MismatchedInputException Java Origin: Application & API Last updated 13 Oct 2026

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList` from Object value

Jackson found JSON whose shape doesn’t match the target Java type (object vs array, string vs number, missing constructor).

Exact message you may see Cannot deserialize value of typeCannot construct instance of
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Seen on: Java

Meaning

Typical: the JSON is wrapped ({"data": [...]}) while you map to a List, a single value where a list is expected, or a class without a default constructor/creator.

Common causes

  • JSON wrapper object around the array
  • Single value instead of array
  • No default constructor (records/immutable classes without creators)
  • Empty body

⚡ Quick fix

  1. Map to a wrapper class or read the subtree
  2. Enable ACCEPT_SINGLE_VALUE_AS_ARRAY when appropriate
  3. Add a no-args constructor or @JsonCreator

Detailed fix by platform

Java

  1. java
    JsonNode root = mapper.readTree(json);
List<Item> items = mapper.convertValue(root.get("data"), new TypeReference<>() {});

How to diagnose

  1. Shape — What does the JSON look like at that path?
  2. Type — What type are you mapping to?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

circular JSON — TypeError: Converting circular structure to JSON (JavaScript)circular JSONEJSONPARSE — npm ERR! code EJSONPARSE — Failed to parse json / Unexpected token in JSON at position (npm)EJSONPARSEentity.parse.failed — API Error: Malformed JSON in request body (entity.parse.failed / JSON parse error) (API)entity.parse.failedHttpMessageNotReadableException — HttpMessageNotReadableException: JSON parse error / Required request body is missing (Java)HttpMessageNotReadabl…InvalidDefinitionException — com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class X and no properties discovered (Java)InvalidDefinitionExce…is an invalid start of a value — System.Text.Json: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0 (.NET)is an invalid start o…json_decode null — json_decode() returns null / JSON_ERROR_SYNTAX: Syntax error (PHP)json_decode nullJSONDecodeError — json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0) (Python)JSONDecodeErrorMismatche…

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