implemented a child first plugin classloader strategy

scm-core/src/main/java/sonia/scm/net/ChildFirstURLClassLoader.java

@@ -0,0 +1,187 @@
+/**
+ * Copyright (c) 2010, Sebastian Sdorra
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ * this list of conditions and the following disclaimer. 2. Redistributions in
+ * binary form must reproduce the above copyright notice, this list of
+ * conditions and the following disclaimer in the documentation and/or other
+ * materials provided with the distribution. 3. Neither the name of SCM-Manager;
+ * nor the names of its contributors may be used to endorse or promote products
+ * derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * http://bitbucket.org/sdorra/scm-manager
+ *
+ */
+
+
+
+package sonia.scm.net;
+
+//~--- JDK imports ------------------------------------------------------------
+
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.net.URLStreamHandlerFactory;
+
+/**
+ * The ChildFirstURLClassLoader alters regular ClassLoader delegation and will
+ * check the URLs used in its initialization for matching classes before
+ * delegating to it's parent. Sometimes also referred to as a
+ * ParentLastClassLoader.
+ *
+ * @author Sebastian Sdorra
+ * @since 1.29
+ */
+public class ChildFirstURLClassLoader extends URLClassLoader
+{
+
+  /**
+   * Constructs a new ChildFirstURLClassLoader for the specified URLs using the
+   * default delegation parent ClassLoader.
+   *
+   * @param urls the URLs from which to load classes and resources
+   */
+  public ChildFirstURLClassLoader(URL[] urls)
+  {
+    super(urls);
+  }
+
+  /**
+   * Constructs a new ChildFirstURLClassLoader for the specified URLs using the
+   * given parent ClassLoader for delegation.
+   *
+   * @param urls the URLs from which to load classes and resources
+   * @param parent the parent class loader for delegation
+   */
+  public ChildFirstURLClassLoader(URL[] urls, ClassLoader parent)
+  {
+    super(urls, parent);
+  }
+
+  /**
+   * Constructs a new URLClassLoader for the specified URLs, parent class
+   * loader, and URLStreamHandlerFactory. The parent argument will be used as
+   * the parent class loader for delegation. The factory argument will be used
+   * as the stream handler factory to obtain protocol handlers when creating
+   * new URLs.
+   *
+   * @param urls the URLs from which to load classes and resources
+   * @param parent the parent class loader for delegation
+   * @param factory the URLStreamHandlerFactory to use when creating URLs
+   */
+  public ChildFirstURLClassLoader(URL[] urls, ClassLoader parent,
+    URLStreamHandlerFactory factory)
+  {
+    super(urls, parent, factory);
+  }
+
+  //~--- methods --------------------------------------------------------------
+
+  /**
+   * Loads the class with the specified name. The default implementation of this
+   * method searches for classes in the following order:
+   * <br />
+   * <br />
+   * <ol>
+   *   <li>
+   *     Invoke the {@link ClassLoader#findClass(String)} method to find the
+   *     class.
+   *   </li>
+   *   <li>
+   *     Invoke {@link ClassLoader#findLoadedClass(String)} to check if
+   *     the class has already been loaded.
+   *   </li>
+   *   <li>
+   *     Invoke the {@link ClassLoader#loadClass(String)} method on the parent
+   *     class loader. If the parent is null the class loader built-in to the
+   *     virtual machine is used, instead.
+   *   </li>
+   * </ol>
+   *
+   * If the class was found using the above steps, and the {@code resolve} flag
+   * is true, this method will then invoke the {@link resolveClass(Class)}
+   * method on the resulting Class object. Subclasses of ClassLoader are
+   * encouraged to override {@link ClassLoader#findClass(String)}, rather than
+   * this method.
+   *
+   * @param name the name of the class
+   * @param resolve if true then resolve the class
+   *
+   * @return the resulting {@code Class} object
+   *
+   * @throws ClassNotFoundException if the class could not be found
+   */
+  @Override
+  @SuppressWarnings("unchecked")
+  public synchronized Class loadClass(String name, boolean resolve)
+    throws ClassNotFoundException
+  {
+    Class clazz = findLoadedClass(name);
+
+    if (clazz == null)
+    {
+      try
+      {
+        clazz = findClass(name);
+      }
+      catch (ClassNotFoundException e)
+      {
+        ClassLoader parent = getParent();
+
+        if (parent != null)
+        {
+          clazz = parent.loadClass(name);
+        }
+        else
+        {
+          clazz = getSystemClassLoader().loadClass(name);
+        }
+      }
+    }
+
+    if (resolve)
+    {
+      resolveClass(clazz);
+    }
+
+    return clazz;
+  }
+
+  //~--- get methods ----------------------------------------------------------
+
+  /**
+   * {@inheritDoc }
+   */
+  @Override
+  public URL getResource(String name)
+  {
+    URL resource = findResource(name);
+
+    if (resource == null)
+    {
+      ClassLoader parent = getParent();
+
+      if (parent != null)
+      {
+        resource = parent.getResource(name);
+      }
+    }
+
+    return resource;
+  }
+}

scm-webapp/src/main/java/sonia/scm/boot/BootstrapListener.java

@@ -47,7 +47,6 @@ import java.io.File;
 
 import java.net.MalformedURLException;
 import java.net.URL;
-import java.net.URLClassLoader;
 
 import java.util.LinkedList;
 import java.util.List;
@@ -224,7 +223,7 @@ public class BootstrapListener implements ServletContextListener
       }
     }
 
-    return new URLClassLoader(classpathURLs.toArray(new URL[0]),
+    return BootstrapUtil.createClassLoader(classpathURLs,
       getParentClassLoader());
   }
 

scm-webapp/src/main/java/sonia/scm/boot/BootstrapUtil.java

@@ -35,11 +35,20 @@ package sonia.scm.boot;
 
 //~--- non-JDK imports --------------------------------------------------------
 
+import com.google.common.base.Strings;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import sonia.scm.net.ChildFirstURLClassLoader;
+
 //~--- JDK imports ------------------------------------------------------------
 
+import java.net.URL;
+import java.net.URLClassLoader;
+
+import java.util.List;
+
 import javax.servlet.ServletContext;
 
 /**
@@ -52,6 +61,16 @@ public final class BootstrapUtil
   /** Field description */
   public static final String CLASSLOADER = "sonia.scm.BoostrapClassLoader";
 
+  /** Field description */
+  private static final String STRATEGY =
+    "sonia.scm.plugin.classloader.strategy";
+
+  /** Field description */
+  private static final String STRATEGY_CHILDFIRST = "child-first";
+
+  /** Field description */
+  private static final String STRATEGY_PARENTFIRST = "parent-first";
+
   /** the logger for BootstrapUtil */
   private static final Logger logger =
     LoggerFactory.getLogger(BootstrapUtil.class);
@@ -66,6 +85,46 @@ public final class BootstrapUtil
 
   //~--- methods --------------------------------------------------------------
 
+  /**
+   * Method description
+   *
+   *
+   * @param classpathURLs
+   * @param parent
+   *
+   * @return
+   */
+  public static ClassLoader createClassLoader(List<URL> classpathURLs,
+    ClassLoader parent)
+  {
+    ClassLoader classLoader = null;
+    URL[] urls = classpathURLs.toArray(new URL[classpathURLs.size()]);
+    String strategy = System.getProperty(STRATEGY);
+
+    if (!Strings.isNullOrEmpty(strategy))
+    {
+      if (STRATEGY_CHILDFIRST.equals(strategy))
+      {
+        logger.info("using {} as plugin classloading strategy",
+          STRATEGY_CHILDFIRST);
+        classLoader = new ChildFirstURLClassLoader(urls, parent);
+      }
+      else if (!STRATEGY_PARENTFIRST.equals(strategy))
+      {
+        logger.warn("unknown plugin classloading strategy {}", strategy);
+      }
+    }
+
+    if (classLoader == null)
+    {
+      logger.info("using {} as plugin classloading strategy",
+        STRATEGY_PARENTFIRST);
+      classLoader = new URLClassLoader(urls, parent);
+    }
+
+    return classLoader;
+  }
+
   /**
    * Method description
    *