相信大家在web開發過程中一定遇到過一種情況，Class班級類一對多關聯一個Student學生類，所以為了效能考慮，配置一個lazy-init=true，然後在前臺頁面需要訪問懶載入資料時需要額外配置一個OpenSessionInViewFilter,但是可能並不知道這個過濾器到底做了什麼，遇到Quartz定時中訪問懶載入問題配置這個filter可沒有用了，下面來討論下，怎麼解決這個問題。

@Entity
@Table(name = "clazz")
public class Clazz {
    @Id
    @GenericGenerator(name = "PKUUID", strategy = "uuid2"
 
)
    @GeneratedValue(generator = "PKUUID")
    @Column(length = 36)
    private String id;
    private String clazzName;
    private String clazzNumber;

    @OneToMany(fetch = FetchType.LAZY,mappedBy = "clazz",cascade = CascadeType.ALL)
    private List<Student> students;

    get/set...
}

Student類省略，我們現在資料手動插入一條記錄，待會我們在定時器裡面查詢他。

定時任務程式碼如下：

public class DemoJob {

    @Autowired
    ClazzDao clazzDao;

    public static final Logger logger = LoggerFactory.getLogger(DemoJob.class);

    public DemoJob() {
        System.out.println("DemoJob=====>init");
    }

    public void run() {

        Clazz clazz = clazzDao.findOne("ba9071fb-f2da-481a-9a90-371022cda195"
 
);
        System.out.println(clazz.getClazzName());
        System.out.println(clazz.getClazzNumber());
        System.out.println(clazz.getStudents());
        logger.info("[email protected]" + new Date());

    }

}

我們先不做額外配置看看執行結果

計算機
131
2016-11-04 10:16:50.213 ERROR 6692 --- [ryBean_Worker-1] org.quartz.core.JobRunShell              : Job job_work.job_name threw an unhandled Exception: 

org.springframework.scheduling.quartz.JobMethodInvocationFailedException: Invocation of method 'run' on target class [class com.example.quartz.job.DemoJob] failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.example.quartz.common.entity.Clazz.students, could not initialize proxy - no Session

可以看到，非懶載入資料可以順利查出來，但是懶載入資料會報no session

很明顯，session已經關閉了。

解決方案1
關閉懶載入，好吧，當我沒講
解決方案2
給定時器排程的方案用事務包裹
原理就是，用事務包裹之後，會當做一個整體，沒有提交時session不會交給SessionFactory管理

解決方案3
既然session關閉了，那就開啟它就行了，類似於OpensessionInView,不過要取決於你的持久化方案來決定到底開啟什麼，一般是SessionFactory或者EntityManager,由於我是採用的JPA，所以使用後者

配置一個任務監聽器，用於開啟session

public class OpenEntityManagerJobListener extends JobListenerSupport implements ApplicationContextAware {

    @Override
    public String getName() {
        return "OpenEntityManagerJobListener";
    }

    EntityManagerFactory entityManagerFactory;

    @Override
    public void jobToBeExecuted(JobExecutionContext context) {
        entityManagerFactory = applicationContext.getBean(EntityManagerFactory.class);
        EntityManager entityManager = entityManagerFactory.createEntityManager();
        EntityManagerHolder emHolder = new EntityManagerHolder(entityManager);
        TransactionSynchronizationManager.bindResource(entityManagerFactory, emHolder);
    }

    @Override
    public void jobWasExecuted(JobExecutionContext context, JobExecutionException jobException) {
        EntityManagerHolder emHolder = (EntityManagerHolder) TransactionSynchronizationManager.unbindResource(entityManagerFactory);
        EntityManagerFactoryUtils.closeEntityManager(emHolder.getEntityManager());
    }


    ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
        if(this.applicationContext ==null) throw new RuntimeException("applicationContext is null");
    }
}

之後記得給排程器註冊全域性任務監聽器，對所有任務監聽

//排程工廠
    @Bean
    public SchedulerFactoryBean schedulerFactoryBean() {
        SchedulerFactoryBean factoryBean = new SchedulerFactoryBean();
        factoryBean.setTriggers(triggerFactoryBeans().getObject());
        factoryBean.setGlobalJobListeners(openEntityManagerJobListener());
        return factoryBean;
    }

再來試試執行一下定時任務

2016-11-04 10:29:12.234  INFO 11188 --- [           main] com.example.quartz.CommonApp             : Started CommonApp in 12.067 seconds (JVM running for 12.675)
計算機
131
Hibernate: select students0_.clazz_id as clazz_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.clazz_id as clazz_id4_1_1_, students0_.stu_name as stu_name2_1_1_, students0_.stu_numer as stu_numer3_1_1_ from student students0_ where students0_.clazz_id=?
[com.example.quartz.common.entity.Student@2cd010f8, com.example.quartz.common.entity.Student@100a20c4]
2016-11-04 10:29:12.336  INFO 11188 --- [ryBean_Worker-1] com.example.quartz.job.DemoJob           : [email protected]