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Auto-generating frontend and backend validation for Angular/Java

I hope my question is not too broad.
Our application has an Angular frontend and a Java backend and
we need both frontend and backened validation. At the moment, we manually code
validators in Typescript and Java that basically do the same thing.
Since that is not only tedious but also error-prone, we would like to define once (maybe in a JSON file)
which input fields have which constraints and ideally use a piece of software that auto-generates
Java and Typescript code based on that definition.
My question is: are there any open-source/commercial software solutions that take care of this?
It seems like a common enough problem, but I haven't come across
a proper solution yet. JSON schema seemed promising and since we're using swagger
anyway, I'd hoped that swagger-codegen would automatically take the supported JSON-schema keywords as constraints into account,
but I haven't figured out how it works. When I generate Angular code, constraints such as 'minimum' and 'maximum' are completely ignored,
and when I generate Java SpringBoot code, they only show up as annotations.

This is exactly what JSON Schema is designed for. Open API (swagger) modifies JSON Schema for it's own purposes, which doesn't include validation.
JSON Schema has validator implementations in most major languages allowing you to use the same schema to do the same validation on the front end and the back end. http://json-schema.org/implementations.html
Unfortunately, you likely won't be able to use the same schemas you use for your Open API service definition. I'm not aware of any validator implementations that support Open API's flavor of JSON Schema. I understand that Open API is trying to close the gap between their definition and JSON Schema, but a gap still exists.

Java library to assist in XSD creation?

Is anyone aware of a library that makes the process of creating XSDs in Java a little easier than using something like a DocumentBuilder? Something like a helper or utility class. I came across the org.eclipse.xsd maven jar but I'm having ClassNotFoundException issues when working with it in Eclipse and I'm not entirely sure it's meant to be used as a standalone kind of thing. This is a bit difficult to Google for as well since there are lot of search results around automatic generation/translation from Java to XSD and vice versa.
Essentially what I need to do is to programmatically create an XSD from a certain source of data -- not Java classes.

Apache XMLSchema is a lightweight Java object model that can be used to manipulate and generate XML schema representations. You can use it to read XML Schema (xsd) files into memory and analyze or modify them, or to create entirely new schemas from scratch.
The fact that with this API one can create an XSD from scratch, it sounds as a starting point to achieve the ask; as to the fitness, it depends on what that "certain source of data" is.

Generic XML to Java bean Conversion

I want to create a Java bean from the XML file which I will get as input at runtime. Can anyone please suggest me the generic way for doing the XML to Java bean conversion?

Typically java classes can be generated from XSD. So, if you do not have it yet you should first write it manually or generate from your XML sample. There are many tools that do this. I know for example XMLSpy But I believe that a lot of others exist.
Then you should generate your java classes. The are many ways to do this too. For example JAXB (as it was mentioned by #Chris Jester-Young) or Castor.

What is best practice in converting XML to Java object?

I need to convert XML data to Java objects. What would be best practice to convert this XML data to object?
Idea is to fetch data via a web service (it doesn't use WSDL, just HTTP GET queries, so I cannot use any framework) and answers are in XML. What would be best practice to handle this situation?

JAXB is a standard API for doing this: http://java.sun.com/developer/technicalArticles/WebServices/jaxb/

Have a look at XStream. It might not be the quickest, but it is one of the most user friendly and straightforward converters in Java, especially if your model is not complex.

For a JMS project we were marshalling and unmarshalling (going from java to xml and xml to java) XML embedded in TextMessages (string property). We tried JAXB, Jibx, and XMLBeans. We found that XMLBeans worked best for us. Fast, easily configurable, good documentation, and easy Maven integration.

I have used and will continue to use JDOM -> www.jdom.org

Another option is a Sax Parser. It is procedural - i.e. a visitor pattern - but if the xml is fairly lightweight, (and even medium weight) I have found it to be very useful for this.

JAXB API which comes in Java(In built).

I have used JIBX in MQ module. It works very well. Ant config is simple. Used Xsd2Jibx converter to generate the binding files and Java beans from XML schema. Marshalling and un-marshalling allow to specify character-set parameter. It was useful in my project to handle custom character-set. But I found an issue in the binding compiler. If the Java bean has lengthier path name, it generates class file with lengthier file name which will cause issue in Windows XP(it has a maximum file length limit).
I haven't used other APIs. So I am not trying to compare with others. If you decided to use JIBX, I hope this will be helpful.
More details, please refer JIBX website

I've used XStream as well, it is easy to use and customizable. You can add your own custom converters and that was very handy for me...

So surprised more people have not mentioned Jibx. Amazing lib and i think a lot simpler to use than Jaxb. Performance is also fab!

For this you can also consider apache's bitwixt and simple framework for xml

How to create WSDL file given SOAP WSDL operations

I haven't had any experience with web service related development. So, any ideas will be greatly appreciated.
Suppose, I have a file listing draft specification of WSDL operations. Following is one example. How would I go about creating the WSDL file. Is notepad sufficient or do I need to have WSDL editor?
getHostSystemInfo
Returns detailed information about host systems specified via given IDs.
input HostSystemIdCollection(Collection of Strings)
Output HostSystemInfoCollection
HostSystemInfo
Id: mandatory
Properties: Following properties should be provided for host systems
HostSystemName
HostSystemProperty1
HostSystemProperty2
HostSystemProperty3
....
....

If the question is just "how do I create the WSDL" then you could indeed use Notepad and just write it, it's only XML after all. However, writing syntactically correct XML by hand is pretty dull, and error prone. So I would recommend using WSDL aware tooling for example an Eclipse editor
An alternative is to write some Java which expresses the interface, and from it generate the WSDL. There are many ways of doing this, including starting with an EJB and annotating it accordingly. A few googles should help you find what you need.
My experience is that simple POC situations tend to work well starting at the Java. Larger scale projects benfit from considered designs starting at the WSDL.

coding WSDL by hand is a big pain! i used a XML editor for creation of and then generated the stubs with JAXWS. It is important to understand and differences of the WSDL styles, which is not trivial (have a look at WSDL styles). a good help is to import the WSDL schema to your IDE (eclipse, idea) and then work with autocompletion.
just for interest, why are you using WSDL + SOAP. if you have a choice and you use anyway HTTP, have a look at REST. It can make implementation of web-api a LOT easier, both on server side and for api-clients.

If you haven't done any web services before, I would strongly recommend a WSDL Editor. The Netbeans has a plugin that should help.
The other way of doing it, which may be easier is by using the Java annotations defined in JSR 181.

Of course you could use the worst text editor in the world (!) but I'd seriously consider using any decent XML editor or IDE (Eclipse's WSDL support is pretty decent). This will save you a lot of pain and suffer.
Or, if this is an option, you could just annotate a Java class with JAX-WS annotations and have your WSDL dynamically generated from the Java code. Personally, I prefer the WSDL-first approach, the Java-first approach is just a suggestion to get you started.

You could use Axis2 to create that for you.