Bit more work for #3110

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/databind/module/SimpleModule.java

@@ -5,6 +5,7 @@
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import com.fasterxml.jackson.core.Version;
 import com.fasterxml.jackson.databind.*;
@@ -42,6 +43,11 @@ public class SimpleModule
 {
     private static final long serialVersionUID = 1L; // 2.5.0
 
+    // 16-Jun-2021, tatu: For [databind#3110], generate actual unique ids
+    //   for SimpleModule instances (System.identityHashCode(...) is close
+    //   but not quite it...
+    private static final AtomicInteger MODULE_ID_SEQ = new AtomicInteger(1);
+
     protected final String _name;
     protected final Version _version;
 
@@ -114,9 +120,11 @@ public class SimpleModule
      */
     public SimpleModule() {
         // can't chain when making reference to 'this'
-        // note: generate different name for direct instantiation, sub-classing
+        // note: generate different name for direct instantiation, sub-classing;
+        // this to avoid collision in former case while still addressing
+        // [databind#3110]
         _name = (getClass() == SimpleModule.class)
-                ? "SimpleModule-"+System.identityHashCode(this)
+                ? "SimpleModule-"+MODULE_ID_SEQ.getAndIncrement()
                 : getClass().getName();
         _version = Version.unknownVersion();
         // 07-Jun-2021, tatu: [databind#3110] Not passed explicitly so...

src/test/java/com/fasterxml/jackson/databind/module/SimpleModuleTest.java

@@ -355,6 +355,40 @@ public void testGetRegisteredModules()
                 mapper.getRegisteredModuleIds());
     }
 
+    // More [databind#3110] testing
+    public void testMultipleSimpleModules()
+    {
+        final SimpleModule mod1 = new SimpleModule();
+        final SimpleModule mod2 = new SimpleModule();
+        ObjectMapper mapper = JsonMapper.builder()
+                .addModule(mod1)
+                .addModule(mod2)
+                .build();
+        assertEquals(2, mapper.getRegisteredModuleIds().size());
+
+        // Still avoid actual duplicates
+        mapper = JsonMapper.builder()
+                .addModule(mod1)
+                .addModule(mod1)
+                .build();
+        assertEquals(1, mapper.getRegisteredModuleIds().size());
+
+        // Same for (anonymous) sub-classes
+        final SimpleModule subMod1 = new SimpleModule() { };
+        final SimpleModule subMod2 = new SimpleModule() { };
+        mapper = JsonMapper.builder()
+                .addModule(subMod1)
+                .addModule(subMod2)
+                .build();
+        assertEquals(2, mapper.getRegisteredModuleIds().size());
+
+        mapper = JsonMapper.builder()
+                .addModule(subMod1)
+                .addModule(subMod1)
+                .build();
+        assertEquals(1, mapper.getRegisteredModuleIds().size());
+    }
+
     /*
     /**********************************************************
     /* Unit tests; other