JPA中的could not initialize proxy - no Session异常分析与解决
引言： JPA是一种非常流行和常用的持久化框架标准，其下可以对接若干种不同的实现，在不同的父子表管理中，经常会碰到no Session的问题，该如何解决呢？
1. 问题的引出
在进行基于JPA的单元测试中，我们使用JUnit来进行测试数据库的关联表信息读取，结果得到如下错误信息：
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.rain.wx.meal.model.DishCategory.dishes, could not initialize proxy - no Session
at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:587)
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:204)
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:566)
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:135)
at org.hibernate.collection.internal.PersistentBag.get(PersistentBag.java:449)
at com.rain.wx.meal.service.DishServiceTest.testDishes(DishServiceTest.java:86)
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:497)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:252)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)2. 代码实现分析
让我们来看看具体的代码吧
@Entity
@Table(name="dish_category")
@Data
@EqualsAndHashCode(callSuper=false)
@JsonRootName(value="category")
public class DishCategory extends BaseEntity {
private static final long serialVersionUID = -7189824224534351030L;
@Column
private String name;
@Column
private String description;
@OneToMany
@JoinColumn(name="category_id",referencedColumnName="id")
private List dishes;
}
@Entity
@Table(name = "dish")
@JsonRootName(value="dish")
//@Lazy(value=false)
public class MealDish extends BaseEntity {
private static final long serialVersionUID = -3982356728880195795L;
@Column
private String name;
@Column
private float price;
@Column(name = "img_url")
private String imgUrl;
@Column(name="category_id")
private long categoryId;
@Column
private boolean active;
// 销售数量
@Column
private int soldCount;
.......
}@Service
public class DishServiceImpl implements DishService {
@Transactional
@Override
public List getDishCategory() {
return this.dishCategoryRepo.findAll();
}
........
}
单元测试的代码内容：
@Test
public void testDishes() throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
List categories = this.dishService.getDishCategory();
for (DishCategory category : categories) {
// log.debug(String.valueOf(category.getDishes().get(0)));
}
String jsonStr = mapper.writeValueAsString(categories);
log.info(jsonStr);
} 基于对Hibernate和JPA的理解，在ORM中，其为了提升性能使用了Lazy加载，就是在使用的时候，才会加载额外的数据，故导致了在使用之时再加载数据之时， session失效的问题出现。所以问题的目标点实现提前加载数据。
4. 问题的解决
尝试1： 在Service方法中新增了@Transactional进行事务添加
结果1： 无效
尝试2： 在@OneToMany的方法上，使用@Lazy(false)
结果2： 无效
尝试3： 在@OneToMany的参数中使用fetch=FetchType=Eager
结果3: 问题解决
尝试4： 在application.properties的配置文件中新增spring.jpa.open-in-view=true
结果4： 问题解决
方法3的正确的代码内容：
@Entity
@Table(name="dish_category")
@Data
@EqualsAndHashCode(callSuper=false)
@JsonRootName(value="category")
public class DishCategory extends BaseEntity {
private static final long serialVersionUID = -7189824224534351030L;
@Column
private String name;
@Column
private String description;
@OneToMany(fetch=FetchType.EAGER)
@JoinColumn(name="category_id",referencedColumnName="id")
private List dishes;
}
方法4的解释与说明：
这个设置选项:
spring.jpa.open-in-view=true其实是之前的 openEntityManagerInViewInterceptor，解决在Spring MVC与JPA之间Session的声明周期问题
5. 总结
核心问题在于解决延迟加载为及时加载，及时加载会消耗一定的资源，将其程序的性能，请注意这个问题。