Check a system property before EntityManager injection outside work unit happens

extensions/persist/src/com/google/inject/persist/jpa/JpaPersistService.java

@@ -36,6 +36,7 @@
 /** @author Dhanji R. Prasanna (dhanji@gmail.com) */
 @Singleton
 class JpaPersistService implements Provider<EntityManager>, UnitOfWork, PersistService {
+  private static final String SYSTEM_PROPERTY_PROHIBIT_EM_INJECTION_OUTSIDE_WORK_UNIT = "com.google.inject.persist.jpa.prohibitEntityManagerInjectionOutsideWorkUnit";
   private final ThreadLocal<EntityManager> entityManager = new ThreadLocal<EntityManager>();
 
   private final String persistenceUnitName;
@@ -51,6 +52,10 @@ public JpaPersistService(
   @Override
   public EntityManager get() {
     if (!isWorking()) {
+      Preconditions.checkState(!isEntityManagerInjectionOutsideWorkUnitProhibited(),
+              "Requested EntityManager outside work unit. "
+                      + "Try calling UnitOfWork.begin() first, or use a PersistFilter if you "
+                      + "are inside a servlet environment.");
       begin();
     }
 
@@ -64,6 +69,10 @@ public EntityManager get() {
     return em;
   }
 
+  private static boolean isEntityManagerInjectionOutsideWorkUnitProhibited() {
+    return Boolean.valueOf(System.getProperty(SYSTEM_PROPERTY_PROHIBIT_EM_INJECTION_OUTSIDE_WORK_UNIT));
+  }
+
   public boolean isWorking() {
     return entityManager.get() != null;
   }

extensions/persist/test/com/google/inject/persist/jpa/JpaPersistServiceTest.java

@@ -32,6 +32,7 @@ public class JpaPersistServiceTest extends TestCase {
 
   private static final String PERSISTENCE_UNIT_NAME = "test_persistence_unit_name";
   private static final Properties PERSISTENCE_PROPERTIES = new Properties();
+  private static final String SYSTEM_PROPERTY_PROHIBIT_EM_INJECTION_OUTSIDE_WORK_UNIT = "com.google.inject.persist.jpa.prohibitEntityManagerInjectionOutsideWorkUnit";
 
   private final JpaPersistService sut =
       new JpaPersistService(PERSISTENCE_UNIT_NAME, PERSISTENCE_PROPERTIES);
@@ -60,5 +61,29 @@ public void test_givenErrorOnEntityManagerClose_whenEndIsCalled_thenEntityManage
     }
   }
 
+  public void test_givenEntityManagerInjectionOutsideWorkUnitProhibited_whenEntityManagerAboutToInject_thenExceptionIsThrown() {
+    System.setProperty(SYSTEM_PROPERTY_PROHIBIT_EM_INJECTION_OUTSIDE_WORK_UNIT, "true");
+    sut.start(factory);
+
+    try {
+      sut.get();
+      fail("Exception expected");
+    }
+    catch (IllegalStateException expected) {
+      // nop
+    }
+  }
+
+  public void test_givenEntityManagerInjectionOutsideWorkUnitNotProhibited_whenEntityManagerAboutToInject_thenInjectionSucceeds() {
+    sut.start(factory);
+
+    assertNotNull(sut.get());
+  }
+
+  @Override
+  public void tearDown() throws Exception {
+    System.clearProperty(SYSTEM_PROPERTY_PROHIBIT_EM_INJECTION_OUTSIDE_WORK_UNIT);
+  }
+
   private static class SimulatedException extends RuntimeException {}
 }