Java project structure explained for newbies?

I come from a .NET background and am completely new to Java and am trying to get my head around the Java project structure.

My typical .NET solution structure contains projects that denote logically distinct components, usually named using the format:

MyCompany.SomeApplication.ProjectName

The project name usually equals the root namespace for the project. I might break the namespace down further if it's a large project, but more often than not I see no need to namespace any further.

Now in Java, you have applications consisting of projects, and then you have a new logical level - the package. What is a package? What should it contain? How do you namespace within this App.Project.Package structure? Where do JARs fit into all this? Basically, can someone provide a newbies intro to Java application structure?

Thanks!

Edit: Some really cracking answers thanks guys. A couple of followup questions then:

• Do .JAR files contain compiled code? Or just compressed source code files?
• Is there a good reason why package names are all lower case?
• Can Packages have 'circular dependencies'? In other words, can Package.A use Package.B and vice versa?
• Can anyone just show the typical syntax for declaring a class as being in a package and declaring that you wish to reference another package in a class (a using statement maybe?)

"Simple" J2SE projects

As cletus explained, source directory structure is directly equivalent to package structure, and that's essentially built into Java. Everything else is a bit less clear-cut.

A lot of simple projects are organized by hand, so people get to pick a structure they feel OK with. What's often done (and this is also reflected by the structure of projects in Eclipse, a very dominant Java tool) is to have your source tree begin in a directory called src. Your package-less source files would sit directly in src, and your package hierarchy, typically starting with a com directory, would likewise be contained in src. If you CD to the src directory before firing up the javac compiler, your compiled .class files will end up in the same directory structure, with each .class file sitting in the same directory and next to its .java file.

If you have a lot of source and class files, you'll want to separate them out from each other to reduce clutter. Manual and Eclipse organization often place a bin or classes directory parallel to src so the .class files end up in a hierarchy that mirrors that of src.

If your project has a set of .jar files to deliver capability from third-party libraries, then a third directory, typically lib, is placed parallel to src and bin. Everything in lib needs to be put on the classpath for compilation and execution.

Finally, there's a bunch of this and that which is more or less optional:

• docs in doc
• resources in resources
• data in data
• configuration in conf...

You get the idea. The compiler doesn't care about these directories, they're just ways for you to organize (or confuse) yourself.

J2EE projects

J2EE is roughly equivalent to ASP.NET, it's a massive (standard) framework for organizing Web applications. While you can develop your code for J2EE projects any way you like, there is a firm standard for the structure that a Web container will expect your application delivered in. And that structure tends to reflect back a bit to the source layout as well. Here is a page that details project structures for Java projects in general (they don't agree very much with what I wrote above) and for J2EE projects in particular:

<http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html>

Maven projects

Maven is a very versatile project build tool. Personally, my build needs are nicely met by ant, which roughly compares with nmake. Maven, on the other hand, is complete-lifecyle build management with dependency management bolted on. The libs and source for most of the code in the Java world is freely available in the 'net, and maven, if asked nicely, will go crawling it for you and bring home everything your project needs without you needing to even tell it to. It manages a little repository for you, too.

The downside to this highly industrious critter is the fact that it's highly fascist about project structure. You do it the Maven way or not at all. By forcing its standard down your throat, Maven manages to make projects worldwide a bit more similar in structure, easier to manage and easier to build automatically with a minimum of input.

Should you ever opt for Maven, you can stop worrying about project structure, because there can only be one. This is it: <http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html>

A package in Java is very similar to a namespace in .Net. The name of the package essentially creates a path to the classes that live inside it. This path can be thought of as the class's namespace (in .Net terms) because it is the unique identifier for the specific class you want to use. For example if you have a package named:

org.myapp.myProject

And inside it you had a bunch of classes:

MyClass1
MyClass2

To specifically refer to those classes you would use:

org.myapp.myProject.MyClass1
org.myapp.myProject.MyClass2

The only real difference between this and .Net (that I know of) is that Java organizes its "namespaces" structurally (each package is a distinct folder) whereas .Net allows you to scope classes using the namespace keyword and ignores where the document actually lives.

A JAR file is roughly analogous to a DLL in most cases. It is a compressed file (you can open them with 7zip) that contains source code from other projects that can be added as dependencies in your application. Libraries are generally contained in JARs.

The thing to remember about Java is that is is very structural; WHERE files live is important. Of course there is more to the story then what I posted but I think this should get you started.

A package is much like a .Net namespace. The general convention in Java is to use your reversed domain name as a package prefix so if your company is example.com your packages will probably be:

com.example.projectname.etc...

It can be broken down to many levels rather than just one (projectname) but usually one is sufficient.

Inside your project structure classes are usually divided into logical areas: controllers, models, views, etc. It depends on the type of project.

There are two dominant build systems in Java: Ant and Maven.

Ant is basically a domain-specific scripting language and quite flexible but you end up writing a lot of boilerplate stuff yourself (build, deploy, test, etc tasks). It's quick and convenient though.

Maven is more modern and more complete and is worth using (imho). Maven is different to Ant in that Maven declares that this project is a "Web application project" (called an archetype). Once that is declared the directory structure is mandated once you specify your groupId (com.example) and artifactId (project name).

You get a lot of stuff for free this way. The real bonus of Maven is that it manages your project dependencies for you so with a pom.xml (Maven project file) and correctly configured Maven you can give that to someone else (with your source code) and they can build, deploy, test and run your project with libraries being downloaded automatically.

Ant gets something like this with Ivy.

Here are some notes about Java packages that should get you started:

The best practice with Java package names is to use the domain name of the organisation as the start of the package, but in reverse, e.g. if your company owns the domain "bobswidgets.com", you would start your package off with "com.bobswidgets".

The next level down will often be the application or library level, so if it's your ecommerce libraries, it could be something like "com.bobswidgets.ecommerce".

Further down than that often represents the architecture of your application. Classes and interfaces that are core to the project reside in the "root" e.g. com.bobswidgets.ecommerce.InvalidRequestException.

Using packages to subdivide functionality further is common. usually the pattern is to put interfaces and exceptions into whatever the root of the subdivision is and the implementation into sub packages e.g.

com.bobswidgets.ecommerce.payment.PaymentAuthoriser (interface)
com.bobswidgets.ecommerce.payment.PaymentException
com.bobswidgets.ecommerce.payment.paypal.PaypalPaymentAuthoriser (implementation)

This makes it pretty easy to pull the "payment" classes and packages into their own project.

Some other notes:

Java packages are tightly coupled to directory structure. So, within a project, a class with a Package of com.example.MyClass will invariably be in com/example/MyClass.java. This is because when it is packaged up into a Jar, the class file will definitely be in com/example/MyClass.class.

Java packages are loosely coupled to projects. It is quite common that projects will have their own distinct package names e.g. com.bobswidgets.ecommerce for ecommerce, com.bobswidgets.intranet for the intranet project.

Jar files will container the class files that are the result of compiling your .java code into bytecodes. They are just zip files with .jar extension. The root of the Jar file is the root of the namespace hierarchy e.g. com.bobswidgets.ecommerce will be /com/bobswidgets/ecommerce/ in the Jar file. Jar files can also container resources e.g. property files etc.

A package is a grouping of source files that lets them see each others' package-private methods and variables, so that that group of classes can access things in each other that other classes can't.

The expectation is that all java classes have a package that is used to disambiguate them. So if you open a jar file in your project, like spring, every package starts with org.springframework. The classloaders don't know about the jarfile name, they use only the package.

There's a common practice of breaking things down by type of object or function, not everybody agrees about this. Like Cletus posted here, there's a tendency to group web controllers, domain objects, services, and data access objects into their own packages. I think some Domain-Driven Design people do not think this is a good thing. It does have the advantage that typically everything in your package shares the same kind of dependencies (controllers might depend on services and domain objects, services depend on domain objects and data access objects, etc.) so that can be convenient.

Okay so in java you have three different types of http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html">access</a> to a classes member functions and variables

public protected package-private and private

All classes in the same package can see each others public, protected, and package-private elements.

Packages are not hierarchical in the system. Usually they are organized in a hierarchical way, but as far as runtime is concerned com.example.widgets is a completely different package from com.example.widgets.cogs

Packages are arranged as directories, which helps keep things organized: your file structure is always similar to your package structure.

They are planning on adding a module system to Java in JDK7 (called http://openjdk.java.net/projects/jigsaw/">Project Jigsaw) and there is an existing module system called http://www.osgi.org/About/Technology">OSGi</a>;. These module systems will/can give you a lot more flexibility and power then the simple package system.

Also, package names are usually all lower case. :)

To answer the example sub-question:

package com.smotricz.goodfornaught;

import java.util.HashMap;
import javax.swing.*;

public class MyFrame extends JFrame {

   private HashMap myMap = new HashMap();

   public MyFrame() {
      setTitle("My very own frame");
   }

}

> Do .JAR files contain compiled code? Or just compressed source code files?

They might contain both, or even totally different kinds of files like pictures. It's a zip archive first of all. Most often you would see JARs that contain class files, and those which contain source files (handy for debugging in your IDE if you use third party code) or those that contain javadoc (sourcecode documentatin), also handy if your IDE supports tooltipping the documentation when you access the lib's functions.

>Is there a good reason why package names are all lower case?

Yes there is a good reason for package names to be written in lowercase letters: There is a guideline which says that only classnames are written with a capital letter in front.

>Can Packages have 'circular dependencies'? In other words, can Package.A use Package.B and vice versa?

Packages do not use each other. Only classes do. And yes that might be possible but bad practice.

> Can anyone just show the typical syntax for declaring a class as being in a package and declaring that you wish to reference another package in a class (a using statement maybe?)

Let's assume you want to use the ArrayList class from package java.util, either use

 import java.util.ArrayList;
 ArrayList myList = new ArrayList(); 

or use without import (say you use two different classes named ArrayList from different packages)

 java.util.ArrayList myList = new java.util.ArrayList();
 your.package.ArrayList mySecondList = new your.package.ArrayList();

From Wikipedia:

> A Java package is a mechanism for > organizing Java classes into > namespaces

and

> Java packages can be stored in > compressed files called JAR files

So for package a.b.c, you could have Java classes in the a, a.b, and a.b.c packages. Generally you group classes inside the same package when they represent related functionality. Functionally, the only difference between classes in the same package and classes in different package is that the default access level for members in Java is "package-protected", which means that other classes in the same package have access.

For a class a.b.c.MyClass, if you want to use MyClass in your project you would import a.b.c.MyClass or, less recommended, import a.b.c.* Also, for MyClass to reside in package a.b.c in the first place, you would declare it in the first line of MyClass.java: package a.b.c;.

To do this you could JAR up the whole package (including packages b and c and class MyClass) and put this JAR into your $CLASSPATH; this would make it accessible for your other source code to use (via the aforementioned import statement).

While it is not as easy to make circular dependent classes work, it may not be impossible. I did get it to work in one case. class A and class B depended on each other and wouldn't compile from scratch. but realizing that a part of class A didn't need class B, and that part was what class B needed to compile completely, I rem'd out that part of class A, not needed by class B, and the remaining part of class A was able to compile, then I was able to compile class B. I was then able to un-rem that section of class A that needed class B, and was able to compile the full class A. Both classes then functioned properly. While it is not typical, if the classes are tied together like this, it is kosher and at times possibly necessary. Just make sure you leave yourself special compile instructions for future updates.