如何访问默认包中的 java 类？

Question

I'm working now together with others in a grails project. I have to write some Java-classes. But I need access to an searchable object created with groovy. It seems, that this object has to be placed in the default-package.

My question is: Is there a way to access this object in the default-package from a Java-class in a named package?

Answer 1

You can't use classes in the default package from a named package.
( Technically you can, as shown in Sharique Abdullah's answer through reflection API, but classes from the unnamed namespace are not in scope in an import declaration )

Prior to J2SE 1.4 you could import classes from the default package using a syntax like this:

import Unfinished;

That's no longer allowed . So to access a default package class from within a packaged class requires moving the default package class into a package of its own.

If you have access to the source generated by groovy, some post-processing is needed to move the file into a dedicated package and add this "package" directive at its beginning.

---

Update 2014: bug 6975015 , for JDK7 and JDK8, describe an even stricter prohibition against import from unnamed package.

The TypeName must be the canonical name of a class type, interface type, enum type, or annotation type.
The type must be either a member of a named package , or a member of a type whose outermost lexically enclosing type is a member of a named package , or a compile-time error occurs .

---

Andreas points out in the comments :

"why is [the default package] there in the first place? design error?"

No, it's deliberate.
JLS 7.4.2. Unnamed Packages says: "Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications or when just beginning development".

Answer 2

In fact, you can.

Using reflections API you can access any class so far. At least I was able to :)

Class fooClass = Class.forName("FooBar");
Method fooMethod = fooClass.getMethod("fooMethod", String.class);

String fooReturned = (String)fooMethod.invoke(fooClass.newInstance(), "I did it");

Answer 3

Use jarjar to repackage the jar file with the following rule:

rule * <target package name>.@1

All classes in the default package of the source jar file will move to the target package, thus are able to access.

Answer 4

You can use packages in the Groovy code, and things will work just fine.

It may mean a minor reorganization of code under grails-app and a little bit of a pain at first, but on a large grails project, it just make sense to organize things in packages. We use the Java standard package naming convention com.foo.<app>.<package> .

Having everything in the default package becomes a hindrance to integration, as you're finding.

Controllers seem to be the one Grails artifact (or artefact) that resists being put in a Java package. Probably I just haven't figured out the Convention for that yet. ;-)

Answer 5

just to complete the idea:

From inside default-package you can access objects resided in named packages.