org.postgresql.util.PSQLException: ERROR: relation "app_user" does not exist

PostgreSQL is following the SQL standard and in that case that means that identifiers (table names, column names, etc) are forced to lowercase, except when they are quoted. So when you create a table like this:

CREATE TABLE APP_USER ...

you actually get a table app_user. You apparently did:

CREATE TABLE "APP_USER" ...

and then you get a table "APP_USER".

In Spring, you specify a regular string for the table name, in capital letters, but that gets spliced into a query to the PostgreSQL server without quotes. You can check this by reading the PostgreSQL log files: it should show the query that Spring generated followed by the error at the top of your message.

Since you have very little control over how Spring constructs queries from entities, you are better off using SQL-standard lower-case identifiers.

Give schema name also along with table name. It solved the issue for me.

@Entity
@Table(name = "APP_USER",schema="xxxxx")

public class User implements Serializable {

    private static final long serialVersionUID = -1152779434213289790L;

    @Id
    @Column(name="ID", nullable = false, updatable = false)
    @GeneratedValue(strategy=GenerationType.AUTO) 
    private long id;

    @Column(name="NAME", nullable = false)
    private String name;

    @Column(name="USER_NAME", nullable = false, unique = true)
    private String username;

    @Column(name="PASSWORD", nullable = false)
    private String password;

    @Column(name="EMAIL", nullable = false, unique = true)
    private String email;

    @Column(name="ROLE", nullable = false)
    private RoleEnum role;