Improve error message when abstract class cannot be constructed (#1814)

Author: Marcono1234

gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java

@@ -18,6 +18,7 @@
 
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Modifier;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.ArrayDeque;
@@ -99,6 +100,11 @@ public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
   }
 
   private <T> ObjectConstructor<T> newDefaultConstructor(Class<? super T> rawType) {
+    // Cannot invoke constructor of abstract class
+    if (Modifier.isAbstract(rawType.getModifiers())) {
+      return null;
+    }
+
     final Constructor<? super T> constructor;
     try {
       constructor = rawType.getDeclaredConstructor();

gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java

@@ -0,0 +1,59 @@
+package com.google.gson.internal;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
+import java.lang.reflect.Type;
+import java.util.Collections;
+import java.util.Map;
+
+import org.junit.Test;
+
+import com.google.gson.InstanceCreator;
+import com.google.gson.reflect.TypeToken;
+
+public class ConstructorConstructorTest {
+  private static final Map<Type, InstanceCreator<?>> NO_INSTANCE_CREATORS = Collections.emptyMap();
+
+  private abstract static class AbstractClass {
+    @SuppressWarnings("unused")
+    public AbstractClass() { }
+  }
+  private interface Interface { }
+
+  /**
+   * Verify that ConstructorConstructor does not try to invoke no-arg constructor
+   * of abstract class.
+   */
+  @Test
+  public void testGet_AbstractClassNoArgConstructor() {
+    ConstructorConstructor constructorFactory = new ConstructorConstructor(NO_INSTANCE_CREATORS, true);
+    ObjectConstructor<AbstractClass> constructor = constructorFactory.get(TypeToken.get(AbstractClass.class));
+    try {
+      constructor.construct();
+      fail("Expected exception");
+    } catch (RuntimeException exception) {
+      assertEquals(
+        "Unable to create instance of class com.google.gson.internal.ConstructorConstructorTest$AbstractClass. "
+        + "Registering an InstanceCreator or a TypeAdapter for this type, or adding a no-args constructor may fix this problem.",
+        exception.getMessage()
+      );
+    }
+  }
+
+  @Test
+  public void testGet_Interface() {
+    ConstructorConstructor constructorFactory = new ConstructorConstructor(NO_INSTANCE_CREATORS, true);
+    ObjectConstructor<Interface> constructor = constructorFactory.get(TypeToken.get(Interface.class));
+    try {
+      constructor.construct();
+      fail("Expected exception");
+    } catch (RuntimeException exception) {
+      assertEquals(
+        "Unable to create instance of interface com.google.gson.internal.ConstructorConstructorTest$Interface. "
+        + "Registering an InstanceCreator or a TypeAdapter for this type, or adding a no-args constructor may fix this problem.",
+        exception.getMessage()
+      );
+    }
+  }
+}