fix Use constructor element and discriminator case element is resultMap property NPE

src/main/java/org/apache/ibatis/mapping/ResultMap.java

@@ -122,7 +122,7 @@ public ResultMap build() {
       if (resultMap.idResultMappings.isEmpty()) {
         resultMap.idResultMappings.addAll(resultMap.resultMappings);
       }
-      if (!constructorArgNames.isEmpty()) {
+      if (!constructorArgNames.isEmpty() && resultMap.type != null) {
         final List<String> actualArgNames = argNamesOfMatchingConstructor(constructorArgNames);
         if (actualArgNames == null) {
           throw new BuilderException("Error in result map '" + resultMap.id

src/test/java/org/apache/ibatis/builder/BlogMapper.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <!--
 
-       Copyright 2009-2019 the original author or authors.
+       Copyright 2009-2021 the original author or authors.
 
        Licensed under the Apache License, Version 2.0 (the "License");
        you may not use this file except in compliance with the License.
@@ -94,6 +94,15 @@
     </discriminator>
   </resultMap>
 
+  <resultMap id="joinedConstructorPost" type="org.apache.ibatis.domain.blog.Post">
+    <constructor>
+      <idArg column="post_id" name="id"/>
+    </constructor>
+    <discriminator javaType="int" column="draft">
+      <case value="1" resultMap="draftPost"/>
+    </discriminator>
+  </resultMap>
+
   <resultMap id="draftPost" type="org.apache.ibatis.domain.blog.DraftPost" extends="joinedPost"/>
 
   <resultMap id="blogJoinedWithPostsAndAuthor" type="Blog">
@@ -149,6 +158,40 @@
     where B.id = #{id}
   </select>
 
+  <select id="selectBlogJoinedConstructorWithPostsAndAuthor" parameterType="int" resultMap="joinedConstructorPost">
+    select
+      B.id as blog_id,
+      B.title as blog_title,
+      B.author_id as blog_author_id,
+      A.id as author_id,
+      A.username as author_username,
+      A.password as author_password,
+      A.email as author_email,
+      A.bio as author_bio,
+      A.favourite_section as author_favourite_section,
+      P.id as post_id,
+      P.blog_id as post_blog_id,
+      P.author_id as post_author_id,
+      P.created_on as post_created_on,
+      P.section as post_section,
+      P.subject as post_subject,
+      P.draft as draft,
+      P.body as post_body,
+      C.id as comment_id,
+      C.post_id as comment_post_id,
+      C.name as comment_name,
+      C.comment as comment_text,
+      T.id as tag_id,
+      T.name as tag_name
+    from Blog B
+    left outer join Author A on B.author_id = A.id
+    left outer join Post P on B.id = P.blog_id
+    left outer join Comment C on P.id = C.post_id
+    left outer join Post_Tag PT on PT.post_id = P.id
+    left outer join Tag T on PT.tag_id = T.id
+    where B.id = #{id}
+  </select>
+
   <select id="selectAllPosts" resultType="hashmap">
     select * from post order by id
   </select>

src/test/java/org/apache/ibatis/builder/xsd/BlogMapper.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <!--
 
-       Copyright 2009-2019 the original author or authors.
+       Copyright 2009-2021 the original author or authors.
 
        Licensed under the Apache License, Version 2.0 (the "License");
        you may not use this file except in compliance with the License.
@@ -93,6 +93,15 @@
     </discriminator>
   </resultMap>
 
+  <resultMap id="joinedConstructorPost" type="org.apache.ibatis.domain.blog.Post">
+    <constructor>
+      <idArg column="post_id" name="id"/>
+    </constructor>
+    <discriminator javaType="int" column="draft">
+      <case value="1" resultMap="draftPost"/>
+    </discriminator>
+  </resultMap>
+
   <resultMap id="draftPost" type="org.apache.ibatis.domain.blog.DraftPost" extends="joinedPost"/>
 
   <resultMap id="blogJoinedWithPostsAndAuthor" type="Blog">
@@ -148,6 +157,40 @@
     where B.id = #{id}
   </select>
 
+  <select id="selectBlogJoinedConstructorWithPostsAndAuthor" parameterType="int" resultMap="joinedConstructorPost">
+    select
+    B.id as blog_id,
+    B.title as blog_title,
+    B.author_id as blog_author_id,
+    A.id as author_id,
+    A.username as author_username,
+    A.password as author_password,
+    A.email as author_email,
+    A.bio as author_bio,
+    A.favourite_section as author_favourite_section,
+    P.id as post_id,
+    P.blog_id as post_blog_id,
+    P.author_id as post_author_id,
+    P.created_on as post_created_on,
+    P.section as post_section,
+    P.subject as post_subject,
+    P.draft as draft,
+    P.body as post_body,
+    C.id as comment_id,
+    C.post_id as comment_post_id,
+    C.name as comment_name,
+    C.comment as comment_text,
+    T.id as tag_id,
+    T.name as tag_name
+    from Blog B
+    left outer join Author A on B.author_id = A.id
+    left outer join Post P on B.id = P.blog_id
+    left outer join Comment C on P.id = C.post_id
+    left outer join Post_Tag PT on PT.post_id = P.id
+    left outer join Tag T on PT.tag_id = T.id
+    where B.id = #{id}
+  </select>
+
   <select id="selectAllPosts" resultType="hashmap">
     select * from post order by id
   </select>

src/test/java/org/apache/ibatis/domain/blog/DraftPost.java

@@ -16,4 +16,11 @@
 package org.apache.ibatis.domain.blog;
 
 public class DraftPost extends Post {
+
+  public DraftPost() {
+  }
+
+  public DraftPost(int id) {
+    super(id);
+  }
 }

src/test/java/org/apache/ibatis/domain/blog/Post.java

@@ -30,6 +30,13 @@ public class Post {
   private List<Comment> comments;
   private List<Tag> tags;
 
+  public Post() {
+  }
+
+  public Post(int id) {
+    this.id = id;
+  }
+
   public int getId() {
     return id;
   }

src/test/java/org/apache/ibatis/domain/blog/mappers/BlogMapper.java

@@ -28,4 +28,6 @@ public interface BlogMapper {
 
   List<Map> selectAllPosts(RowBounds rowBounds, Object param);
 
+  List<Map> selectBlogJoinedConstructorWithPostsAndAuthor(int id);
+
 }

src/test/java/org/apache/ibatis/session/SqlSessionTest.java

@@ -661,6 +661,15 @@ void shouldLimitResultsUsingMapperClass() {
     }
   }
 
+  @Test
+  void shouldNotNPEResultsUsingMapperClass() {
+    try (SqlSession session = sqlMapper.openSession()) {
+      BlogMapper mapper = session.getMapper(BlogMapper.class);
+      List<Map> postsAndAuthor = mapper.selectBlogJoinedConstructorWithPostsAndAuthor(1);
+      assertNotNull(postsAndAuthor);
+    }
+  }
+
   private static class TestResultHandler implements ResultHandler {
     int count = 0;
     @Override