 自定义类加载器

• /
  public class FileSystemClassLoader extends ClassLoader{
  private String root; // 类路径

  public FileSystemClassLoader(String root){

    this.root = root;

  }

  /**

  • Finds the class with the specified binary name.

  • This method should be overridden by class loader implementations that

  • follow the delegation model for loading classes, and will be invoked by

  • the {@link #loadClass <tt>loadClass</tt>} method after checking the

  • parent class loader for the requested class. The default implementation

  • throws a <tt>ClassNotFoundException</tt>.

  • @param name The binary name of the class

  • @return The resulting <tt>Class</tt> object

  • @throws ClassNotFoundException If the class could not be found

  • @since 1.2

  • /
    @Override
    protected Class findClass(String name) throws ClassNotFoundException { Class c = findLoadedClass(name); // 查找对应name的Class

    if (c != null){ // 找到直接返回

      return c;

    } else{

      ClassLoader parent = this.getParent();
      try {
          c = parent.loadClass(name); // 委派给父类
      } catch (ClassNotFoundException e) {
          e.printStackTrace();
      }
    
      if (c != null){
          return c;
      } else{
          byte[] classData = getClassData(name); // 返回字节数据
          if (classData != null){
              c = defineClass(name,classData,0,classData.length);
          } else{
              throw new ClassNotFoundException();
          }
      }

    }
    // return super.findClass(name);
    return c;

    }

    private byte[] getClassData(String classname){
    String path = root+"/"+classname.replace('.','/')+".class";

    // 将流中的数据转成字节数组
    try(InputStream is = new FileInputStream(path); ByteArrayOutputStream baos = new ByteArrayOutputStream();) {

      byte[] buffer = new byte[1024];
      int temp = 0;
      while((temp=is.read(buffer))!=-1){ // 读取数据并写入
          baos.write(buffer,0,temp);
      }
      return baos.toByteArray();

    } catch (Exception e){

      e.printStackTrace();
      return null;

    }
    }
    }