Spring Boot Exception Handling

Spring?is a popular Java application framework and?Spring Boot?is an evolution of Spring that helps create stand-alone, production-grade Spring based applications easily.

@ExceptionHandler?is an annotation for handling exceptions in specific handler classes or handler methods. In Servlet environments, we can combine the?@ExceptionHandler?annotation with?@ResponseStatus?to define the response status for the HTTP response.

Spring Boot @ExceptionHandler example

In the following application, we demonstrate the usage of the?@ExceptionHandler. A HTML link in the home page calls a controller's method, which either returns data or throws an exception.

pom.xml
src
├── main
│   ├── java
│   │   └── com
│   │       └── anjum
│   │           ├── Application.java
│   │           ├── controller
│   │           │   └── MyController.java
│   │           ├── exception
│   │           │   └── MyDataException.java
│   │           └── service
│   │               ├── IDataService.java
│   │               └── MyDataService.java
│   └── resources
│       ├── static
│       │   ├── index.html
│       │   └── showError.html
│       └── templates
│           └── showData.ftl
└── test
    └── java        

This is the project structure of the Spring application.

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="https://maven.apache.org/POM/4.0.0"
         xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="https://maven.apache.org/POM/4.0.0
https://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>com.anjum</groupId>
    <artifactId>springbootexceptionhandlerex</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.1.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-freemarker</artifactId>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>        

com/anjum/controller/MyController.java

package com.aanjum.controller;

import com.anjum.exception.MyDataException;
import com.anjum.service.IDataService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.ModelAndView;

import java.util.HashMap;
import java.util.Map;

@Controller
public class MyController {

    @Autowired
    private IDataService dataService;

    @RequestMapping(value = "/getData")
    public ModelAndView getData() {

        var data = dataService.findAll();

        Map<String, Object> params = new HashMap<>();
        params.put("values", data);

        return new ModelAndView("showData", params);
    }

    @ExceptionHandler(MyDataException.class)
    public String handleError(MyDataException e) {

        return "redirect:/showError.html";
    }
}        

The?MyController's?getData()?method calls a service method and stores the retrieved data into a list. The data is sent to the?showData?view. In case of a?MyDataException, the controller redirects to an error page.

@ExceptionHandler(MyDataException.class)
public String handleError(MyDataException e) {

    return "redirect:/showError.html";
}        

The?handleError()?is decorated with?@ExceptionHandler. The handler is activated for the?MyDataException. In the body of the method, we redirect to the?showError.html?page

com/anjum/exception/MyDataException.java

package com.anjum.exception;

public class MyDataException extends RuntimeException {

    public MyDataException(String message) {
        super(message);
    }
}        

We define a custom?MyDataException.

com/anjum/service/IDataService.java

package com.anjum.service;

import java.util.List;

public interface IDataService {

    List<String> findAll();
}        

IDataService?contains the contract method.

com/anjum/service/MyDataService.java

package com.anjum.service;

import com.anjum.exception.MyDataException;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import org.springframework.stereotype.Service;

@Service
public class MyDataService implements IDataService {

    @Override
    public List<String> findAll() {

        var r = new Random();

        if (r.nextBoolean()) {

            throw new MyDataException("Failed to retrieve data");
        }

        var data = new ArrayList<String>();

        data.add("yellow moon");
        data.add("brisk pace");
        data.add("empty bottle");
        data.add("beautiful weather");

        return data;
    }
}        

MyDataService?implements?IDataService's?findAll()?method. The method either returns data or throws a?MyDataException.

var r = new Random();

if (r.nextBoolean()) {

    throw new MyDataException("Failed to retrieve data");
}        

The?findAll()?method randomly throws?MyDataException. The exception is then handled in the controller.

var data = new ArrayList<>();

data.add("yellow moon");
data.add("brisk pace");
data.add("empty bottle");
data.add("beautiful weather");

return data;        

When there is no exception, we return a list of strings.

resources/static/index.html

<!DOCTYPE html>
<html>
    <head>
        <title>Home page</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
        <a href="/getData">Get data</a>
    </body>
</html>        

This is the home page. It contains a link that calls our controller method to fetch some data.

resources/static/showError.html

<!DOCTYPE html>
<html>
    <head>
        <title>Error</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
        <p>Failed to retrieve data</p>
    </body>
</html>        

This is an error page. It is shown when?MyDataException?is thrown.

resources/templates/showData.ftl

<!DOCTYPE html>
<html>
    <head>
        <title>Data</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>

    <body>

        <h2>Data</h2>

        <ul>
            <#list values as val>
                <li>${val}</td>
            </#list>
        </ul>
    
    </body>
</html>        

The?showData.ftl?is a Freemarker template file which shows all retrieved data in an HTML list.

com/anjum/Application.java

package com.anjum;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application  {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}        

Application?is the entry point which bootstraps Spring Boot application.

In this tutorial, we have shown how to handle exceptions in a Spring application with?@ExceptionHandler