union-type-wrappers

name	union-type-wrappers
description	Add typed getters and setters over BinaryData properties that represent TypeSpec union types in generated Java models. Use when generated classes expose BinaryData for union-typed fields and you need ergonomic, type-safe accessors instead.

Union Type Wrappers for Generated Java Models

When the Java codegen encounters a TypeSpec union type (e.g. string | SomeModel), it emits the property as BinaryData. This skill replaces public BinaryData accessors with typed setters and getters for each union variant, following the same pattern established for PromptAgentDefinition.toolChoice.

Preconditions

• You must be in a Java SDK module directory containing tsp-location.yaml and pom.xml.
• TypeSpec sources must be available in TempTypeSpecFiles/. If missing, run tsp-client sync first.
• The project must compile before starting.

Important: BinaryData.writeTo(JsonWriter) semantics

Understanding how BinaryData writes to JSON is critical for choosing the correct factory method:

Factory method	Content type	writeTo(JsonWriter) calls	JSON output
BinaryData.fromString("auto")	StringContent	jsonWriter.writeString("auto")	"auto" (quoted)
BinaryData.fromObject("auto")	SerializableContent	jsonWriter.writeRawValue(...)	"auto" (quoted via Jackson)
BinaryData.fromObject(42.0)	SerializableContent	jsonWriter.writeRawValue(...)	42.0 (raw)
BinaryData.fromObject(true)	SerializableContent	jsonWriter.writeRawValue(...)	true (raw)
BinaryData.fromObject(jsonSerializable)	SerializableContent	jsonWriter.writeRawValue(...)	{...} (JSON object via JacksonAdapter)

Key: JacksonAdapter has special handling for JsonSerializable types — BinaryData.fromObject() and BinaryData.toObject() both work correctly with Azure JsonSerializable models.

Setter factory method rules

Union variant	Factory method	Reason
String value (ID, enum token)	BinaryData.fromString(value)	Writes as JSON string via writeString()
Numeric / boolean primitive	BinaryData.fromObject(value)	Writes as raw JSON value
Azure JsonSerializable model	BinaryData.fromObject(value)	JacksonAdapter handles serialization
Stainless (openai-java) type	BinaryData.fromObject(value)	Jackson handles serialization

Getter deserialization rules

Union variant	Deserialization	Notes
String	this.field.toObject(String.class)	Consistent regardless of how BinaryData was created (fromString vs fromObject during deserialization)
Primitive (Number, Boolean)	this.field.toObject(Double.class) etc.	Jackson deserializes raw JSON values
Azure JsonSerializable model	this.field.toObject(ModelClass.class)	JacksonAdapter calls fromJson()
Stainless type	this.field.toObject(StainlessType.class)	Jackson deserializes natively

Workflow

1. Scan for BinaryData properties

Find all BinaryData fields in generated model classes:

grep -rn "private\s\+\(final\s\+\)\?BinaryData\s\+" src/main/java/ --include="*.java"

Exclude List<BinaryData> and Map<..., BinaryData> from this pass — those are collection-of-union patterns that require separate handling.

2. Cross-reference against TypeSpec

For each BinaryData property found, determine whether it comes from a union type or an unknown type:

# Search for the property name (use the wire name, e.g. tool_choice, not toolChoice)
grep -rn "<wire_name>" TempTypeSpecFiles/ --include="*.tsp"

• Union type (type_a | type_b): Proceed with this skill.
• unknown type: Leave as BinaryData — this is the correct representation.

Also check client.tsp for any @@changePropertyType overrides that may have already flattened the union to string (like tool_choice).

3. Identify the union variants

For each union type, determine what types the property can hold. Sources:

1. Local TSP files — look at the type definition in TempTypeSpecFiles/.
2. Stainless SDK JAR — if the union comes from OpenAI.*, inspect the Stainless types:

   jar tf ~/.m2/repository/com/openai/openai-java-core/<version>/openai-java-core-<version>.jar | grep "<TypeName>"
   javap -cp <jar> "com.openai.models.responses.<OuterClass>\$<InnerUnion>"
   

3. Generated Azure models — check if the Azure SDK already generates the variant model classes (e.g. AutoCodeInterpreterToolParam, McpToolFilter).

4. Apply changes to the model class

For each union-typed BinaryData property, apply the following pattern:

4a. Leave the property field as-is

Do not modify the property declaration. Keep @Generated, the block comment, and the visibility exactly as the codegen produced them. The field is already private — there is nothing to change.

/*
 * Original generated block comment.
 */
@Generated
private BinaryData myField;

4b. Make the existing getter and setter package-private

• Remove @Generated.
• Change visibility to package-private (no access modifier). This keeps them hidden from SDK consumers but accessible to unit tests in the same package.
• Keep the original method name — do NOT rename to *Internal.
• Keep the original javadoc intact.
• Add // AI Tooling: union type as the first line inside the method body.

/**
 * Get the myField property: original description.
 *
 * @return the myField value.
 */
BinaryData getMyField() {
    // AI Tooling: union type
    return this.myField;
}

/**
 * Set the myField property: original description.
 *
 * @param myField the myField value to set.
 * @return the MyClass object itself.
 */
MyClass setMyField(BinaryData myField) {
    // AI Tooling: union type
    this.myField = myField;
    return this;
}

For @Immutable classes (value is a constructor param):

• Change the BinaryData constructor visibility to package-private (keep for fromJson deserialization).
• Add public constructor overloads for each variant.
• Make the BinaryData getter private.

/**
 * Creates an instance of MyFilter class.
 *
 * @param type the type value to set.
 * @param key the key value to set.
 * @param value the value value to set.
 */
MyFilter(MyFilterType type, String key, BinaryData value) {
    // AI Tooling: union type
    this.type = type;
    this.key = key;
    this.value = value;
}

public MyFilter(MyFilterType type, String key, String value) {
    this.type = type;
    this.key = key;
    this.value = BinaryData.fromObject(value);
}

4c. Add typed setters (one per union variant)

Naming convention: set<PropertyName>(<VariantType> value) — use method overloading.

Copy the javadoc from the original generated setter, adapting the @param description to the specific variant type. Add // AI Tooling: union type as the first line inside the method body.

/**
 * Set the myField property: original description.
 *
 * @param myField the string value to set.
 * @return the MyClass object itself.
 */
public MyClass setMyField(String myField) {
    // AI Tooling: union type
    this.myField = BinaryData.fromString(myField);
    return this;
}

/**
 * Set the myField property: original description.
 *
 * @param myField the SomeModel value to set.
 * @return the MyClass object itself.
 */
public MyClass setMyField(SomeModel myField) {
    // AI Tooling: union type
    this.myField = BinaryData.fromObject(myField);
    return this;
}

When overloading isn't possible (e.g. two different String meanings), disambiguate with parameter names and javadoc.

For List<String> variants:

/**
 * Set the allowedTools property: original description.
 *
 * @param allowedTools the list of tool name strings to set.
 * @return the McpTool object itself.
 */
public McpTool setAllowedTools(List<String> allowedTools) {
    // AI Tooling: union type
    this.allowedTools = BinaryData.fromObject(allowedTools);
    return this;
}

4d. Add typed getters (one per union variant)

Naming convention: get<PropertyName>As<TypeName>()

Copy the javadoc from the original generated getter, adapting the @return description. Add // AI Tooling: union type as the first line inside the method body.

/**
 * Get the myField property as a String: original description.
 *
 * @return the myField value as a String.
 */
public String getMyFieldAsString() {
    // AI Tooling: union type
    if (this.myField == null) {
        return null;
    }
    return this.myField.toObject(String.class);
}

/**
 * Get the myField property as a {@link SomeModel}: original description.
 *
 * @return the myField value as a SomeModel.
 */
public SomeModel getMyFieldAsSomeModel() {
    // AI Tooling: union type
    if (this.myField == null) {
        return null;
    }
    return this.myField.toObject(SomeModel.class);
}

For List<String> variants:

/**
 * Get the allowedTools property as a list of tool name strings: original description.
 *
 * @return the allowedTools value as a list of Strings.
 */
@SuppressWarnings("unchecked")
public List<String> getAllowedToolsAsStringList() {
    // AI Tooling: union type
    if (this.allowedTools == null) {
        return null;
    }
    return this.allowedTools.toObject(List.class);
}

5. Update callers

Search for existing code that uses the old BinaryData API:

grep -rn "\.setMyField(BinaryData\|\.getMyField()" src/ --include="*.java"

Update samples, tests, and internal code to use the new typed API. Remove unused BinaryData imports where applicable.

6. Write unit tests

Create a test class per model under src/test/java/.../models/<ModelName>SerializationTests.java.

Each test class must include:

1. Serialization tests — one per union variant. Construct the model with the typed setter, serialize to JSON, assert the JSON contains the expected field and value.
2. Deserialization tests — one per union variant. Parse a JSON string, assert the typed getter returns the correct value.
3. Null/absent tests — verify getters return null when the field is not set or absent from JSON.
4. Round-trip tests — serialize → deserialize → assert values match.

Use these helpers:

private String serializeToJson(MyModel model) throws IOException {
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    try (JsonWriter jsonWriter = JsonProviders.createWriter(outputStream)) {
        model.toJson(jsonWriter);
    }
    return outputStream.toString("UTF-8");
}

private MyModel deserializeFromJson(String json) throws IOException {
    try (JsonReader jsonReader = JsonProviders.createReader(json)) {
        return MyModel.fromJson(jsonReader);
    }
}

7. Compile and run tests

mvn compile -Dbuildhelper.addtestsource.skip=true -Dbuildhelper.addtestresource.skip=true \
    -Dcodesnippet.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true

mvn "-Dtest=*SerializationTests" test \
    -Dcodesnippet.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true

All tests must pass before finishing.

Checklist

Before reporting completion, verify:

• Every BinaryData property was classified as union or unknown
• Property fields left exactly as generated (no modifications)
• Original BinaryData getter/setter made package-private, name kept, @Generated removed, javadoc preserved
• // AI Tooling: union type placed inside the body of every modified or added getter/setter
• Typed setters added for each union variant with javadoc copied from original
• Typed getters added for each union variant (get*As*()) with javadoc copied from original
• All callers (samples, tests, internal code) updated to use new API
• Unused BinaryData imports removed from callers
• Unit tests written and passing for serialization, deserialization, null, and round-trip
• Full compilation succeeds (main + test + samples)

Troubleshooting

Symptom	Cause	Fix
BinaryData.fromObject(jsonSerializable) produces wrong JSON	JacksonAdapter not on classpath	Verify azure-core dependency includes JacksonAdapter
toObject(AzureModel.class) fails	JacksonAdapter doesn't find fromJson	Use BinaryData.toObject() which delegates to JacksonAdapter.deserialize() — confirm azure-core ≥ 1.51
Setter creates StringContent but expects raw JSON	Wrong factory method	Use fromString() only for string tokens; use fromObject() for primitives and objects
Test fails on deserialized value comparison	Asymmetry between fromString/fromObject for string values	Deserialization always uses fromObject(readUntyped()), producing SerializableContent. Use toObject(String.class) in string getters — never toString() — to normalize both paths.
Compilation error: cannot find List	Missing import after adding List<String> setter	Add import java.util.List;
@SuppressWarnings needed	Unchecked cast on toObject(List.class)	Add @SuppressWarnings("unchecked") to the method