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Exchangeable Swing Element - how to implement? [duplicate]

How would you implement a Plugin-system for your Java application?
Is it possible to have an easy to use (for the developer) system which achieves the following:
Users put their plugins into a subdirectory of the app
The Plugin can provide a configuration screen
If you use a framework, is the license compatible with commercial developement?

First you need an interface that all plugins need to implement, e.g.
public interface Plugin {
public void load(PluginConfiguration pluginConfiguration);
public void run();
public void unload();
public JComponent getConfigurationPage();
}
Plugin authors should then bundle their plugins into JAR files. Your applications opens the JAR file and could then use an attribute from JAR manifest or the list of all files in the JAR file to find the class that implements your Plugin interface. Instantiate that class, the plugin is ready to go.
Of course you may also want to implement some kind of sandboxing so that the plugin is restricted in what it can and can not do. I have created a small test application (and blogged about it) that consists of two plugins, one of which is denied access to local resources.

Use OSGi.
It is the foundation of the Eclipse plug-in system. Equinox is Eclipse's implementation (licensed EPL) and Felix is the Apache Project's implementation (licensed Apache Public License).
Eclipse provides a concrete example that OSGi can cover the points you mentioned (or you could just build your application on top of Eclipse RCP if you want a full Eclipse/SWT/JFace stack).

Since 1.6, there's been java.util.ServiceLoader which can be used if you want to code your own simple system.
But if you want anything more than basic features, use one of the existing frameworks.

Use PF4J.
It has support for Web, Spring and Wicket.
Easy to use and build the applications

There is also JPF (Java Plugin Framework).

I worked on OSGi for a week--an intense, nothing but OSGi week. At the end it was like a bad dream but I learned a lot.
I was able to get OSGi working (not easy, all examples are out of date, everything on the net is at least three years old if not five), but I had serious trouble getting it integrated into an existing project because of issues with the jar manifests.
In short, there are only a few obscure tools used for building manifests and they are not well documented (BND Tools is hardly obscure, but it is designed for a certain process in Eclipse). Also, most of the OSGi information available is not targeted towards application developers who have an existing desktop application.
This makes a lot of the context for the information foggy or inappropriate. Neil Bartlett's blog posts were the biggest help, but even those failed to get a working system (I grabbed some code from the Felix tutorial and pieced it together to get the embedded framework rolling). I found his book draft that he posted for free years ago, which is excellent, but the examples in Eclipse do not work because of changes in Eclipse OSGi support.

I think that recommending OSGi for solving the above stated problem is extremely poor advice. OSGi is "the right choice" but for a scenario as the one above, I think either JPF or some homegrown minimalistic framework is sufficient.

Years ago I started a project like that and I hope soon will be ready.I got inspired by projects like NetBeans and Eclipse but meanwhile it changed to something a little bit different. OSGi looks like a good choice now, but I didn't had a chance to compare it with my project.It is similar with JPF mentioned above, but in the same time different in many ways.
The basic idea which motivated me is to be as easy as possible to build Java application, with no separation between web applications, desktop applications or applet/JWS applications(of course this doesn't cover the UI - yet) as a core functionality.
I built the project with a few goals in my mind :
it doesn't matter if you build a web application or a desktop application you should start the application in the same way, a plain main method, No fancy web.xml declaration(not that I'm against having a standard web descriptor, but it doesn't go well with a plug-in system, where you add "servlets" - I call them RequestHandler(s) - dynamic at your will).
easy to plug in "extensions" around an "extension point" - something from Eclipse but a different approach.
self-deployable, since all the plugins are registered(XML files) the application must be self-deployable independent of the build system - of course there is an Ant task and a Maven MOJO which are the links with the ourside world, but in the end it calls the application and instruct it to self-deploy itself at a specific location.
borrowed from Maven, it can download code from repositories(including Maven 1 & 2 repositories) so your application can be deployed as a single small jar as long as you have access to the repositories(useful sometime, and basically this provides support for auto-updates - don't you love the idea to be notified by your web application that there is a newer version, it was downloaded and it just needs your permission to install it? I know I love that).
basic application monitoring about system health, email notifications in case of failures

How to add a plugin-architecture in my Java application? [duplicate]

How would you implement a Plugin-system for your Java application?
Is it possible to have an easy to use (for the developer) system which achieves the following:
Users put their plugins into a subdirectory of the app
The Plugin can provide a configuration screen
If you use a framework, is the license compatible with commercial developement?

First you need an interface that all plugins need to implement, e.g.
public interface Plugin {
public void load(PluginConfiguration pluginConfiguration);
public void run();
public void unload();
public JComponent getConfigurationPage();
}
Plugin authors should then bundle their plugins into JAR files. Your applications opens the JAR file and could then use an attribute from JAR manifest or the list of all files in the JAR file to find the class that implements your Plugin interface. Instantiate that class, the plugin is ready to go.
Of course you may also want to implement some kind of sandboxing so that the plugin is restricted in what it can and can not do. I have created a small test application (and blogged about it) that consists of two plugins, one of which is denied access to local resources.

Use OSGi.
It is the foundation of the Eclipse plug-in system. Equinox is Eclipse's implementation (licensed EPL) and Felix is the Apache Project's implementation (licensed Apache Public License).
Eclipse provides a concrete example that OSGi can cover the points you mentioned (or you could just build your application on top of Eclipse RCP if you want a full Eclipse/SWT/JFace stack).

Since 1.6, there's been java.util.ServiceLoader which can be used if you want to code your own simple system.
But if you want anything more than basic features, use one of the existing frameworks.

Use PF4J.
It has support for Web, Spring and Wicket.
Easy to use and build the applications

There is also JPF (Java Plugin Framework).

I worked on OSGi for a week--an intense, nothing but OSGi week. At the end it was like a bad dream but I learned a lot.
I was able to get OSGi working (not easy, all examples are out of date, everything on the net is at least three years old if not five), but I had serious trouble getting it integrated into an existing project because of issues with the jar manifests.
In short, there are only a few obscure tools used for building manifests and they are not well documented (BND Tools is hardly obscure, but it is designed for a certain process in Eclipse). Also, most of the OSGi information available is not targeted towards application developers who have an existing desktop application.
This makes a lot of the context for the information foggy or inappropriate. Neil Bartlett's blog posts were the biggest help, but even those failed to get a working system (I grabbed some code from the Felix tutorial and pieced it together to get the embedded framework rolling). I found his book draft that he posted for free years ago, which is excellent, but the examples in Eclipse do not work because of changes in Eclipse OSGi support.

I think that recommending OSGi for solving the above stated problem is extremely poor advice. OSGi is "the right choice" but for a scenario as the one above, I think either JPF or some homegrown minimalistic framework is sufficient.

Years ago I started a project like that and I hope soon will be ready.I got inspired by projects like NetBeans and Eclipse but meanwhile it changed to something a little bit different. OSGi looks like a good choice now, but I didn't had a chance to compare it with my project.It is similar with JPF mentioned above, but in the same time different in many ways.
The basic idea which motivated me is to be as easy as possible to build Java application, with no separation between web applications, desktop applications or applet/JWS applications(of course this doesn't cover the UI - yet) as a core functionality.
I built the project with a few goals in my mind :
it doesn't matter if you build a web application or a desktop application you should start the application in the same way, a plain main method, No fancy web.xml declaration(not that I'm against having a standard web descriptor, but it doesn't go well with a plug-in system, where you add "servlets" - I call them RequestHandler(s) - dynamic at your will).
easy to plug in "extensions" around an "extension point" - something from Eclipse but a different approach.
self-deployable, since all the plugins are registered(XML files) the application must be self-deployable independent of the build system - of course there is an Ant task and a Maven MOJO which are the links with the ourside world, but in the end it calls the application and instruct it to self-deploy itself at a specific location.
borrowed from Maven, it can download code from repositories(including Maven 1 & 2 repositories) so your application can be deployed as a single small jar as long as you have access to the repositories(useful sometime, and basically this provides support for auto-updates - don't you love the idea to be notified by your web application that there is a newer version, it was downloaded and it just needs your permission to install it? I know I love that).
basic application monitoring about system health, email notifications in case of failures

How can I create a "modern looking" Java desktop application?

Similar questions to this are asked periodically, but many of these answers are outdated.
I need to build a cross-platform desktop application in Java with a GUI of comparable quality to contemporary desktop apps.
Swing is the default choice, but I have yet to encounter a Swing application that didn't look, at the very least, quite dated and clunky (subjective, I know, but with GUIs it's hard to avoid aesthetic judgements).
I notice that the new Bitcoin client now uses QT with Java bindings, and does have an attractive user interface, but this has the disadvantage that it is no-longer pure Java.
So much of what I find when I search for Swing-related libraries is 5 years old or older, even though the aesthetics of desktop applications have evolved significantly since then.
If you needed to build a Java desktop application from scratch, what would you use for its GUI?

I can also offer you a new LaF to look into - WebLaF. I'am currently working on it to bring a lot of UI features and make work with Swing much easier for anyone by just using WebLaF library basic features.
A few examples showing how some of WebLaF components look like:
Some of main goals i am targeting:
Great and modern-looking L&F with support for all Swing components
An extensive set of additional components which you won't find in standard Swing
A big set of Utility classes to assist you with writing the code
WebLaF library also suggests a few other advantages and unique features:
It is an open-source project (GPLv3 licensed)
Easy components styling using painters system (specifically with 9-patch files)
Quick and easy customization of the default "Web" style
Lots of features to accelerate and simplify Swing application interface creation
You can try the demo-app to see if it is modern and simple enough :)

Have you looked into JavaFX 2.0? It is designed to interop easily with Swing, and has many modern 'good looking' controls.
Also, as lrAndroid mentions, a Swing app can look like a native app if you set the system look and feel with:
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

Look into changing the Look and Feel of your Java program. This allows you to customize the overall "theme" of your program.
Here is information on changing the LAF.

I've had good experiences with FlatLaf.
It's easy to install, it's cross-platform, it has no external dependency, it's open-source (Apache license), it includes dark modes and there are tons of themes.
After adding :
UIManager.setLookAndFeel( new FlatLightLaf() );
your app looks like it was written during the last decade, and not during the previous millenium.
It also has one high-contrast theme, for accessibility.

Modern Javascript frameworks (ExtJS, Dojo, etc...) offer the same widgets richness or more (Excel like grids for instance), a wider variety of L&F and usually fit better with the OS of the user.
Users are also very comfortable with their browsers and, hey, "modern stuff" is on the web, and the modern web today is HTML+Javascript.
The overhead of converting an app to "web like" is minimal. An embedded Jetty can remain really small and disk space has become much less of an issue.
There are additional benefits going down this route for the future evolutions of the application.
Suddenly, the desktop app becomes a server app, which can be accessed from another desktop. We were able top promote a desktop app to 'portal' in a matter of weeks.
Rewriting the app in terms of (Ajax) web services provides an easy transition to creating a full REST (or SOAP...) services stack. The app can then be integrated to other applications, easily monitored externally, etc...
Support of other devices (smart phones, tablets,...) becomes much easier, by concentrating on the UI layer only
As the app grows, separation of concerns is cleaner; developers working on the UI do not have to mess with low level code.
There are a lot of excellent JS/HTML designers and developers that do not program in Java.
EDIT
3 years later this has become extremely easy thanks to Electron

Try one of these:
JTattoo
JGoodies
Quaqua

What about Nimbus look and feel? Oracle Link
Also take a look at SO-Question

I know this question is old, but if you don't want to use FX and still want to use Swing, then try MacWidgets, i've used it on a couple projects. It's very light, and looks great. Below is an old project i was working on, over time i've perfected using macwidgets and now use it internally in my company.
http://www.digitalhand.net/projects/jdataanalyzer/mainGUI.png

QT is quite extensive but also very big (bloated) and complex. There is also the SWT library being used by open office for instance. SWT uses native UI widgets for buttons, tables etc where as Swing emulates them.
However, the trend is clearly towards writing rich client applications for the browser using HTML and Javascript. Both of these have made huge strides in the past few years.
HTML5 specifically targets rich client applications with features such as better forms and local database to support disconnected scenarios (note that this last feature is not standardized yet but it is implemented by all latest browsers).
Javascript has now powerful libraries with jQuery and its many plugins. There are even languages like Coffeescript which can be used to produce Javascript with a simpler and more powerful syntax.
There's also no need for such apps to connect to an outside server. Small footprint local servers (eg: jetty, node.js, ...) and databases (SQLite, H2,...) can be installed on the client to make a completely self contained application.

Swing is good (stable, documented, supported until 2026). The problem is that it relies on the LookAndFeel system that provide a very limited number of boilerplate choices. It should be easy to change the appearance of each component individually and then unleash its creativity. Unfortunately it is painful. IMHO frameworks should be built on top of Swing to make it possible instead of creating JavaFX.
Take a look at https://github.com/dotxyteam/ReflectionUI.
Ex GUI: http://javacollection.net/reflectionui/wp-content/uploads/2017/08/general.png

Go for Jetpack Compose for Desktop. It's a super modern reactive UI which is nice to work with. It also works the same as it would for Android with very few exceptions. It's Kotlin though instead of Java, but these are both JVM based and it's very easy to make the jump.

What are my options for building a rich web application?

I'm working on a client-side Java application for which I want to create a web-based user interface (which will be served up to a browser from the local app). At the risk of oversimplifying, its a microblogging tool that will support functionality similar to Twitter and Facebook. Its open source, and you can read more on our wiki.
Its important that this app is extremely easy to use, and I also want it to look good relative to contemporary web applications (such as the examples available on the Chrome web store). Since I am not a web designer, I'm hoping to use tools that give me an attractive UI "out of the box". Oh, and I need to keep the distributable file reasonably small (un him so whatever I use needs to be reasonably lightweight.
So far, I've looked at Sproutcore and GWT.
Sproutcore apps definite have the appearance I'm hoping for, but I'm not that familiar with JavaScript, and after some experimentation is not entirely clear to me how I will achieve some of the more sophisticated functionality that I think I'm going to need.
Being Java, GWT is more within my comfort zone, however I had a hell of a time just getting my development environment set up properly. It seems that GWT wants to dictate the structure of my application, but since the user interface is only a small part of it I'm not willing to build the entire app around GWT.
Something else I've been thinking about is using CoffeeScript (which seems more palatable him and him him him him him than JavaScript, and there are Maven builders for it), perhaps together with JQuery-UI.
What other options should I consider?

To recommend two big javascript frameworks Sencha and Cappuccino.
These have an extensive UI toolkit out of the box. The latter has an MVC framework build in, the former has a general JavaScript library build in. Both come highly recommended.
Also as recommended jQuery UI is a reasonable toolkit. I would personally couple Sencha for its rich UI toolkit and a lightweight MVC framework like Backbone. I'm afraid this does require learning JavaScript well.
You can also instead drive your JavaScript UI toolkit mainly from the server and skip the clientside MVC. For that sencha should surfice as a UI toolkit, but if you want meaningful functionality you will need some more structure. I would also recommend RequireJS for modular file management.

Take a look on Vaadin its based on GWT but simpler to use. I use it since 2 years and i'am not a designer too.

Take a look at jQuery & jQuery UI (jQuery Modile if you are looking to deploy to hand held devices)
Also, since you mentioned that you are not a designer, you might want to implement the front end in an MVVM pattern to separate the UI from the business logic of your applications. Knockout is a nice way to do with with decelerative data binding

I don't like writing plain JavaScript, so I'll tell you little bit about Java based frameworks. Recently, I've picked Google Web Toolkit.
Pros:
plenty of materials
Eclipse + Google plugin
Very intuitive API
3rd party libraries
Fancy widgets + optimized JavaScript
lot of generics
very active community
awesome async callbacks
Cons:
very slow debugging, that will drive you mad sooner or later
lot of deprecated code for so young framework
Mixed feelings:
You don't have to write declarative UI (you can do Swing-like development), but your code grows bigger and less maintainable, so it is quite probable that you'll eventually learn some declarative UI like UI Binder
Visual designer for GWT is somehow nice, but very unstable (eclipse) :(
Some considerations:
Do some prototyping with Vaadin. Try some debugging. Then try GWT and debugging on similar sized project. Consider debugging as the VERY important decision factor, because GWT debugging was the most frustrating thing for me.
Consider JSF + some nice rich component framework for JSF like RichFaces, IceFaces, PrimeFaces etc (but you app won't be "light" and "small" anymore if you really care). Be sure to be perfectly comfortable with environment (IDE + plugins), nothing beats JSF there (I'd pick RichFaces + Seam today, if full Java EE is option).

If you don't want to learn javascript I recommend you GWT with Sencha-GWT (GXT), you can develop all your app in java language, and also debug your code in java.
It is very easy to create a web app with this technology, and provides almost all funcionality available in javascript version.
Also you can mix it with hibernate and giled to persist your data, and giled helps you to serialize hibernate objects.

I have been writing an application in SproutCore for the front-end and a Java backend. In my mind this works perfectly.
The backend can stop worrying about HTML and JavaScript contents on the client side, if the request is XHR or plain request/response. I can implement the backends using plain old Java Servlets, and only worry about the data that is passed between the client and the server (JSON for the most part).
The client becomed fast and responsive, and SproutCore has a rich amount of components that you can mix and match.
So in other words with SproutCore (and Cappuccino and other full-blows MVC JavaScript frameworks) you can let your client do what it does best : Render and display HTML, CSS and JavaScript, while leaving your server to worry about what it does best: retrieve, store and update data. If you are interested, the source code for this project is available with the GPLv3 licence: https://github.com/joachimhs/EurekaJ
I have written an article about the differnt approaches between server-side MVC and client-side MVC here: http://haagen.name/2011/05/24/The_RIA_MVC_Model.html.
Also SproutCore comes with the MIT licence, so its avaiable for use without (m)any restrictions.

.NET, Java to JavaScript compiler

I am interested to create a drag-and-drop layout designer using only JavaScript, HTML and CSS. The designer will allow the user to drag the page elements from one place to another (something like Blogger's layout designer) to create a site layout. But I don't want to hand code everything in JavaScript, I would prefer to write my application in .NET (preferably) or Java and rely on a compiler to compile it to JavaScript and HTML.
What are the .NET or Java to JavaScript compilers that you have used and can recommend? For Java to JavaScript I know GWT is available. What about .NET to JavaScript? Microsoft did come out with Volta, but the project seems to be no longer available.

Look no further, you already mentioned GWT pick that!
It has a very good API and many good applications have use them.
Even JavaScript frameworks like http://extjs.com/ have GWT support.
I use it for an small JavaScript calendar recently.
To be honest, I don't really like JavaScript that much. Most of the times the errors are hard to track (specially for a non JavaScript guy as me) and the workarounds included some plug-ins for the explorer just to get exactly what a compiler should do. Catch silly error early.
In the other hand I'm very familiar with the Java Programming language, and many of the libraries (if not the most important) such as java.lang and java.util have been ported to GWT.
Plus, the guy who wrote relevant parts of java.util is the same behind GWT (google Joshua Bloch.)

Check out Nikhil Khotari's Script# project. It allows you to write C# code and compiles it to JavaScript.

Script# has already been mentioned. It hasn't been updated since August 2008.
Milescript is another, but also has seen very little for 6 months.
Extsharp for the Ext library. Adds Ext support for Script#
Javascript compiler to Java (going the wrong way for you)
Java to script Eclipse plugin
My issues with Script# (a known issue) is it doesn't support jQuery yet. However it comes with a very lightweight library to tie in with the .NET framework, in Nikhil's sscorlib.js file and ssfx.core.js files. And also has support for lots of other Javascript APIs (mostly Microsoft, seeing as he is in the ASP.NET team).
I'd love to see a Script# extension for jQuery (I'm thinking about writing it if it's easy enough). As it stands, most don't provide full compilation yet but they're certainly getting there.
Update: I wrote a small extension to enable JQuery support Script# a few months ago. The project can be found here.

I'm going to second the use of GWT. I've used it for several projects and, when used in combination with a modern editor like Eclipse or IDEA, it really makes the mess manageable.
It's important to note that not only does it allow you to write in Java and have that transformed into optimized and obfuscated Javascript it also comes with a substantial subset of the core Java API. In addition to this they provide lots of additional classes for doing things like parsing and working with JSON and XML and communicating with a server via asynchronous HTTP. You can check out the docs to get an idea of what else they offer.
Another feature that might be of special interest to you for implementing drag and drop functionality is it's integration with javascript libraries like Ext and scriptaculous. Either through pre-built interfaces or via JSNI

Also for Java there is J2S.
Java2Script (J2S) Pacemaker provides
an Eclipse Java to JavaScript compiler
plugin and an implementation of
JavaScript version of Eclipse Standard
Widget Toolkit (SWT) with other common
utilities, such as java.lang.* and
java.util.*. You can convert your
SWT-base Rich Client Platform (RCP)
into Rich Internet Application (RIA)
by Java2Script Pacemaker.
This means that if you use the SWT IDE (drag and drop) you can then convert the generated code to JS + HTML.

I wouldn't hand write any Javascript for UI. This can lead to maintenance disaster. jQuery is what I am using but I still wouldn't use it to write full UI Javascript code. ExtJS is also another good option if you plan to write in Javascript. In general what I am saying here is that it's so much easier to main in Java/C# than Javascript. Check out cappuccino framework and Atlas. Never used GWT. Script# is similar to GWT but for ASP.NET framework. Also depends on the requirement, if your site is public facing then RIA isn't a good option. It's all about which extreme end you pursuit (hand written and web standard, or RAD or libraries like jQuery/ExtJS as the middle option).

Check out Axial, a .NET to JavaScript converter that works well in ASP.NET. It supports WebForms, jQuery and canvas. It's not very mature, but it's worth a look.

http://jsc.sourceforge.net/ is a C# to JavaScript, Java, Flash and PHP compiler.

JscriptSuite offers another free .NET to Javascript compiler. There is a big difference to Saltarelle (jsc, SharpKit# etc.). Developer write down and debug only C# code (or any other .NET langauge), like in GWT. Javascript will be generated für deployment only.