package org.apache.ibatis.io;

import java.io.InputStream;
import java.net.URL;

/**
 * A class to wrap access to multiple class loaders making them work as one
 */
public class ClassLoaderWrapper {

  ClassLoader defaultClassLoader;
  ClassLoader systemClassLoader;

  ClassLoaderWrapper() {
    try {
      systemClassLoader = ClassLoader.getSystemClassLoader();
    } catch (SecurityException ignored) {
      // AccessControlException on Google App Engine   
    }
  }
  
  /**
   * Get a resource as a URL using the current class path
   *
   * @param resource - the resource to locate
   * @return the resource or null
   */
  public URL getResourceAsURL(String resource) {
    return getResourceAsURL(resource, getClassLoaders(null));
  }

  /**
   * Get a resource from the classpath, starting with a specific class loader
   *
   * @param resource    - the resource to find
   * @param classLoader - the first classloader to try
   * @return the stream or null
   */
  public URL getResourceAsURL(String resource, ClassLoader classLoader) {
    return getResourceAsURL(resource, getClassLoaders(classLoader));
  }

  /**
   * Get a resource from the classpath
   *
   * @param resource - the resource to find
   * @return the stream or null
   */
  public InputStream getResourceAsStream(String resource) {
    return getResourceAsStream(resource, getClassLoaders(null));
  }

  /**
   * Get a resource from the classpath, starting with a specific class loader
   *
   * @param resource    - the resource to find
   * @param classLoader - the first class loader to try
   * @return the stream or null
   */
  public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
    return getResourceAsStream(resource, getClassLoaders(classLoader));
  }

  /**
   * Find a class on the classpath (or die trying)
   *
   * @param name - the class to look for
   * @return - the class
   * @throws ClassNotFoundException Duh.
   */
  public Class<?> classForName(String name) throws ClassNotFoundException {
    return classForName(name, getClassLoaders(null));
  }

  /**
   * Find a class on the classpath, starting with a specific classloader (or die trying)
   *
   * @param name        - the class to look for
   * @param classLoader - the first classloader to try
   * @return - the class
   * @throws ClassNotFoundException Duh.
   */
  public Class<?> classForName(String name, ClassLoader classLoader) throws ClassNotFoundException {
    return classForName(name, getClassLoaders(classLoader));
  }

  /**
   * Try to get a resource from a group of classloaders
   *
   * @param resource    - the resource to get
   * @param classLoader - the classloaders to examine
   * @return the resource or null
   */
  InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
    for (ClassLoader cl : classLoader) {
      if (null != cl) {

        // try to find the resource as passed
        InputStream returnValue = cl.getResourceAsStream(resource);

        // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
        if (null == returnValue) returnValue = cl.getResourceAsStream("/" + resource);

        if (null != returnValue) return returnValue;
      }
    }
    return null;
  }

  /**
   * Get a resource as a URL using the current class path
   *
   * @param resource    - the resource to locate
   * @param classLoader - the class loaders to examine
   * @return the resource or null
   */
  URL getResourceAsURL(String resource, ClassLoader[] classLoader) {

    URL url;

    for (ClassLoader cl : classLoader) {

      if (null != cl) {

        // look for the resource as passed in...
        url = cl.getResource(resource);

        // ...but some class loaders want this leading "/", so we'll add it
        // and try again if we didn't find the resource
        if (null == url) url = cl.getResource("/" + resource);

        // "It's always in the last place I look for it!"
        // ... because only an idiot would keep looking for it after finding it, so stop looking already.
        if (null != url) return url;

      }

    }

    // didn't find it anywhere.
    return null;

  }

  /**
   * Attempt to load a class from a group of classloaders
   *
   * @param name        - the class to load
   * @param classLoader - the group of classloaders to examine
   * @return the class
   * @throws ClassNotFoundException - Remember the wisdom of Judge Smails: Well, the world needs ditch diggers, too.
   */
  Class<?> classForName(String name, ClassLoader[] classLoader) throws ClassNotFoundException {

    for (ClassLoader cl : classLoader) {

      if (null != cl) {

        try {

          Class<?> c = Class.forName(name, true, cl);

          if (null != c) return c;

        } catch (ClassNotFoundException e) {
          // we'll ignore this until all classloaders fail to locate the class
        }

      }

    }

    throw new ClassNotFoundException("Cannot find class: " + name);

  }

  ClassLoader[] getClassLoaders(ClassLoader classLoader) {
    return new ClassLoader[]{
        classLoader, 
        defaultClassLoader, 
        Thread.currentThread().getContextClassLoader(), 
        getClass().getClassLoader(), 
        systemClassLoader}; 
  }

}
