1. Multi-Dispatch in the Java™ Virtual Machine
Design, Implementation, and Evaluation
Christopher Dutchyn
Computing Science
University of Alberta
11/8/2018
Multi-Dispatch Java

2. The Problem
Production OO languages (C++, Java) select the method to run based on the dynamic type of a single argument only:
We call this uni-dispatch }
Color
HPJet
PScript
.draw(Shape)
The process of selecting code based on types is called “dispatch”. When the compile-time type is used, we call it “static dispatch”, when the execution-time type is used, we call it “dynamic dispatch”.
11/8/2018
Multi-Dispatch Java

3. The Goal
We want method selection based on the dynamic types of more than one, and possibly all of the arguments:
We call this multiple dispatch ( ) }
Circle
Ellipse
Square
Rectangle
Color
HPJet
PScript
.print
We need multiple dispatch. It appears in many situations:
binary operations: equals, compareTo
graphical user interfaces: components & events
drag and drop: document type & target
cut and paste: clip type & target application
11/8/2018
Multi-Dispatch Java

4. Double Dispatch Solutions
Type fields – switch on constant integers
maintain types encoded as obscure numbers
risk that some type fields might be omitted
Typecases – if …instanceof …else if…else…
risk that some types might not be tested for
instanceof tests contain order dependencies
Visitor pattern – sequence of uni-dispatches
duplicates dispatcher code into many classes
They encode the type hierarchy of the program into many different locations in the program, resulting in brittle, difficult to maintain, and error-prone code.
11/8/2018
Multi-Dispatch Java

5. Another Solution
Perform a single dynamic dispatch operation that considers the dynamic types of all of the arguments
We call this
multi-dispatch.
The code is cleaner; the language runtime is responsible for executing the correct implementation.
11/8/2018
Multi-Dispatch Java

6. Research Contributions
Added dynamic multi-dispatch to Java
without changing syntax or compiler,
allowing programmer to select classes supporting multi-dispatch,
without penalizing existing uni-dispatch,
and maintaining reflection and existing APIs.
Realizes the following benefits
shorter, simpler, and more maintainable code,
with equal or better performance than complex, error-prone double dispatch.
Published COOTS 2001
Best student paper award.
11/8/2018
Multi-Dispatch Java

7. Multi-Dispatch At a Call Site
Look up the uni-dispatch method
If multi-dispatchable
Walk the operand stack to obtain precise types HPJet and Circle instead of Printer and Shape
Select the method that most closely matches the argument types:
Verify return types conform
Invoke new method
We have to wait until uni-dispatch because (a) we need to know the number of arguments and the size of each one, and (b) we want to ensure that if a subclass implements VMD but superclass does not, then the subclass method invocations still are multi-dispatched.
HPJet.print(Circle)
11/8/2018
Multi-Dispatch Java

8. Evaluation: Two Criteria
Compatibility with uni-dispatch
Do existing uni-dispatch Java programs continue to run correctly?
What performance penalty does our additions impose on pure uni-dispatch programs?
Performance versus double dispatch
How does multi-dispatch compare with existing double dispatch techniques?
Does multi-dispatch scale to full applications?
11/8/2018
Multi-Dispatch Java

9. Uni-Dispatch Java Support
The java compiler, javac, is a large Java program that runs over a Java Virtual Machine.
As part of constructing the JDK, javac runs on the just-constructed JVM to compile classes comprising the Java Class Libraries.
Our multi-dispatch JVMs host those compilations.
11/8/2018
Multi-Dispatch Java

10. Uni-Dispatch Overhead
individual uni-dispatch times
multi-invoker adds zero
Inlined tests adds 2.5%
sun.tools.* compile
multi-dispatch adds 2-3%
cache contention due to larger data structures
class load overhead exaggerated by repeated loading
11/8/2018
Multi-Dispatch Java

11. Double vs. Multi-Dispatch
Using existing event processing kernel from sun.awt.component
7 event types
7 components
Comparison of
original kernel
typecases
type fields
Visitor
multi-dispatch (SRP)
11/8/2018
Multi-Dispatch Java

12. Multi-Swing (and AWT) Modified 92 of 846 classes (11%)
Replaced 171 conditionals (5%)
Mean number of decision points reduced from 3.8 to 2.0 per method
Added 57 new event subclasses
Added 123 new multimethods
Working with this real-world application reinforced
1) some of the issues regarding null parameters, return types, visibility, and type promotion.
11/8/2018
Multi-Dispatch Java

13. Multi-Swing Results
The multi-dispatch version executes 8% fewer dispatches and the multi-dispatches represent approximately 7.7% of the dispatches.
Total execution time is 0.3 seconds shorter (2%).
11/8/2018
Multi-Dispatch Java

14. Research Contributions
Added dynamic multi-dispatch to Java
without changing syntax or compiler,
allowing programmer to select classes supporting multi-dispatch,
without penalizing existing uni-dispatch,
and maintaining reflection and existing APIs.
Realizes the following benefits
shorter, simpler, and more maintainable code,
with equal or better performance than complex, error-prone double dispatch.
Published COOTS 2001
Best student paper award.
11/8/2018
Multi-Dispatch Java