How can I create Java Pojo at runtime? ASM or BCEL is required for this?
I am reading a XML file and I want to generate Pojo according to this xml.
There are lot of libraries available to generate classes in runtime. If you want to create a class and write it back to disk, BCEL and CGLIB is good. If you want most of them for runtime only, CGLIB is probably the best
You might find that generating the code in memory is much easier to work with. (ASM is very good otherwise) With generated code you just have to create the java you would need.
You can use tools like BeanShell or the Compiler API
There's a nice open source library for reading xml to objects called Xstream. Use that and you don't need to worry about manually parsing the XML or manually creating objects.
Related
Using Java to write xml strikes me as perhaps not the best match. Is this just because it's new and unfamiliar, perhaps?
Rather than generating JAXB source code from xml might it not be more flexible to work with xml files directly? What I mean is, cannot the middle-man be cut out?
If JAXB can generate source code for the class files to manipulate xml might there a tool which uses JAXB without creating concrete boiler-plate?
If there's simply a mis-match making this impossible please elaborate. Or, if it's simply out of reach, what are the obstacles?
I have an assignment where i need to write a Java program that parses a .class file and retrieves things like :
1.name of the .java file
2.implemented interfaces
3.variables
4.constructors
5.methods
I don't have any ideeas where to begin from?
For example,what kind of Data I/O structure should I use?
You can you ClassParser which is available in Apache commons library. you can read the Javadoc here. You can download apache commons from here
You can also use Java reflection API which provides method such as getDeclaredFileds, getDeclaredMethods etc.
There are already several libraries for classfile parsing out there. Objectweb ASM is the most popular.
If you have to do it from scratch, that I'd recommend starting by the JVM specification, which explains the binary layout of classfiles in detail. After that, parsing is just a simple matter of programming. I've written a classfile parser before, it's not that hard.
You don't need any external library, just use java.lang.Class. Write the name of your class:
[NameOfMyClass].class.getDeclaredFields();
[NameOfMyClass].class.getDeclaredConstructors();
[NameOfMyClass].class.getDeclaredMethods();
It's the same for interfaces and many other attributes.
You can use Java Reflection. Here is a good tutorial -->
Java Reflection Tutorial
OpenJDK actually comes with an API that lets you parse and manipulate class files programmatically that most programmers don't know about. It is located at the package com.sun.org.apache.bcel.internal.
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.
I have a project which I need to manipulate files. things like: create new file by a defined structure(header,data,trail). and then I need to things like search/validate/create/read.
basically I want to map the files to objects and vise versa.(I am willing to map them to objects coz it will be much more comfortable for me to manipulate the fields inside each file via object)
I wonder if any of you deal with such things before? and maybe could recommend me on libraries which could easy my work.
thanks,
ray.
You may want to look at serialization and de-serialization
If you want custom mapping, you need custom coding. I would suggest you look at DataInputStream and DataOutputStream.
Using these you can control the header, records and footer in any binary format you want.
I suggest you generate your serialization (if you need to have the afstest possible speed) or use reflections to do the translation. Just using reflection is pretty fast and much simpler than generating code. ;)
In the end I Found a ORM framework called Canyon which mapping Files to Objects. but still had difficulties. so I have implemented my own ORM file to objects and vise versa.
If you have a defined file layout with different content you should consider to use a template engine like FreeMarker or Velocity to generate your files.
You can define templates here which will be filled with your dynamic content which you have to provide. Definitly better than to use System.out (I mean hard code your template text).
A library which helps for basic file manipulation is Apache Commons IO.
If you realy want to map your files to objects than it would be a Serialization/Deserialization as Angelom mentions. Many libraries help you to do this but the file format is fixed:
JSON: Jackson, GSON
XML: JAXB
If you want the file read by 3rd party as well, how about using some popular existing exchange format such as CSV or XML?
XML is fully supported in standard library. There's plenty of CSV libraries out there, including Apache Commons CSV.
I'm working on a project where we're doing a lot of remote object transfer between a Java service and clients written in other various languages. Given our current constraints I've decided to see what it would take to generate code based on an existing Java class. Basically I need to take a .class file (or a collection of them) parse the bytecode to determine all of the data members and perhaps getters/setters and then write something that can output code in a different language to create a class with the same structure.
I'm not looking for standard decompilers such as JAD. I need to be able to take a .class file and create an object model of its data members and methods. Is this possible at all?
I've used BCEL and find it really quite awkward. ASM is much better. It very extensively uses visitors (which can be a little confusing) and does not create an object model. Not creating an object model turns out to be a bonus, as any model you do want to create is unlikely to look like a literal interpretation of all the data.
I have used BCEL in the past and it was pretty easy to use. It was a few years ago so there may be something better now.
Apache Jakarta BCEL
From your description, it sounds like simple reflection would suffice. You can discover all of the static structure of the class, as well as accessing the fields of a particular instance.
I would only move on to BCEL if you are trying to translate method instructions. (And if that's what you're trying to automate, good luck!)
I'm shocked that no one has mentioned ASM yet. It's the best bytecode library your money can buy. Well, ok it's free.
JAD is a java decompiler that doesn't allow programmatic access. It isn't readily available anymore, and probably won't work for newer projects with Java7 bytecodes.
I think javassist might help you too.
http://www.jboss.org/javassist/
I have never had the need of using it, but if you give it a try, would you let us know your comments about it?
Although I think it is more for bytecode manipulation than .class inspection.