Java Package Names and File Structure Relationship

javapackages

I'm currently refreshing my Java knowledge and came across a question I couldn't answer yet:

How does the Java compiler know where the (possibly custom) packages are located on disk? I'd assume that using built-in libraries it looks up the java library folder – but how does it find the custom ones?

Here's the file's locations for the following example:

// part of the package
com/usuallyNot/example.java

// not part of the package
com/example2.java

Is the naming convention to be seen as relative path? If it's this, why do we need to write "import package com.usuallyNot.example;" (instead of "import package "usuallyNot.example;") even if the file we write it in resides in the "com" folder?

Best Answer

The answer is the classpath:

Classpath is a parameter—set either on the command-line, or through an environment variable—that tells the Java Virtual Machine or the Java compiler where to look for user-defined classes and packages.

There are instructions from Oracle how to set your classpath.

The classpath is searched in order of the folders that appear for a class matching the import. There are lots of details in how multiple classes of the same name can co-exist.

The classpath is set either by %CLASSPATH% (Windows DOS shell, $env:CLASSPATH in PowerShell) or $CLASSPATH in Unix or in the manifest of a Jar file.

You can see the future of classpath from the 2015 JavaOne keynot (starting @ 17 minutes).

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