mybatis#2618 When javaType has not (or partially) been specified, try to determine the best matching constructor

- falls back to current behaviour if any ambiguity is detected
- remove setters from immutable `ResultMapping`

Author: epochcoder

src/main/java/org/apache/ibatis/builder/MapperBuilderAssistant.java

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 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.
@@ -15,8 +15,10 @@
  */
 package org.apache.ibatis.builder;
 
+import java.lang.reflect.Constructor;
 import java.sql.ResultSet;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -25,6 +27,7 @@
 import java.util.Properties;
 import java.util.Set;
 import java.util.StringTokenizer;
+import java.util.stream.Collectors;
 
 import org.apache.ibatis.cache.Cache;
 import org.apache.ibatis.cache.decorators.LruCache;
@@ -467,4 +470,66 @@ private Class<?> resolveParameterJavaType(Class<?> resultType, String property,
     return javaType;
   }
 
+  /**
+   * Attempts to assign a {@code javaType} to result mappings when it has been omitted, this is done based on matching
+   * constructors of the specified {@code resultType}
+   *
+   * @param resultType
+   *          the result type of the object to be built
+   * @param resultMappings
+   *          the current mappings
+   *
+   * @return null if there are no missing javaType mappings, or if no suitable mapping could be determined
+   */
+  public List<ResultMapping> autoTypeResultMappingsForUnknownJavaTypes(Class<?> resultType,
+      List<ResultMapping> resultMappings) {
+    // check if we have any undefined java types present, and try to set them automatically
+    if (resultMappings.stream().noneMatch(resultMapping -> Object.class.equals(resultMapping.getJavaType()))) {
+      return null;
+    }
+
+    final List<List<Class<?>>> matchingConstructors = Arrays.stream(resultType.getDeclaredConstructors())
+        .map(Constructor::getParameterTypes).filter(parameters -> parameters.length == resultMappings.size())
+        .map(Arrays::asList).collect(Collectors.toList());
+
+    final List<Class<?>> typesToMatch = resultMappings.stream().map(ResultMapping::getJavaType)
+        .collect(Collectors.toList());
+
+    List<Class<?>> matchingTypes = null;
+
+    outer: for (final List<Class<?>> actualTypes : matchingConstructors) {
+      for (int j = 0; j < typesToMatch.size(); j++) {
+        final Class<?> type = typesToMatch.get(j);
+        // pre-filled a type, check if it matches the constructor
+        if (!Object.class.equals(type) && !type.equals(actualTypes.get(j))) {
+          continue outer;
+        }
+      }
+
+      if (matchingTypes != null) {
+        // multiple matches found, abort as we cannot reliably guess the correct one.
+        matchingTypes = null;
+        break;
+      }
+
+      matchingTypes = actualTypes;
+    }
+
+    if (matchingTypes == null) {
+      return null;
+    }
+
+    final List<ResultMapping> adjustedAutoTypeResultMappings = new ArrayList<>();
+    for (int i = 0; i < resultMappings.size(); i++) {
+      ResultMapping otherMapping = resultMappings.get(i);
+      Class<?> identifiedMatchingJavaType = matchingTypes.get(i);
+
+      // given that we selected a new java type, overwrite the currently
+      // selected type handler so it can get retrieved again from the registry
+      adjustedAutoTypeResultMappings
+          .add(new ResultMapping.Builder(otherMapping).javaType(identifiedMatchingJavaType).typeHandler(null).build());
+    }
+
+    return adjustedAutoTypeResultMappings;
+  }
 }

src/main/java/org/apache/ibatis/builder/annotation/MapperAnnotationBuilder.java

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 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.
@@ -29,6 +29,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Properties;
 import java.util.Set;
@@ -514,6 +515,7 @@ private boolean hasNestedSelect(Result result) {
   }
 
   private void applyConstructorArgs(Arg[] args, Class<?> resultType, List<ResultMapping> resultMappings) {
+    final List<ResultMapping> mappings = new ArrayList<>();
     for (Arg arg : args) {
       List<ResultFlag> flags = new ArrayList<>();
       flags.add(ResultFlag.CONSTRUCTOR);
@@ -527,8 +529,11 @@ private void applyConstructorArgs(Arg[] args, Class<?> resultType, List<ResultMa
           nullOrEmpty(arg.column()), arg.javaType() == void.class ? null : arg.javaType(),
           arg.jdbcType() == JdbcType.UNDEFINED ? null : arg.jdbcType(), nullOrEmpty(arg.select()),
           nullOrEmpty(arg.resultMap()), null, nullOrEmpty(arg.columnPrefix()), typeHandler, flags, null, null, false);
-      resultMappings.add(resultMapping);
+      mappings.add(resultMapping);
     }
+
+    final List<ResultMapping> autoMappings = assistant.autoTypeResultMappingsForUnknownJavaTypes(resultType, mappings);
+    resultMappings.addAll(Objects.requireNonNullElse(autoMappings, mappings));
   }
 
   private String nullOrEmpty(String value) {

src/main/java/org/apache/ibatis/builder/xml/XMLMapperBuilder.java

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 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.
@@ -23,6 +23,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Properties;
 
 import org.apache.ibatis.builder.BaseBuilder;
@@ -267,14 +268,21 @@ protected Class<?> inheritEnclosingType(XNode resultMapNode, Class<?> enclosingT
 
   private void processConstructorElement(XNode resultChild, Class<?> resultType, List<ResultMapping> resultMappings) {
     List<XNode> argChildren = resultChild.getChildren();
+
+    final List<ResultMapping> mappings = new ArrayList<>();
     for (XNode argChild : argChildren) {
       List<ResultFlag> flags = new ArrayList<>();
       flags.add(ResultFlag.CONSTRUCTOR);
       if ("idArg".equals(argChild.getName())) {
         flags.add(ResultFlag.ID);
       }
-      resultMappings.add(buildResultMappingFromContext(argChild, resultType, flags));
+
+      mappings.add(buildResultMappingFromContext(argChild, resultType, flags));
     }
+
+    final List<ResultMapping> autoMappings = builderAssistant.autoTypeResultMappingsForUnknownJavaTypes(resultType,
+        mappings);
+    resultMappings.addAll(Objects.requireNonNullElse(autoMappings, mappings));
   }
 
   private Discriminator processDiscriminatorElement(XNode context, Class<?> resultType,

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

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 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.
@@ -15,6 +15,13 @@
  */
 package org.apache.ibatis.mapping;
 
+import org.apache.ibatis.annotations.Param;
+import org.apache.ibatis.builder.BuilderException;
+import org.apache.ibatis.logging.Log;
+import org.apache.ibatis.logging.LogFactory;
+import org.apache.ibatis.reflection.ParamNameUtil;
+import org.apache.ibatis.session.Configuration;
+
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Constructor;
 import java.util.ArrayList;
@@ -24,13 +31,6 @@
 import java.util.Locale;
 import java.util.Set;
 
-import org.apache.ibatis.annotations.Param;
-import org.apache.ibatis.builder.BuilderException;
-import org.apache.ibatis.logging.Log;
-import org.apache.ibatis.logging.LogFactory;
-import org.apache.ibatis.reflection.ParamNameUtil;
-import org.apache.ibatis.session.Configuration;
-
 /**
  * @author Clinton Begin
  */
@@ -137,7 +137,7 @@ public ResultMap build() {
         if (actualArgNames == null) {
           throw new BuilderException("Error in result map '" + resultMap.id + "'. Failed to find a constructor in '"
               + resultMap.getType().getName() + "' with arg names " + constructorArgNames
-              + ". Note that 'javaType' is required when there is no writable property with the same name ('name' is optional, BTW). There might be more info in debug log.");
+              + ". Note that 'javaType' is required when there is ambiguous constructors or there is no writable property with the same name ('name' is optional, BTW). There might be more info in debug log.");
         }
         resultMap.constructorResultMappings.sort((o1, o2) -> {
           int paramIdx1 = actualArgNames.indexOf(o1.getProperty());

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

@@ -1,5 +1,5 @@
 /*
- *    Copyright 2009-2024 the original author or authors.
+ *    Copyright 2009-2025 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.
@@ -15,16 +15,16 @@
  */
 package org.apache.ibatis.mapping;
 
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.List;
-import java.util.Set;
-
 import org.apache.ibatis.session.Configuration;
 import org.apache.ibatis.type.JdbcType;
 import org.apache.ibatis.type.TypeHandler;
 import org.apache.ibatis.type.TypeHandlerRegistry;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+
 /**
  * @author Clinton Begin
  */
@@ -72,6 +72,24 @@ public Builder(Configuration configuration, String property) {
       resultMapping.lazy = configuration.isLazyLoadingEnabled();
     }
 
+    public Builder(ResultMapping otherMapping) {
+      this(otherMapping.configuration, otherMapping.property);
+
+      resultMapping.flags.addAll(otherMapping.flags);
+      resultMapping.composites.addAll(otherMapping.composites);
+
+      resultMapping.column = otherMapping.column;
+      resultMapping.javaType = otherMapping.javaType;
+      resultMapping.jdbcType = otherMapping.jdbcType;
+      resultMapping.typeHandler = otherMapping.typeHandler;
+      resultMapping.nestedResultMapId = otherMapping.nestedResultMapId;
+      resultMapping.nestedQueryId = otherMapping.nestedQueryId;
+      resultMapping.notNullColumns = otherMapping.notNullColumns;
+      resultMapping.columnPrefix = otherMapping.columnPrefix;
+      resultMapping.resultSet = otherMapping.resultSet;
+      resultMapping.foreignColumn = otherMapping.foreignColumn;
+    }
+
     public Builder javaType(Class<?> javaType) {
       resultMapping.javaType = javaType;
       return this;
@@ -243,18 +261,10 @@ public String getForeignColumn() {
     return foreignColumn;
   }
 
-  public void setForeignColumn(String foreignColumn) {
-    this.foreignColumn = foreignColumn;
-  }
-
   public boolean isLazy() {
     return lazy;
   }
 
-  public void setLazy(boolean lazy) {
-    this.lazy = lazy;
-  }
-
   public boolean isSimple() {
     return this.nestedResultMapId == null && this.nestedQueryId == null && this.resultSet == null;
   }

src/test/java/org/apache/ibatis/submitted/auto_type_from_non_ambiguous_constructor/Account.java

@@ -0,0 +1,20 @@
+/*
+ *    Copyright 2009-2025 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.
+ *    You may obtain a copy of the License at
+ *
+ *       https://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.ibatis.submitted.auto_type_from_non_ambiguous_constructor;
+
+public record Account(long accountId, String accountName, String accountType) {
+
+}

src/test/java/org/apache/ibatis/submitted/auto_type_from_non_ambiguous_constructor/Account2.java

@@ -0,0 +1,23 @@
+/*
+ *    Copyright 2009-2025 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.
+ *    You may obtain a copy of the License at
+ *
+ *       https://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.ibatis.submitted.auto_type_from_non_ambiguous_constructor;
+
+public record Account2(long accountId, String accountName, String accountType) {
+
+  public Account2(long accountId, String accountName, String accountType, String extraInfo) {
+    this(accountId, accountName, accountType);
+  }
+}

src/test/java/org/apache/ibatis/submitted/auto_type_from_non_ambiguous_constructor/Account3.java

@@ -0,0 +1,27 @@
+/*
+ *    Copyright 2009-2025 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.
+ *    You may obtain a copy of the License at
+ *
+ *       https://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.ibatis.submitted.auto_type_from_non_ambiguous_constructor;
+
+public record Account3(long accountId, String accountName, String accountType) {
+
+  public Account3(long accountId, int mismatch, String accountType) {
+    this(accountId, "MismatchedAccountI", accountType);
+  }
+
+  public Account3(long accountId, long mismatch, String accountType) {
+    this(accountId, "MismatchedAccountL", accountType);
+  }
+}