Why do java source files require package declarati

问题:

I think I am failing to understand java package structure, it seemed redundant to me that java files have a package declaration within, and then are also required to be present in a directory that matches the package name. For example, if I have a MyClass.java file:

package com.example;

public class MyClass {
    public static void main(String[] args) {
        System.out.println("Hello, World");
    }
}

Then I would be required to have this file located in com/example, relative to the base directory, and I would execute java com.example.MyClass from the base directory to run it.

Why wouldn't the compiler be able to infer the package name by looking at the directory structure? For example, if I compiled the file from the base directory javac com\example\MyClass.java, I am not understanding why the MyClass.java wouldn't implicity belong to the com.example package.

I understand there is a default package, but it still seems that the package declaration in the source file is redundant information?

回答1:

As you (implicitly) acknowledged, you are not required to declare the name of a package ... in the case if the default package.

Ignoring that quibble ... the reason for this seeming redundancy is that without the requirement for a package declaration, the meaning of Java source code would be ambiguous. For example, a source file whose pathname was "/home/steve/project/src/com/example/Main.java" could have 7 different fully qualified names ... depending on how you compiled the code. Most likely, only one of those will be the "correct" one. But you wouldn't be able to tell which one is correct by looking at (just) the source code file.

It should also be noted that the Java language specification does not require you to organize the source code tree according to the packages. That is a requirement of a (large) family of Java compilers, but a conformant compiler could be written that did not require this. In that eventuality, the package declaration would not be duplicative.

回答2:

Turn the question on its head:

Assume that the package statement is the important thing - It represents the namespace of the class and belongs in the class file.

So now the question is - Why do classes have to be in folders that match their package?

The answer is that it makes finding them much easier - it is just a good way to organize them.

Does that help?

回答3:

You have to keep in mind that packages do not just indicate the folder structure. The folder structure is the convention Java adopted to match the package names, just like the convention that the class name must match the filename.

A package is required to disambiguate a class from other classes with the same name. For instance java.util.Date is different from java.sql.Date. The package also gives access to methods or members which are package-private, to other classes in the same package.

You have to see it the other way round. The class has all the information about itself, the class name and the package name. Then when the program needs it, and the class is not loaded yet, the JVM knows where to look for it by looking at the folder structure that matches the package name and the class with the filename matching its class name.

回答4:

In fact there's no such obligation at all.

Oracle JDKs javac (and I believe most other implementations too) will happily compile your HelloWorld class, no matter what directory it is in and what package you declare in the source file.

Where the directory structure comes into the picture is when you compile multiple source files that refer to each other. At this point the compiler must be able to look them up somehow. But all it has in the source code is the fully qualified name of the referred class (which may not even have been compiled yet).

At runtime the story is similar: when a class needs to be loaded, its fully qualified name is the starting point. Now the class loader's job is to find a .class file (or an entry in a ZIP file, or any other imaginable source) based on the FQN alone, and again the simplest thing in a hierarchical file system is to translate the package name into a directory structure.

The only difference is that at runtime your "standalone" class too has to be loaded by the VM, therefore it needs to be looked up, therefore it should be in the correct folder structure (because that's how the bootstrap class loader works).