Fixes #422 Avoid NPE when non-local parent POM is present

src/it/it-display-plugin-updates-009-issue-422/invoker.properties

@@ -0,0 +1 @@
+invoker.goals=${project.groupId}:${project.artifactId}:${project.version}:display-plugin-updates

src/it/it-display-plugin-updates-009-issue-422/pom.xml

@@ -0,0 +1,52 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+      <groupId>org.apache</groupId>
+      <artifactId>apache</artifactId>
+      <version>23</version>
+      <relativePath/>
+  </parent>
+
+  <groupId>localhost</groupId>
+  <artifactId>it-101</artifactId>
+  <version>1.0</version>
+  <packaging>pom</packaging>
+  <name>display-plugin-updates</name>
+
+  <description>Make sure display-plugin-updates works if a non-local parent is referenced.</description>
+
+  <prerequisites>
+    <maven>2.0.6</maven>
+  </prerequisites>
+
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <groupId>localhost</groupId>
+          <artifactId>dummy-maven-plugin</artifactId>
+          <version>1.0</version>
+        </plugin>
+        <plugin>
+          <artifactId>maven-clean-plugin</artifactId>
+          <version>2.2</version>
+        </plugin>
+        <plugin>
+          <artifactId>maven-deploy-plugin</artifactId>
+          <version>2.3</version>
+        </plugin>
+        <plugin>
+          <artifactId>maven-install-plugin</artifactId>
+          <version>2.2</version>
+        </plugin>
+        <plugin>
+          <artifactId>maven-site-plugin</artifactId>
+          <version>2.0</version>
+        </plugin>
+      </plugins>
+    </pluginManagement>
+  </build>
+
+</project>

src/it/it-display-plugin-updates-009-issue-422/verify.bsh

@@ -0,0 +1,25 @@
+import java.io.*;
+import org.codehaus.plexus.util.FileUtils;
+import java.util.regex.*;
+
+try
+{
+    File file = new File( basedir, "build.log" );
+    String buf = FileUtils.fileRead( file );
+
+    Pattern p = Pattern.compile( "\\Qlocalhost:dummy-maven-plugin\\E\\s*\\.*\\s*1\\.0\\s+->\\s+3\\.1" );
+    Matcher m = p.matcher( buf.toString() );
+    if ( !m.find() )
+    {
+        System.out.println( "Did not suggest updating dummy-maven-plugin to version 3.1" );
+        return false;
+    }
+    System.out.println( m.group( 0 ) );
+}
+catch( Throwable t )
+{
+    t.printStackTrace();
+    return false;
+}
+
+return true;

src/main/java/org/codehaus/mojo/versions/DisplayPluginUpdatesMojo.java

@@ -70,6 +70,8 @@
 
 import javax.xml.stream.XMLStreamException;
 import javax.xml.stream.events.XMLEvent;
+
+import java.io.File;
 import java.io.IOException;
 import java.io.Reader;
 import java.io.StringWriter;
@@ -769,7 +771,7 @@ private Map<String, String> getParentsPlugins( List<MavenProject> parents )
                 try
                 {
                     Set<String> withVersionSpecified =
-                        findPluginsWithVersionsSpecified( new StringBuilder( writer.toString() ), parentProject.getFile().getAbsolutePath() );
+                        findPluginsWithVersionsSpecified( new StringBuilder( writer.toString() ), getSafeProjectPathInfo(parentProject) );
 
                     Map<String, String> map = getPluginManagement( interpolatedModel );
                     map.keySet().retainAll( withVersionSpecified );
@@ -797,6 +799,17 @@ private Map<String, String> getParentsPlugins( List<MavenProject> parents )
         }
         return parentPlugins;
     }
+
+    private String getSafeProjectPathInfo(MavenProject project) {
+        File file = project.getFile();
+        if (file != null) {
+            return file.getAbsolutePath();
+        }
+        else {
+            // path is used only as information in error message, we can fallback to project artifact info here
+            return project.toString();
+        }
+    }
 
     private boolean isMavenPluginProject()
     {
@@ -850,7 +863,7 @@ public String toString()
     private Set<String> findPluginsWithVersionsSpecified( MavenProject project )
         throws IOException, XMLStreamException
     {
-        return findPluginsWithVersionsSpecified( PomHelper.readXmlFile( project.getFile() ), project.getFile().getAbsolutePath() );
+        return findPluginsWithVersionsSpecified( PomHelper.readXmlFile( project.getFile() ), getSafeProjectPathInfo(project) );
     }
 
     /**