Inexact approximation of UNO Value Semantics

OpenOffice.org Developer's Guide

Professional UNO

< Previous Section

Next Section >

• Introduction
• API Concepts

• Data Types

• Simple Types
• The Any Type
• Interfaces
• Services
• Structs
• Predefined Values
• Sequences
• Modules
• Exceptions
• Singletons

• Understanding the API Reference

• UNO Concepts

• UNO Interprocess Connections

• Starting OpenOffice.org in Listening Mode
• Importing a UNO Object
• Characteristics of the Interprocess Bridge
• Opening a Connection
• Creating the Bridge
• Closing a Connection
• Example: A Connection Aware Client

• Service Manager and Component Context

• Service Manager
• Component Context

• Using UNO Interfaces
• Properties
• Collections and Containers
• Event Model
• Exception Handling
• Lifetime of UNO objects

• acquire() and release()
• The XComponent Interface
• Children of the XEventListener Interface
• Weak Objects and References
• Differences Between the Lifetime of C++ and Java Objects

• Object Identity

• UNO Language Bindings

• Java Language Binding

• Getting a Service Manager
• Transparent Use of Office UNO Components
• Handling Interfaces
• Type Mappings

• Mapping of Sequence Types
• Mapping of Enum Types
• Mapping of Struct Types
• Mapping of Exception Types
• Mapping of Interface Types
• Mapping of UNOIDL
• Mapping of Services
• Mapping of Singletons
• Inexact approximation of UNO Value Semantics

• C++ Language Binding

• Establishing Interprocess Connections
• Transparent Use of Office UNO Components
• Type Mappings

• Mapping of Struct Types
• Mapping of Interface Types
• Mapping of Sequence Types
• Mapping of Services
• Mapping of Singletons

• Using Weak References
• Exception Handling in C++

• OpenOffice.org Basic

• Handling UNO Objects

• Instantiating UNO Services
• Getting Information about UNO Objects

• Mapping of UNO and Basic Types

• Mapping of Simple Types
• Mapping of Sequences and Arrays
• Mapping of Structs
• Mapping of Enums and Constant Groups

• Case Sensitivity
• Exception Handling
• Listeners

• Automation Bridge

• The Service Manager Component
• Using UNO from Automation

• Calling Functions and Accessing Properties
• Return Values
• Usage of Types

• Using Automation Objects from UNO
• Type Mappings

• Default Mappings

• Mapping of Simple Types
• Mapping of Hyper and Decimal
• Mapping of String
• Mapping of Interfaces and Structures
• Mapping of Sequence
• Mapping of Type

• Conversion Mappings
• Client-Side Conversions
• Value Objects
• Exceptions and Errorcodes

• Automation Objects with UNO Interfaces
• DCOM
• The Bridge Services
• Unsupported COM Features

• CLI Language Binding

• The Language Binding DLLs
• Type Mapping

• Type Name Decoration
• Type Mappings
• Additional Structures

• Lifetime Management and Obtaining Interfaces
• Writing Client Programs

• The Override Problem
• Important Interfaces and Implementations

< Previous Page

Next Page >

Some UNO types that are generally considered to be value types are mapped to reference types in Java. Namely, these are the UNO types string, type, any, and the UNO sequence, enum, struct, and exception types. The problem is that when a value of such a type (a Java object) is used:

• As the value stored in an any
• As the value of a sequence component
• As the value of a struct or exception member
• As the value of an interface attribute
• As an argument or return value in an interface method invocation
• As an argument in a service constructor invocation
• As a raised exception

then Java does not create a clone of that object, but instead shares the object via multiple references to it. If the object is now modified through any one of its references, all other references observe the modification, too. This violates the intended value semantics.

The solution chosen in the Java language binding is to forbid modification of any Java objects that are used to represent UNO values in any of the situations listed above. Note that for Java objects that represent values of the UNO type string, or a UNO enum type, this is trivially warranted, as the corresponding Java types are immutable. This would also hold for the UNO type type, if the Java class com.sun.star.Type were final.

In the sense used here, modifying a Java object A includes modifying any other Java object B that is both (1) reachable from A by following one or more references, and (2) used to represent a UNO value in any of the situations listed above. For a Java object that represents a UNO any value, the restriction to not modify it only applies to a wrapping object of type com.sun.star.uno.Any (which should really be immutable), or to an unwrapped object that represents a UNO value of type string or type, or of a sequence, enum, struct or exception type.

Note that the types java.lang.Boolean, java.lang.Byte, java.lang.Short, java.lang.Integer, java.lang.Long, java.lang.Float, java.lang.Double, and java.lang.Character, used to represent certain UNO values as any values or as parametric members of instantiated polymorphic struct types, are immutable, anyway, and so need not be considered specially here.

Content on this page is licensed under the Public Documentation License (PDL).