获取对象字段以前的值休眠JPA
假设我有这个课程:
@EntityListeners({MyListener.class})
class MyClass {
String name;
String surname;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return name;
}
public void setSurname(String name) {
this.name = name;
}
public void save() {
JPA.em().persist(this);
return this;
}
public void update() {
JPA.em().merge(this);
}
public static MyClass findById(Long id) {
return JPA.em().find(MyClass.class, id);
}
}
现在在我的 MyListener
类中,我试图找出以前的值 MyClass
实例与即将保存(更新)到数据库的新值.我使用更新前方法来做到这一点:
Now in my MyListener
class I'm trying to figure out the previous value MyClass
instance versus the new value that is about to get saved(updated) to database. I do this with preupdate metdhod :
@PreUpdate
public void preUpdate(Object entity) {
...some logic here...unable to get the old object value in here
}
所以假设我有一个带有姓名和姓氏的 MyClass
对象实例:
So assuming I have a MyClass
objects instance with name and surname :
MyClass mycls = new MyClass();
mycls.setName("Bob");
mycls.setSurname("Dylan");
mycls.save();
这没有被听众接收到,这没关系,因为我只收听更新.现在,如果我要像这样更新这个实例:
This wasn't picked up by listener which is ok because I'm listening only to updates. Now if I were to update this instance like so :
MyClass cls = MyClass.findById(someLongId)
cls.setSurname("Marley");
cls.update();
所以这会触发 mylistener 中的 preUpdate
方法,当我尝试时:
So this triggers the preUpdate
method in mylistener and when I try to :
MyClass.findById(someLongId);
当我调试时,我已经获得了新更新的实例,但更新尚未发生,因为当我在 surname 列中检入数据库时,它仍然是 Dylan
.
When I debug I already get the newly updated instance but the update hasn't happened yet because when I check in database in column surname it's still Dylan
.
如何在我的 preUpdate
方法中而不是我刚刚更新的方法中从数据库中获取值?
How do I get the value from database in my preUpdate
method and not the one I just updated?
推荐答案
我认为一种简单的方法是将先前的值保存在 JPA 不会持久的瞬态变量中.因此,只需引入一个变量 previousSurname 并保存实际值,然后再将其覆盖在 setter 中.
I think an easy way is to save the previous value in a transient variable that JPA will not persist. So just introduce a variable previousSurname and save the actual value before you overwrite it in the setter.
如果你想保存多个属性,如果你的类 MyClass
是 Serializable.
If you want to save multiple properties it would be easy if your class MyClass
is Serializable.
如果是这样,添加一个后加载监听器
If so add a post load listener
public class MyClass implements Serializable {
@Transient
private transient MyClass savedState;
@PostLoad
private void saveState(){
this.savedState = SerializationUtils.clone(this); // from apache commons-lang
}
}
但请注意,savedState
是一个分离的实例.
But be aware that the savedState
is a detached instance.
然后您可以在 EntityListener
中访问之前的状态.
You can then access the previous state in your EntityListener
.
您还可以将 PostLoad
侦听器移动到 EntityListener
类.但是您需要访问 savedState
字段.我建议将其设置为包作用域或使用包作用域访问器并将 MyClass
和 MyListener
放在同一个包中,
You can also move the PostLoad
listener to the EntityListener
class. But then you need access to the savedState
field. I recommend to make it either package scoped or use a package scoped accessor and put MyClass
and MyListener
in the same package,
public class MyListener {
@PostLoad
private void saveState(MyClass myClass){
myClass.saveState(SerializationUtils.clone(myClass)); // from apache commons-lang
}
}
public class MyClass implements Serializable {
@Transient
private transient MyClass savedState;
void saveState(MyClass savedState){
this.savedState = savedState;
}
}
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