jpa的级联删除 在SSH2框架内执行失败，求指导啊解决方案

jpa的级联删除 在SSH2框架内执行失败，求指导啊~~
城市类

Java code

package wxm.domain;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToMany;//城市类@Entitypublic class City {    @Id    @GeneratedValue    private Integer cId;    @Column(length = 20)    private String cName;    // CascadeType属性有四个值，其中REMOVE属性是实现级联删除，要实现级联删除    // 在父栏必需添加CascadeType.REMOVE标注，这是级联删除的关键    @OneToMany(cascade = { CascadeType.REMOVE }, mappedBy = "city")    private Set<Garage> garages = new HashSet<Garage>(0);    public Integer getcId() {        return cId;    }    public void setcId(Integer cId) {        this.cId = cId;    }    public String getcName() {        return cName;    }    public void setcName(String cName) {        this.cName = cName;    }    public Set<Garage> getGarages() {        return garages;    }    public void setGarages(Set<Garage> garages) {        this.garages = garages;    }}

车库类

Java code

package wxm.domain;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.ManyToOne;import javax.persistence.OneToMany;//车库类@Entitypublic class Garage {    private Integer gid;    private String garagenum;    private City city;    private Set<Auto> autos = new HashSet<Auto>();    @Id    @GeneratedValue    public Integer getGid() {        return gid;    }    public void setGid(Integer gid) {        this.gid = gid;    }    @Column(length = 20)    public String getGaragenum() {        return garagenum;    }    public void setGaragenum(String garagenum) {        this.garagenum = garagenum;    }    // CascadeType属性有四个值，其中REMOVE属性是实现级联删除，要实现级联删除    // 在父栏必需添加CascadeType.REMOVE标注，这是级联删除的关键    @OneToMany(cascade = { CascadeType.REMOVE }, mappedBy = "garage")    public Set<Auto> getAutos() {        return autos;    }    public void setAutos(Set<Auto> autos) {        this.autos = autos;    }    @ManyToOne()    @JoinColumn(name = "cityId")    public City getCity() {        return city;    }    public void setCity(City city) {        this.city = city;    }    public void addGarageAuto(Auto auto) {        auto.setGarage(this);        this.autos.add(auto);    }}

汽车类 

Java code

package wxm.domain;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.ManyToOne;//汽车类 @Entitypublic class Auto {    private Integer autoId;    private String autotype;    private String autonum;    private Garage garage;    @Id    @GeneratedValue    public Integer getAutoId() {        return autoId;    }    public void setAutoId(Integer autoId) {        this.autoId = autoId;    }    public String getAutotype() {        return autotype;    }    public void setAutotype(String autotype) {        this.autotype = autotype;    }    public String getAutonum() {        return autonum;    }    public void setAutonum(String autonum) {        this.autonum = autonum;    }    @ManyToOne()    @JoinColumn(name = "garageid")    public Garage getGarage() {        return garage;    }    public void setGarage(Garage garage) {        this.garage = garage;    }} 
 

dao层

Java code

package wxm.dao.impl;import javax.persistence.EntityManager;import javax.persistence.PersistenceContext;import org.springframework.stereotype.Repository;import wxm.dao.CityDao;import wxm.domain.City;@Repositorypublic class CityDaoBean implements CityDao {    @PersistenceContext    private EntityManager em;    @Override    public void addCity(City city) {        em.persist(city);    }    @Override    public void delCity(City city) {        em.remove(city);    }    @Override    public City findCityById(Integer cityId) {        return em.find(City.class, cityId);    }}

其他两个类，写法一样这里就省略了 

service层

Java code

package wxm.service.impl;import javax.annotation.Resource;import org.springframework.stereotype.Service;import org.springframework.transaction.annotation.Transactional;import wxm.dao.CityDao;import wxm.domain.City;import wxm.service.CityService;@Service@Transactionalpublic class CityServiceBean implements CityService {    @Resource    private CityDao cityDao;    @Override    public void addCity(City city) {        cityDao.addCity(city);    }    @Override    public void delCity(City city) {        cityDao.delCity(city);    }    @Override    public City findCityById(Integer cityId) {        return cityDao.findCityById(cityId);    }}

junit测试方法

Java code

package junit;import org.junit.BeforeClass;import org.junit.Test;import org.springframework.context.ApplicationContext;import org.springframework.context.support.ClassPathXmlApplicationContext;import wxm.domain.City;import wxm.service.AutoService;import wxm.service.CityService;import wxm.service.GarageService;public class CT {    private static AutoService autoService;    private static CityService cityService;    private static GarageService garageService;    @BeforeClass    public static void setUpBeforeClass() throws Exception {        try {            ApplicationContext ctx = new ClassPathXmlApplicationContext(                    "applicationContext.xml");            autoService = (AutoService) ctx.getBean("autoServiceBean");            cityService = (CityService) ctx.getBean("cityServiceBean");            garageService = (GarageService) ctx.getBean("garageServiceBean");        } catch (Exception e) {            e.printStackTrace();        }    }    @Test    public void test() {        City city = new City();        city.setcName("北京");        cityService.addCity(city);    }        @Test    public void del()    {        City city = cityService.findCityById(2);        cityService.delCity(city);    }}

除了删除所有都可以正常执行。 
如果不使用框架 直接用entityManager来手动控制事务也能执行成功。
报错语句：java.lang.IllegalArgumentException: Removing a detached instance wxm.domain.City#2
改了很多地方都不对，救命啊高手…… 要怎么才能让它在框架内执行成功，并能级联删除……

[解决办法]
em.remove(city);
 改成 em.remove(em.merge(city));

 或者方法改成根据ID删除。

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