Coercion bug

[Ivan-Shaml]

After update from Jackson 2 to Jackson 3, and respectively updating from com.networknt:json-schema-validator:2.0.1 to 3.0.2, the following issue occurred and one unit test started failing.

Exception:

tools.jackson.databind.exc.JsonNodeException: 'ObjectNode' method `asString()` cannot coerce value {...(1 properties}] to `java.lang.String`: value type not coercible

Full stack trace: stack_trace.txt
Repository with example test to reproduce the issue: repo

[stevehu]

• Root cause confirmed: this is a regression from the Jackson 3 migration, specifically in Dialect.readText.

In 2.0.1, Dialect.readText used fieldNode.textValue(), which returns null for non-text nodes. In 3.0.2/current master, it was changed to fieldNode.asString() at json-schema-validator/
src/main/java/com/networknt/schema/dialect/Dialect.java:417. Jackson 3 throws when asString() is called on an ObjectNode.

Why the reproducer hits it:

• The repro validates Swagger 2.0 using SpecificationVersion.DRAFT_4.
• Draft 4’s id keyword is plain id, not $id, from json-schema-validator/src/main/java/com/networknt/schema/dialect/Draft4.java:16.
• The Swagger schema has refs like http://json-schema.org/draft-04/schema#/properties/title.
• While resolving that ref, json-schema-validator/src/main/java/com/networknt/schema/Schema.java:394 walks the fragment and calls resolveSchemaId on each intermediate node.
• The intermediate draft-04 meta-schema node /properties contains an entry named "id": { "type": "string" }.
• That "id" is a property schema, not a schema id, but Dialect.readId sees the field name id and calls asString() on its object value. That causes the reported exception.

So this is not really a user schema validation failure. It is the validator treating a non-schema container object as though it might have a draft-04 id, combined with the Jackson 3
change from text-only access to coercive/string conversion.

I reproduced it with the issue repo using:

mvn -Dtest=Example test

It fails with the same ObjectNode.asString() stack trace. I did not make any code changes. The likely fix is to restore text-only behavior in Dialect.readText, probably back to
fieldNode.textValue() or an explicit fieldNode.isString() ? fieldNode.asString() : null, then add this issue as a regression test.

[stevehu]

• Fixed locally in json-schema-validator.

Changed json-schema-validator/src/main/java/com/networknt/schema/dialect/Dialect.java:417 so schema metadata fields like draft-04 id, $anchor, and $dynamicAnchor are only read when the
node is actually a string. Non-string nodes now return null instead of calling Jackson 3 asString() and throwing on ObjectNode.

Added json-schema-validator/src/test/java/com/networknt/schema/Issue1246Test.java:1, a focused draft-04 regression test that resolves
http://json-schema.org/draft-04/schema#/properties/title, which hits the same failure path as the Swagger reproducer.

Verified:

mvn -q -Dtest=Issue1246Test test
mvn -q -Dtest=Issue994Test,RefTest,SchemaTest test
mvn -q test

Full test suite passed. Maven printed JDK/GraalVM native-access deprecation warnings, but no failures.

[stevehu]

This fix is perfectly correct. Using isString() safely ensures that Dialect.java ignores any non-string id properties (which usually indicate nested object schemas) instead of throwing value type not coercible in Jackson 3.

I went ahead and ran the entire mvn test suite on this updated branch. All 8,444 tests passed successfully without a single error.

The fix is robust, compliant with Draft-04 semantics, and correctly adapts to the new strictness of Jackson 3. It looks fantastic and is ready to merge! Let me know if you need me to commit or review anything else.