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<td class="page-title-pattern-first-line " style="padding: 0; border-collapse: collapse; font-family: Arial, sans-serif; font-size: 14px; padding-top: 10px"> <a href="https://hibernate.atlassian.net/browse/HHH" style="color: #3b73af; text-decoration: none">Hibernate ORM</a> / <a href="https://hibernate.atlassian.net/browse/HHH-12353" style="color: #3b73af; text-decoration: none"><img src="cid:jira-generated-image-avatar-50719bec-75b5-4e7c-a840-4ccc1784233a" height="16" width="16" border="0" align="absmiddle" alt="Bug" style="vertical-align: text-bottom"></a> <a href="https://hibernate.atlassian.net/browse/HHH-12353" style="color: #3b73af; text-decoration: none">HHH-12353</a> </td>
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<td style="vertical-align: top;; padding: 0; border-collapse: collapse; padding-right: 5px; font-size: 20px; line-height: 30px; mso-line-height-rule: exactly" class="page-title-pattern-header-container"> <span class="page-title-pattern-header" style="font-family: Arial, sans-serif; padding: 0; font-size: 20px; line-height: 30px; mso-text-raise: 2px; mso-line-height-rule: exactly; vertical-align: middle"> <a href="https://hibernate.atlassian.net/browse/HHH-12353" style="color: #3b73af; text-decoration: none">Document that Session.getReference not always returns a T</a> </span> </td>
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<td class="has-icon" style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <img src="cid:jira-generated-image-avatar-50719bec-75b5-4e7c-a840-4ccc1784233a" height="16" width="16" border="0" align="absmiddle" alt="Bug" style="vertical-align: text-bottom"> Bug </td>
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<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> 5.2.14 </td>
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<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> Unassigned </td>
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<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> documentation </td>
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<th style="color: #707070; font: normal 14px/20px Arial, sans-serif; text-align: left; vertical-align: top; padding: 2px 0">Created:</th>
<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> 05/Mar/2018 04:00 AM </td>
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<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <a class="user-hover" rel="rolandillig" id="email_rolandillig" href="https://hibernate.atlassian.net/secure/ViewProfile.jspa?name=rolandillig" style="color:#6c797f;; color: #3b73af; text-decoration: none">Roland Illig</a> </td>
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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0; border-collapse: collapse; padding: 0 0 10px"> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0; margin-top: 0">The type signature for <tt>Session.getReference</tt> is inherited from <tt>EntityManager</tt> and promises to return a <tt>T</tt>. The <a href="http://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html#entity-proxy" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">User Guide</a> contains an example snippet where this guarantee is violated.</p> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Since this behavior is surprising, it should be documented at the Javadoc of that method.</p> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Is this surprise also the reason that the unit test uses the complicated <tt>Identifiable.class.isInstance(book)</tt> instead of the simpler <tt>book instanceof Identifiable</tt>? When I write the latter in IntelliJ, it warns me that the expression is always true (which it is). It also warns me that <tt>book instanceof Book</tt> is always true because of the method's type signature. But that warning is wrong since the JVM does not ensure the correct type in this situation. Minimal example:</p>
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<pre class="code-java" style="margin: 10px 0 0; margin-top: 0; max-height: 30em; overflow: auto; white-space: pre-wrap; word-wrap: normal">
<span class="code-keyword" style="color: #000091">public</span> <span class="code-keyword" style="color: #000091">class </span>TypeInferenceDemo {
<span class="code-keyword" style="color: #000091">interface</span> HasId {
<span class="code-object" style="color: #910091">Long</span> getId();
}
<span class="code-keyword" style="color: #000091">static</span> <span class="code-keyword" style="color: #000091">class </span>Entity <span class="code-keyword" style="color: #000091">implements</span> HasId {
<span class="code-keyword" style="color: #000091">public</span> <span class="code-object" style="color: #910091">Long</span> getId() {
<span class="code-keyword" style="color: #000091">return</span> 1L;
}
}
<span class="code-keyword" style="color: #000091">interface</span> Session {
<T> T getReference(<span class="code-object" style="color: #910091">Class</span><T> clazz, <span class="code-object" style="color: #910091">Object</span> id);
}
<span class="code-keyword" style="color: #000091">static</span> <span class="code-keyword" style="color: #000091">class </span>SessionImpl <span class="code-keyword" style="color: #000091">implements</span> Session {
<span class="code-keyword" style="color: #000091">public</span> <T> T getReference(<span class="code-object" style="color: #910091">Class</span><T> clazz, <span class="code-object" style="color: #910091">Object</span> id) {
<span class="code-comment" style="color: #808080">// The type signature promises to <span class="code-keyword" style="color: #000091; color: #808080">return</span> a T, but <span class="code-keyword" style="color: #000091; color: #808080">this</span> is not checked at runtime.
</span> <span class="code-keyword" style="color: #000091">return</span> (T)(HasId)(() -> 1L);
}
}
<span class="code-keyword" style="color: #000091">static</span> <span class="code-keyword" style="color: #000091">class </span>Application {
<span class="code-keyword" style="color: #000091">public</span> <span class="code-keyword" style="color: #000091">static</span> void main(<span class="code-object" style="color: #910091">String</span>[] args) {
Session session = <span class="code-keyword" style="color: #000091">new</span> SessionImpl();
HasId entity = session.getReference(Entity.class, 1L);
<span class="code-object" style="color: #910091">System</span>.out.println(entity);
}
}
}
</pre>
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</div> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Almost every Java developer and also the IDE developers assume that a method declared as returning a <tt>T</tt> indeed returns a <tt>T</tt>. Since <tt>Session.getReference</tt> violates this assumption in an actual documented use case, this should be documented.</p> </td>
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