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    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  package org.apache.maven.exception;
  
  import java.io.IOException;
  import java.net.ConnectException;
  import java.net.UnknownHostException;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.IdentityHashMap;
  import java.util.List;
  import java.util.Set;
  
  import org.apache.maven.lifecycle.LifecycleExecutionException;
  import org.apache.maven.model.building.ModelProblem;
  import org.apache.maven.model.building.ModelProblemUtils;
  import org.apache.maven.plugin.AbstractMojoExecutionException;
  import org.apache.maven.plugin.MojoExecutionException;
  import org.apache.maven.plugin.MojoFailureException;
  import org.apache.maven.plugin.PluginContainerException;
  import org.apache.maven.plugin.PluginExecutionException;
  import org.apache.maven.project.ProjectBuildingException;
  import org.apache.maven.project.ProjectBuildingResult;
  import org.codehaus.plexus.component.annotations.Component;
  import org.codehaus.plexus.util.StringUtils;
  
  /*
  
  - test projects for each of these
  - how to categorize the problems so that the id of the problem can be match to a page with descriptive help and the test
    project
  - nice little sample projects that could be run in the core as well as integration tests
  
  All Possible Errors
  - invalid lifecycle phase (maybe same as bad CLI param, though you were talking about embedder too)
  - <module> specified is not found
  - malformed settings
  - malformed POM
  - local repository not writable
  - remote repositories not available
  - artifact metadata missing
  - extension metadata missing
  - extension artifact missing
  - artifact metadata retrieval problem
  - version range violation
  - circular dependency
  - artifact missing
  - artifact retrieval exception
  - md5 checksum doesn't match for local artifact, need to redownload this
  - POM doesn't exist for a goal that requires one
  - parent POM missing (in both the repository + relative path)
  - component not found
  
  Plugins:
  - plugin metadata missing
  - plugin metadata retrieval problem
  - plugin artifact missing
  - plugin artifact retrieval problem
  - plugin dependency metadata missing
  - plugin dependency metadata retrieval problem
  - plugin configuration problem
  - plugin execution failure due to something that is know to possibly go wrong (like compilation failure)
  - plugin execution error due to something that is not expected to go wrong (the compiler executable missing)
  - asking to use a plugin for which you do not have a version defined - tools to easily select versions
  - goal not found in a plugin (probably could list the ones that are)
  
   */
  
  // PluginNotFoundException, PluginResolutionException, PluginDescriptorParsingException,
  // CycleDetectedInPluginGraphException;
  
  /**
   * Transform an exception into useful end-user message.
   */
  @Component(role = ExceptionHandler.class)
  public class DefaultExceptionHandler implements ExceptionHandler {
      @Override
      public ExceptionSummary handleException(Throwable exception) {
          return handle("", exception);
      }
  
      private ExceptionSummary handle(String message, Throwable exception) {
          String reference = getReference(Collections.newSetFromMap(new IdentityHashMap<>()), exception);
 
         List<ExceptionSummary> children = null;
 
         if (exception instanceof ProjectBuildingException) {
             List<ProjectBuildingResult> results = ((ProjectBuildingException) exception).getResults();
 
             children = new ArrayList<>();
 
             for (ProjectBuildingResult result : results) {
                 ExceptionSummary child = handle(result);
                 if (child != null) {
                     children.add(child);
                 }
             }
 
             message = "The build could not read " + children.size() + " project" + (children.size() == 1 ? "" : "s");
         } else {
             message = getMessage(message, exception);
         }
 
         return new ExceptionSummary(exception, message, reference, children);
     }
 
     private ExceptionSummary handle(ProjectBuildingResult result) {
         List<ExceptionSummary> children = new ArrayList<>();
 
         for (ModelProblem problem : result.getProblems()) {
             ExceptionSummary child = handle(problem, result.getProjectId());
             if (child != null) {
                 children.add(child);
             }
         }
 
         if (children.isEmpty()) {
             return null;
         }
 
         String message = "\nThe project " + result.getProjectId() + " (" + result.getPomFile() + ") has "
                 + children.size() + " error" + (children.size() == 1 ? "" : "s");
 
         return new ExceptionSummary(null, message, null, children);
     }
 
     private ExceptionSummary handle(ModelProblem problem, String projectId) {
         if (ModelProblem.Severity.ERROR.compareTo(problem.getSeverity()) >= 0) {
             String message = problem.getMessage();
 
             String location = ModelProblemUtils.formatLocation(problem, projectId);
 
             if (StringUtils.isNotEmpty(location)) {
                 message += " @ " + location;
             }
 
             return handle(message, problem.getException());
         } else {
             return null;
         }
     }
 
     private String getReference(Set<Throwable> dejaVu, Throwable exception) {
         String reference = "";
         if (!dejaVu.add(exception)) {
             return reference;
         }
 
         if (exception != null) {
             if (exception instanceof MojoExecutionException) {
                 reference = MojoExecutionException.class.getSimpleName();
 
                 Throwable cause = exception.getCause();
                 if (cause instanceof IOException) {
                     cause = cause.getCause();
                     if (cause instanceof ConnectException) {
                         reference = ConnectException.class.getSimpleName();
                     }
                 }
             } else if (exception instanceof MojoFailureException) {
                 reference = MojoFailureException.class.getSimpleName();
             } else if (exception instanceof LinkageError) {
                 reference = LinkageError.class.getSimpleName();
             } else if (exception instanceof PluginExecutionException) {
                 Throwable cause = exception.getCause();
 
                 if (cause instanceof PluginContainerException) {
                     Throwable cause2 = cause.getCause();
 
                     if (cause2 instanceof NoClassDefFoundError) {
                         String message = cause2.getMessage();
                         if (message != null && message.contains("org/sonatype/aether/")) {
                             reference = "AetherClassNotFound";
                         }
                     }
                 }
 
                 if (StringUtils.isEmpty(reference)) {
                     reference = getReference(dejaVu, cause);
                 }
 
                 if (StringUtils.isEmpty(reference)) {
                     reference = exception.getClass().getSimpleName();
                 }
             } else if (exception instanceof LifecycleExecutionException) {
                 reference = getReference(dejaVu, exception.getCause());
             } else if (isNoteworthyException(exception)) {
                 reference = exception.getClass().getSimpleName();
             }
         }
 
         if (StringUtils.isNotEmpty(reference) && !reference.startsWith("http:")) {
             reference = "http://cwiki.apache.org/confluence/display/MAVEN/" + reference;
         }
 
         return reference;
     }
 
     private boolean isNoteworthyException(Throwable exception) {
         if (exception == null) {
             return false;
         } else if (exception instanceof Error) {
             return true;
         } else if (exception instanceof RuntimeException) {
             return false;
         } else if (exception.getClass().getName().startsWith("java")) {
             return false;
         }
         return true;
     }
 
     private String getMessage(String message, Throwable exception) {
         String fullMessage = (message != null) ? message : "";
 
         // To break out of possible endless loop when getCause returns "this", or dejaVu for n-level recursion (n>1)
         Set<Throwable> dejaVu = Collections.newSetFromMap(new IdentityHashMap<>());
         for (Throwable t = exception; t != null && t != t.getCause(); t = t.getCause()) {
             String exceptionMessage = t.getMessage();
 
             if (t instanceof AbstractMojoExecutionException) {
                 String longMessage = ((AbstractMojoExecutionException) t).getLongMessage();
                 if (StringUtils.isNotEmpty(longMessage)) {
                     if (StringUtils.isEmpty(exceptionMessage) || longMessage.contains(exceptionMessage)) {
                         exceptionMessage = longMessage;
                     } else if (!exceptionMessage.contains(longMessage)) {
                         exceptionMessage = join(exceptionMessage, '\n' + longMessage);
                     }
                 }
             }
 
             if (StringUtils.isEmpty(exceptionMessage)) {
                 exceptionMessage = t.getClass().getSimpleName();
             }
 
             if (t instanceof UnknownHostException && !fullMessage.contains("host")) {
                 fullMessage = join(fullMessage, "Unknown host " + exceptionMessage);
             } else if (!fullMessage.contains(exceptionMessage)) {
                 fullMessage = join(fullMessage, exceptionMessage);
             }
 
             if (!dejaVu.add(t)) {
                 fullMessage = join(fullMessage, "[CIRCULAR REFERENCE]");
                 break;
             }
         }
 
         return fullMessage.trim();
     }
 
     private String join(String message1, String message2) {
         String message = "";
 
         if (StringUtils.isNotEmpty(message1)) {
             message = message1.trim();
         }
 
         if (StringUtils.isNotEmpty(message2)) {
             if (StringUtils.isNotEmpty(message)) {
                 if (message.endsWith(".") || message.endsWith("!") || message.endsWith(":")) {
                     message += " ";
                 } else {
                     message += ": ";
                 }
             }
 
             message += message2;
         }
 
         return message;
     }
 }