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.
70 lines
2.3 KiB
Markdown
70 lines
2.3 KiB
Markdown
# Part 1 — ObjectMapper: reading and writing
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Post: <https://ankurm.com/jackson-objectmapper-guide/>
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## B01 — writing
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[`B01WriteJson.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B01WriteJson.java)
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String, File and pretty-printed output.
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```
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{"articleId":1,"title":"Jackson Deep Dive","tags":["java","json"]}
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file : {"articleId":1,"title":"Jackson Deep Dive","tags":["java","json"]}
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pretty :
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{
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"articleId" : 1,
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"title" : "Jackson Deep Dive",
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"tags" : [ "java", "json" ]
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}
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```
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## B02 — reading
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[`B02ReadJson.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B02ReadJson.java)
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String, File and InputStream. The post's `readValue(new URL(...), ...)` example is
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deliberately not reproduced — it makes a live network call, so it could not be part of
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a reproducible run. The `InputStream` form is what an HTTP client hands you in practice.
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```
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from String : Jackson Deep Dive
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from File : 1
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from Stream : [java, json]
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```
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## B03 — generic collections
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[`B03GenericCollections.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B03GenericCollections.java)
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`TypeReference` lives in `tools.jackson.core.type` in Jackson 3. The example also runs
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the wrong version — `readValue(json, List.class)` — to show what it actually produces.
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```
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size : 2
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element class : Article
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first title : First
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raw element : LinkedHashMap <- not Article
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cast fails : ClassCastException, as expected
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map value : Nine
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```
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The failure is deferred, not immediate: the call succeeds and returns a
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`List<LinkedHashMap>`. You find out at the first cast, often far from the parse site.
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## B04 — property ordering
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[`B04PropertyOrdering.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B04PropertyOrdering.java)
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Beyond the post, but it explains why your first output does not look like the article's.
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```
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POJO : {"listPrice":79.99,"productId":1,"productName":"Mechanical Keyboard"}
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record : {"productId":1,"productName":"Mechanical Keyboard","listPrice":79.99}
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ordered : {"productId":1,"productName":"Mechanical Keyboard","listPrice":79.99}
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```
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Getter-based POJOs serialise alphabetically; records follow declaration order;
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`@JsonPropertyOrder` overrides both. Worth pinning explicitly if the JSON is
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cached, signed, or diffed against a fixture.
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