Am i right that #Id annotation add two constraints in database:
nullable=false
unique=true
?
I saw a lot of examples in the Internet with syntax like
#Id
#Column(name="xxx",nullable=false)
BigInteger id
It is correct? Do i really need this nullable=false?
Yes you are right. If you use hibernate schema generation mechanism, all #Id columns in the database will be NOT NULL and have unique index by default.
In the other hand, #Column(nullable=false) declaration is absolutely meaningless if you create the schema any other way.
One reason you might see the two together is for the name attribute on #Column. It's name attribute lets you explicitly choose the name of the column in the resulting table, in cases where the default name that JPA provides. I will at times use #Column solely for that purpose, just so I can give my column a certain name.
As for the nullable attribute, I agree with you. It's worthless in that case.
Related
I am looking for an answer if it is possible or not in hibernate. What I am trying to achieve is that if a field exists in a particular table then only it should insert it. Else just ignore the field in the #Entity class.
I want this as a new field is going to introduce in one of the table we are using and there are many dependent components which right now insert the data into that table. I don't want a big bang release. Want something like it doesn't impact the older version as well as when the upgrade happens and a new column introduced then also it should work.
For example -
#Entity
#Table(name = "EMPLOYEE_RECORDS")
public class Employee
{
#Id
#Column(name = "employee_id")
private Integer employeeId;
#Column(name = "employee_name")
private String employeeName;
#Column(name="address")
private String address;
}
What if I only want to insert address field into DB only when column(address) exists in the table EMPLOYEE_RECORDS. Please forgive me if this is something obvious, as I am not very proficient in Hibernate.
Also let me explain what have I thought of (But not sure if it will also work) -
1. Create 2 different #Entity classes. Try to insert and if the insertion failed then at runtime switch the #Entity and use without address.
2. Check if field exist in the table by simple query if it fails use #Entity without address else use without address.
I'm very confused about the scenario - It seems like there were deeper issues regarding decoupling of components in your system.
Nevertheless, you can add the column in the database, but you don't need to declare the field in the hibernate entity. On the other hand there is no way you can have an optional field in an hibernate entity. Either a field is mapped or it is not mapped.
Suppose I am having 'subject' table
CREATE TABLE subject (id int PRIMARY KEY, name VARCHAR(255) **UNIQUE**)
and associated Mapped Object,
#Entity
#Table(name="subject")
public class SubjectDO {
#Id
#Column(name="id")
int id;
#Column(name="name", unique=true)
String name;
...
// Getter-Setter methods
}
When I try to save object having duplicate 'name' with and without 'unique=true' defined, I am getting similar behavior (same exception.) And it is obvious that JPA implementation can't really do anything unless reaching out to DB for checking.
What is the real use case for it?
(I am assuming here, unique constraint is defined at Database level too.)
unique in #Column is used only if you let your JPA provider create the database for you - it will create the unique constraint on the specified column. But if you already have the database, or you alter it once created, then unique doesn't have any effect.
unique=true in #Column annotation will be used only in DDL generation, it doesn't have any impact during runtime. The actual uniqueness checks happens in the database.
There's a situation that if the varchar field is too long, say 255 length with utf8mb4 encoded, then the unique constaint won't work even JPA re-create the table with unique-true. Try to set the field with #Column(length=128, unique=ture) and re-create table again to verify. See the top answer in this question for more accurate explanation.
I have MySql database with several tables. Every field in each table is not nullable. That's why I am forced to use #NotNull annotation to each field in all my Java classes marked with #Entity. Do I really have to do this or is there a way to tell JPA/Hibernate to treat every field NotNullable on default?
edit:
I am aware of #Column(name="something", nullable=false) too. But still, it doesn't solve any problem - you have to write nullable=false
There are no such possibility. Not Nullable constraint is not what you always expect from a field, although it is used quite often. It is convenient when you can look at the attribute definition and tell everything out of it, without addressing to some high-level settings like "every field should be #NotNull".
It would be rather confusing to see such entity definition with this setting hidden elsewhere.
And, one more thing. #NotNull annotation and #Column(name="something", nullable=false) are not the same. More details here :
Confusion: #NotNull vs #Column(nullable = false)
I'm currently coming (back) up to speed with EJB and while I was away it changed drastically (so far for the better). However, I've come across a concept that I am struggling with and would like to understand better as it seems to be used in our (where I work, not me and all the voices in my head) code quite a bit.
Here's the example I've found in a book. It's part of an example showing how to use the #EmbeddedId annotation:
#Entity
public class Employee implements java.io.Serializable
{
#EmbeddedId
#AttributeOverrides({
#AttributeOverride(name="lastName", column=#Column(name="LAST_NAME"),
#AttributeOverride(name="ssn", column=#Column(name="SSN"))
})
private EmbeddedEmployeePK pk;
...
}
The EmbeddedEmployeePK class is a fairly straightforward #Embeddable class that defines a pair of #Columns: lastName and ssn.
Oh, and I lifted this example from O'Reilly's Enterprise JavaBeans 3.1 by Rubinger & Burke.
Thanks in advance for any help you can give me.
It's saying that the attributes that make up the embedded id may have predefined (through explicit or implicit mappings) column names. By using the #AttributeOverride you're saying "ignore what other information you have with regard to what column it is stored in, and use the one I specify here".
In the UserDetails class, I have overridden homeAddress & officeAddress with Address
This One Address POJO will act for two table in DB.
DB:
Table1 Table2
STREET_NAME HOME_STREET_NAME
CITY_NAME HOME_CITY_NAME
STATE_NAME HOME_STATE_NAME
PIN_CODE HOME_PIN_CODE
The EmbeddedEmployeePK class is a fairly straightforward #Embeddable class that defines a pair of #Columns: lastName and ssn.
Not quite - EmbeddedEmployeePK defines a pair of properties, which are then mapped to columns. The #AttributeOverride annotations allow you to override the columns to which the embedded class's properties are mapped.
The use case for this is when the embeddable class is used in different situations where its column names differ, and some mechanism is required to let you change those column mappings. For example, say you have an entity which contains two separate instances of the same embeddable - they can't both map to the same column names.
JPA tries to map field names to column names in a datasource, so what you're seeing here is the translation between the name of a field variable to the name of a column in a database.
You can override also other column properties (not just names).
Let's assume that you want to change the length of SSN based on who is embedding your component. You can define an #AttributeOverride for the column like this:
#AttributeOverrides({
#AttributeOverride(name = "ssn", column = #Column(name = "SSN", length = 11))
})
private EmbeddedEmployeePK pk;
See "2.2.2.4. Embedded objects (aka components)" in the Hibernate Annotations documentation.
In order to preserve other #Column properties (such as name and nullable) keep them on the overridden column the same as you specified on the original column.
In JPA, I am confused when to use the attribute optional=false and the annotation #Column(nullable=false). What is the difference?
#Column(nullable=false) is an instruction for generating the schema. The database column generated off the class will be marked not nullable in the actual database.
optional=false is a runtime instruction. The primary functional thing it does is related to Lazy Loading. You can't lazy load a non-collection mapped entity unless you remember to set optional=false (because Hibernate doesn't know if there should be a proxy there or a null, unless you tell it nulls are impossible, so it can generate a proxy.)
Both is used to prevent a null value, but if you mind that null should be blocked in ...
The database layer (and you want to generate the schema using JPA) --> use #Column(nullable=false)
The runtime (and before contacting the database)--> use optional=false (much faster than the first checking).
If you want both abilities, use them both.