Add component annotation and use it in injector class

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

@@ -1,5 +1,9 @@
 package mate.academy.lib;
 
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+@Retention(RetentionPolicy.RUNTIME)
 public @interface Component {
 
 }

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

@@ -1,5 +1,9 @@
 package mate.academy.lib;
 
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+@Retention(RetentionPolicy.RUNTIME)
 public @interface Inject {
 
 }

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

@@ -1,5 +1,18 @@
 package mate.academy.lib;
 
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import mate.academy.service.FileReaderService;
+import mate.academy.service.ProductParser;
+import mate.academy.service.ProductService;
+import mate.academy.service.impl.FileReaderServiceImpl;
+import mate.academy.service.impl.ProductParserImpl;
+import mate.academy.service.impl.ProductServiceImpl;
+
 public class Injector {
     private static final Injector injector = new Injector();
 
@@ -8,6 +21,74 @@ public static Injector getInjector() {
     }
 
     public Object getInstance(Class<?> interfaceClazz) {
-        return null;
+        return getInstance(interfaceClazz, new HashSet<>());
+    }
+
+    private Object getInstance(Class<?> interfaceClazz, Set<Class<?>> instantiationStack) {
+        Class<?> clazz = findImplementation(interfaceClazz);
+
+        if (!clazz.isAnnotationPresent(Component.class)) {
+            throw new RuntimeException(
+                    "Class " + clazz.getName()
+                    + " is not annotated with @Component or no implementation found"
+            );
+        }
+
+        if (instantiationStack.contains(clazz)) {
+            throw new RuntimeException(
+                    "Circular dependency detected for class: " + clazz.getName()
+            );
+        }
+
+        instantiationStack.add(clazz);
+
+        Object clazzImplementationInstance = createNewInstance(clazz);
+
+        Field[] declaredFields = clazz.getDeclaredFields();
+        for (Field field : declaredFields) {
+            if (field.isAnnotationPresent(Inject.class)) {
+                Object fieldInstance = getInstance(field.getType(), instantiationStack);
+
+                try {
+                    field.setAccessible(true);
+                    field.set(clazzImplementationInstance, fieldInstance);
+                } catch (IllegalAccessException e) {
+                    throw new RuntimeException(
+                            "Can't initialize field value. "
+                            + "Class: " + clazz.getName() + ". Field: " + field.getName()
+                    );
+                }
+            }
+        }
+
+        instantiationStack.remove(clazz);
+
+        return clazzImplementationInstance;
+    }
+
+    private Object createNewInstance(Class<?> clazz) {
+        try {
+            Constructor<?> constructor = clazz.getDeclaredConstructor();
+            constructor.setAccessible(true);
+            return constructor.newInstance();
+        } catch (Exception e) {
+            throw new RuntimeException("Failed to create instance of class: " + clazz.getName(), e);
+        }
+    }
+
+    private Class<?> findImplementation(Class<?> interfaceClazz) {
+        Map<Class<?>, Class<?>> interfaceToImplementation = new HashMap<>();
+        interfaceToImplementation.put(FileReaderService.class, FileReaderServiceImpl.class);
+        interfaceToImplementation.put(ProductParser.class, ProductParserImpl.class);
+        interfaceToImplementation.put(ProductService.class, ProductServiceImpl.class);
+
+        Class<?> implementationClass =
+                interfaceToImplementation.get(interfaceClazz);
+        if (implementationClass == null) {
+            throw new RuntimeException(
+                    "No implementation found for interface: " + interfaceClazz.getName()
+            );
+        }
+        return implementationClass;
     }
 }

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