From 0e33ad4bbcfd7f8628f63221db4faa7415ac4384 Mon Sep 17 00:00:00 2001
From: Christian Bandowski <christian@myvm.de>
Date: Sat, 22 Dec 2018 15:19:39 +0100
Subject: [PATCH] #1657 Add Mappers.getMapperClass for getting the class of a
 Mapper

---
 .../java/org/mapstruct/factory/Mappers.java   | 96 +++++++++++++++----
 .../org/mapstruct/factory/MappersTest.java    | 21 +++-
 2 files changed, 97 insertions(+), 20 deletions(-)

diff --git a/core/src/main/java/org/mapstruct/factory/Mappers.java b/core/src/main/java/org/mapstruct/factory/Mappers.java
index ac479d364..05b616fe9 100644
--- a/core/src/main/java/org/mapstruct/factory/Mappers.java
+++ b/core/src/main/java/org/mapstruct/factory/Mappers.java
@@ -53,14 +53,7 @@ public class Mappers {
      */
     public static <T> T getMapper(Class<T> clazz) {
         try {
-            List<ClassLoader> classLoaders = new ArrayList<>( 3 );
-            classLoaders.add( clazz.getClassLoader() );
-
-            if ( Thread.currentThread().getContextClassLoader() != null ) {
-                classLoaders.add( Thread.currentThread().getContextClassLoader() );
-            }
-
-            classLoaders.add( Mappers.class.getClassLoader() );
+            List<ClassLoader> classLoaders = collectClassLoaders( clazz.getClassLoader() );
 
             return getMapper( clazz, classLoaders );
         }
@@ -88,23 +81,88 @@ public class Mappers {
             Class<T> implementation = (Class<T>) classLoader.loadClass( clazz.getName() + IMPLEMENTATION_SUFFIX );
             Constructor<T> constructor = implementation.getDeclaredConstructor();
             constructor.setAccessible( true );
+
             return constructor.newInstance();
         }
         catch (ClassNotFoundException e) {
-            ServiceLoader<T> loader = ServiceLoader.load( clazz, classLoader );
-
-            if ( loader != null ) {
-                for ( T mapper : loader ) {
-                    if ( mapper != null ) {
-                        return mapper;
-                    }
-                }
-            }
-
-            return null;
+            return getMapperFromServiceLoader( clazz, classLoader );
         }
         catch ( InstantiationException | InvocationTargetException | IllegalAccessException e) {
             throw new RuntimeException( e );
         }
     }
+
+    /**
+     * Returns the class of the implementation for the given mapper type.
+     *
+     * @param clazz The type of the mapper to return.
+     * @param <T> The type of the mapper to create.
+     *
+     * @return A class of the implementation for the given mapper type.
+     *
+     * @since 1.3
+     */
+    public static <T> Class<? extends T> getMapperClass(Class<T> clazz) {
+        try {
+            List<ClassLoader> classLoaders = collectClassLoaders( clazz.getClassLoader() );
+
+            return getMapperClass( clazz, classLoaders );
+        }
+        catch ( ClassNotFoundException e ) {
+            throw new RuntimeException( e );
+        }
+    }
+
+    private static <T> Class<? extends T> getMapperClass(Class<T> mapperType, Iterable<ClassLoader> classLoaders)
+        throws ClassNotFoundException {
+
+        for ( ClassLoader classLoader : classLoaders ) {
+            Class<? extends T> mapperClass = doGetMapperClass( mapperType, classLoader );
+            if ( mapperClass != null ) {
+                return mapperClass;
+            }
+        }
+
+        throw new ClassNotFoundException( "Cannot find implementation for " + mapperType.getName() );
+    }
+
+    @SuppressWarnings("unchecked")
+    private static <T> Class<? extends T> doGetMapperClass(Class<T> clazz, ClassLoader classLoader) {
+        try {
+            return (Class<? extends T>) classLoader.loadClass( clazz.getName() + IMPLEMENTATION_SUFFIX );
+        }
+        catch ( ClassNotFoundException e ) {
+            T mapper = getMapperFromServiceLoader( clazz, classLoader );
+            if ( mapper != null ) {
+                return (Class<? extends T>) mapper.getClass();
+            }
+
+            return null;
+        }
+    }
+
+    private static <T> T getMapperFromServiceLoader(Class<T> clazz, ClassLoader classLoader) {
+        ServiceLoader<T> loader = ServiceLoader.load( clazz, classLoader );
+
+        for ( T mapper : loader ) {
+            if ( mapper != null ) {
+                return mapper;
+            }
+        }
+
+        return null;
+    }
+
+    private static List<ClassLoader> collectClassLoaders(ClassLoader classLoader) {
+        List<ClassLoader> classLoaders = new ArrayList<>( 3 );
+        classLoaders.add( classLoader );
+
+        if ( Thread.currentThread().getContextClassLoader() != null ) {
+            classLoaders.add( Thread.currentThread().getContextClassLoader() );
+        }
+
+        classLoaders.add( Mappers.class.getClassLoader() );
+
+        return classLoaders;
+    }
 }
diff --git a/core/src/test/java/org/mapstruct/factory/MappersTest.java b/core/src/test/java/org/mapstruct/factory/MappersTest.java
index 507eb08d0..60371cada 100644
--- a/core/src/test/java/org/mapstruct/factory/MappersTest.java
+++ b/core/src/test/java/org/mapstruct/factory/MappersTest.java
@@ -25,18 +25,37 @@ public class MappersTest {
         assertThat( mapper ).isNotNull();
     }
 
+    @Test
+    public void shouldReturnImplementationClass() {
+
+        Class<? extends Foo> mapperClass = Mappers.getMapperClass( Foo.class );
+        assertThat( mapperClass ).isNotNull();
+        assertThat( mapperClass ).isNotExactlyInstanceOf( Foo.class );
+    }
+
     /**
      * Checks if an implementation of a nested mapper can be found. This is a special case since
      * it is named
      */
     @Test
-    public void findsNestedMapperImpl() throws Exception {
+    public void findsNestedMapperImpl() {
         assertThat( Mappers.getMapper( SomeClass.Foo.class ) ).isNotNull();
         assertThat( Mappers.getMapper( SomeClass.NestedClass.Foo.class ) ).isNotNull();
     }
 
+    @Test
+    public void findsNestedMapperImplClass() {
+        assertThat( Mappers.getMapperClass( SomeClass.Foo.class ) ).isNotNull();
+        assertThat( Mappers.getMapperClass( SomeClass.NestedClass.Foo.class ) ).isNotNull();
+    }
+
     @Test
     public void shouldReturnPackagePrivateImplementationInstance() {
         assertThat( Mappers.getMapper( PackagePrivateMapper.class ) ).isNotNull();
     }
+
+    @Test
+    public void shouldReturnPackagePrivateImplementationClass() {
+        assertThat( Mappers.getMapperClass( PackagePrivateMapper.class ) ).isNotNull();
+    }
 }