Java 类加载机制详细介绍

The ClassLoader class uses a delegation model to search for classes and resources. Each instance
 of ClassLoader has an associated parent class loader. When requested to find a class or resource,
 a ClassLoader instance will delegate the search for the class or resource to its parent class loader
 before attempting to find the class or resource itself. The virtual machine's built-in class loader,
 called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a
 ClassLoader instance.

public class Test {
}
 
public class TestMain {
  public static void main(String[] args) {
 
    ClassLoader loader = Test.class.getClassLoader();
    while (loader!=null){
      System.out.println(loader);
      loader = loader.getParent();
    }
  }
}

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
  {
    synchronized (getClassLoadingLock(name)) {
      // First, check if the class has already been loaded
      Class<?> c = findLoadedClass(name);
      if (c == null) {
        long t0 = System.nanoTime();
        try {
          if (parent != null) {
            c = parent.loadClass(name, false);
          } else {
            c = findBootstrapClassOrNull(name);
          }
        } catch (ClassNotFoundException e) {
          // ClassNotFoundException thrown if class not found
          // from the non-null parent class loader
        }
 
        if (c == null) {
          // If still not found, then invoke findClass in order
          // to find the class.
          long t1 = System.nanoTime();
          c = findClass(name);
 
          // this is the defining class loader; record the stats
          sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
          sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
          sun.misc.PerfCounter.getFindClasses().increment();
        }
      }
      if (resolve) {
        resolveClass(c);
      }
      return c;
    }
  }

package com.paddx.test.classloading;
 
import java.io.*;
 
/**
 * Created by liuxp on 16/3/12.
 */
public class MyClassLoader extends ClassLoader {
 
  private String root;
 
  protected Class<?> findClass(String name) throws ClassNotFoundException {
    byte[] classData = loadClassData(name);
    if (classData == null) {
      throw new ClassNotFoundException();
    } else {
      return defineClass(name, classData, 0, classData.length);
    }
  }
 
  private byte[] loadClassData(String className) {
    String fileName = root + File.separatorChar
        + className.replace('.', File.separatorChar) + ".class";
    try {
      InputStream ins = new FileInputStream(fileName);
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      int bufferSize = 1024;
      byte[] buffer = new byte[bufferSize];
      int length = 0;
      while ((length = ins.read(buffer)) != -1) {
        baos.write(buffer, 0, length);
      }
      return baos.toByteArray();
    } catch (IOException e) {
      e.printStackTrace();
    }
    return null;
  }
 
  public String getRoot() {
    return root;
  }
 
  public void setRoot(String root) {
    this.root = root;
  }
 
  public static void main(String[] args) {
 
    MyClassLoader classLoader = new MyClassLoader();
    classLoader.setRoot("/Users/liuxp/tmp");
 
    Class<?> testClass = null;
    try {
      testClass = classLoader.loadClass("com.paddx.test.classloading.Test");
      Object object = testClass.newInstance();
      System.out.println(object.getClass().getClassLoader());
    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    } catch (InstantiationException e) {
      e.printStackTrace();
    } catch (IllegalAccessException e) {
      e.printStackTrace();
    }
  }
}