Java XML-RPC API Design Guideline

Overview

Design XML-RPC APIs with clear exception handling, proper return types, and interoperable serialization.

Core principle: Exceptions signal failure, return values signal success. Use JAXB for cross-language compatibility.

Quick Reference

Scenario	Pattern
API interface method	ReturnType method(Param p) throws XmlRpcException
Void-like operation	Return int, always 0 (value is meaningless, workaround for spec limitation)
Success result	Return value (DTO, primitive, etc.)
Failure result	Throw XmlRpcException
DTO serialization	Use JAXB annotations (@XmlRootElement, @XmlAttribute)

Exception Handling

All API Methods Must Declare XmlRpcException

Every interface method must include throws XmlRpcException:

public interface SomeApi {
    SomeDto someAction(SomeParameterDto request) throws XmlRpcException;
}

Why: XML-RPC library can transmit one exception type to client. Most projects use extension features enabling this. The exception serializes as:

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse xmlns:ex="http://ws.apache.org/xmlrpc/namespaces/extensions">
    <fault>
        <value>
            <struct>
                <member>
                    <name>faultCode</name>
                    <value><i4>1</i4></value>
                </member>
                <member>
                    <name>faultString</name>
                    <value>failed to execute api</value>
                </member>
            </struct>
        </value>
    </fault>
</methodResponse>

Avoid: enabledForException feature. It serializes Java exceptions including chained exceptions, but only works with Java clients.

Return Type Guidelines

Void Methods Must Return int

XML-RPC library doesn't support void return type. Use int instead:

public interface ServiceControlApi {
    int start(String name) throws XmlRpcException;
}

Rules:

• Always return 0 (regardless of success or failure)
• Signal failure by throwing XmlRpcException
• The return value has no meaning - it exists only because XML-RPC spec doesn't support void

Why: This is purely a workaround for XML-RPC library limitation. If the spec supported void, we would use void. The int return is meaningless; failure is communicated exclusively through exceptions.

Success vs Failure: Clear Separation

Outcome	How to Signal
Operation succeeded	Return value
Query found nothing	Return empty/false (this is success)
Operation failed	Throw XmlRpcException

Example - Query API:

public interface PublicIpApi {
    boolean isExists(InetAddress address) throws XmlRpcException;
}

• IP exists → return true
• IP doesn't exist → return false (success case - query worked)
• System error during query → throw XmlRpcException

Example - Action API:

public interface ServiceControlApi {
    int start(String name) throws XmlRpcException;
}

• Service started → return 0
• Service already running → return 0 (value is meaningless)
• Service failed to start → throw XmlRpcException (this is how failure is signaled)

Why this matters:

• Returning error codes in response (like -1 or error field in DTO) makes XML-RPC request appear successful
• Server logs show success, no stack trace
• Client must inspect response to detect failure
• Debugging becomes difficult

DTO Serialization

Use JAXB, Not Serializable

DTOs must use XML structures via JAXB for cross-language compatibility:

// GOOD: JAXB DTO
@XmlRootElement
public class ComplexJaxbDto implements Element {
    @XmlAttribute
    private String type;
    
    @XmlAttribute
    private String number;
    
    private List<NestedDto> nestedDtos;
    
    private ComplexJaxbDto() { }
    
    public static final class NestedDto {
        private String value;
        private NestedDto() { }
    }
}

// BAD: Serializable DTO (Java-only)
public class SerializableDto implements Serializable {
    private String value;
    public SerializableDto(String value) {
        this.value = value;
    }
}

Serialization Comparison

Serializable output (unreadable, Java-only):

<ex:serializable>rO0ABXNyACx0aWwueG1scnBj...</ex:serializable>

JAXB output (readable, cross-language):

<ex:jaxb>
    <complexJaxbDto number="number" type="type">
        <nestedDtos>
            <value>test</value>
        </nestedDtos>
        <nestedDtos>
            <value>test2</value>
        </nestedDtos>
    </complexJaxbDto>
</ex:jaxb>

Why JAXB:

• XML-RPC is designed for interoperability
• Serializable limits clients to Java
• JAXB produces human-readable XML
• Long-term maintainability

Common Mistakes

Mistake	Problem	Fix
Missing throws XmlRpcException	Client can't receive errors	Add to all API methods
Using void return type	XML-RPC library doesn't support it	Use int, always return 0
Returning -1 or other codes	Meaningless, creates confusion	Always return 0, use exception for failure
Error codes in DTO fields	Request appears successful	Throw XmlRpcException
Using Serializable	Java-only, unreadable	Use JAXB annotations
Using enabledForException	Java-only	Avoid, use standard faults

Idempotency Decisions

For action APIs, decide if operation should be idempotent:

Idempotent approach:

• start() on running service → return 0
• Easier for clients, more forgiving

Strict approach:

• start() on running service → throw XmlRpcException
• Explicit about state transitions

Document your choice in the API contract. Either is valid - consistency matters.

Checklist

Before completing XML-RPC API design:

• All interface methods declare throws XmlRpcException
• No void return types (use int, always return 0)
• Success cases return values (for non-void methods)
• Failure cases throw exceptions (never use return codes)
• For void-like methods: return value is always 0, failure via exception only
• Idempotency behavior documented