Handle Exception in SpringBoot

In SpringBoot, how to handle exceptions?

Before 3.2,


1. The controller level @ExceptionHandler

drawback: it is defined within a controller, and it works only for that controller.

2. The HandlerExceptionResolver:

It offers a uniform exception handler, which is enabled by default Dispatch servlet.

@ExceptionHandler: since Spring 3.1
@ResponseStatus: since Spring 3.0, it is a ResponseStatusExceptionResolver, mapping an Exception to a HttpStatus code.

drawback: 1) it maps an exception to an HTTP status code in the response but without a body massage. 2) one ResponseStatus is able to handle only one Exception.

3. The new one:  @ControllerAdvice
Since Spring 3.2 above, it allows scattered handlers resided in one handler, and it allows to customize response body.


Reference:
Error Handling for REST with Spring

Spring Data Rest

Spring-Data-Rest builds on top of Spring-Data-Repository. it may automatically analyze underlying data model, and bring out a REST API layer in a hypermedia format.  It, implicitly, merged Rest-API within the repository layer. By this way, it removes controller and service layer.

In Spring boot, it is quite simple to initiate a Spring-Data-Rest layer by adding a starter, i.e.

 org.springframework.boot
 spring-boot-starter-data-rest




Certainly, it also needs to include following for repository objects, i.e.

 org.springframework.boot
 spring-boot-starter-data-jpa
 

Checked and Unchecked Exception

Java exceptions are categorized into 3 groups.

(1) checked exception: those are verified during compile time.
(2) unchecked exception: those are not verified during compile time.
(3) error (unchecked)

(1): A Checked exception can be predicted by the programmer.
(2): A Unchecked exception is a runtime exception. it may not be predicted as coding it, fx: index out of bound etc.
(3): Errors made by JVM, for instance, run out of memory etc.

An exception extends a Throwable interface.  All exceptions are throwables.
(1): extends from Exception extends Throwable;  fx:  IOException
(2): extends from RuntimeException extends Throwable; for instance: ArithmeticException
(3): extends from Error extends Throwable

(1) must be handled by a developer by try catch finally, or throws to a caller.
(2) not an error; a developer may handle it or not; whatever it will be handled by the system.
(3) Errors come from the system; they cannot be handled.


Rule in inheritance

class A{
doIt(){ };  //overriden method
}

class B extends A{
@override
doIt() throws RuntimeException{ //overriding method
}
}

In general,  an overriding method cannot throw an exception more generic than the overridden method.  The case above is valid,  only because a RuntimeException is an unchecked exception, so the overriding is valid.

How to print 1 to 100 without using loop

recursion call works the same as a loop.

/**
 *
 * @author YNZ
 */
public class PrintOneToNWithoutLoop {

    public static void main(String[] args) {
        printN(1);
        System.out.println("");
    }

    public static void printN(int n) {
        if (n <= 100) {
            System.out.print(" " + n);
            n++;
            printN(n);
        }

    }

}

Using Lombok to create Builder Pattern

Applying the builder pattern to a data model is a trivial task.  Now it is able to use Lombok annotation to auto-generate it.

import lombok.Getter;
import lombok.Setter;
import lombok.NoArgsConstructor;
import java.io.Serializable;
import java.time.ZonedDateTime;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;
import javax.validation.constraints.NotNull;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.EqualsAndHashCode;
import lombok.ToString;
import org.springframework.format.annotation.DateTimeFormat;

/**
 *
 * @author YNZ
 */
@Entity
@Table(name = "SHARE_ORDER")
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Builder
@ToString
@EqualsAndHashCode
public class Order implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ORDER_PK")
    protected long id;

    @OneToOne
    @JoinColumn(name = "LISTER_PK_FK", referencedColumnName = "USER_PK")
    @NotNull
    protected Lister lister;

    @OneToOne
    @JoinColumn(name = "SEARCHER_PK_FK", referencedColumnName = "USER_PK")
    protected Searcher searcher;

    @Column(name = "CREATION_TIME")
    @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
    @Builder.Default
    protected ZonedDateTime createdDateTime = ZonedDateTime.now();

    @Column(name = "ACTIVATED")
    protected Boolean activated;

    @Column(name = "ORDER_START_TIME")
    @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
    protected ZonedDateTime startTime;

    @Column(name = "ORDER_END_TIME")
    @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
    protected ZonedDateTime endTime;

}