package com.atlassian.plugin.util;
   
   import com.google.common.base.Predicates;
   import com.google.common.collect.ImmutableList;
   import com.google.common.collect.Iterables;
   import org.slf4j.Logger;
   import org.slf4j.LoggerFactory;
   
   import java.io.IOException;
  import java.io.InputStream;
  import java.net.URL;
  import java.util.Collections;
  import java.util.Enumeration;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.NoSuchElementException;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  import static com.google.common.base.Preconditions.checkState;
  
  /**
   * A class loader that delegates to a list of class loaders. The order is important as classes and resources will be
   * loaded from the first classloader that can load them.  This class is optimized for a small number of classloaders.
   *
   * @since 2.6.0
   */
  public class ChainingClassLoader extends ClassLoader {
      private static final Logger log = LoggerFactory.getLogger(ChainingClassLoader.class);
  
      /**
       * The list of classloader to delegate to.
       */
      private final List<ClassLoader> classLoaders;
  
      /**
       * Map of which resources are overridden by other resources
       */
      private final Map<String, String> resourceRedirects;
  
      /**
       * Constructs a chaining classloader
       *
       * @param classLoaders The classloaders to delegate to, in order
       */
      public ChainingClassLoader(ClassLoader... classLoaders) {
          this(Collections.<String, String>emptyMap(), classLoaders);
      }
  
      /**
       * Constructs a classloader that overrides certain resources
       *
       * @param resourceRedirects The map of resources to redirect
       * @param classLoaders      The classloaders to delegate to, in order
       */
      public ChainingClassLoader(Map<String, String> resourceRedirects, ClassLoader... classLoaders) {
          super(null);
          this.resourceRedirects = checkNotNull(resourceRedirects);
          this.classLoaders = ImmutableList.copyOf(classLoaders);
          checkState(!Iterables.any(this.classLoaders, Predicates.isNull()), "ClassLoader arguments cannot be null");
      }
  
      @Override
      public Class loadClass(String name) throws ClassNotFoundException {
          for (ClassLoader classloader : classLoaders) {
              try {
                  return classloader.loadClass(name);
              } catch (ClassNotFoundException e) {
                  // ignoring until we reach the end of the list since we are chaining
              }
          }
          throw new ClassNotFoundException(name);
      }
  
      @Override
      public Enumeration<URL> getResources(String name) throws IOException {
          return new ResourcesEnumeration(getAlternativeResourceName(name), classLoaders);
      }
  
      @Override
      public URL getResource(String name) {
          final String realResourceName = getAlternativeResourceName(name);
          for (ClassLoader classloader : classLoaders) {
              final URL url = classloader.getResource(realResourceName);
              if (url != null) {
                  return url;
              }
          }
          return null;
      }
  
      @Override
      public InputStream getResourceAsStream(String name) {
          final String realResourceName = getAlternativeResourceName(name);
          for (ClassLoader classloader : classLoaders) {
              final InputStream inputStream = classloader.getResourceAsStream(realResourceName);
              if (inputStream != null) {
                  return inputStream;
              }
         }
 
         if (!name.equals(realResourceName)) {
             //looks like we didn't find anything with the alternate resourcename.  Lets fall back to the
             //original resource name!
             log.debug("No resource found with alternate resourceName '{}'. Falling back to original name '{}'.", realResourceName, name);
             for (ClassLoader classloader : classLoaders) {
                 final InputStream inputStream = classloader.getResourceAsStream(name);
                 if (inputStream != null) {
                     return inputStream;
                 }
             }
         }
         return null;
     }
 
     private String getAlternativeResourceName(String name) {
         String resultName = name;
         if (resourceRedirects.containsKey(name)) {
             String redirectedName = resourceRedirects.get(name);
             log.debug("Redirecting resource '{}' to '{}'", name, redirectedName);
             resultName = redirectedName;
         }
         return resultName;
     }
 
     @Override
     public synchronized void setDefaultAssertionStatus(boolean enabled) {
         for (ClassLoader classloader : classLoaders) {
             classloader.setDefaultAssertionStatus(enabled);
         }
     }
 
     @Override
     public synchronized void setPackageAssertionStatus(String packageName, boolean enabled) {
         for (ClassLoader classloader : classLoaders) {
             classloader.setPackageAssertionStatus(packageName, enabled);
         }
     }
 
     @Override
     public synchronized void setClassAssertionStatus(String className, boolean enabled) {
         for (ClassLoader classloader : classLoaders) {
             classloader.setClassAssertionStatus(className, enabled);
         }
     }
 
     @Override
     public synchronized void clearAssertionStatus() {
         for (ClassLoader classloader : classLoaders) {
             classloader.clearAssertionStatus();
         }
     }
 
     private static final class ResourcesEnumeration implements Enumeration<URL> {
         private final List<Enumeration<URL>> resources;
         private final String resourceName;
 
         ResourcesEnumeration(String resourceName, List<ClassLoader> classLoaders) throws IOException {
             this.resourceName = resourceName;
             this.resources = new LinkedList<Enumeration<URL>>();
             for (ClassLoader classLoader : classLoaders) {
                 resources.add(classLoader.getResources(resourceName));
             }
         }
 
         public boolean hasMoreElements() {
             for (Enumeration<URL> resource : resources) {
                 if (resource.hasMoreElements()) {
                     return true;
                 }
             }
 
             return false;
         }
 
         public URL nextElement() {
             for (Enumeration<URL> resource : resources) {
                 if (resource.hasMoreElements()) {
                     return resource.nextElement();
                 }
             }
             throw new NoSuchElementException(resourceName);
         }
     }
 }