How does a java compiler resolve a non-imported name

Question

Consider I use a type X in my java compilation unit from package foo.bar and X is not defined in the compilation unit itself nor is it directly imported. How does a java compiler resolve X now efficiently? There are a few possibilities where X could reside:

1. X might be imported via a star import a.b.*
2. X might reside in the same package as the compilation unit
3. X might be a language type, i.e. reside in java.lang

The problem I see is especially (2.). Since X might be a package-private type, it is not even required that X resides in a compilation unit that is named X.java. Thus, the compiler must look into all entries of the class path and search for any classes in a package foo.bar, it then must read every class that is in package foo.bar to check whether X is included.

That sounds very expensive. Especially when I compile only a single file, the compiler has to read dozens of class files only to find a type X. If I use a lot of star imports, this procedure has to be repeated for a lot of types (although class files won't be read twice, of course).

So is it advisable to import also types from the same package to speed up the compilation process? Or is there a faster method for resolving an unimported type X which I was not able to find?

Answer 1

That sounds very expensive.

If the compiler did it that way, it would be expensive.

But what actually happens is that it builds an in-memory data structure containing all of the class names on the classpath, bootclasspath and sourcepath, and uses that for all of the classname resolutions in all classes that are compiled in the javac run.

So is it advisable to import also types from the same package to speed up the compilation process? Or is there a faster method for resolving an unimported type X which I was not able to find?

No, and no. It will make almost no difference. And besides, this is unlikely to be a significant bottleneck if you use the compiler how it was designed to be used.

It is better to do the imports in a way that gives the most readable and reliable code.

Answer 2

Thus, the compiler must look into all entries of the class path and search for any classes in a package foo.bar

That's just Class.forName().

it then must read every class that is in package foo.bar to check whether X is included.

I don't know what this means.

After looking for a .class file via Class.forName(), it then has to look on the sourcepath in the current package for a file by the appropriate name, which is just new File(...).exists() etc.

I'm not convinced that it looks inside each file for non-public classes by that name: you would have to try it. If it does, that is indeed an expensive step, but I'm not convinced it's taken.