How to create a class dynamically in java

Question

I don't know if this is even possible. Anyway, here is my problem: I want to create a Class having a database table schema, for example suppose that I have a table like

id - unsigned int 
username - varchar(128)
password - varchar(128)

and let's assume I can query this data from my db. What I want to do is to dynamically create (and, of course, instantiate) a Java class that should look like this:

public class User{
    private unsigned int id;
    private String username;
    private String password;
}

(actually an ActiveRecord for my table)

Can you help me starting this? Tnks

Michael Borgwardt · Accepted Answer · 2011-01-25 14:47:07Z

18

What would you do with a dynamically created and instantiated class that none of your other code knows about?

For a stically typed language like Java, it makes little sense to have such classes. On the other hand, most OR Mappers like Hibernate come with tools that allow you to statically generate classes from a database schema.

answered Jan 25, 2011 at 14:47

Michael Borgwardt

347k81 gold badges491 silver badges726 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Snekse Over a year ago

One use case would be generating a class that implements an interface which your static code knows about. I'm researching this topic because we have an application that runs against mixed database schema, so columns may or may not exist.

Michael Borgwardt Over a year ago

@Snekse: dynamically implementing interfaces is quite easy, using java.lang.reflect.Proxy

Snekse Over a year ago

I was just trying to answer the question of what you might do with such a dynamically generated class. We're about to attempt something similar, but I think we're going to investigate Groovy before going down the .reflect.Proxy path or the like.

dtortola Over a year ago

another case would be to generate a class annotated with something, for instance to use with an ApplicationContext as an additional configuration class

Troyseph Over a year ago

You could do a lot with dynamically generated classes, you code would know about it through inheritance. Arguably you could probably do anything that you could think of through more traditional means, but they would require a lot more future-proofing. I'm thinking in-game scripting / game modding here.

|

Bozho · Accepted Answer · 2011-01-25 15:12:08Z

18

Technically, you can, via a bytecode manipulation library - CGLIB, javassist, asm, bcel and the likes.

However, this is not the Java "philosophy". Java is statically-typed, so you'd better create the classes before runtime.

Take a look at hibernate / eclipseLink for Java ORM - a way of mapping tables to objects.

edited Jan 25, 2011 at 15:12

answered Jan 25, 2011 at 14:43

Bozho

599k147 gold badges1.1k silver badges1.2k bronze badges

Comments

Pointy · Accepted Answer · 2011-01-25 14:45:02Z

6

I think what you want is the facility provided by java.lang.reflect.Proxy and related classes.

answered Jan 25, 2011 at 14:45

Pointy

415k62 gold badges600 silver badges631 bronze badges

4 Comments

Michael Borgwardt Over a year ago

That requires an interface to exist, so quite useless in this scenario.

Pointy Over a year ago

Well, I agree that that's true, but the question isn't exactly rock-solid.

Sean Patrick Floyd Over a year ago

I think you confused the JDK proxy mechanism with bytecode manipulation libs like asm or cglib

Stijn de Witt Over a year ago

Not necessarily wrong; Think about it. What is the point of having a real method if it does not exist in any interface? How would you ever use it in any way that gets you any benefit of the fact that it's a real statically typed method?

pablochan · Accepted Answer · 2011-01-25 14:45:19Z

4

This is a good article to start with, but are you sure you need to actually create a new class? Maybe you could just use a Map?

answered Jan 25, 2011 at 14:45

pablochan

5,72530 silver badges42 bronze badges

1 Comment

EGHM Over a year ago

Using CodeModel to narrow to a Class that I haven't created yet was problematic, this article trivially solves my problem, via Javassist: ClassPool.getDefault().makeClass(clazzString).toClass();

Riduidel · Accepted Answer · 2011-01-25 14:50:45Z

1

Like @Bozho states, Java is a statically typed language for which generating classes at runtime can only lead to mayhem.

In our world, it is far more convenient to generate classes at build time, that's to say during compilation. Typycally, using Hibernate reverse engineering, you can build your Java classes from your DB schema at build time, and deploy those classes in your application, which give you authentical Java code to read, with the guarantee that your code will be bound to your DB schema

answered Jan 25, 2011 at 14:50

Riduidel

22.4k15 gold badges91 silver badges194 bronze badges

Comments

Andreas Dolk · Accepted Answer · 2011-01-25 15:12:11Z

1

The Article about the "new" Compiler API and the java doc for JavaCompiler show a way on how to compile java source from String objects. (I don't know if we can compile to output streams and load the class files in memory yet...)

You can load the class files later on with a URLClassLoader and create instances (reclection/invocation API)

answered Jan 25, 2011 at 15:12

Andreas Dolk

115k20 gold badges185 silver badges275 bronze badges

Comments

Sergei Tachenov · Accepted Answer · 2011-01-25 14:55:13Z

0

You could generate Java source text and use javax.tools package to compile it and a class loader to load it. Googling yields some examples how it can be done, but I never tried anything like that, so I don't know what problems you may encounter. Clearly, Java wasn't designed for such things.

answered Jan 25, 2011 at 14:55

Sergei Tachenov

25k8 gold badges61 silver badges74 bronze badges

Comments

Hrabosch · Accepted Answer · 2018-04-09 11:22:34Z

Old question and if it is possible, you should avoid to generate class during runtime, but sometimes you have to do that. So you can use Javassist and here is example...

I created a small example here: http://hrabosch.com/2018/04/08/generate-class-during-runtime-with-javassist/

But here is main point:

public static Class generateClass(String className, String methodName, String methodBody)
  throws CannotCompileException {
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.makeClass(className);
StringBuffer method = new StringBuffer();
method.append("public void ")
      .append(methodName)
      .append("() {")
      .append(methodBody)
      .append(";}");
cc.addMethod(CtMethod.make(method.toString(), cc));
return cc.toClass();
}

So what I did... Via Javassist I made a class in ClassPool. Also I added a method inside this class and via reflection I invoked it.

Hope it helps.

Just keep on mind whatever you want to use in generated class, there are NOT imports, so you have to use fully-qualified names.

Collectives™ on Stack Overflow

How to create a class dynamically in java

8 Answers 8

7 Comments

Comments

4 Comments

1 Comment

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

7 Comments

Comments

4 Comments

1 Comment

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related