How eclipse executes a Java application?

Question

I have a very simple .java file in Eclipse Java Project that creates a .txt file.

package mproject.assgn.q12;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;

public class ColumnPattern {

    public static void main(String[] args) {
        System.out.println(getInputFile().getAbsolutePath());
    }

    private static File getInputFile() {

        String content = "110\t100\t255\t122\n"
                + "100\t109\t180\t111\n"
                + "22\t101\t122\t110\n"
                + "211\t122\t177\t101";

        File file = new File("file2.txt");
        try {

            if(!file.exists()){

            System.out.println("Creating Input File since it does'nt exist");
            file.createNewFile();

            }

            FileWriter fw = new FileWriter(file.getAbsoluteFile());
            BufferedWriter bw = new BufferedWriter(fw);

            bw.write(content);
            bw.flush();
            bw.close();

            return file;
        }

        catch(IOException e){
            e.printStackTrace();
            System.err.println("Error during File I/O");
        }

        return null;
    }

}

What I want to know is why the file that is created is found under /projectFolder/, while the .java resides in /projectName/src and the class file resides in /projectFolder/bin. Shouldn't the text file be created under the /bin folder where the .class file resides ?

How does eclipse execute the .java file when we select "Run As Java Application" ?

Does it compile it to a jar and then execute it ?

Answer 1

Why the text file is not under /bin
Eclipse creates hidden by default project files - .project. The directory where this file is created is considered the root directory for the project. When using relative file paths, they are always resolved against the project root.

How does Eclipse run the application

Eclipse compiles the classes under /bin (unless specified otherwise) and executes the class with main method. Considering you have more than one class with a main method, you have to choose after selecting Run As Java Application.

Also, a side note regarding the jar files - those are just archives containing classes and other files. When executing a jar file, you are basically executing the main method of the main class, declared in the META-INF/Manifest.mf file.