Dynamic Class Loading

 This article was originally posted on JRoller on June 30, 2005.

The other day, I wanted to write an Eclipse plugin (maybe more about that in a different post), in which I need to read a selected class file from the project I am working on and execute a method in it. Since I can not have my project in the classpath, I found out that the only solution is to have the class loaded dynamically. If there is a better solution in Eclipse, please somebody tell me.

Before starting to write my plugin, I decided to write a small test application, because I never used class loading before. So here is the class I want to load:

package hello;

public class HelloWorld
{
  public void run()
  {
    System.out.println ("Hello World!");
  }
}

To load it and execute the run method, you can then use the following lines of code:

        ClassLoader loader = new ClassLoader(getClass().getClassLoader())
        {
            public Class findClass(String name) {
                try
                {
                    String path = "C:\\mypath\\hello";
                    File file = new File(path, name + ".class");
                    RandomAccessFile raf = new RandomAccessFile(file, "r");
                    byte[] content = new byte[(int)file.length()];
                    raf.readFully(content);

                    return defineClass("hello." + name, content, 0, content.length);
                }
                catch (Exception e)
                {
                    e.printStackTrace();
                }
                
                return null;
            }
        };
        
        try
        {
            Class helloClass = loader.loadClass("HelloWorld");
            Object hello = helloClass.newInstance();
            Method m = helloClass.getMethod("run", new Class[0]);
            m.invoke(hello);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

 I did not try the code on more recent java, but since the whole Class Loader API was in the process of being removed, I guess there are other ways to perform this nowadays. I tried asking ChatGPT to produce this code, and the result is quite similar, except it was using the URLClassLoader object which handles reading the file content for us.

Python: ruamel.yaml lib has a problem handling comments

 In our project, we are using the ruamel.yaml library for handling reading/writing YAML files. The reason we are not using yaml basic lib from Python is that ruamel handles better yaml standard, keeps the comments and formatting, and always dumps the keys in the same order.

However, since version 0.18.3, we had some strange behavior in our file dump. Some newlines were removed from some files. I opened ticket #492, with the following code that replicates the problem:

import ruamel.yaml

y = ruamel.yaml.YAML()

with open("organizational_units.yaml", "r") as file:

    ou = y.load(file)

with open("organizational_units.yaml", "r") as file:

    content = y.load(file)

content["organizational_units"] = ou["organizational_units"]

with open("test.yaml", "w") as file:

    y.dump(content, file)

with open("test.yaml", "w") as file:

    y.dump(content, file)

with open("test.yaml", "r") as file:

    y.load(file)

It is of course an oversimplified version of what we are doing in our project. We are normally loading several YAML files and combine them into one big model. Then, when we need to save changes into one file, we first reload it into memory in order to retrieve the original comments at the beginning of the file before replacing the old content with the new one.

Then you can see that we are saving our file twice. In fact, we are really performing a first save into an in-memory string stream, before logging the content in the file (at least in debug mode). Then we are saving it. Again, this code here is a simplification just to display the problem.

The problem occurs on the second save. The first works fine. Using this file as an example:

# Organizational Unit Specification

organizational_units:

- Name: root

  Accounts:

  - FirstAccount

  - SecondAccount

After the second save, we have this result:

# Organizational Unit Specification

organizational_units: -

  Name: root

  Accounts:

  - FirstAccount

  - SecondAccount

Noticed the missing newlines?

The last line of the code is loading the resulting file, just to show that we can not read it back.

After opening the ticket, I got the answer (on the same day, nice reactivity!) that it is in fact the duplicate of ticket #410. The #410 is a bit different, because it duplicates the complete structure, while we are only replacing a part of it. So maybe that is why our code was still working. I think the part that broke it is coming from this fix: "fix issue with spurious newline on first item after comment + nested block sequence".

As the developer explains, the issue is coming from the way the library is storing comments internally. It seems that comments are stored in different places, with the same reference. And when they are dumped, to avoid saving them several times, there is some internal bookkeeping going on. When we replaced reference to the top key, we broke some comments reference.

As a workaround, I restored comments reference around the top key:

comments = content["organizational_units"].ca.comment

content["organizational_units"] = ou["organizational_units"]

content["organizational_units"].ca.comment = comments

Worked for me...

JComboBox Editor Listening

This article was posted originally on JRoller June 3, 2005

To listen to edition event in the editor component of a JComboBox:

((JTextComponent)comboBox.getEditor().getEditorComponent()).getDocument().addDocumentListener(listener);