Olingo: Odata: Create new entity

In my previous posts, I explained how to read all the entities, single entity and property of an entity.

In this tutorial, I am going to explain how to create an entity. This is continuation to my previous post, I will provide you the source code, that is required to create a product entity.

To support creation of objects, we need to implement 'EntityProcessor' interface and provide implementation to 'createEntity' method.

Step 1: Identify the type of entity from the request uri.

EdmEntitySet edmEntitySet = EntityUtil.getEntitySetFromURI(uriInfo);

EdmEntityType edmEntityType = edmEntitySet.getEntityType();

Step 2: Read the information from request body and deserialize the information to Entity.

InputStream requestInputStream = request.getBody();

ODataDeserializer deserializer = this.odata.createDeserializer(requestFormat);

DeserializerResult result = deserializer.entity(requestInputStream, edmEntityType);

Entity requestEntity = result.getEntity();

Step 3: Extract the information from Entity object and create new entity depends on the type.

EdmEntityType edmEntityType = edmEntitySet.getEntityType();

// actually, this is only required if we have more than one Entity Type

if (edmEntityType.getName().equals(DemoEdmProvider.ET_PRODUCT_NAME)) {

         return ProductUtil.createProduct(edmEntityType, entityToCreate);

}

Step 4: Serialize the created entity and send the response.

Project structure looks like below.

Find the below working application.

Product.java

package com.app.model;

public class Product {
 private int id;
 private String name;
 private String description;
 private double price;

 public Product(int id, String name, String description, double price){
  this.id = id;
  this.name = name;
  this.description = description;
  this.price = price;
 }

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 public double getPrice() {
  return price;
 }

 public void setPrice(double price) {
  this.price = price;
 }

}

DemoEdmPrimitiveProcessor.java

package com.app.service;

import java.io.InputStream;
import java.util.List;
import java.util.Locale;

import org.apache.olingo.commons.api.data.ContextURL;
import org.apache.olingo.commons.api.data.Entity;
import org.apache.olingo.commons.api.data.Property;
import org.apache.olingo.commons.api.edm.EdmEntitySet;
import org.apache.olingo.commons.api.edm.EdmPrimitiveType;
import org.apache.olingo.commons.api.edm.EdmProperty;
import org.apache.olingo.commons.api.format.ContentType;
import org.apache.olingo.commons.api.http.HttpHeader;
import org.apache.olingo.commons.api.http.HttpStatusCode;
import org.apache.olingo.server.api.OData;
import org.apache.olingo.server.api.ODataApplicationException;
import org.apache.olingo.server.api.ODataLibraryException;
import org.apache.olingo.server.api.ODataRequest;
import org.apache.olingo.server.api.ODataResponse;
import org.apache.olingo.server.api.ServiceMetadata;
import org.apache.olingo.server.api.processor.PrimitiveProcessor;
import org.apache.olingo.server.api.serializer.ODataSerializer;
import org.apache.olingo.server.api.serializer.PrimitiveSerializerOptions;
import org.apache.olingo.server.api.serializer.SerializerResult;
import org.apache.olingo.server.api.uri.UriInfo;
import org.apache.olingo.server.api.uri.UriParameter;
import org.apache.olingo.server.api.uri.UriResource;
import org.apache.olingo.server.api.uri.UriResourceEntitySet;
import org.apache.olingo.server.api.uri.UriResourceProperty;

import com.app.util.EntityUtil;

public class DemoEdmPrimitiveProcessor implements PrimitiveProcessor {

 private OData odata;
 private ServiceMetadata serviceMetadata;

 @Override
 public void init(OData odata, ServiceMetadata serviceMetadata) {
  this.odata = odata;
  this.serviceMetadata = serviceMetadata;
 }

 @Override
 public void readPrimitive(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType responseFormat)
   throws ODataApplicationException, ODataLibraryException {

  List<UriResource> resourceParts = uriInfo.getUriResourceParts();

  UriResourceEntitySet uriEntityset = (UriResourceEntitySet) resourceParts.get(0);
  EdmEntitySet edmEntitySet = uriEntityset.getEntitySet();

  List<UriParameter> keyPredicates = uriEntityset.getKeyPredicates();

  // Retrieve the requested (Edm) property. The last segment is the
  // Property
  UriResourceProperty uriProperty = (UriResourceProperty) resourceParts.get(resourceParts.size() - 1);
  EdmProperty edmProperty = uriProperty.getProperty();

  String edmPropertyName = edmProperty.getName();
  EdmPrimitiveType edmPropertyType = (EdmPrimitiveType) edmProperty.getType();

  Entity entity = EntityUtil.readEntityData(edmEntitySet, keyPredicates);
  if (entity == null) { 
   throw new ODataApplicationException("Entity not found", HttpStatusCode.NOT_FOUND.getStatusCode(),
     Locale.ENGLISH);
  }

  Property property = entity.getProperty(edmPropertyName);
  if (property == null) {
   throw new ODataApplicationException("Property not found", HttpStatusCode.NOT_FOUND.getStatusCode(),
     Locale.ENGLISH);
  }

  Object value = property.getValue();

  if (value == null) {
   response.setStatusCode(HttpStatusCode.NO_CONTENT.getStatusCode());
   return;
  }

  ODataSerializer serializer = odata.createSerializer(responseFormat);

  ContextURL contextUrl = ContextURL.with().entitySet(edmEntitySet).navOrPropertyPath(edmPropertyName).build();
  PrimitiveSerializerOptions options = PrimitiveSerializerOptions.with().contextURL(contextUrl).build();

  SerializerResult serializerResult = serializer.primitive(serviceMetadata, edmPropertyType, property, options);
  InputStream propertyStream = serializerResult.getContent();

  response.setContent(propertyStream);
  response.setStatusCode(HttpStatusCode.OK.getStatusCode());
  response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());

 }

 @Override
 public void updatePrimitive(ODataRequest request, ODataResponse response, UriInfo uriInfo,
   ContentType requestFormat, ContentType responseFormat)
   throws ODataApplicationException, ODataLibraryException {
  // TODO Auto-generated method stub

 }

 @Override
 public void deletePrimitive(ODataRequest request, ODataResponse response, UriInfo uriInfo)
   throws ODataApplicationException, ODataLibraryException {
  // TODO Auto-generated method stub

 }

}

DemoEdmProvider.java

package com.app.service;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import org.apache.olingo.commons.api.edm.EdmPrimitiveTypeKind;
import org.apache.olingo.commons.api.edm.FullQualifiedName;
import org.apache.olingo.commons.api.edm.provider.CsdlAbstractEdmProvider;
import org.apache.olingo.commons.api.edm.provider.CsdlEntityContainer;
import org.apache.olingo.commons.api.edm.provider.CsdlEntityContainerInfo;
import org.apache.olingo.commons.api.edm.provider.CsdlEntitySet;
import org.apache.olingo.commons.api.edm.provider.CsdlEntityType;
import org.apache.olingo.commons.api.edm.provider.CsdlProperty;
import org.apache.olingo.commons.api.edm.provider.CsdlPropertyRef;
import org.apache.olingo.commons.api.edm.provider.CsdlSchema;
import org.apache.olingo.commons.api.ex.ODataException;

/**
 * Class to provide OData services. Edm stands for Entity Data Model.
 * 
 * @author Krishna
 *
 */
public class DemoEdmProvider extends CsdlAbstractEdmProvider {
 // Service Namespace
 public static final String NAMESPACE = "OData.Demo";

 // EDM Container
 public static final String CONTAINER_NAME = "Container";
 public static final FullQualifiedName CONTAINER = new FullQualifiedName(NAMESPACE, CONTAINER_NAME);

 // Entity Types Names
 public static final String ET_PRODUCT_NAME = "Product";
 public static final FullQualifiedName ET_PRODUCT_FQN = new FullQualifiedName(NAMESPACE, ET_PRODUCT_NAME);

 // Entity Set Names
 public static final String ES_PRODUCTS_NAME = "Products";

 @Override
 public CsdlEntityContainer getEntityContainer() throws ODataException {
  // create EntitySets
  List<CsdlEntitySet> entitySets = new ArrayList<CsdlEntitySet>();
  entitySets.add(getEntitySet(CONTAINER, ES_PRODUCTS_NAME));

  // create EntityContainer
  CsdlEntityContainer entityContainer = new CsdlEntityContainer();
  entityContainer.setName(CONTAINER_NAME);
  entityContainer.setEntitySets(entitySets);

  return entityContainer;
 }

 @Override
 public CsdlEntityContainerInfo getEntityContainerInfo(FullQualifiedName entityContainerName) throws ODataException {
  // This method is invoked when displaying the Service Document at e.g.
  // http://localhost:8080/DemoService/DemoService.svc
  if (entityContainerName == null || entityContainerName.equals(CONTAINER)) {
   CsdlEntityContainerInfo entityContainerInfo = new CsdlEntityContainerInfo();
   entityContainerInfo.setContainerName(CONTAINER);
   return entityContainerInfo;
  }

  return null;
 }

 @Override
 public CsdlEntitySet getEntitySet(FullQualifiedName entityContainer, String entitySetName) throws ODataException {
  if (entityContainer.equals(CONTAINER)) {
   if (entitySetName.equals(ES_PRODUCTS_NAME)) {
    CsdlEntitySet entitySet = new CsdlEntitySet();
    entitySet.setName(ES_PRODUCTS_NAME);
    entitySet.setType(ET_PRODUCT_FQN);

    return entitySet;
   }
  }

  return null;
 }

 @Override
 public CsdlEntityType getEntityType(FullQualifiedName entityTypeName) throws ODataException {
  if (entityTypeName.equals(ET_PRODUCT_FQN)) {

   // create EntityType properties
   CsdlProperty id = new CsdlProperty().setName("ID")
     .setType(EdmPrimitiveTypeKind.Int32.getFullQualifiedName());
   CsdlProperty name = new CsdlProperty().setName("Name")
     .setType(EdmPrimitiveTypeKind.String.getFullQualifiedName());
   CsdlProperty description = new CsdlProperty().setName("Description")
     .setType(EdmPrimitiveTypeKind.String.getFullQualifiedName());
   CsdlProperty price = new CsdlProperty().setName("Price")
     .setType(EdmPrimitiveTypeKind.Double.getFullQualifiedName());

   // create CsdlPropertyRef for Key element
   CsdlPropertyRef propertyRef = new CsdlPropertyRef();
   propertyRef.setName("ID");

   // configure EntityType
   CsdlEntityType entityType = new CsdlEntityType();
   entityType.setName(ET_PRODUCT_NAME);
   entityType.setProperties(Arrays.asList(id, name, description, price));
   entityType.setKey(Collections.singletonList(propertyRef));

   return entityType;
  }

  return null;
 }

 @Override
 public List<CsdlSchema> getSchemas() throws ODataException {
  // create Schema
  CsdlSchema schema = new CsdlSchema();
  schema.setNamespace(NAMESPACE);

  // add EntityTypes
  List<CsdlEntityType> entityTypes = new ArrayList<CsdlEntityType>();
  entityTypes.add(getEntityType(ET_PRODUCT_FQN));
  schema.setEntityTypes(entityTypes);

  // add EntityContainer
  schema.setEntityContainer(getEntityContainer());

  // finally
  List<CsdlSchema> schemas = new ArrayList<CsdlSchema>();
  schemas.add(schema);

  return schemas;
 }

}

DemoEntityCollectionProcessor.java

package com.app.service;

import java.util.List;

import org.apache.olingo.commons.api.data.ContextURL;
import org.apache.olingo.commons.api.data.EntityCollection;
import org.apache.olingo.commons.api.edm.EdmEntitySet;
import org.apache.olingo.commons.api.edm.EdmEntityType;
import org.apache.olingo.commons.api.format.ContentType;
import org.apache.olingo.commons.api.http.HttpHeader;
import org.apache.olingo.commons.api.http.HttpStatusCode;
import org.apache.olingo.server.api.OData;
import org.apache.olingo.server.api.ODataApplicationException;
import org.apache.olingo.server.api.ODataLibraryException;
import org.apache.olingo.server.api.ODataRequest;
import org.apache.olingo.server.api.ODataResponse;
import org.apache.olingo.server.api.ServiceMetadata;
import org.apache.olingo.server.api.processor.EntityCollectionProcessor;
import org.apache.olingo.server.api.serializer.EntityCollectionSerializerOptions;
import org.apache.olingo.server.api.serializer.ODataSerializer;
import org.apache.olingo.server.api.serializer.SerializerResult;
import org.apache.olingo.server.api.uri.UriInfo;
import org.apache.olingo.server.api.uri.UriResource;
import org.apache.olingo.server.api.uri.UriResourceEntitySet;

import com.app.util.EntityUtil;

public class DemoEntityCollectionProcessor implements EntityCollectionProcessor {
 private OData odata;
 private ServiceMetadata serviceMetadata;

 public void init(OData odata, ServiceMetadata serviceMetadata) {
  this.odata = odata;
  this.serviceMetadata = serviceMetadata;
 }

 public void readEntityCollection(ODataRequest request, ODataResponse response, UriInfo uriInfo,
   ContentType responseFormat) throws ODataApplicationException, ODataLibraryException {

  List<UriResource> resourcePaths = uriInfo.getUriResourceParts();
  UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) resourcePaths.get(0);
  EdmEntitySet edmEntitySet = uriResourceEntitySet.getEntitySet();

  EntityCollection entitySet = EntityUtil.getData(edmEntitySet);

  ODataSerializer serializer = odata.createSerializer(responseFormat);

  EdmEntityType edmEntityType = edmEntitySet.getEntityType();
  ContextURL contextUrl = ContextURL.with().entitySet(edmEntitySet).build();

  final String id = request.getRawBaseUri() + "/" + edmEntitySet.getName();
  EntityCollectionSerializerOptions opts = EntityCollectionSerializerOptions.with().id(id).contextURL(contextUrl)
    .build();
  SerializerResult serializedContent = serializer.entityCollection(serviceMetadata, edmEntityType, entitySet,
    opts);

  response.setContent(serializedContent.getContent());
  response.setStatusCode(HttpStatusCode.OK.getStatusCode());
  response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());

 }

}

DemoEntityProcessor.java

package com.app.service;

import java.io.InputStream;
import java.util.List;

import org.apache.olingo.commons.api.data.ContextURL;
import org.apache.olingo.commons.api.data.Entity;
import org.apache.olingo.commons.api.edm.EdmEntitySet;
import org.apache.olingo.commons.api.edm.EdmEntityType;
import org.apache.olingo.commons.api.format.ContentType;
import org.apache.olingo.commons.api.http.HttpHeader;
import org.apache.olingo.commons.api.http.HttpStatusCode;
import org.apache.olingo.server.api.OData;
import org.apache.olingo.server.api.ODataApplicationException;
import org.apache.olingo.server.api.ODataLibraryException;
import org.apache.olingo.server.api.ODataRequest;
import org.apache.olingo.server.api.ODataResponse;
import org.apache.olingo.server.api.ServiceMetadata;
import org.apache.olingo.server.api.deserializer.DeserializerResult;
import org.apache.olingo.server.api.deserializer.ODataDeserializer;
import org.apache.olingo.server.api.processor.EntityProcessor;
import org.apache.olingo.server.api.serializer.EntitySerializerOptions;
import org.apache.olingo.server.api.serializer.ODataSerializer;
import org.apache.olingo.server.api.serializer.SerializerResult;
import org.apache.olingo.server.api.uri.UriInfo;
import org.apache.olingo.server.api.uri.UriParameter;
import org.apache.olingo.server.api.uri.UriResource;
import org.apache.olingo.server.api.uri.UriResourceEntitySet;

import com.app.util.EntityUtil;

public class DemoEntityProcessor implements EntityProcessor {
 private OData odata;
 private ServiceMetadata serviceMetadata;

 @Override
 public void init(OData odata, ServiceMetadata serviceMetadata) {
  this.odata = odata;
  this.serviceMetadata = serviceMetadata;
 }

 @Override
 public void readEntity(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType responseFormat)
   throws ODataApplicationException, ODataLibraryException {
  // 1. retrieve the Entity Type
  List<UriResource> resourcePaths = uriInfo.getUriResourceParts();

  // Note: only in our example we can assume that the first segment is the
  // EntitySet
  UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) resourcePaths.get(0);
  EdmEntitySet edmEntitySet = uriResourceEntitySet.getEntitySet();

  // 2. retrieve the data from backend
  List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
  Entity entity = EntityUtil.readEntityData(edmEntitySet, keyPredicates);

  // 3. serialize
  EdmEntityType entityType = edmEntitySet.getEntityType();

  ContextURL contextUrl = ContextURL.with().entitySet(edmEntitySet).build();
  // expand and select currently not supported
  EntitySerializerOptions options = EntitySerializerOptions.with().contextURL(contextUrl).build();

  ODataSerializer serializer = odata.createSerializer(responseFormat);
  SerializerResult serializerResult = serializer.entity(serviceMetadata, entityType, entity, options);
  InputStream entityStream = serializerResult.getContent();

  // 4. configure the response object
  response.setContent(entityStream);
  response.setStatusCode(HttpStatusCode.OK.getStatusCode());
  response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());
 }

 @Override
 public void createEntity(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType requestFormat,
   ContentType responseFormat) throws ODataApplicationException, ODataLibraryException {
  EdmEntitySet edmEntitySet = EntityUtil.getEntitySetFromURI(uriInfo);

  EdmEntityType edmEntityType = edmEntitySet.getEntityType();

  InputStream requestInputStream = request.getBody();
  ODataDeserializer deserializer = this.odata.createDeserializer(requestFormat);
  DeserializerResult result = deserializer.entity(requestInputStream, edmEntityType);
  Entity requestEntity = result.getEntity();

  Entity createdEntity = EntityUtil.createEntityData(edmEntitySet, requestEntity);

  ContextURL contextUrl = ContextURL.with().entitySet(edmEntitySet).build();

  EntitySerializerOptions options = EntitySerializerOptions.with().contextURL(contextUrl).build();

  ODataSerializer serializer = this.odata.createSerializer(responseFormat);
  SerializerResult serializedResponse = serializer.entity(serviceMetadata, edmEntityType, createdEntity, options);

  response.setContent(serializedResponse.getContent());
  response.setStatusCode(HttpStatusCode.CREATED.getStatusCode());
  response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());
 }

 @Override
 public void updateEntity(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType requestFormat,
   ContentType responseFormat) throws ODataApplicationException, ODataLibraryException {
  // TODO Auto-generated method stub

 }

 @Override
 public void deleteEntity(ODataRequest request, ODataResponse response, UriInfo uriInfo)
   throws ODataApplicationException, ODataLibraryException {
  // TODO Auto-generated method stub

 }

}

DemoServlet.java

package com.app.servlet;

import java.io.IOException;
import java.util.ArrayList;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.olingo.server.api.OData;
import org.apache.olingo.server.api.ODataHttpHandler;
import org.apache.olingo.server.api.ServiceMetadata;
import org.apache.olingo.server.api.edmx.EdmxReference;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.app.service.DemoEdmPrimitiveProcessor;
import com.app.service.DemoEdmProvider;
import com.app.service.DemoEntityCollectionProcessor;
import com.app.service.DemoEntityProcessor;

public class DemoServlet extends HttpServlet {

 private static final long serialVersionUID = 1L;
 private static final Logger LOG = LoggerFactory.getLogger(DemoServlet.class);

 @Override
 protected void service(final HttpServletRequest req, final HttpServletResponse resp)
   throws ServletException, IOException {

  try {
   OData odata = OData.newInstance();
   
   ServiceMetadata edm = odata.createServiceMetadata(new DemoEdmProvider(), new ArrayList<EdmxReference>());
   
   ODataHttpHandler handler = odata.createHandler(edm);
   handler.register(new DemoEntityCollectionProcessor());
   handler.register(new DemoEntityProcessor());
   handler.register(new DemoEdmPrimitiveProcessor());
   
   handler.process(req, resp);

  } catch (RuntimeException e) {
   LOG.error("Server Error occurred in ExampleServlet", e);
   throw new ServletException(e);
  }
 }
}

EntityUtil.java

package com.app.util;

import java.util.List;

import org.apache.olingo.commons.api.data.Entity;
import org.apache.olingo.commons.api.data.EntityCollection;
import org.apache.olingo.commons.api.edm.EdmEntitySet;
import org.apache.olingo.commons.api.edm.EdmEntityType;
import org.apache.olingo.server.api.ODataApplicationException;
import org.apache.olingo.server.api.uri.UriInfo;
import org.apache.olingo.server.api.uri.UriParameter;
import org.apache.olingo.server.api.uri.UriResource;
import org.apache.olingo.server.api.uri.UriResourceEntitySet;

import com.app.service.DemoEdmProvider;

/**
 * Responsible to perform CRUD operations on entities.
 * 
 * @author Krishna
 *
 */
public class EntityUtil {

 public static EntityCollection getData(EdmEntitySet edmEntitySet) {
  if (DemoEdmProvider.ES_PRODUCTS_NAME.equals(edmEntitySet.getName())) {
   return ProductUtil.getProductsData();
  }

  return null;
 }

 public static Entity readEntityData(EdmEntitySet edmEntitySet, List<UriParameter> keyParams)
   throws ODataApplicationException {

  EdmEntityType edmEntityType = edmEntitySet.getEntityType();

  // actually, this is only required if we have more than one Entity Type
  if (edmEntityType.getName().equals(DemoEdmProvider.ET_PRODUCT_NAME)) {
   return ProductUtil.getProduct(edmEntityType, keyParams);
  }

  return null;
 }

 public static EdmEntitySet getEntitySetFromURI(UriInfo uriInfo) {
  List<UriResource> resourcePaths = uriInfo.getUriResourceParts();

  UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) resourcePaths.get(0);
  EdmEntitySet edmEntitySet = uriResourceEntitySet.getEntitySet();

  return edmEntitySet;
 }

 public static Entity createEntityData(EdmEntitySet edmEntitySet, Entity entityToCreate) {

  EdmEntityType edmEntityType = edmEntitySet.getEntityType();

  // actually, this is only required if we have more than one Entity Type
  if (edmEntityType.getName().equals(DemoEdmProvider.ET_PRODUCT_NAME)) {
   return ProductUtil.createProduct(edmEntityType, entityToCreate);
  }

  return null;
 }

}

ProductUtil.java

package com.app.util;

import java.net.URI;
import java.net.URISyntaxException;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import org.apache.olingo.commons.api.data.Entity;
import org.apache.olingo.commons.api.data.EntityCollection;
import org.apache.olingo.commons.api.data.Property;
import org.apache.olingo.commons.api.data.ValueType;
import org.apache.olingo.commons.api.edm.EdmEntityType;
import org.apache.olingo.commons.api.edm.EdmPrimitiveType;
import org.apache.olingo.commons.api.edm.EdmPrimitiveTypeException;
import org.apache.olingo.commons.api.edm.EdmProperty;
import org.apache.olingo.commons.api.edm.EdmType;
import org.apache.olingo.commons.api.ex.ODataRuntimeException;
import org.apache.olingo.commons.api.http.HttpStatusCode;
import org.apache.olingo.server.api.ODataApplicationException;
import org.apache.olingo.server.api.uri.UriParameter;

import com.app.model.Product;

public class ProductUtil {
 private static List<Product> products = new ArrayList<>();
 public static int idCounter = 3;

 static {
  Product prod1 = new Product(1, "Washing Machine by AVC", "Good for morth east countries", 876544);
  Product prod2 = new Product(2, "XYZ Camera", "Catch pics while travelling", 965987);

  products.add(prod1);
  products.add(prod2);
 }

 public static int getId() {
  return idCounter++;
 }

 private static Entity getProductEntity(final Product product) {
  Entity entity = new Entity().addProperty(new Property(null, "ID", ValueType.PRIMITIVE, product.getId()))
    .addProperty(new Property(null, "Name", ValueType.PRIMITIVE, product.getName()))
    .addProperty(new Property(null, "Description", ValueType.PRIMITIVE, product.getDescription()))
    .addProperty(new Property(null, "Price", ValueType.PRIMITIVE, product.getPrice()));
  entity.setId(createId("Products", product.getId()));
  return entity;
 }

 public static EntityCollection getProductsData() {
  EntityCollection dataToSend = new EntityCollection();
  List<Entity> productList = dataToSend.getEntities();

  for (Product prod : products) {
   productList.add(getProductEntity(prod));
  }

  return dataToSend;
 }

 private static URI createId(final String entitySetName, final Object id) {
  try {
   return new URI(entitySetName + "(" + String.valueOf(id) + ")");
  } catch (URISyntaxException e) {
   throw new ODataRuntimeException("Unable to create id for entity: " + entitySetName, e);
  }
 }

 public static Entity getProduct(EdmEntityType edmEntityType, List<UriParameter> keyParams)
   throws ODataApplicationException {
  EntityCollection entitySet = getProductsData();

  Entity requestedEntity = findEntity(edmEntityType, entitySet, keyParams);

  if (requestedEntity == null) {
   throw new ODataApplicationException("Entity for requested key doesn't exist",
     HttpStatusCode.NOT_FOUND.getStatusCode(), Locale.ENGLISH);
  }

  return requestedEntity;
 }

 public static Entity findEntity(EdmEntityType edmEntityType, EntityCollection entitySet,
   List<UriParameter> keyParams) throws ODataApplicationException {

  List<Entity> entityList = entitySet.getEntities();

  for (Entity entity : entityList) {
   boolean foundEntity = entityMatchesAllKeys(edmEntityType, entity, keyParams);
   if (foundEntity) {
    return entity;
   }
  }

  return null;
 }

 private static String getValueOfTheProperty(EdmEntityType edmEntityType, Entity entity, String propertyName)
   throws ODataApplicationException {
  EdmProperty edmKeyProperty = (EdmProperty) edmEntityType.getProperty(propertyName);

  /*
   * Get some basic information to convert the property value of this
   * entity to string
   */
  Boolean isNullable = edmKeyProperty.isNullable();
  Integer maxLength = edmKeyProperty.getMaxLength();
  Integer precision = edmKeyProperty.getPrecision();
  Boolean isUnicode = edmKeyProperty.isUnicode();
  Integer scale = edmKeyProperty.getScale();

  EdmType edmType = edmKeyProperty.getType();
  Object valueObject = entity.getProperty(propertyName).getValue();

  try {
   EdmPrimitiveType edmPrimitiveType = (EdmPrimitiveType) edmType;
   return edmPrimitiveType.valueToString(valueObject, isNullable, maxLength, precision, scale, isUnicode);
  } catch (EdmPrimitiveTypeException e) {
   throw new ODataApplicationException("Failed to retrieve String value",
     HttpStatusCode.INTERNAL_SERVER_ERROR.getStatusCode(), Locale.ENGLISH, e);
  }
 }

 public static boolean entityMatchesAllKeys(EdmEntityType edmEntityType, Entity entity, List<UriParameter> keyParams)
   throws ODataApplicationException {

  /* We need to make sure that all the predicates are matched */
  for (final UriParameter key : keyParams) {
   String keyName = key.getName();
   String keyText = key.getText();

   String valueAsString = getValueOfTheProperty(edmEntityType, entity, keyName);

   if (valueAsString == null || !valueAsString.equals(keyText)) {
    return false;
   }
  }

  return true;
 }

 public static Entity createProduct(EdmEntityType edmEntityType, Entity entity) {

  int newId = ProductUtil.getId();

  Property idProperty = entity.getProperty("ID");

  if (idProperty != null) {
   idProperty.setValue(ValueType.PRIMITIVE, Integer.valueOf(newId));
  } else {
   entity.getProperties().add(new Property(null, "ID", ValueType.PRIMITIVE, newId));
  }

  String name = (String) entity.getProperty("Name").getValue();
  String description = (String) entity.getProperty("Description").getValue();
  double price = (Double) entity.getProperty("Price").getValue();

  entity.setId(createId("Products", newId));

  Product prod = new Product(newId, name, description, price);

  products.add(prod);

  return entity;

 }

}

web.xml

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
 <display-name>Archetype Created Web Application</display-name>


 <servlet>
  <servlet-name>DemoServlet</servlet-name>
  <servlet-class>com.app.servlet.DemoServlet</servlet-class>
  <load-on-startup>1</load-on-startup>
 </servlet>

 <servlet-mapping>
  <servlet-name>DemoServlet</servlet-name>
  <url-pattern>/DemoService.svc/*</url-pattern>
 </servlet-mapping>

</web-app>

index.jsp

<html>
<body>
 <h2>Hello World!</h2>
</body>
</html>

Run the application on server.

Hit below request from Postman or any other tool.

Method: POST

URL: http://localhost:8080/olingoWriteData/DemoService.svc/Products

BODY:

{

  "Name":"ABC Laser printer",

  "Description":"Laser Printer",

  "Price": 786591

}

Click on Send button to send the POST request.

You can see below response.

Now hit get request 'http://localhost:8080/olingoWriteData/DemoService.svc/Products', you can confirm that new object is added to existing list.

http://localhost:8080/olingoWriteData/DemoService.svc/Products

{
    "@odata.context": "$metadata#Products",
    "value": [
        {
            "ID": 1,
            "Name": "Washing Machine by AVC",
            "Description": "Good for morth east countries",
            "Price": 876544
        },
        {
            "ID": 2,
            "Name": "XYZ Camera",
            "Description": "Catch pics while travelling",
            "Price": 965987
        },
        {
            "ID": 3,
            "Name": "ABC Laser printer",
            "Description": "Laser Printer",
            "Price": 786591
        }
    ]
}

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