asmhatre
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.
65 lines
3.0 KiB
Java
65 lines
3.0 KiB
Java
package com.ankurm.jackson3.part0setup;
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import com.fasterxml.jackson.annotation.JsonInclude;
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import tools.jackson.databind.DeserializationFeature;
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import tools.jackson.databind.json.JsonMapper;
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import java.time.LocalDate;
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/**
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* Post: Jackson 101 — https://ankurm.com/jackson-java-tutorial/
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* Section: "The ObjectMapper Lifecycle — The One Rule That Matters Most"
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*
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* Build the mapper ONCE at startup and share it. In Jackson 3 this is enforced
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* by the API rather than by convention: the built mapper has no setters at all.
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*/
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public class A02SharedMapperConfiguration {
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public record Invoice(Long invoiceId, String customerName, LocalDate issuedOn, String note) { }
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/**
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* The Jackson 3 equivalent of the classic Spring @Bean ObjectMapper.
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*
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* Three of the four settings people habitually copy from Jackson 2 tutorials
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* are unnecessary or wrong in Jackson 3 — the comments say which and why.
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*/
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static JsonMapper buildSharedMapper() {
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return JsonMapper.builder()
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// NOT NEEDED: FAIL_ON_UNKNOWN_PROPERTIES already defaults to false in
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// Jackson 3. Listed here only because it is the single most-copied line
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// from Jackson 2 configuration; deleting it changes nothing.
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.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
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// NOT NEEDED: java.time support is built into jackson-databind 3.x and
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// dates already serialise as ISO-8601 strings. There is no
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// SerializationFeature.WRITE_DATES_AS_TIMESTAMPS in Jackson 3 — the flag
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// moved to tools.jackson.databind.cfg.DateTimeFeature and is off by
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// default. See beyond/Y03DateTimeDefaults for proof.
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// Skip null fields. Jackson 2's setSerializationInclusion(...) and the
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// builder's serializationInclusion(...) do NOT exist in Jackson 3.
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// The real API is changeDefaultPropertyInclusion.
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.changeDefaultPropertyInclusion(
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incl -> incl.withValueInclusion(JsonInclude.Include.NON_NULL))
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.build();
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}
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public static void main(String[] args) {
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JsonMapper mapper = buildSharedMapper();
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Invoice invoice = new Invoice(500L, "Alice", LocalDate.of(2026, 4, 9), null);
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System.out.println("configured : " + mapper.writeValueAsString(invoice));
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JsonMapper plain = JsonMapper.builder().build();
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System.out.println("defaults : " + plain.writeValueAsString(invoice));
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// Proof that the mapper is immutable: ObjectMapper in Jackson 3 exposes no
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// set*() mutators at all, so there is no way to reconfigure a shared instance.
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long setterCount = java.util.Arrays.stream(
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tools.jackson.databind.ObjectMapper.class.getMethods())
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.filter(m -> m.getName().startsWith("set"))
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.count();
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System.out.println("ObjectMapper set*() methods in Jackson 3: " + setterCount);
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}
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}
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