Add `FunctionalScalarDeserializer` for functional-style deserialization by jhan0121 · Pull Request #5595 · FasterXML/jackson-databind

jhan0121 · 2026-01-18T04:31:17Z

Implements: #4004

This PR add functional-style deserializer extending StdScalarDeserializer that allows using Function<String, T> or BiFunction<JsonParser, DeserializationContext, T> for simple scalar-to-object conversions.

…FasterXML#4004) Add functional-style deserializer extending StdScalarDeserializer that allows using Function<String, T> or BiFunction<JsonParser, DeserializationContext, T> for simple scalar-to-object conversions.

github-actions · 2026-01-18T05:11:24Z

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github-actions · 2026-01-19T01:22:55Z

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github-actions · 2026-01-19T01:29:24Z

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JooHyukKim · 2026-01-19T16:06:32Z

src/main/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializer.java

+ * Usage examples:
+ * <pre>
+ * // Simple case - method reference
+ * new FunctionalScalarDeserializer&lt;&gt;(Bar.class, Bar::of)


Let's fix the brackets, and ones below.

Suggested change

* new FunctionalScalarDeserializer<>(Bar.class, Bar::of)

* new FunctionalScalarDeserializer<>(Bar.class, Bar::of)

JooHyukKim · 2026-01-19T16:09:06Z

src/test/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializerTest.java

+/**
+ * Tests for {@link FunctionalScalarDeserializer}.
+ */
+// [databind#4004]


Let's add more meaningful information, see this test class

JooHyukKim · 2026-01-19T16:09:14Z

src/test/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializerTest.java

+ * Tests for {@link FunctionalScalarDeserializer}.
+ */
+// [databind#4004]
+public class FunctionalScalarDeserializerTest


Suggested change

public class FunctionalScalarDeserializerTest

public class FunctionalScalarDeserializer4004Test

JooHyukKim · 2026-01-19T16:09:34Z

src/test/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializerTest.java

+        Bar result = mapper.readValue("\"\"", Bar.class);
+        assertNull(result);
+    }
+}


Suggested change

}

}

JooHyukKim · 2026-01-19T16:11:18Z

src/main/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializer.java

+     * Subclasses may override to provide custom behavior.
+     * Default implementation returns null.
+     */
+    protected Object _deserializeFromEmptyStringDefault(DeserializationContext ctxt)


Does this need to be protected why not private?

Good point. I've reconsidered the design.

The core intent of FunctionalScalarDeserializer is to accept a function to deserialize into a user-specified type. Users are expected to customize behavior by passing a Function or BiFunction, rather than by subclassing.

Given this, I believe we should start with the simplest form. If subclassing becomes necessary in the future, we can easily expand visibility from private to protected without breaking backward compatibility.

I'll apply the following changes:

_deserializeFromEmptyStringDefault: change to private

_deserializeFromEmptyString: change to private

Remove the copy constructor (as it is currently unused)

JooHyukKim · 2026-01-19T16:13:04Z

src/test/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializerTest.java

+    @Test
+    public void testSimpleFunctionFromString() throws Exception
+    {
+        SimpleModule module = new SimpleModule("test", Version.unknownVersion());


Since version value is fixed toVersion.unknownVersion() in all places, let's replace ALL SimpleModule declarations with

Suggested change

SimpleModule module = new SimpleModule("test", Version.unknownVersion());

SimpleModule module = new SimpleModule("test");

src/main/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializer.java

JooHyukKim · 2026-01-19T16:19:51Z

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Also @jhan0121 could this PR result in non-negative test coverage change? :-) tnx

jhan0121 · 2026-01-20T06:39:25Z

Thanks for the detailed review, @JooHyukKim! I have addressed your comments and updated the code accordingly.

github-actions · 2026-01-21T04:08:34Z

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cowtowncoder · 2026-01-21T04:17:05Z

src/main/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializer.java

+        }
+
+        try {
+            return _function.apply(p, ctxt);


Ok this is problematic: this ignores text altogether; and will fail for

text = ctxt.extractScalarFromObject(p, this, _valueClass);

case from above. I think it will be necessary to keep track of type of original Function being passed and:

In case of "complex" 2-arg function, just invoke that function without doing about any of things in this method

In case of "simple" from-string function, use code here and just pass text to function

Thanks for catching this! You're right, the extracted text was being ignored.

I've fixed it by separating the storage into two distinct fields to handle each case correctly:

_biFunction: invoked directly without preprocessing.

_stringFunction: receives the extracted text directly.

github-actions · 2026-01-21T04:20:10Z

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src/main/java/tools/jackson/databind/deser/std/FunctionalScalarDeserializer.java

cowtowncoder

Looks good, almost ready to merge. Couple of minor comments added.

github-actions · 2026-01-22T00:03:21Z

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…ckson-databind into 4004-functional-deserializer

cowtowncoder

LGTM

github-actions · 2026-01-22T03:48:17Z

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cowtowncoder added the cla-needed PR looks good (although may also require code review), but CLA needed from submitter label Jan 18, 2026

cowtowncoder added cla-received PR already covered by CLA (optional label) and removed cla-needed PR looks good (although may also require code review), but CLA needed from submitter labels Jan 18, 2026

Merge branch '3.x' into 4004-functional-deserializer

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Merge branch '3.x' into 4004-functional-deserializer

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JooHyukKim reviewed Jan 19, 2026

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Refine FunctionalScalarDeserializer per review feedback

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Merge branch '3.x' into 4004-functional-deserializer

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Refine FunctionalScalarDeserializer per review feedback

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Merge branch '3.x' into 4004-functional-deserializer

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cowtowncoder changed the title ~~Implement #4004: Add FunctionalScalarDeserializer for functional-style deserialization~~ Add FunctionalScalarDeserializer for functional-style deserialization Jan 22, 2026

cowtowncoder added 2 commits January 21, 2026 19:44

Minor tweaks from code review; release notes

b9bb79c

Merge branch '4004-functional-deserializer' of github.com:jhan0121/ja…

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…ckson-databind into 4004-functional-deserializer

cowtowncoder self-requested a review January 22, 2026 03:47

cowtowncoder approved these changes Jan 22, 2026

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cowtowncoder merged commit 08df310 into FasterXML:3.x Jan 22, 2026
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cowtowncoder mentioned this pull request Jan 22, 2026

Custom static factory-based deserializer? #4004

Closed

JooHyukKim mentioned this pull request Jan 24, 2026

Confusing error-handling logic in FunctionalScalarDeserializer (Wrapped with exceptions) #5608

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cowtowncoder added a commit that referenced this pull request Jan 25, 2026

Fix foe #5595 wrt #5608: support wrap-error config setting

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cowtowncoder added a commit that referenced this pull request Jan 25, 2026

Fix for #5595 wrt #5608: support wrap-error config setting (#5610)

5407220

	* new FunctionalScalarDeserializer<>(Bar.class, Bar::of)
	* new FunctionalScalarDeserializer<>(Bar.class, Bar::of)

	public class FunctionalScalarDeserializerTest
	public class FunctionalScalarDeserializer4004Test

	SimpleModule module = new SimpleModule("test", Version.unknownVersion());
	SimpleModule module = new SimpleModule("test");

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jhan0121 commented Jan 18, 2026

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JooHyukKim commented Jan 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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JooHyukKim commented Jan 19, 2026 •

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