Implement #5064 (automatic type detection)

src/main/java/tools/jackson/databind/ObjectMapper.java

@@ -7,6 +7,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -2230,6 +2231,45 @@ public ObjectReader readerForMapOf(Class<?> type) {
                 null, _injectableValues);
     }
 
+    /**
+     * Factory method for constructing {@link ObjectReader} that will
+     * read or update instances of auto-detected type; inferred from compiled
+     * info. Note that the nominal parameter MUST be an empty array;  
+     * otherwise an exception will be thrown.
+     * Usage:
+     *<pre>
+     *  MyType result = objectMapper.readerForDetectedType().readValue(json);
+     *</pre>
+     *
+     * @since 3.0
+     */
+    public <T> ObjectReader readerForDetectedType(T... reified) {
+        // Should be auto-generated empty array; verify
+        Objects.requireNonNull(reified, "reified");
+        if (reified.length != 0) {
+            throw new IllegalArgumentException(String.format(
+"argument \reified\" should be empty; has %d elements", reified.length));
+        }
+        Type type = _getClassOf(reified);
+System.err.println("DEBUG: reified type: "+type);
+        _assertNotNull("type", type);
+        return _newReader(deserializationConfig(), _typeFactory.constructType(type), null,
+                null, _injectableValues);
+    }
+
+    /**
+     * Utility method to get the class type from a varargs array.
+     *
+     * @param <T> the generic type
+     * @param array the varargs array
+     * @return the class of the array component
+     *
+     * @since 3.0
+     */
+    private static <T> Class<T> _getClassOf(T[] array) {
+        return (Class<T>) array.getClass().getComponentType();
+    }
+
     /**
      * Factory method for constructing {@link ObjectReader} that will
      * use specified {@link JsonNodeFactory} for constructing JSON trees.

src/test/java/tools/jackson/databind/ObjectReaderTest.java

@@ -90,7 +90,7 @@ public void testReaderForArrayOf() throws Exception
         assertEquals(ABC.C, abcs[1]);
     }
 
-    // [databind#2693]: convenience read methods:
+    // [databind#2693]: convenience read methods
     @Test
     public void testReaderForListOf() throws Exception
     {
@@ -100,7 +100,7 @@ public void testReaderForListOf() throws Exception
         assertEquals(Arrays.asList(ABC.B, ABC.C), value);
     }
 
-    // [databind#2693]: convenience read methods:
+    // [databind#2693]: convenience read methods
     @Test
     public void testReaderForMapOf() throws Exception
     {
@@ -110,6 +110,29 @@ public void testReaderForMapOf() throws Exception
         assertEquals(Collections.singletonMap("key", ABC.B), value);
     }
 
+    // [databind#5064]: auto-detection of type
+    @Test
+    public void testReaderForDetectedType() throws Exception
+    {
+        // First, regular "untyped" (java.lang.Object)
+        List<Integer> ob = MAPPER.readerForDetectedType()
+                .readValue("[1]");
+        assertNotNull(ob);
+        assertEquals(1, ob.get(0));
+
+        // then simple pojo
+        Point point = MAPPER.readerForDetectedType().readValue("{\"x\":1,\"y\":2}");
+        assertNotNull(point);
+        assertEquals(1, point.x);
+        assertEquals(2, point.y);
+
+        // and more complex
+        POJO pojo = MAPPER.readerForDetectedType().readValue("{\"name\":{\"value\":123}}");
+        assertNotNull(pojo);
+        assertNotNull(pojo.name);
+        assertEquals(Map.of("value", 123), pojo.name);
+    }
+
     /*
     /**********************************************************
     /* Test methods, some alternative JSON settings