JAVA EE: Optional Dependencies

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Thursday, 13 September 2018

How to enable or disable the Spring Dependency checking using @Autowired annotation

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Overview: Dependency Checking with @Autowired(required = ...)

By default, the @Autowired annotation in the Spring Framework enforces strict dependency checking. If Spring cannot locate a matching bean in the application context to satisfy an autowired field, constructor, or setter, it throws a NoSuchBeanDefinitionException at startup. However, in scenarios where a dependency is optional, Spring allows you to disable mandatory dependency checking using the required attribute.

In this guide, you will learn how to enable or disable dependency checking using @Autowired(required = false) and handle optional dependencies cleanly in Spring applications.

Understanding the 'required' Attribute in @Autowired

The required parameter provides flexible control over bean injection at initialization time:

Mandatory by Default: @Autowired behaves as @Autowired(required = true) by default. If the matching bean does not exist, initialization fails immediately.
Disabling Dependency Checking: Setting @Autowired(required = false) instructs Spring to inject the bean if present, or simply leave the target field or property as null if absent.
Graceful Fallbacks: Allows you to supply optional components (like audit loggers, custom cache engines, or notification handlers) without causing container boot failures.
Safe Invocation: When disabling dependency checking, ensure your application logic performs appropriate null-checks or provides default fallback instances before calling methods on the dependency.

Step-by-Step Implementation

Annotate the Dependency: Mark your field or setter method with @Autowired(required = false):
@Autowired(required = false) private NotificationService notificationService;
Handle Null Values Safely: Within your business methods, check whether the optional bean is injected before interacting with it:
if (notificationService != null) { notificationService.sendNotification(); }
Run the Application Context: If no bean definition exists for the dependency, the application context starts up smoothly without exceptions.

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How to enable or disable the Spring Dependency checking using @Autowired annotation

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0%20http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ram.core</groupId>

    <artifactId>SpringDemo</artifactId>

    <packaging>jar</packaging>

    <version>1.0-SNAPSHOT</version>

    <name>SpringDemo</name>

    <url>http://maven.apache.org</url>

    <properties>

        <spring.version>5.0.2.RELEASE</spring.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>junit</groupId>

            <artifactId>junit</artifactId>

            <version>3.8.1</version>

            <scope>test</scope>

        </dependency>

        <!-- Spring 5 dependencies -->

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-core</artifactId>

            <version>${spring.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-context</artifactId>

            <version>${spring.version}</version>

        </dependency>

    </dependencies>

</project>

Address.java

package com.ram.model;

public class Address

{

    private String city;

    private String state;

    private String country;

    public Address(String city, String state, String country)

    {

        this.city = city;

        this.state = state;

        this.country = country;

    }

    @Override

    public String toString()

    {

        return "Address [city=" + city + ", state=" + state + ", country="

                + country + "]";

    }

}

Employee.java

package com.ram.model;

import org.springframework.beans.factory.annotation.Autowired;

public class Employee

{

    private String name;

    @Autowired(required=false)

    private Address address;

    public Employee(String name)

    {

        System.out.println("Employee(String name) constructor "

                + "is called by the Spring container");

        this.name = name;

    }

    public void displayEmployeeDetails()

    {

        System.out.println(

                "Employee [name=" + name + ", address=" + address + "]");

    }

}

applicationContext.xml

<beans xmlns="http://www.springframework.org/schema/beans"

    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 

    xmlns:context="http://www.springframework.org/schema/context"

    xsi:schemaLocation="http://www.springframework.org/schema/beans

    http://www.springframework.org/schema/beans/spring-beans-4.3.xsd

    http://www.springframework.org/schema/context

    http://www.springframework.org/schema/context/spring-context-4.3.xsd">

    <context:annotation-config />

    <!-- <bean id="address" class="com.ram.model.Address">

        <constructor-arg value="Chennai"></constructor-arg>

        <constructor-arg value="Tamil Nadu"></constructor-arg>

        <constructor-arg value="India"></constructor-arg>

    </bean> -->

    <bean id="employee" class="com.ram.model.Employee">

        <constructor-arg value="Peter"></constructor-arg>

    </bean>

</beans>

App.java

package com.ram.core;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.ram.model.Employee;

public class App

{

    public static void main(String[] args)

    {

        ApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

        System.out.println("---------------------------------------");

        Employee employee = (Employee) context.getBean("employee");

        System.out.println(

                "Got employee object from the ApplicationContext[Spring Container]");

        employee.displayEmployeeDetails();

    }

}

Output

Aug 20, 2018 9:42:47 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5a2e4553: startup date [Mon Aug 20 09:42:47 IST 2018]; root of context hierarchy

Aug 20, 2018 9:42:47 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

Employee(String name) constructor is called by the Spring container

---------------------------------------

Got employee object from the ApplicationContext[Spring Container]

Employee [name=Peter, address=null]