What Is the Easiest Way to Ignore a JPA Field During Persistence

What is the easiest way to ignore a JPA field during persistence?

@Transient complies with your needs.

is it possible to ignore some field when mapping in annotation,Entity class

Use the @Transient annotation:

This annotation specifies that the property or field is not persistent. It is used to annotate a property or field of an entity class, mapped superclass, or embeddable class.

i.e.

@Column(name="skills")
public String getSkills() {
    return skills;
}

@Transient
public int getRowCount() {
    return rowCount;
}

JPA: ignore field on save, but fetch on select

From the excellent book Pro JPA 2 :

JPA defines options to set individual mappings to be read-only using
the insertable and updatable elements of the @Column and @JoinColumn
annotations. These two settings default to true but can be set to
false if we want to ensure that the provider will not insert or update
information in the table in response to changes in the entity
instance. If the data in the mapped table already exists and we want
to ensure that it will not be modified at runtime, then the
insertable and updatable elements can be set to false, effectively
preventing the provider from doing anything other than reading the
entity from the database.

@Column(insertable = false, updatable = false)
private String readOnlyField;

Make Hibernate ignore instance variables that are not mapped

JPA will use all properties of the class, unless you specifically mark them with @Transient:

@Transient
private String agencyName;

The @Column annotation is purely optional, and is there to let you override the auto-generated column name. Furthermore, the length attribute of @Column is only used when auto-generating table definitions, it has no effect on the runtime.

Hibernate - Ignore non annotated field without the need to add @Transient

It doesn't look like it's possible with Hibernate annotations, according to the Hibernate Reference:

In the annotations world, every non static non transient property
(field or method depending on the access type) of an entity is
considered persistent, unless you annotate it as @Transient. Not
having an annotation for your property is equivalent to the
appropriate @Basic annotation.

Of course, with XML mappings you could just only add the properties that will be mapped.

How to exclude an entity field when doing an update with JPA

You need to set updatable attribute of the @Column annotation to false:

@Column(name = "USER_NAME", nullable = false, length = 75, updatable= false)
private String userName;

The updatable attribute instruct Hibernate to omit this column from the generated UPDATE SQL statement.

I removed the @Transient and the @Id annotations.

If this column is your PK (mapped to the entity identifier), then you can only set it during INSERT, since Hibernate doesn't allow you to update an entity identifier (the updatable attribute being redundant in this case).

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