Ankur
c438afc33b
37 runnable examples covering the eight feature posts on ankurm.com, verified against Jackson 3.2.1 on Temurin 21.0.5. Every output committed under docs/ was produced by run-all.sh. Also documents 11 places where the published snippets do not compile or do not behave as printed against a real Jackson 3 build - most notably that writeValueAsString(List<Base>) silently drops the polymorphic type discriminator, so the post's serialised output cannot be read back.
112 lines
4.5 KiB
Markdown
112 lines
4.5 KiB
Markdown
# Part 5 — Polymorphic deserialisation
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Post: <https://ankurm.com/jackson-polymorphic-deserialization/>
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This part contains the most consequential correction in the whole repo, so it gets the
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most space.
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## The defect
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The post's "Serialising a Mixed List" section shows:
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```java
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List<PaymentMethod> payments = List.of(card, bank);
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String jsonOutput = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(payments);
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```
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and prints output containing `"paymentType" : "credit_card"`.
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It does not produce that. `writeValueAsString(Object)` sees only the runtime class of
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the argument — `ImmutableCollections.ListN` — which carries no element type. Without a
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declared element type Jackson never engages the polymorphic type serialiser, and the
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discriminator is silently omitted from every element. No exception, no warning.
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The consequence is not cosmetic. That JSON cannot be read back: the deserialiser has
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no type id to dispatch on and throws `InvalidTypeIdException`. A service that writes
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with the post's code and reads with the post's code does not round-trip.
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## F01 — the defect and both fixes
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[`F01SerialiseMixedList.java`](../src/main/java/com/ankurm/jackson3/part5polymorphic/F01SerialiseMixedList.java)
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```
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--- 1. single element: discriminator present ---
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{"paymentType":"credit_card","amountDue":99.99,"cardNetwork":"VISA","cardNumberLastFour":"4242","paymentId":1}
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--- 2. BROKEN: writeValueAsString(List) drops paymentType ---
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[{"amountDue":99.99,"cardNetwork":"VISA","cardNumberLastFour":"4242","paymentId":1},{"amountDue":250.0,"bankAccountIban":"GB29NWBK60161331926819","bankName":"National Bank","paymentId":2}]
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--- 3. FIX A: writerFor(TypeReference) ---
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[ {
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"paymentType" : "credit_card",
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"amountDue" : 99.99,
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"cardNetwork" : "VISA",
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"cardNumberLastFour" : "4242",
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"paymentId" : 1
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}, {
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"paymentType" : "bank_transfer",
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"amountDue" : 250.0,
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"bankAccountIban" : "GB29NWBK60161331926819",
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"bankName" : "National Bank",
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"paymentId" : 2
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} ]
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--- 4. FIX B: a typed array carries its component type ---
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[{"paymentType":"credit_card","amountDue":99.99,"cardNetwork":"VISA","cardNumberLastFour":"4242","paymentId":1},{"paymentType":"bank_transfer","amountDue":250.0,"bankAccountIban":"GB29NWBK60161331926819","bankName":"National Bank","paymentId":2}]
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```
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Block 1 shows a single element serialising correctly, which is why this is easy to miss
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in a unit test that only checks one object. Block 2 is the collection, discriminator
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absent. Blocks 3 and 4 are the two fixes:
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- `mapper.writerFor(new TypeReference<List<PaymentMethod>>() {})` — declares the element
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type on the writer.
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- `payments.toArray(new PaymentMethod[0])` — an array carries its component type at
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runtime, so no extra declaration is needed.
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## F02 — round-trip proof
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[`F02DeserialiseMixedList.java`](../src/main/java/com/ankurm/jackson3/part5polymorphic/F02DeserialiseMixedList.java)
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```
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Card ending: 4242
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Bank: National Bank
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lossy JSON round-trip -> InvalidTypeIdException
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correct JSON round-trip -> 2 payments, CreditCardPayment first
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```
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Deserialisation itself works exactly as the post describes; it is the serialisation side
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that is wrong. The middle two lines are the demonstration: lossy output fails, correctly
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written output survives.
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## F03 — the four include strategies
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[`F03IncludeStrategies.java`](../src/main/java/com/ankurm/jackson3/part5polymorphic/F03IncludeStrategies.java)
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The post gives these as a table. Here they are executed, with the read-back to confirm
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each wire format is symmetric.
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```
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PROPERTY : {"kind":"card","amountDue":99.0}
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WRAPPER_OBJECT : {"card":{"amountDue":99.0}}
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WRAPPER_ARRAY : ["card",{"amountDue":99.0}]
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EXISTING_PROPERTY : {"kind":"card","amountDue":99.0}
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read PROPERTY -> PropCard[amountDue=99.0]
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read WRAPPER_OBJECT -> WrapObjCard[amountDue=99.0]
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read WRAPPER_ARRAY -> WrapArrCard[amountDue=99.0]
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```
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`EXISTING_PROPERTY` needs `visible = true` and a real field of that name on the type;
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without it the discriminator is written but not populated back onto the object.
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## F04 — bad discriminators
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[`F04UnknownTypeId.java`](../src/main/java/com/ankurm/jackson3/part5polymorphic/F04UnknownTypeId.java)
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```
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unknown logical name -> InvalidTypeIdException
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attacker-supplied class name -> InvalidTypeIdException
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missing discriminator -> InvalidTypeIdException
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```
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All three fail the same way, which is the reassuring answer: an unregistered logical
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name, an attacker-supplied fully qualified class name, and a missing discriminator are
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indistinguishable to the type resolver, and none of them instantiate anything.
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