13

C H A P T E R

Binary Compatibility

J

AVA development tools should support automatic recompilation as necessary

whenever source code is available. Particular implementations of Java may also

store the source and binary of types in a versioning database and implement a

ClassLoader

 ( 20.14) that uses integrity mechanisms of the database to prevent

linkage errors by providing binary compatible versions of types to clients.

Developers of packages and classes that are to be widely distributed face a

different set of problems. In the Internet, which is our favorite example of a

widely distributed system, it is often impractical or impossible to automatically

recompile the pre existing binaries that directly or indirectly depend on a type that

is to be changed. Instead, Java defines a set of changes that developers are permit 

ted to make to a package or to a class or interface type while preserving (not

breaking) compatibility with existing binaries.

The paper quoted above appears in

Proceedings of OOPSLA '95

, published as

ACM SIGPLAN Notices

, Volume 30, Number 10, October 1995, pages 426 438.

Within the framework of that paper, Java binaries are binary (release to release)

compatible under all relevant transformations that the authors identify. Using their

scheme, here is a list of some important binary compatible changes that Java sup 

ports:

Reimplementing existing methods, constructors, and initializers to improve

performance.

Changing methods or constructors to return values on inputs for which they

previously either threw exceptions that normally should not occur or failed by

going into an infinite loop or causing a deadlock.

Adding new fields, methods, or constructors to an existing class or interface.

Deleting

private

 fields, methods, or constructors of a class or interface.

When an entire package is updated, deleting default (package only) access

fields, methods, or constructors of classes and interfaces in the package.

Reordering the fields, methods, or constructors in an existing type declaration.

237