Solution

src/main/java/mate.academy/Main.java

@@ -8,9 +8,8 @@
 public class Main {
 
     public static void main(String[] args) {
-        // Please test your Injector here. Feel free to push this class as a part of your solution
         Injector injector = Injector.getInjector();
-        ProductService productService = null;
+        ProductService productService = (ProductService) injector.getInstance(ProductService.class);
         List<Product> products = productService.getAllFromFile("products.txt");
         products.forEach(System.out::println);
     }

src/main/java/mate.academy/lib/Component.java

@@ -1,5 +1,11 @@
 package mate.academy.lib;
 
-public @interface Component {
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+public @interface Component {
 }

src/main/java/mate.academy/lib/Inject.java

@@ -1,5 +1,11 @@
 package mate.academy.lib;
 
-public @interface Inject {
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.FIELD)
+public @interface Inject {
 }

src/main/java/mate.academy/lib/InjectionException.java

@@ -0,0 +1,11 @@
+package mate.academy.lib;
+
+public class InjectionException extends RuntimeException {
+    public InjectionException(String message) {
+        super(message);
+    }
+
+    public InjectionException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}

src/main/java/mate.academy/lib/Injector.java

@@ -1,13 +1,108 @@
 package mate.academy.lib;
 
+import java.io.File;
+import java.io.IOException;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.net.URL;
+import java.util.ArrayList;
+import java.util.Enumeration;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
 public class Injector {
     private static final Injector injector = new Injector();
+    private final Map<Class<?>, Object> instances = new HashMap<>();
+    private final Map<Class<?>, Class<?>> interfaceImplementations = new HashMap<>();
+
+    private Injector() {
+        try {
+            List<Class<?>> classes = getClasses("mate.academy");
+            for (Class<?> clazz : classes) {
+                if (clazz.isAnnotationPresent(Component.class)) {
+                    Class<?>[] interfaces = clazz.getInterfaces();
+                    if (interfaces.length > 0) {
+                        interfaceImplementations.put(interfaces[0], clazz);
+                    }
+                }
+            }
+        } catch (Exception e) {
+            throw new RuntimeException("Can't initialize injector", e);
+        }
+    }
 
     public static Injector getInjector() {
         return injector;
     }
 
     public Object getInstance(Class<?> interfaceClazz) {
-        return null;
+        if (instances.containsKey(interfaceClazz)) {
+            return instances.get(interfaceClazz);
+        }
+        Class<?> clazz = findImplementation(interfaceClazz);
+        if (clazz == null || !clazz.isAnnotationPresent(Component.class)) {
+            throw new InjectionException("Missing @Component annotation on class "
+                    + "or no implementation for " + interfaceClazz.getName());
+        }
+        try {
+            Constructor<?> constructor = clazz.getConstructor();
+            Object instance = constructor.newInstance();
+            Field[] fields = clazz.getDeclaredFields();
+            for (Field field : fields) {
+                if (field.isAnnotationPresent(Inject.class)) {
+                    Object fieldInstance = getInstance(field.getType());
+                    field.setAccessible(true);
+                    field.set(instance, fieldInstance);
+                }
+            }
+            instances.put(interfaceClazz, instance);
+            return instance;
+        } catch (ReflectiveOperationException e) {
+            throw new InjectionException("Can't create instance of " + clazz.getName(), e);
+        }
+    }
+
+    private Class<?> findImplementation(Class<?> interfaceClazz) {
+        if (interfaceClazz.isInterface()) {
+            return interfaceImplementations.get(interfaceClazz);
+        }
+        return interfaceClazz;
+    }
+
+    private static List<Class<?>> getClasses(String packageName)
+            throws ClassNotFoundException, IOException {
+        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
+        String path = packageName.replace('.', '/');
+        Enumeration<URL> resources = classLoader.getResources(path);
+        List<File> dirs = new ArrayList<>();
+        while (resources.hasMoreElements()) {
+            URL resource = resources.nextElement();
+            dirs.add(new File(resource.getFile()));
+        }
+        ArrayList<Class<?>> classes = new ArrayList<>();
+        for (File directory : dirs) {
+            classes.addAll(findClasses(directory, packageName));
+        }
+        return classes;
+    }
+
+    private static List<Class<?>> findClasses(File directory, String packageName)
+            throws ClassNotFoundException {
+        List<Class<?>> classes = new ArrayList<>();
+        if (!directory.exists()) {
+            return classes;
+        }
+        File[] files = directory.listFiles();
+        for (File file : files) {
+            if (file.isDirectory()) {
+                classes.addAll(findClasses(file, packageName + "." + file.getName()));
+            } else if (file.getName().endsWith(".class")) {
+                String className = packageName + '.'
+                        + file.getName().substring(0, file.getName().length() - 6);
+                classes.add(Class.forName(className));
+            }
+        }
+        return classes;
     }
 }

src/main/java/mate.academy/service/impl/FileReaderServiceImpl.java

@@ -4,8 +4,10 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.util.List;
+import mate.academy.lib.Component;
 import mate.academy.service.FileReaderService;
 
+@Component
 public class FileReaderServiceImpl implements FileReaderService {
     @Override
     public List<String> readFile(String fileName) {

src/main/java/mate.academy/service/impl/ProductParserImpl.java

@@ -1,9 +1,11 @@
 package mate.academy.service.impl;
 
 import java.math.BigDecimal;
+import mate.academy.lib.Component;
 import mate.academy.model.Product;
 import mate.academy.service.ProductParser;
 
+@Component
 public class ProductParserImpl implements ProductParser {
     public static final int ID_POSITION = 0;
     public static final int NAME_POSITION = 1;

src/main/java/mate.academy/service/impl/ProductServiceImpl.java

@@ -2,13 +2,18 @@
 
 import java.util.List;
 import java.util.stream.Collectors;
+import mate.academy.lib.Component;
+import mate.academy.lib.Inject;
 import mate.academy.model.Product;
 import mate.academy.service.FileReaderService;
 import mate.academy.service.ProductParser;
 import mate.academy.service.ProductService;
 
+@Component
 public class ProductServiceImpl implements ProductService {
+    @Inject
     private ProductParser productParser;
+    @Inject
     private FileReaderService fileReaderService;
 
     @Override

src/main/java/mate/academy/lib/Component.java

@@ -0,0 +1 @@
+

src/main/java/mate/academy/lib/Inject.java

@@ -0,0 +1 @@
+

src/main/java/mate/academy/lib/InjectionException.java

@@ -0,0 +1 @@
+

src/main/java/mate/academy/lib/Injector.java

@@ -0,0 +1 @@
+