From 2bc8b693b89d31fd59e29c8db9f667b786e59907 Mon Sep 17 00:00:00 2001
From: Marcono1234 <Marcono1234@users.noreply.github.com>
Date: Thu, 5 Nov 2020 15:25:59 +0100
Subject: [PATCH] Improve error message when abstract class cannot be
 constructed

---
 .../gson/internal/ConstructorConstructor.java |  7 +++
 .../internal/ConstructorConstructorTest.java  | 59 +++++++++++++++++++
 2 files changed, 66 insertions(+)
 create mode 100644 gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java

diff --git a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java b/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
index 5fab460105..860d3af053 100644
--- a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
+++ b/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;
@@ -97,6 +98,11 @@ 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;
+    }
+
     try {
       final Constructor<? super T> constructor = rawType.getDeclaredConstructor();
       if (!constructor.isAccessible()) {
@@ -225,6 +231,7 @@ private <T> ObjectConstructor<T> newUnsafeAllocator(
           Object newInstance = unsafeAllocator.newInstance(rawType);
           return (T) newInstance;
         } catch (Exception e) {
+          // TODO: JsonParseException ?
           throw new RuntimeException(("Unable to invoke no-args constructor for " + type + ". "
               + "Registering an InstanceCreator with Gson for this type may fix this problem."), e);
         }
diff --git a/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java b/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java
new file mode 100644
index 0000000000..ca5974bb8c
--- /dev/null
+++ b/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);
+    ObjectConstructor<AbstractClass> constructor = constructorFactory.get(TypeToken.get(AbstractClass.class));
+    try {
+      constructor.construct();
+      fail("Expected exception");
+    } catch (RuntimeException exception) {
+      assertEquals(
+        "Unable to invoke no-args constructor for " + AbstractClass.class
+        + ". Registering an InstanceCreator with Gson for this type may fix this problem.",
+        exception.getMessage()
+      );
+    }
+  }
+
+  @Test
+  public void testGet_Interface() {
+    ConstructorConstructor constructorFactory = new ConstructorConstructor(NO_INSTANCE_CREATORS);
+    ObjectConstructor<Interface> constructor = constructorFactory.get(TypeToken.get(Interface.class));
+    try {
+      constructor.construct();
+      fail("Expected exception");
+    } catch (RuntimeException exception) {
+      assertEquals(
+        "Unable to invoke no-args constructor for " + Interface.class
+        + ". Registering an InstanceCreator with Gson for this type may fix this problem.",
+        exception.getMessage()
+      );
+    }
+  }
+}