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Apache Fory™ xlang serialization is the Java wire mode for payloads that must be read by Python, Rust, Go, JavaScript/TypeScript, C++, C#, Swift, Dart, Scala, Kotlin, or another non-Java Fory runtime. Java defaults to xlang mode with compatible schema evolution, but examples set the mode explicitly so the payload contract is visible in code.

Create an Xlang Runtime

Use one long-lived Fory or ThreadSafeFory instance per configuration. Creating a runtime is expensive because Fory caches type metadata and generated serializers.

import org.apache.fory.Fory;

Fory fory = Fory.builder()
    .withXlang(true)
    .requireClassRegistration(true)
    .withRefTracking(true)
    .build();

withRefTracking(true) is required only when the cross-language data model includes shared object identity or cycles. Disable it for value-shaped schemas.

Use Native Serialization instead when every writer and reader is Java and the payload should preserve Java-specific object behavior.

Register Types

Types must be registered with consistent IDs or names across all languages. Fory supports two registration methods.

Register by ID (Recommended for Performance)

public record Person(String name, int age) {}

// Numeric ID registration is compact and fast.
fory.register(Person.class, 1);

Person person = new Person("Alice", 30);
byte[] bytes = fory.serialize(person);
// bytes can be deserialized by Python, Rust, Go, etc.

Benefits: faster serialization and smaller binary size.

Trade-off: every service must coordinate IDs so the same logical type uses the same number.

Register by Name (Recommended for Flexibility)

public record Person(String name, int age) {}

// Namespace/type-name registration is easier to coordinate across teams.
fory.register(Person.class, "example", "Person");

Person person = new Person("Alice", 30);
byte[] bytes = fory.serialize(person);
// bytes can be deserialized by Python, Rust, Go, etc.

Benefits: less risk of numeric ID conflicts and easier management across independently owned services.

Trade-off: the payload includes string identity, so it is larger than ID-based registration.

The Java API also supports a single string type name, such as fory.register(Person.class, "example.Person"). Use the same logical identity on every runtime.

Java To Python Example

Java (Serializer)

import org.apache.fory.Fory;

public record Person(String name, int age) {}

public class Example {
  public static void main(String[] args) {
    Fory fory = Fory.builder()
        .withXlang(true)
        .withRefTracking(true)
        .build();

    // Register with the same logical name used by Python.
    fory.register(Person.class, "example.Person");

    Person person = new Person("Bob", 25);
    byte[] bytes = fory.serialize(person);

    // Send bytes to Python by your service transport.
  }
}

Python (Deserializer)

import pyfory
from dataclasses import dataclass

@dataclass
class Person:
    name: str
    age: pyfory.Int32

fory = pyfory.Fory(xlang=True, ref=True)

# Register with the same name as Java.
fory.register_type(Person, typename="example.Person")

person = fory.deserialize(bytes_from_java)
print(f"{person.name}, {person.age}")  # Output: Bob, 25

Handling Circular and Shared References

Xlang mode supports circular and shared references when reference tracking is enabled:

public class Node {
  public String value;
  public Node next;
  public Node parent;
}

Fory fory = Fory.builder()
    .withXlang(true)
    .withRefTracking(true)
    .build();

fory.register(Node.class, "example.Node");

Node node1 = new Node();
node1.value = "A";
Node node2 = new Node();
node2.value = "B";
node1.next = node2;
node2.parent = node1;

byte[] bytes = fory.serialize(node1);
// Python/Rust/Go can correctly deserialize this with circular references preserved

Type Mapping Considerations

Not all Java types have equivalents in other languages. When using xlang mode:

  • Use primitive types (int, long, double, String) for maximum compatibility.
  • Use standard collections (List, Map, Set) instead of language-specific collections.
  • Use reduced-precision carriers (Float16, BFloat16, Float16List, BFloat16List) for 16-bit float payloads.
  • Treat Float16[], BFloat16[], Float16List, and BFloat16List as list<T> carriers by default; use @ArrayType when the schema must be array<float16> or array<bfloat16>.
  • Avoid Java-specific types like Optional, BigDecimal, and EnumSet unless every target runtime has an agreed mapping.
  • See Type Mapping Guide for the complete compatibility matrix.

Lists and Dense Arrays

Java primitive arrays are dense array<T> carriers, except plain byte[], which defaults to bytes. General Java collections and Fory primitive-list carriers such as Int32List, Float16List, and BFloat16List use list<T> unless the field has explicit @ArrayType metadata.

Fory schemaJava field shape
list<int32>List<Integer> or Int32List
array<bool>boolean[]
array<int8>@Int8Type byte[] type-use
array<int16>short[]
array<int32>int[]
array<int64>long[]
array<uint8>@UInt8Type byte[] type-use
array<uint16>@UInt16Type short[] type-use
array<uint32>@UInt32Type int[] type-use
array<uint64>@UInt64Type long[] type-use
array<float16>Float16Array or @Float16Type short[]
array<bfloat16>BFloat16Array or @BFloat16Type short[]
array<float32>float[]
array<float64>double[]

Prefer type-use syntax for primitive-array annotations:

private @UInt32Type int[] ids;
private @BFloat16Type short[] values;

Compatible Types

public record UserData(
    String name,           // compatible
    int age,               // compatible
    List<String> tags,     // compatible
    Map<String, Integer> scores  // compatible
) {}

Problematic Types

public record UserData(
    Optional<String> name,    // not cross-language compatible
    BigDecimal balance,       // limited support
    EnumSet<Status> statuses  // Java-specific collection
) {}

Performance Considerations

Xlang mode has additional overhead compared to Java native mode:

  • Type metadata encoding: Adds extra bytes per type
  • Type resolution: Requires name/ID lookup during deserialization

For best performance:

  • Use ID-based registration when possible (smaller encoding)
  • Disable reference tracking if you don't need circular references (withRefTracking(false))
  • Use native mode (withXlang(false)) when only Java serialization is needed

Best Practices

  1. Use explicit type IDs or namespace/type names for every user type.
  2. Keep compatible mode for independently deployed services.
  3. Test payloads through every runtime before relying on a schema in production.
  4. Use native serialization for Java-only traffic that needs Java-specific object behavior.

Troubleshooting

“Type not registered” errors

  • Verify type is registered with same ID/name on both sides
  • Check if type name has typos or case differences

“Type mismatch” errors

Data corruption or unexpected values

  • Verify both sides use xlang payloads
  • Ensure both sides have compatible Fory versions

See Also

Related Topics