I'm writing scala-js frontend framework, the key feature of which is server-side rendering. The idea was that there are components that manipulate dom with document.createElement, element.appendChild and others. On the server I'd subclass HTMLDocument, Element and others, override their methods with server dom implementation that can be converted to plain string html. So I added scalajs-dom_sjs dependency to the server module and tried to do that. But HTMLDocument, Element and most likely other classes have calls to js.native inside their constructors which throw exceptions saying "use JVM version of the library". Which doesn't exist obviously. I could use the other way and implement my own dom library, but that is twice as much work, cause I'd have to implement it on server and client, while using the first approach I'd implement it only once on server.
So my question is: why is it forbidden to use scala-js library versions on server so strictly and is there a work around it?
The reason this is forbidden is that, as you noticed, the DOM API is full of js.natives. These classes are not implemented in Scala. They are part of the browser's DOM API, which does not have an equivalent on the JVM. You cannot use the types defined in scalajs-dom on the JVM and expect them to do anything useful. Where would the implementations of the methods come from?
You will indeed need to implement your own DOM-like library for the JVM side. If you do not want to "reimplement" it on the client side, you could reuse the org.scalajs.dom namespace for your classes, and give them exactly the same structure and types as in scalajs-dom (except they won't extend js.Any, obviously).
Note that this is semantically dubious. Types extending js.Any do not have the same semantics as normal Scala types. You might be able to come up with some "compatible enough" API for normal use, but it's still dubious.
Usually, to enable so-called isomorphic DOM manipulations on server and client, one would write a DOM-agnostic cross-compiling library. On the client side, it would offer a "rendering" function to actual DOM nodes; and on the server side, it would render to strings to be sent to the client in the HTML.
This is precisely what Scalatags does.
We are currently using PostSharp for its standard functionality (logging, caching, transactions, and so on).
We also generate dynamically, at runtime, some custom classes, using Reflection.Emit. This obviously slows startup, and as we need to add more dynamic type generation, I am wondering, since all the information for the dynamic types is known at build time, whether we can use PostSharp to do this.
So, the question itself is, can I use PostSharp to achieve what I can do with Reflection.Emit, but at build time?
Regards
The PostSharp itself is using PostSharp.Sdk to manipulate the binary code, but this API is not publicly documented and supported at the moment. So, it's not future-proof to rely on it in your project.
The closest you can get with the documented API is probably by introducing interfaces, methods and properties: http://doc.postsharp.net/content/code-injections
Is there any way to have the same class implemented differently on the client vs the server?
To avoid the "Why do you want to do that?" question.. I will elaborate
I am converting a very large Java client/server application. Currently it uses a Swing GUI client and talks to the server via Spring remoting (RPC). Using GWT RPC with Spring services is not a problem, there are several excellent examples available and the all seem to work well.
Several classes that are common to both the client and the server contain data that is passed back and forth. These classes also contain some behavior that is implemented by using the standard JRE classes. For example, one class contains, parses and formats date and time, including time zone, DST, etc. in a locale specific way. I could rewrite/refactor it but the application is over 10 million SLOC, resulting in literally millions of references to this class alone, so a major rewrite is not cost effective.
To use this as an example, GWT provides excellent i18n support for parsing and formatting dates. But the implementation is different to the way the JRE does it.
So I'm looking for a cleaver way where by I can inject an implementation into the shell of my DateTime class, depending on whether it is in the client (using GWT and native JS) or in the server (using the JRE). Is there a cunning way to do this? Perhaps using the module file XXXXX.gwt.xml. I'm looking for a generic solution.
You'd want to use the <super-source> for overriding one package implementation with another.
This is what GWT uses to emulate the Java Runtime classes, and (among others) provide different implementations for client and server of the com.google.gwt.regexp.shared.* classes.
I think what You are looking for, is this: <source path="client" /> in your project gwt.xml file. It tells the GWT generator where to look for client side code to convert to JS. In my project I have it set up this way:
<source path="client" />
<source path="shared" />
Basically client code is in the client directory, and in shared we keep beans and some data wrappers for client and server side.
What you could do, is to add the packages you want to convert to client with the source path like above. But you must remember, that the classes you are going to convert, can be composed only of objects and properties which GWT generator can convert into client-side java script. I'm not sure also if more accurate path can be put in the source path, like:
<source path="shared/beans/whatever" />
Another drawback is, that if you use GWT i18n support, it handles different locale in time of compilation by its own - which is good. If you decide to use your own mechanism, your classes must contain some logic to be aware of locale being used currently, which must be compatible with GWT.
My question is very simple, my intention is to generate a repository with your responses so it could serve to the community when selecting frameworks for developing enterprise general purpose applications.
This could apply very well for general purpose languages such as C++, C# or Java.
What Framework do you recommend for generating Layered Architectures?
Based on you experience why do you prefer the usage of some Framework versus your own architecture?
How long do you believe your selected Framework will stay as a preferred option in the software development industry?
This is indeed an overly general question, especially since there are so many interpretations of the very word framework, and within the world of frameworks many different kinds for different tasks. Nevertheless, I'll give it a shot for Java.
Java
Java EE
The default overall enterprise framework of Java is called Java EE. Java EE strongly emphasis a layered architecture. It's a quite large framework and learning every aspect of it can take some time. It supports several types of applications. Extremely small and simple ones may only use JSP files with some scriptlets, while larger ones may use much more.
Java EE doesn't really enforce you to use all parts of it, but you pick and choose what you like.
Top down it consists of the following parts:
Web layer
For the web layer Java EE primarily defines a component and MVC based Web Framework called JSF - JavaServer Faces. JSF utilizes an XML based view description language (templating language) called Facelets. Pages are created by defining templates and letting template clients provide content for them, including other facelets and finally placing components and general markup on them.
JSF provides a well defined life-cyle for doing all the things that every web app should do: converting request values, validating them, calling out to business logic (the model) and finally delegating to a (Facelets) view for rendering.
For a more elaborate description look up some of the articles by BalusC here, e.g. What are the main disadvantages of Java Server Faces 2.0?
Business layer
The business layer in the Java EE framework is represented by a light-weight business component framework called EJB - Enterprise JavaBeans. EJBs are supposed to contain the pure business logic of an application. Among others EJBs take care of transactions, concurrency and when needed remoting.
An ordinary Java class becomes an EJB by applying the #Stateless annotation. By default, every method of that bean is then automatically transactional. Meaning, if the method is called and no transaction is active one is started, otherwise one is joined. If needed this behavior can be tuned or even disabled. In the majority of cases transactions will be transparent to the programmer, but if needed there is an explicit API in Java EE to manage them manually. This is the JTA API - Java Transaction API.
Methods on an EJB can easily be made to execute asynchronous by using the #Asynchronous annotation.
Java EE explicitly supports layering via the concept of a separate module specifically for EJBs. This isolates those beans and prevents them from accessing their higher layer. See this Packaging EJB in JavaEE 6 WAR vs EAR for a more elaborate explanation.
Persistence layer
For persistence the Java EE framework comes with a standard ORM framework called JPA - Java Persistence API. This is based on annotating plain java classes with the #Entity annotation and a property or field on them with #Id. Optionally (if needed) further information can be specified via annotations on how objects and object relations map to a relational database.
JPA heavily emphasizes slim entities. This means the entities themselves are as much as possible POJOs that can be easily send to other layers and even remote clients. An entity in Java EE typically does not take care of its own persistence (i.e. it does not hold any references to DB connections and such). Instead, a separate class called the EntityManager is provided to work with entities.
The most convenient way of working with this EntityManager is from within an EJB bean, which makes obtaining an instance and the handling of transactions a breeze. However, using JPA in any other layer, even outside the framework (e.g. in Java SE) is supported as well.
These are the most important services related to the traditional layers in a typical enterprise app, but the Java EE framework supports a great many additional services. Some of which are:
Messaging
Messaging is directly supported in the Java EE framework via the JMS API - Java Messaging Service. This allows business code to send messages to so-called queues and topics. Various parts of the application or even remote applications can listen to such a queue or topic.
The EJB component framework even has a type of bean that is specifically tailored for messaging; the message driven bean which has a onMessage method that is automatically invoked when a new message for the queue or topic that the bean is listening to comes in.
Next to JMS, Java EE also provides an event-bus, which is a simple light-weight alternative to full blown messaging. This is provided via the CDI API, which is a comprehensive API that among others provides scopes for the web layer and takes care of dependency injections. Being a rather new API it currently partially overlaps with EJB and the so-called managed beans from JSF.
Remoting
Java EE provides a lot of options for remoting out of the box. EJBs can be exposed to external code willing and able to communicate via a binary protocol by merely letting them implement a remote interface.
If binary communication is not an option, Java EE also provides various web service implementations. This is done via among others JAX-WS (web services, soap) and JAX-RS (Rest).
Scheduling
For scheduling periodic or timed jobs, Java EE offers a simple timer API. This API supports CRON-like timers using natural language, as well as timers for delayed execution of code or follow up checks.
This part of Java EE is usable but as mentioned fairly basic.
There are quite some more things in Java EE, but I think this about covers the most important things.
Spring
An alternative enterprise framework for Java is Spring. This is a proprietary, though fully open source framework.
Just as the Java EE framework, the Spring framework contains a web framework (called Spring MVC), a business component framework (simply called Spring, or Core Spring Framework) and a web services stack (called Spring Web Services).
Although many parts of the Java EE framework can be used standalone, Spring puts more emphasis on building up your own stack than Java EE does.
The choice of Java EE vs Spring is often a religiously influenced one. Technically both frameworks offer a similar programming model and a comparable amount of features. Java EE may be seen as slightly more light-weight (emphasis convention over configuration) and having the benefit of type-safe injections, while Spring may offer more of those smaller convenience methods that developers often need.
Additionally Spring offers a more thoroughly and directly usable security API (called Spring Security), where Java EE leaves a lot of security details open to (third party) vendors.
To specifically answer the second question:
Developing your own framework gives you the burden of having to maintain it and educating new developers in using it.
The larger your framework becomes, the more time you have to devote specifically to it and the less time you thus have to solve your actual business problem. This is okay if your business problem is the framework, but otherwise it can become a bit of a problem, even for very large companies that can dedicate a group of people to such a framework.
If you're a smaller company (say ~15 developer max) this can really become a huge burden.
Additionally, if your own framework is the kind of framework that can take advantage of third party developments (e.g. third parties can develop components for JSF), then your own framework obviously won't be able to take advantage of that.
Unless of course you open source your own framework, but this will only significantly increase the burden of supporting it. Just dumping your source code on sourceforge does not really count. You will have to actively support it. All of a sudden your framework becomes their framework with maybe 'weird' feature requests and awkward error reports for environments that you have no personal interest in.
This also assumes that your framework will actually be used by external users. Unless it's really very, very, good and you put lots of energy in it, this will probably not happen if it's simply the umpteenth Java web- or ORM framework.
Obviously, some people have to take up the job of creating new frameworks, otherwise the industry just stagnates, but if your prime concern is your business problem I would really think twice of starting your own framework.
Very vague question, I'm not really sure it's ever a good idea to "write your own" at this point for a work project (unless writing your own, IS the project). If it's a learning exercise, fine, but otherwise go use one of the libraries written by people who have been doing it far longer. If you really want to get involved, read their code, try and contribute patches etc.
For .Net there is Sharp Architecture Which is a pretty popular framework for layered applications.
Here's some of the stuff I use (I don't use Sharp Architecture)
First, the infrastructure stuff
For Dependency Injection, I use StructureMap. I use it because it's way more robust and performant than anything I would or could write, and it's very well supported within the .Net community. It also sticks to being DI, and doesn't venture out into other things that I might want to use other libs for (AOP etc). The fluent configuration is fantastic (but many .Net DI Tools have that now)
For AOP, I use Linfu Dynamic Proxy. I know a lot of people that like the code weaver variety for performance reasons, but that's always seemed a bit like premature optimization to me.
For a DataMapper, I use AutoMapper. This is one where I'm on again off again. If you can do your mappings based just on convention, then great, I'll use it. Once I have to start tweaking the configuration to do special things.... to me that starts to get into the gray area where the code might be more clear with just some left=>right wrapped in a function.
Web/UI
Asp.Net MVC. Although to be quite honest, I'm having a falling out lately and may soon be moving to FubuMvc. Asp.Net MVC seems like it has split personalities in terms of API design (dynamic over here, static over there, using blocks to render forms, but System.Actions to render other things etc). Combine that with the fact that it's not really OSS (you can't submit a patch), and to me there's a compelling reason why the community should come up with something better that's OSS.
Persistence
NHibernate, Specifically Fluent NHibernate. Sure I'd love to write my own OR/M, but at the same time I'm certain that the hordes of developers who have worked on NHibernate are way smarter than me.
Services/Distribution etc
WCF for Synchronous calls
NServiceBus for Messaging and most async calls.
Most of this stuff is OSS, so how long will it be around, well, I would imagine a good long while.
This question doesn't work very well. Selecting frameworks is difficult, and very context specific. For each selection process you might end up with a simple shortlist and a simple list of questions to answer, but those lists do not transfer well to other selections.
The number of parameters and the parameter sensitivity influencing a decision is very large, and at enterprise level a lot of them are not technical.
Currently, there are no frameworks available that are ready to support these near-term enterprise needs:
the switch for most of the workforce from pc to tablet and phone;
the switch from web client and rdbms to p2p/disconnected based storage and distribution
I accidentally used HashSet and HashMap defined under the package com.google.gwt.dev.util.collect in the client side code. Found out the package does not have a module xml file and hence these collection classes are not meant to be used on the client side.
What is the purpose of having these classes in the GWT SDK, if these aren't supposed to be used within the client package? There definitely has to be some benefit from these classes to merit inclusion in the SDK.
What am I missing?
Those collection implementations were written by Google engineers to improve the performance (specifically memory usage) of the GWT internals, such as the Java-to-JavaScript compiler. They are implementation details not intended to be part of the public API and should not be used by GWT developers.