When serializing a Map via Converter(StdDelegatingSerializer), a NullPointerException is thrown due to missing key serializer

[k163377]

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Describe the bug

For a class that wraps a Map, registering and serializing a StdDelegatingSerializer with a Converter that unwraps it will throw the following error

com.fasterxml.jackson.databind.JsonMappingException: Cannot invoke "com.fasterxml.jackson.databind.JsonSerializer.serialize(Object, com.fasterxml.jackson.core.JsonGenerator, com.fasterxml.jackson.databind.SerializerProvider)" because "keySerializer" is null (through reference chain: java.util.ImmutableCollections$Map1["a"])

	at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:400)
	at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:359)
	at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:324)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:810)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeWithoutTypeInfo(MapSerializer.java:763)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:719)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:34)
	at com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer.serialize(StdDelegatingSerializer.java:166)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:503)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:342)
	at com.fasterxml.jackson.databind.ObjectMapper._writeValueAndClose(ObjectMapper.java:4838)
	at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:4079)
	at com.fasterxml.jackson.databind.Kotlin873Test.directTest(Kotlin873Test.java:61)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
	at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:232)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:55)
Caused by: java.lang.NullPointerException: Cannot invoke "com.fasterxml.jackson.databind.JsonSerializer.serialize(Object, com.fasterxml.jackson.core.JsonGenerator, com.fasterxml.jackson.databind.SerializerProvider)" because "keySerializer" is null
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:796)
	... 34 more

Version Information

Confirmed on 2.18(7144db0) and 2.19(6729641) branches.

Reproduction

You can run it by pasting it into src/test/java/com/fasterxml/jackson/databind.

package com.fasterxml.jackson.databind;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.module.SimpleSerializers;
import com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer;
import com.fasterxml.jackson.databind.util.StdConverter;
import org.junit.Test;

import java.util.Map;

public class Kotlin873Test {
    // region: DTO Definitions
    static class MapWrapper {
        private final Map<String, Object> value;

        MapWrapper(Map<String, Object> value) {
            this.value = value;
        }

        public Map<String, Object> getValue() {
            return value;
        }
    }
    // endregion

    // region: Jackson settings
    static class WrapperConverter extends StdConverter<MapWrapper, Object> {
        @Override
        public Object convert(MapWrapper value) {
            return value.getValue();
        }
    }

    static class MySerializers extends SimpleSerializers {
        @Override
        public JsonSerializer<?> findSerializer(SerializationConfig config, JavaType type, BeanDescription beanDesc) {
            Class<?> rawClass = type.getRawClass();
            if (MapWrapper.class.isAssignableFrom(rawClass)) {
                return new StdDelegatingSerializer(new WrapperConverter());
            }
            return super.findSerializer(config, type, beanDesc);
        }
    }

    private final ObjectMapper mapper;

    public Kotlin873Test() {
        SimpleModule sm = new SimpleModule();
        sm.setSerializers(new MySerializers());
        mapper = new ObjectMapper().registerModule(sm);
    }
    // endregion

    private final MapWrapper wrapper = new MapWrapper(Map.of("a", 1));

    @Test
    public void directTest() throws JsonProcessingException {
        String json = mapper.writeValueAsString(wrapper);
        System.out.println(json);
    }
}

Expected behavior

At least it shouldn't be a NullPointerException.

Additional context

This error can be suppressed by changing the definition of WrapperConverter as follows to make the specification to generics concrete.

static class WrapperConverter extends StdConverter<MapWrapper, Map<String, Object>> {
    @Override
    public Map<String, Object> convert(MapWrapper value) {
        return value.getValue();
    }
}

This was not done because I was trying to reproduce a problem that had been reported to kotlin-module.

[cowtowncoder]

Agreed, at very least should not throw NPE. Hoping to look into this soon.