为以下内容生成的null id：class com.jpa.project.model.domain.Item

Question

我没有生成，如何生成？

@Id
@GenericGenerator(name = "ID_GENERATOR", strategy = "com.ItemIdGenerator")
@GeneratedValue(generator = "ID_GENERATOR")
protected Long id;

ItemIdGenerator

public class ItemIdGenerator implements IdentifierGenerator {

    @Override
    public Serializable generate(SharedSessionContractImplementor sharedSessionContractImplementor, Object object)
            throws HibernateException {
        String prefix = "item_";
        String generatedId = null;

        Connection connection = sharedSessionContractImplementor.connection();

        try {
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement
                    .executeQuery("Select count(id) as id from com.jpa.project.model.domain.Item");
            if (resultSet.next()) {
                int id = resultSet.getInt(1) + 999;
                generatedId = (prefix + String.valueOf(id)).toString();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

        return generatedId;

    }

}

异常

FAILED: createTest
javax.persistence.PersistenceException: org.hibernate.id.IdentifierGenerationException: null id generated for:class com.jpa.project.model.domain.Item
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:147)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:155)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:162)
    at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:769)
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:742)
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:747)
    at com.jpa.project.model.domain.test.ItemTest.createTest(ItemTest.java:25)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:86)
    at org.testng.internal.Invoker.invokeMethod(Invoker.java:643)
    at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:820)
    at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1128)
    at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:129)
    at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:112)
    at org.testng.TestRunner.privateRun(TestRunner.java:782)
    at org.testng.TestRunner.run(TestRunner.java:632)
    at org.testng.SuiteRunner.runTest(SuiteRunner.java:366)
    at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:361)
    at org.testng.SuiteRunner.privateRun(SuiteRunner.java:319)
    at org.testng.SuiteRunner.run(SuiteRunner.java:268)
    at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
    at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
    at org.testng.TestNG.runSuitesSequentially(TestNG.java:1244)
    at org.testng.TestNG.runSuitesLocally(TestNG.java:1169)
    at org.testng.TestNG.run(TestNG.java:1064)
    at org.testng.remote.AbstractRemoteTestNG.run(AbstractRemoteTestNG.java:132)
    at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:236)
    at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:81)
Caused by: org.hibernate.id.IdentifierGenerationException: null id generated for:class com.jpa.project.model.domain.Item
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:107)
    at org.hibernate.jpa.event.internal.core.JpaPersistEventListener.saveWithGeneratedId(JpaPersistEventListener.java:67)
    at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:189)
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:132)
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:58)
    at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:762)
    ... 27 more