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Lecture 9, 10 and 11 : Command Handlers
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‎Initial_Projects/NoBS/src/main/java/com/example/demo/Command.java‎

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package com.example.demo;
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import org.springframework.http.ResponseEntity;
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public interface Command <E, T> {
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// <E, T> = Entity, T is generic type in java
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ResponseEntity <T> execute(E entity);
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// we can also name the method handle.. since its a handler.. but execute is also used..
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}

‎Initial_Projects/NoBS/src/main/java/com/example/demo/Product/Model/UpdateProductCommand.java‎

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package com.example.demo.Product.Model;
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import lombok.Data;
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@Data
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public class UpdateProductCommand {
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private int id;
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private Product product;
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public UpdateProductCommand(int id, Product product) {
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this.id = id;
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this.product = product;
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}
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}

‎Initial_Projects/NoBS/src/main/java/com/example/demo/Product/ProductController.java‎

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package com.example.demo.Product;
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import com.example.demo.Product.Model.Product;
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import com.example.demo.Product.Model.UpdateProductCommand;
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import com.example.demo.Product.commandHandlers.CreateProductCommandHandler;
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import com.example.demo.Product.commandHandlers.DeleteProductCommandHandler;
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import com.example.demo.Product.commandHandlers.UpdateProductCommandHandler;
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import com.example.demo.Product.queryHandlers.GetAllProductsQueryHandler;
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import com.example.demo.Product.queryHandlers.GetProductQueryHandler;
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import org.apache.coyote.Response;
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return getProductQueryHandler.execute(id);
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}
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// Lecture - 5: POST, PUT and DELETE MAPPING
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// // Lecture - 5: POST, PUT and DELETE MAPPING
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// @PostMapping
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// // @RequestBody will tell that in the HTTP request body look for a product.
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// public ResponseEntity createProduct(@RequestBody Product product) {
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// // Accept a product through JSON
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// // Convert it to a Product
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// // Save it in the database
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// productRepository.save(product); // Currently we do not have any data validation.
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// return ResponseEntity.ok().build(); // This wont return anything in the body for now. Only returns a 200
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// // response code.
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// }
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// Lecture 9 : Command Handler
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@Autowired
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private CreateProductCommandHandler createProductCommandHandler;
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@PostMapping
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// @RequestBody will tell that in the HTTP request body look for a product.
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public ResponseEntity createProduct(@RequestBody Product product) {
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// Accept a product through JSON
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// Convert it to a Product
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// Save it in the database
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productRepository.save(product); // Currently we do not have any data validation.
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return ResponseEntity.ok().build(); // This wont return anything in the body for now. Only returns a 200
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// response code.
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public ResponseEntity<String> createProduct(@RequestBody Product product) {
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return createProductCommandHandler.execute(product);
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}
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// PUT is kind of a combination of GET AND POST.
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// @PutMapping("/{id}")
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// public ResponseEntity updateProduct(@PathVariable Integer id, @RequestBody Product product) {
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// // We would require the id to update the product
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// // Again we are not doing any validation here.
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// // We are doing this id thing manually just to see. We could have passed the ID
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// // in the product itself.
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// product.setId(id); // Helps set the id so that retrieval / update becomes easy.
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// productRepository.save(product); // updates the given product for the id set above.
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// return ResponseEntity.ok().build();
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// }
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// Lecture 10 : Command Handler 2
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@Autowired
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UpdateProductCommandHandler updateProductCommandHandler;
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@PutMapping("/{id}")
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public ResponseEntity updateProduct(@PathVariable Integer id, @RequestBody Product product) {
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// We would require the id to update the product
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// Again we are not doing any validation here.
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// We are doing this id thing manually just to see. We could have passed the ID
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// in the product itself.
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product.setId(id); // Helps set the id so that retrieval / update becomes easy.
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productRepository.save(product); // updates the given product for the id set above.
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return ResponseEntity.ok().build();
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UpdateProductCommand updateProductCommand = new UpdateProductCommand(id, product);
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return updateProductCommandHandler.execute(updateProductCommand);
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}
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// @DeleteMapping("/{id}")
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// public ResponseEntity deleteProduct(@PathVariable Integer id) {
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// Product product = productRepository.findById(id).get();
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// productRepository.delete(product);
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//
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// // OR..
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// // productRepository.deleteById(id);
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//
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// return ResponseEntity.ok().build();
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// }
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// Lecture 11 : Command Handler 3
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@Autowired
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private DeleteProductCommandHandler deleteProductCommandHandler;
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@DeleteMapping("/{id}")
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public ResponseEntity deleteProduct(@PathVariable Integer id) {
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Product product = productRepository.findById(id).get();
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productRepository.delete(product);
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// OR..
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// productRepository.deleteById(id);
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return ResponseEntity.ok().build();
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// Not fully done yet as we have not handled the Optional (nullable) yet... in case we don't find the product with the given id.
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// It'd be done in further classes with custom exception...
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return deleteProductCommandHandler.execute(id);
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}
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}

‎Initial_Projects/NoBS/src/main/java/com/example/demo/Product/commandHandlers/CreateProductCommandHandler.java‎

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package com.example.demo.Product.commandHandlers;
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import com.example.demo.Command;
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import com.example.demo.Product.Model.Product;
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import com.example.demo.Product.ProductRepository;
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import io.micrometer.common.util.StringUtils;
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import org.springframework.beans.factory.annotation.Autowired;
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import org.springframework.http.ResponseEntity;
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import org.springframework.stereotype.Service;
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// Lecture 9 : Command Handler
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@Service
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public class CreateProductCommandHandler implements Command<Product, String> {
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@Autowired // allows us to inject the product repository and access it in the command handler.
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private ProductRepository productRepository;
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@Override
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public ResponseEntity<String> execute(Product product) {
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// Validate then save...
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checkValidation(product);
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// Validation provides the safety for our app here...
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// The validation won't allow to save a wrong product and will throw an exception and stop execution.
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productRepository.save(product);
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return ResponseEntity.ok().build();
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}
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private void checkValidation(Product product) {
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// name
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if (StringUtils.isBlank(product.getName())) { // micrometer utils... check import
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throw new RuntimeException("Product name cannot be empty");
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}
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// description
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if(StringUtils.isBlank(product.getDescription())) {
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throw new RuntimeException("Description cannot be empty");
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}
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// price
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if(product.getPrice() <= 0.0) {
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throw new RuntimeException("Price cannot be negative or 0");
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}
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// quantity
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if(product.getQuantity() <= 0) {
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throw new RuntimeException("Quantity cannot be negative or 0");
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}
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}
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}

‎Initial_Projects/NoBS/src/main/java/com/example/demo/Product/commandHandlers/DeleteProductCommandHandler.java‎

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package com.example.demo.Product.commandHandlers;
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import com.example.demo.Command;
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import com.example.demo.Product.Model.Product;
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import com.example.demo.Product.ProductRepository;
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import org.springframework.beans.factory.annotation.Autowired;
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import org.springframework.http.ResponseEntity;
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import org.springframework.stereotype.Service;
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@Service
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public class DeleteProductCommandHandler implements Command<Integer, ResponseEntity> {
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@Autowired
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private ProductRepository productRepository;
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@Override
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public ResponseEntity<ResponseEntity> execute(Integer id) {
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Product product = productRepository.getReferenceById(id);
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productRepository.delete(product);
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return ResponseEntity.ok().build();
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}
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}

‎Initial_Projects/NoBS/src/main/java/com/example/demo/Product/commandHandlers/UpdateProductCommandHandler.java‎

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package com.example.demo.Product.commandHandlers;
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import com.example.demo.Command;
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import com.example.demo.Product.Model.Product;
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import com.example.demo.Product.Model.UpdateProductCommand;
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import com.example.demo.Product.ProductRepository;
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import io.micrometer.common.util.StringUtils;
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import org.springframework.beans.factory.annotation.Autowired;
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import org.springframework.http.ResponseEntity;
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import org.springframework.stereotype.Service;
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@Service
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public class UpdateProductCommandHandler implements Command<UpdateProductCommand, ResponseEntity> {
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@Autowired
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private ProductRepository productRepository;
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@Override
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public ResponseEntity execute(UpdateProductCommand command) {
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Product product = command.getProduct();
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// validate product
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checkValidation(product);
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product.setId(command.getId());
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// If this is not done and the entry does not already exist in the database
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// then it will work just like POST command else it will create a new entry.
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productRepository.save(product);
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return ResponseEntity.ok().build();
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}
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// This method could be abstracted even more as it is being used in multiple classes...
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// by making something like another utility class
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private void checkValidation(Product product) {
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// name
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if (StringUtils.isBlank(product.getName())) { // micrometer utils... check import
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throw new RuntimeException("Product name cannot be empty");
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}
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// description
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if(StringUtils.isBlank(product.getDescription())) {
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throw new RuntimeException("Description cannot be empty");
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}
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// price
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if(product.getPrice() <= 0.0) {
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throw new RuntimeException("Price cannot be negative or 0");
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}
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// quantity
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if(product.getQuantity() <= 0) {
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throw new RuntimeException("Quantity cannot be negative or 0");
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}
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}
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}

‎Initial_Projects/db/NoBS.sql‎

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use nobs;
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/*
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CREATE TABLE product (
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id INT AUTO_INCREMENT PRIMARY KEY,
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name VARCHAR(255),
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description VARCHAR(255),
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price double,
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quantity int
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);
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*/
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select * from product;
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