Recipe UseMapOf generates Map.of() with nullable values or duplicate keys, causing runtime NullPointerException

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@henriquejsza is already working on this.

Since Oct 30, 2026.

  • #1382 by @henriquejsza — open

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
java
Domain
backend

Research direction

Start with the UseMapOf recipe and UseMapOfTest.java, which the issue names, and trace how put arguments are checked before conversion to Map.of. Review the existing null check and add coverage for nullable values and duplicate keys; the issue includes a parameter example and expected behavior. Done means unsafe maps are left unchanged while safe maps still rewrite, with the regression tests passing.

Written by the indexing model from the issue text.

Description

bug

I checked out all relevant OpenRewrite repo's, fed the AI with existing recipe name and a very careful description. Antigravity with Vertex / Gemini was then tasked to check and test. .Also to create a ticket for it, using your default template. The result is shown beneath.

What version of OpenRewrite are you using?

  • rewrite-core: 8.93.0
  • rewrite-maven-plugin: 6.50.0

How are you running OpenRewrite?

Maven plugin with recipe org.openrewrite.java.migrate.util.UseMapOf.

What is the smallest, simplest way to reproduce the problem?

UseMapOf currently only checks !J.Literal.isLiteralValue(arg, null). When parameters, method calls, or duplicate keys are passed to .put(k, v), it rewrites to Map.of(...), which strictly rejects nulls (NPE per JEP 269) and duplicate keys (IllegalArgumentException):

import java.util.HashMap;
import java.util.Map;

class Test {
    Map<String, Object> create(String dataOwner) {
        Map<String, Object> map = new HashMap<>();
        map.put("owner", dataOwner); // nullable parameter
        map.put("id", "123");
        return map;
    }
}

What did you expect to see?

Do not rewrite into Map.of(...) unless keys and values are statically guaranteed non-null, and keys are distinct.

What did you see instead?

Rewritten to new HashMap<>(Map.of("owner", dataOwner, "id", "123")), throwing NullPointerException at runtime when dataOwner is null:

java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "val" is null
    at java.base/java.util.KeyValueHolder.<init>(KeyValueHolder.java:54)
    at java.base/java.util.Map.of(Map.java:1320)
Solution in UseMapOf.java

Abort rewrite unless both key and value satisfy isKnownNonNull(Expression) and keys are unique:

if (!isKnownNonNull(key) || !isKnownNonNull(value)) {
    hasUnsafePut = true;
    break;
}

isKnownNonNull returns true only for:

  • Non-null J.Literal, primitives, J.NewClass, J.NewArray, String concatenation (+)
  • Class literals (X.class), Enums, and non-null static final constants
Regression Test in UseMapOfTest.java
@Test
void parameterInPutNotRewritten() {
    rewriteRun(
      java(
        """
          import java.util.HashMap;
          import java.util.Map;

          class Test {
              void m(String dataOwner) {
                  Map<String, Object> map = new HashMap<>();
                  map.put("dataOwner", dataOwner);
                  map.put("id", "6");
              }
          }
          """
      )
    );
}
Dominant language
Java
Stars
162
Forks
139
Avg merge
10h 28m
Merged PRs (30d)
121

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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