Question

I want to write my own simple DI framework.
I want that it perform only this simple case like Spring does:

public interface IWriter {
    public void writer(String s);
}

@Service
public class MySpringBeanWithDependency {
    private IWriter writer;

    @Autowired
    public void setWriter(IWriter writer) {
        this.writer = writer;
    }

    public void run() {
        String s = "This is my test";
        writer.writer(s);
    }
}

@Service
public class NiceWriter implements IWriter {
    public void writer(String s) {
        System.out.println("The string is " + s);
    }
}

public class Main extends TestCase {

    @Test
    public void test() {

        ApplicationContext context = new ClassPathXmlApplicationContext(
                "META-INF/beans.xml");
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) context
                .getBean("mySpringBeanWithDependency");
        test.run();
    }
}

The same case, but mb without an XML-file.

Can somebody explain the concept of this type of frameworks and write some code.

Answer 1

Guice is open source. You can browse the code here:
http://code.google.com/p/google-guice/source/browse/

Spring is also open source. You can download it here:
http://www.springsource.org/download

Browsing through either of these should satisfy the “write some code” part of your question.

EDIT: It seems you are looking for the “magic”. It boils down to reflection. Spring and/or Guice use Java Reflection to find the appropriate setters (or fields) on the class and set the values. That’s it. Everything else is glue to make the whole system work.