`@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&#39;t lazy load a non-collection mapped entity unless you remember to set optional=false (because Hibernate doesn&#39;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) --&gt; use `@Column(nullable=false)`  

The **runtime** (and before contacting the database)--&gt; use `optional=false` (much faster than the first checking).

If you want both abilities, use them both.

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Go string to ascii byte array

How can I compile/install node.js on Ubuntu? It failed with an error about `cxx compiler`.



How to compile/install node.js(could not configure a cxx compiler!) (Ubuntu). 

In JPA, I am confused when to use the attribute `optional=false` and the annotation `@Column(nullable=false)`. What is the difference?

What is the difference between @ManyToOne(optional=false) vs. @Column(nullable=false)

<p>In JPA, I am confused when to use the attribute <code>optional=false</code> and the annotation <code>@Column(nullable=false)</code>. What is the difference?</p>


I&#39;m finally making the voyage back to IntelliJ via Eclipse.  Currently my Eclipse is set up so that if I currently have a statement such as this (where ^ denotes where my cursor currently sits):

    foo.bar(&quot;hello&quot;^)

and I hit the semi-colon (;) key, it will automatically put the semi-colon at the end of the statement:

    foo.bar(&quot;hello&quot;);

Currently IntelliJ does this:

    foo.bar(&quot;hello&quot;;)

Meaning I will have to explicitly type the closing bracket before typing the semi-colon.

Not a huge problem obviously but I have found myself putting the semi-colon in the wrong place a few times today as I make the transition back to IntelliJ and thought that it may be more efficient (for both my fingers and brain) not to have to type the closing bracket.

How to add semicolon after method call when inside parameter list in IntelliJ IDEA?

How can I change the Java Runtime Version on Windows.

I installed Java&amp;nbsp;7 for some tests, and now I need the old java6 as system default, but I don&#39;t want to uninstall the Java&amp;nbsp;7 (I need it for later tests). Can I change the system-used [JRE][1] in the control panel/Java/JRE tab? I can change/edit/add/delete the user-used version, but not the system-used.

  [1]: https://en.wikipedia.org/wiki/Java_virtual_machine


How can I change the Java Runtime Version on Windows (7)?

How can I loop over a class attributes in java dynamically.

For eg :
   
    public class MyClass{
        private type1 att1;
        private type2 att2;
        ...

        public void function(){
            for(var in MyClass.Attributes){
                System.out.println(var.class);
            }
        }
    }

is this possible in Java?


How to loop over a Class attributes in Java?

&gt; **Possible Duplicate:**  
&gt; [What is dependency injection?](https://stackoverflow.com/questions/130794/what-is-dependency-injection)  

&lt;!-- End of automatically inserted text --&gt;

Spring is the framework from where the concept Dependency Injection came to picture.

What is purpose of DI ? How does it benefit ? How is it implemented ?

What is Dependency Injection?

I cannot for the life of me find a definition of what the Java VM flag `CMSClassUnloadingEnabled` actually does, other than some very fuzzy high-level definitions such as &quot;gets rid of your PermGen problems&quot; ([which it doesn&#39;t][1], btw).

I have looked on Sun&#39;s/Oracle&#39;s site, and even [the options list][2] doesn&#39;t actually say what it does.

Based upon the name of the flag, I&#39;m guessing that the CMS Garbage Collector doesn&#39;t by default unload classes, and this flag turns it on - but I can&#39;t be sure.




  [1]: https://stackoverflow.com/questions/3333807/the-various-options-for-solving-permgen-problems/3334397#3334397
  [2]: http://blogs.oracle.com/watt/resource/jvm-options-list.html

What does JVM flag CMSClassUnloadingEnabled actually do?

I am writing a serializer to serialize POJO to JSON but stuck in circular reference problem. In  hibernate bidirectional one-to-many relation, parent references child and child references back to parent and here my serializer dies. (see example code below)  
How to break this cycle? Can we get owner tree of an object to see whether object itself exists somewhere in its own owner hierarchy? Any other way to find if the reference is going to be circular? or any other idea to resolve this problem?

How to solve circular reference in json serializer caused by hibernate bidirectional mapping?

Here is my JPA2 / Hibernate definition:

    Code:
    @Column(nullable = false)
    private boolean enabled;


In MySql this column is resolved to a bit(1) datatype - which does not work for me. For legacy issues I need to map the boolean to a tinyint not to a bit. But I do not see a possibility to change the default datatype. Is there any?

Hibernate JPA, MySQL and TinyInt(1) for Boolean instead of bit or char

Is there a way to specify using JPA that there should be multiple unique constraints on different sets of columns?

    @Entity
    @Table(name=&quot;person&quot;, 
           uniqueConstraints=@UniqueConstraint(columnNames={&quot;code&quot;, &quot;uid&quot;}))
    public class Person {
        // Unique on code and uid
        public String code;
        public String uid;

        // Unique on username
        public String username;

        public String name;
        public String email;
    }

I have seen a hibernate specific annotation but I am trying to avoid vendor specific solutions as we are still deciding between hibernate and datanucleus.

Multiple unique constraints in JPA

How do you define a field, eg `email` as having an index using JPA annotations. We need a non-unique key on `email` because there are literally millions of queries on this field per day, and its a bit slow without the key.

    @Entity
    @Table(name=&quot;person&quot;, 
           uniqueConstraints=@UniqueConstraint(columnNames={&quot;code&quot;, &quot;uid&quot;}))
    public class Person {
        // Unique on code and uid
        public String code;
        public String uid;

        public String username;
        public String name;
        public String email;
    }

I have seen a hibernate specific annotation but I am trying to avoid vendor specific solutions as we are still deciding between hibernate and datanucleus.

UPDATE:

As of JPA 2.1, you can do this. See: https://stackoverflow.com/questions/17620405/the-annotation-index-is-disallowed-for-this-location


Specifying an Index (Non-Unique Key) Using JPA

What&#39;s the exact difference between `@JoinColumn` and `@PrimaryKeyJoinColumn`?

You use `@JoinColumn` for columns that are part of a foreign key. A typical column could look like (e.g. in a join table with additional attributes):

    @ManyToOne
    @JoinColumn(name = &quot;...&quot;)
    private OtherClass oc;

What happens if I promote the column to be a/the PK, too (a.k.a. identifying relationship)? As the column is now the PK, I must tag it with `@Id`:

    @Id
    @ManyToOne
    @JoinColumn(name = &quot;...&quot;)
    private OtherClass oc;

Now the question is:

Are `@Id` + `@JoinColumn` the same as just `@PrimaryKeyJoinColumn`?:

    @ManyToOne
    @PrimaryKeyJoinColumn(name = &quot;...&quot;)
    private OtherClass oc;

If not, what&#39;s `@PrimaryKeyJoinColumn` there for?

JPA: difference between @JoinColumn and @PrimaryKeyJoinColumn?

How do I set a default value in Doctrine 2?

Default value in Doctrine

There&#39;s an Entity Class &quot;A&quot;. Class A might have children of the same type &quot;A&quot;. Also &quot;A&quot; should hold it&#39;s parent if it is a child.  

Is this possible? If so how should I map the relations in the Entity class?
[&quot;A&quot; has an id column.]

JPA: How to have one-to-many relation of the same Entity type

I am wondering if it is possible using annotations to persist the `attributes` map in the following class using JPA2

    public class Example {
        long id;
        // ....
        Map&lt;String, String&gt; attributes = new HashMap&lt;String, String&gt;();
        // ....
    }

As we already have a pre existing production database, so ideally the values of `attributes`  could map to the following existing table:

    create table example_attributes {
        example_id bigint,
        name varchar(100),
        value varchar(100));



Content Type	Original Author	Original Content on Stackoverflow
Question	Truong Ha	View Question on Stackoverflow
Solution 1 - Java	Affe	View Answer on Stackoverflow
Solution 2 - Java	O.Badr	View Answer on Stackoverflow

What is the difference between @ManyToOne(optional=false) vs. @Column(nullable=false)

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How to compile/install node.js(could not configure a cxx compiler!) (Ubuntu).

Go string to ascii byte array

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