{"id":12753,"date":"2013-05-14T16:00:59","date_gmt":"2013-05-14T13:00:59","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/?p=12753"},"modified":"2013-05-14T05:05:40","modified_gmt":"2013-05-14T02:05:40","slug":"jpa-querydsl-projections","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2013\/05\/jpa-querydsl-projections.html","title":{"rendered":"JPA &#8211; Querydsl Projections"},"content":{"rendered":"<p>In my last post: <a href=\"http:\/\/vard-lokkur.blogspot.com\/2013\/05\/jpa-basic-projections.html\" target=\"_blank\">JPA &#8211; Basic Projections<\/a> &#8211; I&#8217;ve mentioned about two basic possibilities of building JPA Projections. This post brings you more examples, this time based on <a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> framework. Note, that I&#8217;m referring <a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> version 3.1.1 here.<\/p>\n<h2>Reinvented constructor expressions<\/h2>\n<p>Take a look at the following code:<br \/>\n&nbsp;<br \/>\n&nbsp;<br \/>\n&nbsp;<br \/>\n&nbsp;<\/p>\n<pre class=\"brush:java\">...\r\nimport static com.blogspot.vardlokkur.domain.QEmployee.employee;\r\n\r\nimport javax.persistence.EntityManager;\r\nimport javax.persistence.PersistenceContext;\r\n\r\nimport org.springframework.beans.factory.annotation.Autowired;\r\n\r\nimport com.blogspot.vardlokkur.domain.EmployeeNameProjection;\r\n\r\nimport com.mysema.query.jpa.JPQLTemplates;\r\nimport com.mysema.query.jpa.impl.JPAQuery;\r\nimport com.mysema.query.types.ConstructorExpression;\r\n...\r\n\r\npublic class ConstructorExpressionExample {\r\n    \r\n    ...\r\n    @PersistenceContext\r\n    private EntityManager entityManager;\r\n\r\n    @Autowired\r\n    private JPQLTemplates jpqlTemplates;\r\n    \r\n    public void someMethod() {\r\n        ...\r\n        final List&lt;EmployeeNameProjection&gt; projections = new JPAQuery(entityManager, jpqlTemplates)\r\n                        .from(employee)\r\n                        .orderBy(employee.name.asc())\r\n                        .list(ConstructorExpression.create(EmployeeNameProjection.class, employee.employeeId,\r\n                                        employee.name));\r\n        ...                                \r\n    }\r\n    ...\r\n}<\/pre>\n<p>The above <a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> construction means: create new JPQL query<sup>[1]<\/sup><sup>[2]<\/sup>, using employee as the data source, order the data using employee name<sup>[3]<\/sup>, and return the list of <i>EmployeeNameProjection<\/i>, built using the 2-arg constructor called with employee ID and name<sup>[4]<\/sup>.\u00a0 This is very similar to the constructor expressions example from my previous post (<a href=\"http:\/\/vard-lokkur.blogspot.com\/2013\/05\/jpa-basic-projections.html\" target=\"_blank\">JPA &#8211; Basic Projections<\/a>), and leads to the following SQL query:<\/p>\n<pre class=\"brush:bash\">>select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc<\/pre>\n<p>As you see above, the main advantage comparing to the JPA constructor expressions is using Java class, instead of its name hard-coded in JPQL query.<\/p>\n<h2>Even more reinvented constructor expressions<\/h2>\n<p><a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> documentation<sup>[4]<\/sup> describes another way of using constructor expressions, requiring <i>@QueryProjection<\/i> annotation and Query Type<sup>[1] <\/sup>usage for projection, see example below. Let&#8217;s start with the projection class modification &#8211; note that I added <i>@QueryProjection<\/i> annotation on the class constructor. <div style=\"display:inline-block; margin: 15px 0;\"> <div id=\"adngin-JavaCodeGeeks_incontent_video-0\" style=\"display:inline-block;\"><\/div> <\/div><\/p>\n<pre class=\"brush:java\">package com.blogspot.vardlokkur.domain;\r\n\r\nimport java.io.Serializable;\r\n\r\nimport javax.annotation.concurrent.Immutable;\r\n\r\nimport com.mysema.query.annotations.QueryProjection;\r\n\r\n@Immutable\r\npublic class EmployeeNameProjection implements Serializable {\r\n\r\n    private final Long employeeId;\r\n\r\n    private final String name;\r\n\r\n    @QueryProjection\r\n    public EmployeeNameProjection(Long employeeId, String name) {\r\n        super();\r\n        this.employeeId = employeeId;\r\n        this.name = name;\r\n    }\r\n\r\n    public Long getEmployeeId() {\r\n        return employeeId;\r\n    }\r\n\r\n    public String getName() {\r\n        return name;\r\n    }\r\n\r\n}\r\n<\/pre>\n<p>Now we may use modified projection class (and corresponding Query Type<sup>[1]<\/sup> ) in following way:<\/p>\n<pre class=\"brush:java\">...\r\nimport static com.blogspot.vardlokkur.domain.QEmployee.employee;\r\n \r\nimport javax.persistence.EntityManager;\r\nimport javax.persistence.PersistenceContext;\r\n \r\nimport org.springframework.beans.factory.annotation.Autowired;\r\n \r\nimport com.blogspot.vardlokkur.domain.EmployeeNameProjection;\r\nimport com.blogspot.vardlokkur.domain.QEmployeeNameProjection;\r\n\r\nimport com.mysema.query.jpa.JPQLTemplates;\r\nimport com.mysema.query.jpa.impl.JPAQuery;\r\n\r\n...\r\n \r\npublic class ConstructorExpressionExample {\r\n    ...\r\n    @PersistenceContext\r\n    private EntityManager entityManager;\r\n \r\n    @Autowired\r\n    private JPQLTemplates jpqlTemplates;\r\n\r\n    public void someMethod() {\r\n        ...\r\n        final List&lt;EmployeeNameProjection&gt; projections = new JPAQuery(entityManager, jpqlTemplates)\r\n            .from(employee)\r\n            .orderBy(employee.name.asc())\r\n            .list(new QEmployeeNameProjection(employee.employeeId, employee.name));\r\n        ...\r\n    }\r\n    ...\r\n}<\/pre>\n<p>Which leads to SQL query:<\/p>\n<pre class=\"brush:bash\">select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc<\/pre>\n<p>In fact, when you take a closer look at the Query Type<sup>[1]<\/sup> generated for <i>EmployeeNameProjection<\/i> (<i>QEmployeeNameProjection<\/i>), you will see it is some kind of &#8220;shortcut&#8221; for creating constructor expression the way described in first section of this post.<\/p>\n<h2>Mapping projection<\/h2>\n<p><a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> provides another way of building projections, using factories based on <i>MappingProjection<\/i>.<\/p>\n<pre class=\"brush:java\">package com.blogspot.vardlokkur.domain;\r\n\r\nimport static com.blogspot.vardlokkur.domain.QEmployee.employee;\r\n\r\nimport com.mysema.query.Tuple;\r\nimport com.mysema.query.types.MappingProjection;\r\n\r\npublic class EmployeeNameProjectionFactory extends MappingProjection&lt;EmployeeNameProjection&gt; {\r\n\r\n    public EmployeeNameProjectionFactory() {\r\n        super(EmployeeNameProjection.class, employee.employeeId, employee.name);\r\n    }\r\n\r\n    @Override\r\n    protected EmployeeNameProjection map(Tuple row) {\r\n        return new EmployeeNameProjection(row.get(employee.employeeId), row.get(employee.name));\r\n    }\r\n\r\n}\r\n<\/pre>\n<p>The above class is a simple factory creating <i>EmployeeNameProjection<\/i> instances using employee ID and name. Note that the factory constructor defines which employee properties will be used for building the projection, and <i>map<\/i> method defines how the instances will be created.<\/p>\n<p>Below you may find an example of using the factory:<\/p>\n<pre class=\"brush:java\">...\r\nimport static com.blogspot.vardlokkur.domain.QEmployee.employee;\r\n \r\nimport javax.persistence.EntityManager;\r\nimport javax.persistence.PersistenceContext;\r\n \r\nimport org.springframework.beans.factory.annotation.Autowired;\r\n \r\nimport com.blogspot.vardlokkur.domain.EmployeeNameProjection;\r\nimport com.blogspot.vardlokkur.domain.EmployeeNameProjectionFactory\r\n \r\nimport com.mysema.query.jpa.JPQLTemplates;\r\nimport com.mysema.query.jpa.impl.JPAQuery;\r\n...\r\n \r\npublic class MappingProjectionExample {\r\n    \r\n    ...\r\n    @PersistenceContext\r\n    private EntityManager entityManager;\r\n \r\n    @Autowired\r\n    private JPQLTemplates jpqlTemplates;\r\n\r\n    public void someMethod() {\r\n        ...\r\n        final List&lt;EmployeeNameProjection&gt; projections = new JPAQuery(entityManager, jpqlTemplates)\r\n                            .from(employee)\r\n                            .orderBy(employee.name.asc())\r\n                            .list(new EmployeeNameProjectionFactory());\r\n        ....\r\n    }\r\n    ...\r\n}<\/pre>\n<p>As you see, the one and only difference here, comparing to constructor expression examples, is the <i>list<\/i> method call.<\/p>\n<p>Above example leads again to the very simple SQL query:<\/p>\n<pre class=\"brush:bash\">select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc<\/pre>\n<p>Building projections this way is much more powerful, and doesn&#8217;t require existence of n-arg projection constructor.<\/p>\n<h2>QBean based projection (JavaBeans strike again)<\/h2>\n<p>There is at least one more possibility of creating projection with <a href=\"http:\/\/www.querydsl.com\/\" target=\"_blank\">Querydsl<\/a> &#8211; QBean based &#8211; in this case we build the result list using:<\/p>\n<pre class=\"brush:bash\">... .list(Projections.bean(EmployeeNameProjection.class, employee.employeeId, employee.name))<\/pre>\n<p>This way requires <i>EmployeeNameProjection<\/i> class to follow JavaBean conventions, which is not always desired in application. Use it if you want, but you have been warned<\/p>\n<h2>Few links for the dessert<\/h2>\n<ol>\n<li><a href=\"http:\/\/www.querydsl.com\/static\/querydsl\/3.1.1\/reference\/html\/ch02.html#d0e184\" target=\"_blank\">Using Query Types<\/a><\/li>\n<li><a href=\"http:\/\/www.querydsl.com\/static\/querydsl\/3.1.1\/reference\/html\/ch02.html#d0e203\" target=\"_blank\">Querying<\/a><\/li>\n<li><a href=\"http:\/\/www.querydsl.com\/static\/querydsl\/3.1.1\/reference\/html\/ch02.html#d0e297\" target=\"_blank\">Ordering<\/a><\/li>\n<li><a href=\"http:\/\/www.querydsl.com\/static\/querydsl\/3.1.1\/reference\/html\/ch03.html#d0e1556\" target=\"_blank\">Constructor projections<\/a><\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<div style=\"border: 1px solid #D8D8D8; background: #FAFAFA; width: 100%; padding-left: 5px;\"><b><i>Reference: <\/i><\/b><a href=\"http:\/\/vard-lokkur.blogspot.com\/2013\/05\/jpa-querydsl-projections.html\">JPA &#8211; Querydsl Projections <\/a> from our <a href=\"http:\/\/www.javacodegeeks.com\/jcg\">JCG partner<\/a> Michal Jastak at the <a href=\"http:\/\/vard-lokkur.blogspot.com\/\">Warlock&#8217;s Thoughts <\/a> blog.<\/div>\n","protected":false},"excerpt":{"rendered":"<p>In my last post: JPA &#8211; Basic Projections &#8211; I&#8217;ve mentioned about two basic possibilities of building JPA Projections. This post brings you more examples, this time based on Querydsl framework. Note, that I&#8217;m referring Querydsl version 3.1.1 here. Reinvented constructor expressions Take a look at the following code: &nbsp; &nbsp; &nbsp; &nbsp; &#8230; import &hellip;<\/p>\n","protected":false},"author":74,"featured_media":12771,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8],"tags":[33,778],"class_list":["post-12753","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-enterprise-java","tag-jpa","tag-querydsl"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>JPA - Querydsl Projections<\/title>\n<meta name=\"description\" content=\"In my last post: JPA - Basic Projections - I&#039;ve mentioned about two basic possibilities of building JPA Projections. 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