- Ensures that Jackson duplicate property detection/rejection is enab…

…led by default.

Fixes #877

extensions/jackson/src/main/java/io/jsonwebtoken/jackson/io/JacksonDeserializer.java

@@ -72,14 +72,8 @@ public JacksonDeserializer() {
      * @param claimTypeMap The claim name-to-class map used to deserialize claims into the given type
      */
     public JacksonDeserializer(Map<String, Class<?>> claimTypeMap) {
-        // DO NOT reuse JacksonSerializer.DEFAULT_OBJECT_MAPPER as this could result in sharing the custom deserializer
-        // between instances
-        this(new ObjectMapper());
-        Assert.notNull(claimTypeMap, "Claim type map cannot be null.");
-        // register a new Deserializer
-        SimpleModule module = new SimpleModule();
-        module.addDeserializer(Object.class, new MappedTypeDeserializer(Collections.unmodifiableMap(claimTypeMap)));
-        objectMapper.registerModule(module);
+        // DO NOT specify JacksonSerializer.DEFAULT_OBJECT_MAPPER here as that would modify the shared instance
+        this(JacksonSerializer.newObjectMapper(), claimTypeMap);
     }
 
     /**
@@ -92,6 +86,46 @@ public JacksonDeserializer(ObjectMapper objectMapper) {
         this(objectMapper, (Class<T>) Object.class);
     }
 
+    /**
+     * Creates a new JacksonDeserializer where the values of the claims can be parsed into given types by registering
+     * a type-converting {@link  com.fasterxml.jackson.databind.Module Module} on the specified {@link ObjectMapper}.
+     * A common usage example is to parse custom User object out of a claim, for example the claims:
+     * <pre>{@code
+     * {
+     *     "issuer": "https://issuer.example.com",
+     *     "user": {
+     *         "firstName": "Jill",
+     *         "lastName": "Coder"
+     *     }
+     * }}</pre>
+     * Passing a map of {@code ["user": User.class]} to this constructor would result in the {@code user} claim being
+     * transformed to an instance of your custom {@code User} class, instead of the default of {@code Map}.
+     * <p>
+     * Because custom type parsing requires modifying the state of a Jackson {@code ObjectMapper}, this
+     * constructor modifies the specified {@code objectMapper} argument and customizes it to support the
+     * specified {@code claimTypeMap}.
+     * <p>
+     * If you do not want your {@code ObjectMapper} instance modified, but also want to support custom types for
+     * JWT {@code Claims}, you will need to first customize your {@code ObjectMapper} instance by registering
+     * your custom types separately and then use the {@link #JacksonDeserializer(ObjectMapper)} constructor instead
+     * (which does not modify the {@code objectMapper} argument).
+     *
+     * @param objectMapper the objectMapper to modify by registering a custom type-converting
+     *                     {@link com.fasterxml.jackson.databind.Module Module}
+     * @param claimTypeMap The claim name-to-class map used to deserialize claims into the given type
+     * @since 0.12.4
+     */
+    //TODO: Make this public on a minor release
+    //      (cannot do that on a point release as that would violate semver)
+    private JacksonDeserializer(ObjectMapper objectMapper, Map<String, Class<?>> claimTypeMap) {
+        this(objectMapper);
+        Assert.notNull(claimTypeMap, "Claim type map cannot be null.");
+        // register a new Deserializer on the ObjectMapper instance:
+        SimpleModule module = new SimpleModule();
+        module.addDeserializer(Object.class, new MappedTypeDeserializer(Collections.unmodifiableMap(claimTypeMap)));
+        objectMapper.registerModule(module);
+    }
+
     private JacksonDeserializer(ObjectMapper objectMapper, Class<T> returnType) {
         Assert.notNull(objectMapper, "ObjectMapper cannot be null.");
         Assert.notNull(returnType, "Return type cannot be null.");

extensions/jackson/src/main/java/io/jsonwebtoken/jackson/io/JacksonSerializer.java

@@ -16,6 +16,7 @@
 package io.jsonwebtoken.jackson.io;
 
 import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.JsonParser;
 import com.fasterxml.jackson.databind.Module;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectWriter;
@@ -41,7 +42,22 @@ public class JacksonSerializer<T> extends AbstractSerializer<T> {
         MODULE = module;
     }
 
-    static final ObjectMapper DEFAULT_OBJECT_MAPPER = new ObjectMapper().registerModule(MODULE);
+    static final ObjectMapper DEFAULT_OBJECT_MAPPER = newObjectMapper();
+
+    /**
+     * Creates and returns a new ObjectMapper with the {@code jjwt-jackson} module registered and
+     * {@code JsonParser.Feature.STRICT_DUPLICATE_DETECTION} enabled (set to true).
+     *
+     * @return and returns a new ObjectMapper with the {@code jjwt-jackson} module registered and
+     * {@code JsonParser.Feature.STRICT_DUPLICATE_DETECTION} enabled (set to true).
+     * @since 0.12.4
+     */
+    // package protected on purpose, do not expose to the public API
+    static ObjectMapper newObjectMapper() {
+        return new ObjectMapper()
+                .registerModule(MODULE)
+                .configure(JsonParser.Feature.STRICT_DUPLICATE_DETECTION, true); // https://github.com/jwtk/jjwt/issues/877
+    }
 
     protected final ObjectMapper objectMapper;
 

extensions/jackson/src/test/groovy/io/jsonwebtoken/jackson/io/JacksonDeserializerTest.groovy

@@ -16,6 +16,7 @@
 //file:noinspection GrDeprecatedAPIUsage
 package io.jsonwebtoken.jackson.io
 
+import com.fasterxml.jackson.core.JsonParseException
 import com.fasterxml.jackson.databind.ObjectMapper
 import io.jsonwebtoken.io.DeserializationException
 import io.jsonwebtoken.io.Deserializer
@@ -120,6 +121,31 @@ class JacksonDeserializerTest {
         assertEquals expected, result
     }
 
+    /**
+     * Asserts https://github.com/jwtk/jjwt/issues/877
+     */
+    @Test
+    void testStrictDuplicateDetection() {
+        // 'bKey' is repeated twice:
+        String json = """
+             {
+                "aKey":"oneValue", 
+                "bKey": 15,
+                "bKey": "hello"
+             }
+            """
+        try {
+            new JacksonDeserializer<>().deserialize(new StringReader(json))
+            fail()
+        } catch (DeserializationException expected) {
+            String causeMsg = "Duplicate field 'bKey'\n at [Source: (StringReader); line: 5, column: 23]"
+            String msg = "Unable to deserialize: $causeMsg"
+            assertEquals msg, expected.getMessage()
+            assertTrue expected.getCause() instanceof JsonParseException
+            assertEquals causeMsg, expected.getCause().getMessage()
+        }
+    }
+
     /**
      * For: https://github.com/jwtk/jjwt/issues/564
      */