获取对象字段以前的值休眠JPA

问题描述

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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