The Java Compat mapping to skeleton classes requires the servant class to inherit from its skeleton class. Occasionally, this creates a problem: some class libraries require you to inherit from a base class in order to access functionality provided by the library; because Java does not support multiple implementation inheritance, this means that you cannot use such a class library to implement your servants because your servants cannot inherit from both the library class and the skeleton class simultaneously.
To allow you to still use such class libraries, Ice provides a way to write servants that replaces inheritance with delegation. This approach is supported by tie classes. The idea is that, instead of inheriting from the skeleton class, you simply create a class (known as an implementation class or delegate class) that contains methods corresponding to the operations of an interface. You use the
["java:tie"] metadata directive to create a tie class. For example, adding this directive in front of the
Node interface we saw previously makes
slice2java emit an additional tie class. For an interface
<interface-name>, the generated tie class has the name
This looks a lot worse than it is: in essence, the generated tie class is simply a servant class (it extends
_NodeDisp) that delegates to your implementation class each invocation of a method corresponding to a Slice operation:
A skeleton class, tie class, and implementation class.
The generated tie class also implements the
Ice.TieBase interface, which defines methods for obtaining and changing the delegate object:
The delegate has type
java.lang.Object in these methods in order to allow a tie object's delegate to be manipulated without knowing its actual type. However, the
ice_delegate modifier raises
ClassCastException if the given delegate object is not of the correct type.
Given this machinery, we can create an implementation class for our
Node interface as follows:
Note that this class is identical to our previous implementation, except that it implements the
_NodeOperations interface and does not extend
_NodeDisp (which means that you are now free to extend any other class to support your implementation).
To create a servant, you instantiate your implementation class and the tie class, passing a reference to the implementation instance to the tie constructor:
Alternatively, you can also default-construct the tie class and later set its delegate instance by calling
When using tie classes, it is important to remember that the tie instance is the servant, not your delegate. Furthermore, you must not use a tie instance to incarnate an Ice object until the tie has a delegate. Once you have set the delegate, you must not change it for the lifetime of the tie; otherwise, undefined behavior results.
You should use the tie approach only when necessary, that is, if you need to extend some base class in order to implement your servants: using the tie approach is more costly in terms of memory because each Ice object is incarnated by two Java objects (the tie and the delegate) instead of just one. In addition, call dispatch for ties is marginally slower than for ordinary servants because the tie forwards each operation to the delegate, that is, each operation invocation requires two function calls instead of one.
Also note that, unless you arrange for it, there is no way to get from the delegate back to the tie. If you need to navigate back to the tie from the delegate, you can store a reference to the tie in a member of the delegate. (The reference can, for example, be initialized by the constructor of the delegate.)