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How to use Java in ASP.NET Core 3.1?

I'm building a REST API with ASP.NET, which should ideally be able to use Java code. I've already managed to create a bridge between .NET and Java by using jni4net in a simple console application. This is absolutely necessary due the non-existing interoperability of .NET and Java.
Nevertheless I can't create a bridge in my ASP.NET project by using jni4net. Apparently this is a common error due the fact that .NET core 2.1 or later versions do not support the methods shown below. I am refering to this question: "Error in the webapp while connecting with JVM using jni4net from C#".
MissingMethodException: Method not found: 'System.Reflection.Emit.AssemblyBuilder System.AppDomain.DefineDynamicAssembly(System.Reflection.AssemblyName, System.Reflection.Emit.AssemblyBuilderAccess)'.
Note that I'm using .NET core 3.1 for my ASP.NET project and jni4net 0.8.8.0. Anyways I'm would change my .NET version if needed. My current implementation in Program.cs looks like this:
using net.sf.jni4net;
public class Program
{
public static void Main(string[] args)
{
var setup = new BridgeSetup();
setup.Verbose = true;
Bridge.CreateJVM(setup); // Error is thrown here
}
}
Does anybody know if there is a way to bypass this error OR how to use Java code in an ASP.NET project? I highly appreciate any help or suggestion, sheers!

An API is a medium that recieves requests from client, processes those requests inside and responds back to the client.
You can probably do what you want to do by creating an API on .NET Core and consume that API in your Java project.
In the simplest way, you can create an API endpoint in your .NET project's controller and send a request to that api on your Java application using the target URL of your API endpoint.
The response from your API is going to be in JSON format.
Please note that you can consume an API regardless of the language since the communication between your application and your API is going to be via HTTP requests and JSON responses.
Some useful video links to start with:
How to build a restful API in .NET Core 3.1 https://www.youtube.com/watch?v=fmvcAzHpsk8&t=11447s&ab_channel=LesJackson
How to consume an API in Java application https://www.youtube.com/watch?v=Fry7waUIJiY&ab_channel=PrakashShindalkar
I also would like to clarify that HTTP is just a a protocol. You can probably achieve what you want with other communication protocols as well but this one is fairly more popular and suitable for your needs.

As far as I know jni4net is not compatible with .NET Core. It's for .NET Framework.
This is absolutely necessary due the non-existing interoperability of .NET and Java.
I don't know the circumstances but it shouldn't be a necessity. Just build a service that exposes API to C# code. Voila you have a bridge. Or you can use CORBA however it's so complex it doesn't worth at all imo.

Is there any use for WSDL file in REST or JSON web service? [duplicate]

This question already has answers here:
RESTful Services - WSDL Equivalent
(8 answers)
Closed 3 years ago.
I am implementing a web service and I have implemented both a REST and SOAP version to see which suited my needs.I have decided to choose REST because of its simplicity and that I will probably be developing an iPhone app to consume it. My question is simple really, is it possible to create a WSDL or WADL for my REST service and is it necessary?
Thanks

With a good RESTful service, it's not necessary to generate WADL (let alone the much-less-well-fitting WSDL) for it because it will self-describe. By “self-describe” I specifically mean that it will deliver documents describing all the (relevant) resources published by the service, and that using a standard HTTP OPTIONS request on any of these will produce basic information about how to interact. The only real benefit to using WADL is that it allows the caller to discover the schemas for the complex documents it needs to work with ahead of time; REST itself provides no help there (and some RESTians believe that doing such things is counter-productive, which I'm not sure I agree with).
Of course, that doesn't capture the deeper interaction patterns, but neither do the vast majority of WSDL descriptions of services so no change there.
For the record, I use Apache CXF to create RESTful services (using JAX-RS) and that publishes WADL for them.

The W3C has made a formal recommendation for a REST documentation standard based on [WSDL 2.0][3]. Here is a quote from the IBM article:
The term Web services is typically associated with operation- or
action-based services using SOAP and the WS* standards, such as
WS-Addressing and WS-Security. The term REST Web services generally
refers to a resource-based Web services architecture that uses HTTP
and XML. Each of these architectural Web service styles has its place,
but until recently, the WSDL standard didn't equally support both
styles. The WSDL 1.1 HTTP binding was inadequate to describe
communications with HTTP and XML, so there was no way to formally
describe REST Web services with WSDL. The publication of WSDL 2.0,
which was designed with REST Web services in mind, as a World Wide Web
Consortium (W3C) recommendation means there is now a language to
describe REST Web services.

As #GiuliaDiFederico said, "of course it's possible" (with WSDL2), showing a good source link about how to do. #DonalFellows, by other hand, does not encouraged the use of WSDL...
I think the use of WSDL is a question of
FORMALIZATION LEVEL: with WSDL you can express more formally all relevant details of your webservice.
STABILITY LEVEL: if you need long term contracts, and avoid risks of changes in the enviroment where your webservice is exposed, WSDL helps to mantain stability.
NEED FOR STANDARDS: if customers prefer webservices that can be said "standard compliant", use standards. The only one is W3C, and W3C requires XML, SOAP and WSDL.

RestDoc tries to create a simple documentation framework for REST resources.
A browser is available via restdoc-renderer.
It also offers Java annotations to enable on-the-fly creation of RestDoc documetnation. Implementations are available for Jersey 1.x and JAX-RS 2.0.

Of course it's possible, but for answer if it is necessary or not, you didn't provide enough info.
I suggest you to take a look on the ibm's developerworks site that provide an interesting article on subject
Strictly, with WSDL 1.0 you can' t, but with WSDL2 you can, because was developed for accept this kind of demand,
"... WSDL 2.0 in a WS-I profile that addressed the requirements
for REST style Web services. The addition of GET in SOAP 1.2 and several
additions in WSDL 2.0 such as operation safety, the ability to describe
messages that refer to other Web services, and the improved HTTP binding
now make it possible to describe REST style Web services.", Arthur Ryman.

I think WSDL is not appropriate for REST and WADL is not necessary. HTTP exposes already what WADL could describe in a separate file. For example the "Allow" header returns allowed HTTP method, and Content Negotiation is for choosing the right format.

WADL is not necessary. But, If Client side code is already present in an application and you want to make a new rest call then it is good practice that you use wadl for generating the client side java stub(POJO). By this way, client side POJOs will be in sync with service side POJOs. For example, If you are replacing EJB/SOAP service call with Rest service call in an existing application then It is very safe and good practice to use WADL.
You can generate client side java stubs from WADL by using wadl2java maven plugin.

Is there a way to build a java rest service so that wsdl can be autogenerated from it?

One problem that continuously comes up at my workplace is creating clients for Java rest services that we create. We have 4 different programming languages to support and it is costly to create clients for each programming language. Often this means our web services aren't available in a cross platform manner since we rarely have time to build all of the clients.
SOAP provides this type of discovery and machine created client mechanism based off of tools that consume WSDL but our architectural direction is to write REST based services in Java instead of SOAP services.
We would also prefer not to write WSDL documents by hand that expose these REST web services. In a .NET environment, WCF and ASMX web services automatically create the WSDL for consumption in other applications, but this is not a direction my team is allowed to pursue.
Can this be done in some fashion for a Java based REST service?
How can this be done without having to invest a lot of manual labor?
We're currently using spring controllers but might be able to argue for a different Java framework if it provides better velocity.

Jersey (JAX-RS implementation) has support for WADL. You can use Jersey to automatically generate a WADL and there is also some support for generating Java clients from the WADL.
That's for Java, and I don't know if it's more than basic support, but I doubt you can pull it off for 4 different programming languages.
You might want to read these first though (tl;dr: REST is more than a CRUD style Web API and is different than SOAP):
Do we need WADL?
Why the slow WADL uptake?

You may want to check third party tools like swagger (http://swagger.io/) and mashery. I know swagger has support for client generation.

Advice on implementing JAX-WS web services to be consumed by non-Java (PHP) endpoint

I am a Java SE programmer and exploring implementing JAX-WS web services for the purpose of integrating with our web server. To this date, I have not had experience with web-services thus would like to get everyone’s expert opinion.
The background is that my company has a POS system developed and hosted in-house using Java SE. We are planning for e-commerce capabilities, which will be implemented in HTML/PHP, via external web development company and hosted externally.
Thus we are exploring implementing JAX-WS web services on our endpoint for the purpose of integrating with our e-commerce server running PHP endpoint.
I’ve done some research and my understanding is that:
it is possible to implement JAX-WS without Java EE containers
JAX-WS Web Services Without Java EE Containers
it is possible to mix end-point technologies, and specifically in my case JAX-WS as our endpoint and PHP SoapClient on our e-commerece end-point
PHP SOAP Client to consume JAX-WS with Basic Http Authentication
Using PHP SoapClient with Java JAX-WS RI (Webservice)
I am now wondering what’s is the proper approach when discussing implementation with the external web development company that is building our e-commerce platform. Most web sites and forums’ examples assumes Java on both endpoints and that both endpoints are implemented by the same developer/team.
Based on my limited understudying, I gather the process would be:
Me/my company creating the web service (coding the web services methods in Java)
Me/my company creating the server program
The wsdl generated from the URI (http://:/md5WebService?wsdl )of my server program is then used as the interface contract between our internal POS system and the external e-commerce platform
The web development company that is implementing the e-commerce platform then uses the wsdl to implement the PHP SoapClient endpoint on their side.
And in the case where our internal POS system need to consume a web services created by the external web development company, they will pass me the wsdl and I use that to make the call to them.
Is this the correct way to do proceed?
Many thanks.
Cheers,
Arthur

In Java you have actually two ways to start your design of your web service. You can either create the WSDL (Web Service Description Language) contract first (contract first approach) and then let Java or some framework tools create Java skeleton classes for you which you can use to implement the logic of each operation or you can start by code first approach and implement each web method and its logic and then let Java or some external framework tools generate the WSDL contract for you.
Either way you start, the result should be very similar and platform independent. The standard message format used for WSDL based web services is SOAP (Simple Object Access Protocol) which is based on XML (eXtensible Markup Language) which is by definition platform and programming language neutral.
So, after implementing your service and starting a server for the WS endpoint adding ?wsdl to the end of the endpoint URL should return the WSDL contract to the invoker, which can be used to create client side stubs for the required programming language which furthermore simplify the sending and receiving of messages from and to the web service. Note however that creating stub files might not be needed as all the information may be parsed from the WSDL contract directly. As of lack of knowledge concerning PHP I can't give details on how to call a WS from PHP directly or if stub file creation is required/recommended.
In order to call an other WS from your service you need to create a WS client within one of your web methods and invoke one or more of the operations offered by the remote WS and process the response within your web method.
As I am not sure if you are using any (Java) frameworks like f.e. Apache CXF I am not giving any code examples here. For integrating external web services within your service you might also have a look at Apache Camel which offers integration support for numerous Java based frameworks including CXF. Here your web service is treated as a Consumer while other external services you need to invoke are handled as Producers. The interaction between your internal and the external services is modeled here within a route where you can apply various Enterprise Integration Patterns (EIP) like splitting multiple elements contained within a response into distinct objects which you furthermore can process in parallel.
In general your enumeration of steps involved does look right if you follow the code first approach but as mentioned earlier you can also start by defining your contract first. Depending on your knowledge of the WSDL/XSD syntax (the less you know the exact syntax the more you should use code first approach), crating the contract first might enable PHP side integration sooner while you still develop the internal logic of your implementation.

it is possible to mix end-point technologies, and specifically in my case JAX-WS as our endpoint and PHP SoapClient on our e-commerece end-point PHP SOAP Client to consume JAX-WS with Basic Http Authentication Using PHP SoapClient with Java JAX-WS RI (Webservice)
This is the exact purpose of introducing webservice concept. You don't have to worry about on which platform or language your client and server is implemented. Client and server will simply exchange xml messages (platform independent) as agreed upon within wsdl.
Go ahead with your understanding.

Consume .NET web services in Java

I have created .NET web services. I want to run it on a remote server and have the Java Applications (clients) contact the server for data. How should I implement the Server such that the Clients can make use of the org.apache.xmlrpc.client.XmlRpcClient package?
I just want the clients to generate a request for data and does not want to have any other dependencies.
Here is a good material on using eclipse for this purpose: http://wso2.org/library/tutorials/creating-web-service-client-3-steps-using-eclipse
Thanks.

Although both are rightfully called "web services" the SOAP based web services usually created in .net are incompatible with web services following the older XML-RPC standard.
In my opinion you can follow 2 routes to solve your problem:
either you go the SOAP route under Java, one of the most common API's for that purpose would be jax-ws - some excellent pointers to tutorials here, in Pascal Thivent's answer
or you transform your .net webservices to XMl-RPC by using xml-rpc.net
Both routes have advantages and disadvantages, it's hard to make that choice for you without knowing more about your project. A priori choosing the SOAP route might look "safer" as there the entire communication will be based on standard components.

If you have the Web Services on the .NET side, you must have a Web Service Description Language (WSDL) (if you are not talking about REST), you can easily create the client classes to consume this Web Service using an IDE, check this link here: http://netbeans.org/kb/docs/websvc/client.html

If you are using Eclipse I suggest you this tutorial to build a simple WS client.
You can adapt this example application to your real needs.
NOTE the example uses an old version of Eclipse, but the wizard is very similar also in newer versions.