Addition and modification of JPA

Problems encountered

​ Usually , When we add and modify, we are based on whether the value passed in to the background has id Value to judge . But now use JPA You will find that even when we make changes, it is transferred to the background id value , A new record will also be added to the database , Instead of modifying . This is mainly because JPA The bottom layer judges the addition and modification based on version Field to judge , But we don't have any in the incoming data version Field , Therefore, the new operation is always performed .

Source code analysis

​ JPA Whether you are executing an add method or a modify method , All the calls are save This method , So this requires save Method body to determine when to add , When to modify . Let's take a look through the debugging program JPA At the bottom save How to achieve .

1、 Get into SimpleJpaRepository Methods save Method .

2、save Method source code

​ By looking at save Method source code can be seen , It judges in the first sentence entity Is it a new test question , If it is a new one, it should be stored persistently , If not, merge the original data . Let's mainly look at isNew Method implementation .

@Transactional
public <S extends T> S save(S entity) {
    

    if (entityInformation.isNew(entity)) {
    
        em.persist(entity);
        return entity;
    } else {
    
        return em.merge(entity);
    }
}

3、 find isNew Method

​ We enter JpaMetamodelEntityInformation Of isNew In the method .

Class inheritance structure diagram

4、isNew Method source code

​ You can find out by looking at the source code , This method is mainly based on version To determine whether to update the data , The main problem is here , If we have to update the data in the entity class version Field , At the same time, when the front end transmits data to the background, you need to bring version Field , If not, it is a new operation . The concrete realization is , This method first determines whether there is version Attributes exist , After that, get the wrapper of the instance bean, By packing bean To determine the version Whether the attribute value is NULL.

@Override
public boolean isNew(T entity) {
    
	// To determine if there is version Attributes exist 
    if (!versionAttribute.isPresent()
        || versionAttribute.map(Attribute::getJavaType).map(Class::isPrimitive).orElse(false)) {
    
        return super.isNew(entity);
    }
	// Get the wrapper of the instance bean
    BeanWrapper wrapper = new DirectFieldAccessFallbackBeanWrapper(entity);
	// By packing bean To determine the version Whether the attribute value is NULL
    return versionAttribute.map(it -> wrapper.getPropertyValue(it.getName()) == null).orElse(true);
}