HHH-14948 - Reduce the size of the imports cache in the metamodel

We have agreed to mitigate a memory issue by introducing a hard-coded
(for the time being) cap on the imports cache in MetamodelImpl. This
would preserve the previously available performance improvement for
the majority of applications using Hibernate (which are expected to have
less than 1000 imports) while mitigating an unbounded memory consumption
issue for applications that exceed this limit when dynamically
generating queries with random entity aliases.

The commit also adds a not-so-beautiful test, which, however, is
beneficial considering the consequences of introduing a regression.

Author: Ivaylo Mitrev

hibernate-core/src/main/java/org/hibernate/metamodel/internal/MetamodelImpl.java

@@ -632,7 +632,13 @@ public String getImportedClassName(String className) {
 				return className;
 			}
 			catch ( ClassLoadingException cnfe ) {
-				imports.put( className, INVALID_IMPORT );
+				// This check doesn't necessarily mean that the map can't exceed 1000 elements because
+				// new entries might be added _while_ performing the check (making it 1000+ since size() isn't
+				// synchronized). Regardless, this would pass as "good enough" to prevent the map from growing
+				// above a certain threshold, thus, avoiding memory issues.
+				if ( imports.size() < 1_000 ) {
+					imports.put( className, INVALID_IMPORT );
+				}
 				return null;
 			}
 		}

hibernate-core/src/test/java/org/hibernate/test/hql/QuerySplitterTest.java

@@ -6,11 +6,15 @@
  */
 package org.hibernate.test.hql;
 
+import java.lang.reflect.Field;
+import java.util.Map;
+import java.util.stream.IntStream;
 import javax.persistence.Entity;
 import javax.persistence.Id;
 import javax.persistence.Table;
 
 import org.hibernate.hql.internal.QuerySplitter;
+import org.hibernate.metamodel.internal.MetamodelImpl;
 
 import org.hibernate.testing.TestForIssue;
 import org.hibernate.testing.junit4.BaseNonConfigCoreFunctionalTestCase;
@@ -47,6 +51,29 @@ public void testQueryWithEntityNameAsStringLiteral2() {
 		);
 	}
 
+	@Test
+	@TestForIssue(jiraKey = "HHH-14948")
+	public void testMemoryConsumptionOfFailedImportsCache() throws NoSuchFieldException, IllegalAccessException {
+
+		IntStream.range( 0, 1001 )
+				.forEach( i -> QuerySplitter.concreteQueries(
+						"from Employee e join e.company" + i,
+						sessionFactory()
+				) );
+
+		MetamodelImpl metamodel = (MetamodelImpl) sessionFactory().getMetamodel();
+
+		Field field = MetamodelImpl.class.getDeclaredField( "imports" );
+		field.setAccessible( true );
+
+		//noinspection unchecked
+		Map<String, String> imports = (Map<String, String>) field.get( metamodel );
+
+		// VERY hard-coded, but considering the possibility of a regression of a memory-related issue,
+		// it should be worth it
+		assertEquals( 1000, imports.size() );
+	}
+
 	@Test
 	@TestForIssue(jiraKey = "HHH-7973" )
 	public void testQueryWithEntityNameAsStringLiteralAndEscapeQuoteChar() {