在java中导入该类时，是导入该类的源代码还是字节码？

Question

我已经开始学习 java 中的包。我了解了 import 关键字。现在我怀疑是 .class 文件还是 .java 文件在我们导入 java 中的任何类时被导入。

Answer 1

An import in a Java files does nothing more than allowing you to refer to a class by it's short name rather than it's fully qualified name.

So if you use a class named some.package.CoolClass then you can either use that full name everywhere in your code or just put import some.package.CoolClass; on the top and call it just CoolClass in your code.

The compiler needs to be able to load the class the you are using (whether or not you imported it) and that means it either needs to find the .class file of that class or also compile the .java file at the same time (ie if you have two classes A and B and compile them both at the same time then each can reference the other, even if there technically aren't any class files of them around at that time).

At runtime (ie when you actually execute your code) you'll need the .class files of each class you use on your classpath. (.java files are not used at runtime). Again, it doesn't matter if you used an import to use the short name or used the fully qualified name in your code.

Answer 2

Does the compiler use .class or .java files for an import?

Answer: .class files, possibly compiling .java files.

This works, even in the following case (when there are no .class files):

// p/a/A.java
package p.a;
import p.b.B;
public class A {
    public static final A DEFAULT_A = new B();
}

// p/b/B.java
package p.b;
import p.a.A;
public class B extends A {
}

The reason that this (a cyclic dependency) can be dealt with, is that .class files are comparable with .obj / .o object files: there is symbolic information (class and method names) in the .class files. And just as the object files are linked with a linker to an executable or library, the java class loader resolves everything.