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Question about the task

What is ClassLoader?

Java Collections, Level 5, Lesson 10

Resolved

Implement the getAllAnimals method.
The method's pathToAnimals parameter is the absolute path to the directory where the compiled classes are saved.
The path doesn't necessarily have a "/" at the end.
Not all classes inherit the Animal interface.
Not all classes have a public no-argument constructor.
Only for classes that both inherit Animal and have a public no-argument constructor, create one object each.
Add the created objects to the resulting set and return it.
The main method is not tested.

Requirements:

The size of the set returned by the getAllAnimals method must be equal to the number of classes that both support the Animal interface and have a public no-argument constructor (from among the classes found in the directory passed as an argument).
The set returned by the getAllAnimals method must include all of the classes that both support the Animal interface and have a public no-argument constructor (from among the classes found in the directory passed as an argument).
The set returned by the getAllAnimals method must NOT contain any classes that don't both support the Animal interface and have a public no-argument constructor (from among the classes found in the directory passed as an argument).
The getAllAnimals method must be static.

package com.codegym.task.task35.task3507; import java.io.File; import java.lang.reflect.Constructor; import java.lang.reflect.Modifier; import java.net.MalformedURLException; import java.net.URISyntaxException; import java.net.URL; import java.net.URLClassLoader; import java.util.*; /* What is ClassLoader? */ public class Solution { public static void main(String[] args) { Set<? extends Animal> allAnimals = getAllAnimals(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + Solution.class.getPackage().getName().replaceAll("[.]", "/") + "/data"); System.out.println(allAnimals); Set<? extends Animal> allAnimals2 = getAllAnimals(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + Solution.class.getPackage().getName().replaceAll("[.]", "/")); System.out.println(allAnimals2); } public static Set<? extends Animal> getAllAnimals(String pathToAnimals) { // 1. get all java class at path // 2. if else ... add class with no argument constructor and inheriting Animal to set File dir = new File(pathToAnimals); Set<Animal> result = new HashSet<>(); try { URL url = dir.toURI().toURL(); URLClassLoader urlClassLoader = URLClassLoader.newInstance(new URL[]{url}); for (String className : walkClassName(dir)) { try { Class<?> clazz = urlClassLoader.loadClass(className); Constructor<?> ct = clazz.getConstructor(); if (Modifier.isPublic(ct.getModifiers()) && Animal.class.isAssignableFrom(clazz)) { Animal animal = (Animal) clazz.newInstance(); result.add(animal); } } catch (InstantiationException | NoSuchMethodException | IllegalAccessException | ClassNotFoundException e) { // e.printStackTrace(); } } } catch (URISyntaxException | MalformedURLException ignored) { } return result; } public static ArrayList<String> walkClassName(File dir) throws URISyntaxException { String path = new File(Solution.class.getProtectionDomain().getCodeSource().getLocation().toURI()).getAbsolutePath(); ArrayList<String> result = new ArrayList<>(); for (File file : Objects.requireNonNull(dir.listFiles())) { if (file.isFile() && (file.getName().endsWith(".class"))) { String filePath = file.getAbsolutePath(); filePath = filePath.substring(0, filePath.length() - 6); result.add(filePath.substring( path.length() + 1 ).replace(File.separator, ".")); } } return result; } public static ArrayList<String> walkAllClassName(File dir) throws URISyntaxException { String path = new File(Solution.class.getProtectionDomain().getCodeSource().getLocation().toURI()).getAbsolutePath(); Stack<File> stack = new Stack<>(); ArrayList<String> result = new ArrayList<>(); stack.add(dir); while (!stack.isEmpty()) { File probe = stack.pop(); File[] files = probe.listFiles(); if (files == null) { continue; } for (File file : files) { if (file.isDirectory()) { stack.add(file); } else if (file.isFile() && file.getName().endsWith(".class")) { String filePath = file.getAbsolutePath(); filePath = filePath.substring(0, filePath.length() - 6); result.add(filePath.substring( path.length() + 1 ).replace(File.separator, ".")); } } } return result; } }

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Wei Cui

Level 39 , Newark, United States

17 April 2020, 18:09

solution

public static class ClassFromPath extends ClassLoader {
        public Class<?> load(Path path, String packageName) {
            try {
                String className = packageName + "." + path.getFileName().toString().replace(".class", "");
                byte[] b = Files.readAllBytes(path);
                return defineClass(className, b, 0, b.length); //here main magic
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
    }

JianQiu Hwang

Level 35 , Washington

19 April 2020, 00:52

Thank for your tips. It is strange that my origin answer above doesn't pass, may be there are some limits which don't on the requirements. And I guess that: 1. should build my own classloader; 2. don't use URLClassLoader; 3. only need to list file in data directory (use `.listFiles()` instead of walking all file)

public static class ClassPathLoader extends ClassLoader {
    public Class<?> loadClass(Path path, String packageName) {
        try {
            String filePath = path.getFileName().toString();
            String className = packageName + "." + filePath.substring(0, filePath.length() - ".class".length());
            byte[] b = Files.readAllBytes(path);
            return defineClass(className, b, 0, b.length);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}
public static Set<? extends Animal> getAllAnimals(String pathToAnimals) {
    Set<Animal> set = new HashSet<>();
    File[] files = new File(pathToAnimals).listFiles();
    ClassPathLoader classLoader = new ClassPathLoader();
    for (File file : files) {
        if (file.isFile() && file.getName().endsWith(".class")) {
            String packageName = Solution.class.getPackage().getName() + ".data";
            Class clazz = classLoader.loadClass(file.toPath(), packageName);
            try {
                Constructor ct = clazz.getConstructor();
                if (Modifier.isPublic(ct.getModifiers()) && Animal.class.isAssignableFrom(clazz)) {
                    Animal animal = (Animal) clazz.newInstance();
                    set.add(animal);
                }
            } catch (NoSuchMethodException | IllegalAccessException | InstantiationException e) {
            }
        }
    }
    return set;
}

Wei Cui

Level 39 , Newark, United States

17 April 2020, 18:09

public static Set<? extends Animal> getAllAnimals(String pathToAnimals) {
        Set<Animal> set = new HashSet<>();

        File[] list = new File(pathToAnimals).listFiles();
        for (File file : list) {
            if (file.isFile() && file.getName().endsWith(".class")) {

                String packageName = Solution.class.getPackage().getName() + ".data"; //some bed solution (Hardcore data)
                Class clazz = new ClassFromPath().load(file.toPath(), packageName); //Loading class from path

                int score = 0;
                //find interface Animal
                Class[] interfaces = clazz.getInterfaces();
                for (Class interf : interfaces)
                    if (interf.getSimpleName().toString().equals("Animal")) {
                        score++;
                        break;
                    }

                //Find default constuctor
                Constructor[] constructors = clazz.getConstructors();
                for (Constructor constructor : constructors)
                    if (constructor.getParameterCount() == 0) {
                        score++;
                    }

                //if all ok, add to set
                if (score == 2)
                    try {
                        Animal animal = (Animal) clazz.newInstance();
                        set.add(animal);
                    } catch (InstantiationException e) {
                        e.printStackTrace();
                    } catch (IllegalAccessException e) {
                        e.printStackTrace();
                    }
            }
        }

        return set;
    }