Add new tutorial example

Author: tboychuk

performance-optimization/pom.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>jpa-hibernate-tutorial</artifactId>
+        <groupId>com.bobocode</groupId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>performance-optimization</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.bobocode</groupId>
+            <artifactId>jpa-hibernate-tutorial-util</artifactId>
+            <version>1.0-SNAPSHOT</version>
+        </dependency>
+    </dependencies>
+
+</project>

performance-optimization/src/main/java/com/bobocode/FetchingDtoProjection.java

@@ -0,0 +1,39 @@
+package com.bobocode;
+
+import com.bobocode.dto.AccountProjection;
+import com.bobocode.model.Account;
+import com.bobocode.util.JpaUtil;
+import com.bobocode.util.TestDataGenerator;
+
+import static com.bobocode.util.JpaUtil.performReturningWithinPersistenceContext;
+import static com.bobocode.util.JpaUtil.performWithinPersistenceContext;
+
+/**
+ * This code example shows how to fetch an entity projection instead of full entity.
+ */
+public class FetchingDtoProjection {
+    public static void main(String[] args) {
+        JpaUtil.init("BasicEntitiesH2");
+        Account account = TestDataGenerator.generateAccount();
+        performWithinPersistenceContext(entityManager -> entityManager.persist(account));
+
+        AccountProjection accountProjection = fetchAccountProjectionById(account.getId());
+        System.out.println(accountProjection);
+        JpaUtil.close();
+    }
+
+    /**
+     * Fetches {@link AccountProjection} by account id. An {@link AccountProjection} is a data transfer object (DTO) for
+     * {@link Account}. (It's a short version that stores some account data, but is not manged by Hibernate)
+     *
+     * @param id account id
+     * @return account projection
+     */
+    private static AccountProjection fetchAccountProjectionById(Long id) {
+        return performReturningWithinPersistenceContext(entityManager ->
+                entityManager.createQuery("select new com.bobocode.dto.AccountProjection(a.id, a.email) " +
+                        "from Account a where a.id = :id", AccountProjection.class)
+                        .setParameter("id", id)
+                        .getSingleResult());
+    }
+}

performance-optimization/src/main/java/com/bobocode/dto/AccountProjection.java

@@ -0,0 +1,15 @@
+package com.bobocode.dto;
+
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+import lombok.Setter;
+import lombok.ToString;
+
+@AllArgsConstructor
+@Getter
+@Setter
+@ToString
+public class AccountProjection {
+    private Long id;
+    private String email;
+}

performance-optimization/src/main/resources/META-INF/persistence.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" version="2.1">
+
+    <persistence-unit name="BasicEntitiesH2">
+        <class>com.bobocode.model.Account</class>
+
+        <properties>
+            <property name="hibernate.connection.url" value="jdbc:h2:mem:bobocode_db;DB_CLOSE_DELAY=-1"/>
+            <property name="hibernate.connection.driver_class" value="org.h2.Driver"/>
+            <property name="hibernate.connection.username" value="bobouser"/>
+            <property name="hibernate.connection.password" value="bobopass"/>
+            <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>
+            <property name="hibernate.show_sql" value="true"/>
+            <property name="hibernate.format_sql" value="true"/>
+            <property name="hibernate.hbm2ddl.auto" value="create"/>
+        </properties>
+    </persistence-unit>
+
+</persistence>

pom.xml

@@ -21,6 +21,7 @@
         <module>dirty-checking-mechanism</module>
         <module>entity-relationships-management</module>
         <module>persistence-context</module>
+        <module>performance-optimization</module>
     </modules>
 
     <dependencies>