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All service implementations must implement If a Java component is derived from a Java helper class that comes with the SDK, it supports XInterface automatically. Otherwise, it is sufficient to add XInterface or any other UNO interface to the implements list. The Java UNO runtime takes care of XInterface. In C++, there are helper classes to inherit that already implement XInterface. However, if XInterface is to be implemented manually, consider the code below.

The IDL specification for looks like this:

  // module com::sun::star::uno
  interface XInterface
      any queryInterface( [in] type aType ); 
      [oneway] void acquire(); 
      [oneway] void release(); 

Requirements for queryInterface()

When queryInterface() is called, the caller asks the implementation if it supports the interface specified by the type argument. The UNOIDL base type stores the name of a type and its The call must return an interface reference of the requested type if it is available or a void any if it is not. There are certain conditions a queryInterface() implementation must meet:

Constant Behaviour

If queryInterface() on a specific object has once returned a valid interface reference for a given type, it must always return a valid reference for any subsequent queryInterface() call for the same type on this object. A query for XInterface must always return the same reference.
If queryInterface() on a specific object has once returned a void any for a given type, it must always return a void any for the same type.


If queryInterface() for XBar on a reference xFoo returns a reference xBar, then queryInterface() on reference xBar for type XFoo must return xFoo or calls made on the returned reference must be equivalent to calls to xFoo.

Object Identity

In C++, two objects are the same if their XInterface are the same. The queryInterface() for XInterface will have to be called on both. In Java, check for the identity by calling the runtime function

The reason for this specifications is that a UNO runtime environment may choose to cache queryInterface() calls. The rules are identical to the rules of the function QueryInterface() in MS COM.

Tip.png If you want to implement queryInterface() in Java, for example, you want to export less interfaces than you implement, your class must implement the Java interface

Reference Counting

The methods acquire() and release() handle the lifetime of the UNO object. This is discussed in detail in chapter Lifetime of UNO objects. Acquire and release must be implemented in a thread-safe fashion. This is demonstrated in C++ in the section about C++ components below.

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