[news.eclipse.tools.jdt] Re: AST and Java Model relation

[Walter Harley <eclipse@xxxxxxxxxxxxxx>]

Aker wrote:
> Hi JDT tool group,
>
> I have just been given an assigment to extend  JDT for a customer. A truly 
> great job.
>
> I have found some very good articels (and postings on this newsgroup) etc, 
> but I'm still a bit confused about the relation
> between the AST and the Java Model. As I understand it the AST is the result 
> of parsing a CompilationUnit
> (e.g. class), but at the same time the Java Model is involved somewhere and 
> I think perhaps the Model keeps all
> the symbols for the entire project and holds references to several AST (per 
> class involved in project). But I'm not sure.
>
> Can someone explain this to me a bit more?

The Java Model is coarser-grained: it has less information.  But its 
information is more readily and cheaply available.  To get an AST, you 
have to compile, which is slow.  To get a type from the Java Model, you 
just need to have it on the build path.

Generally if you are just asking about types (e.g., what methods does a 
class have), the Java Model is what you want.  If you are trying to 
manipulate code or look inside methods, you need an AST.

Also be aware that there are a couple of different typesystems in there, 
partly because of the history of the Eclipse IDE.  So be wary for what 
package a type is coming from and whether it has "internal," 
"compiler.lookup", etc. in the name.  You'll find many reuses of given 
type names and even of hierarchies, in different coexisting packages. 
As I say, this is mainly just because of backwards-compatibility issues 
when bringing together previously-separate projects.