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Question about the task

Mastering ClassLoader and Reflection

Java Collections, Level 6, Lesson 10

Under discussion

I can't figure out how to approach this correctly. Can someone give a hint or two?

The Solution class constructor's parameter is the absolute path to a package.
The package name may include the File.separator.
In addition to compiled classes (.class), the package may have other files (for example: .java).
We know that every class has a no-argument constructor and implements the HiddenClass interface.
Read all the classes from the file system. Create a factory. Implement the getHiddenClassObjectByKey method.
Note: The package can only have one class whose simple name starts with the String key (not case sensitive).

Requirements:

Implement the scanFileSystem method. It must add the classes it finds to the hiddenClasses field.
Implement the getHiddenClassObjectByKey method. It must create a class object as outlined in the task conditions.
Don't change the main method.
The getHiddenClassObjectByKey method must not throw exceptions.

package com.codegym.task.task36.task3606; import java.io.*; import java.net.URL; import java.util.ArrayList; import java.util.List; /* Mastering ClassLoader and Reflection */ public class Solution { private List<Class> hiddenClasses = new ArrayList<>(); private String packageName; public Solution(String packageName) { this.packageName = packageName; } public static void main(String[] args) throws ClassNotFoundException { Solution solution = new Solution(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + "com/codegym/task/task36/task3606/data/second"); solution.scanFileSystem(); System.out.println(solution.getHiddenClassObjectByKey("secondhiddenclassimpl")); System.out.println(solution.getHiddenClassObjectByKey("firsthiddenclassimpl")); System.out.println(solution.getHiddenClassObjectByKey("packa")); } public void scanFileSystem() throws ClassNotFoundException { ClassLoader loader = Solution.class.getClassLoader(); String pack = packageName.replace(".", "/"); String dottedPackage = pack.replaceAll("[/]", "."); URL uPackage = loader.getResource(pack); try { DataInputStream dis = new DataInputStream((InputStream) uPackage.getContent()); String line = null; while ((line = dis.readLine()) != null) { if (line.endsWith(".class")) { hiddenClasses.add(Class.forName(dottedPackage + "." + line.substring(0, line.lastIndexOf('.')))); } } } catch (IOException ignored){} /*URL root = Thread.currentThread().getContextClassLoader().getResource(packageName.replace(".", "/")); File[] files = new File(root.getFile()).listFiles(new FilenameFilter() { public boolean accept(File dir, String name) { return name.endsWith(".class"); } }); for (File file : files) { String className = file.getName().replaceAll(".class$", ""); Class<?> cls = Class.forName(className); hiddenClasses.add(cls); } ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); URL packageURL = classLoader.getResource(packageName.replaceAll("[.]", "/")); try { URI uri = new URI(packageURL.toString()); File folder = new File(uri.getPath()); File[] files = folder.listFiles(); for (File f : files) { hiddenClasses.add(f.getClass()); } } catch (URISyntaxException ignored){}*/ } public HiddenClass getHiddenClassObjectByKey(String key) { HiddenClass hiddenClass = null; for (Class C : hiddenClasses){ if (C.getSimpleName().startsWith(key.toLowerCase())){ try { hiddenClass = (HiddenClass) C.newInstance(); } catch (InstantiationException | IllegalAccessException ignored){} } } return hiddenClass; } }

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Wei Cui

Level 39 , Newark, United States

21 April 2020, 21:52

public void scanFileSystem() throws ClassNotFoundException {
        File[] files = new File(packageName).listFiles();
        ClassLoaderFromPath classLoader = new ClassLoaderFromPath();

        for (File f : files) {
            Class<?> clazz = classLoader.load(f.toPath());
            if (clazz != null)
                hiddenClasses.add(clazz);
        }
    }

    public HiddenClass getHiddenClassObjectByKey(String key) {
        for (Class<?> clazz : hiddenClasses) {
            if (clazz.getSimpleName().toLowerCase().startsWith(key.toLowerCase())) {
                try {
                    Constructor[] constructors = clazz.getDeclaredConstructors();
                    for (Constructor constructor : constructors) {
                        if (constructor.getParameterTypes().length == 0) {
                            constructor.setAccessible(true);
                            return (HiddenClass) constructor.newInstance(null);
                        }
                    }
                } catch (Exception e) {
                    return null;
                }
            }
        }
        return null;
    }


    public static class ClassLoaderFromPath extends ClassLoader {
        public Class<?> load(Path path) {
            try {
                if (path.getFileName().toString().lastIndexOf(".class") == -1)
                    return null;

                byte[] b = Files.readAllBytes(path);
                return defineClass(null, b, 0, b.length); //here main magic
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
    }

+13