JPA之"could not initialize proxy - no Session"的异常解决

引言： 在使用JPA中经常会碰到各类问题，这里列出解决no Session问题的方法。
1.  异常信息的描述
   这里列出异常栈， 关键词是无法初始化proxy，在读取数据库的过程中.
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:148)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:266)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:68)
at com.rain.wx.meal.model.ShopCartOrder_$$_jvst9ff_3.toString(ShopCartOrder_$$_jvst9ff_3.java)
at java.lang.String.valueOf(String.java:2982)
at java.io.PrintStream.println(PrintStream.java:821)
at com.rain.wx.meal.service.OrderServiceTest.testNewShopCartOrder(OrderServiceTest.java:26)
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.  测试代码   这里的测试代码如下：   import javax.transaction.Transactional;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.ActiveProfiles;
import org.springframework.test.context.junit4.SpringRunner;

import com.rain.wx.meal.dao.ShopCartOrderRepository;
import com.rain.wx.meal.model.ShopCartOrder;

@RunWith(SpringRunner.class)
@SpringBootTest
@ActiveProfiles("dev")
public class OrderServiceTest {
@Autowired
private ShopCartOrderRepository shopCartOrderRepo;

@Test
public void testNewShopCartOrder() {
long shopCartOrderId = 5l;

ShopCartOrder shopCartOrder = this.shopCartOrderRepo.getOne(shopCartOrderId);

System.out.println(shopCartOrder);
}
}
 ShopCartRepository的代码：import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;
import org.springframework.stereotype.Repository;

import com.rain.wx.meal.model.ShopCartOrder;

@Repository
public interface ShopCartOrderRepository extends JpaRepository<ShopCartOrder, Long> {
public List<ShopCartOrder> getByUserId(long userId);

/**
* Query the current shop cart order.
*
* @param userId
* @param dishId
* @return
*/
@Query(value="from ShopCartOrder shopCartOrder where shopCartOrder.userId = :userId and shopCartOrder.mealDish.id = :dishId")
public ShopCartOrder getByUserIdAndDishId(@Param("userId") long userId, @Param("dishId") long dishId);
}
 经过分析，可以发现findOne使用的是JpaRepository自带的方法findOne. 那问题出在哪来呢，因为自带的方法一般不会报错的。3.  问题的解决
    鉴于数据库操作中一般会使用事务，是不是事务造成的呢？  添加@Transactional暂且一试。
import javax.transaction.Transactional;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.ActiveProfiles;
import org.springframework.test.context.junit4.SpringRunner;

import com.rain.wx.meal.dao.ShopCartOrderRepository;
import com.rain.wx.meal.model.ShopCartOrder;

@RunWith(SpringRunner.class)
@SpringBootTest
@ActiveProfiles("dev")
public class OrderServiceTest {
@Autowired
private ShopCartOrderRepository shopCartOrderRepo;

@Test
@Transactional
public void testNewShopCartOrder() {
long shopCartOrderId = 5l;

ShopCartOrder shopCartOrder = this.shopCartOrderRepo.getOne(shopCartOrderId);

System.out.println(shopCartOrder);
}
}
@Transactional主要在当下的数据库操作中，添加事务控制。