JAVA EE: Apply @Resource Annotation in constructor in Spring Autowiring

Friday, 21 September 2018

Apply @Resource Annotation in constructor in Spring Autowiring | Spring - @Resource annotation

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Can You Use @Resource with Constructor Injection?

In Spring Framework, dependency injection can be performed using fields, setter methods, or constructors. A frequent question among developers is: Can we apply the @Resource annotation directly to a constructor?

The short answer is no. If you try to use @Resource directly on a constructor, you will encounter a compilation error. Let us dive into why this happens and what alternatives you should use instead.

The Compilation Error Explained

Suppose you create an Employee class and try to inject dependencies through its constructor using @Resource:

public class Employee {
    private String name;
    private Address address;

    // Disallowed! Causes compile-time error
    @Resource
    public Employee(Address address) {
        this.address = address;
    }
}

The Java compiler will fail with an error stating:

"The annotation @Resource is disallowed for this location."

Why Does This Happen?

The @Resource annotation belongs to the Java standard library (javax.annotation.Resource / jakarta.annotation.Resource). If you examine the annotation definition, its target types are restricted to:

TYPE: Classes and interfaces
FIELD: Class properties/fields
METHOD: Setter and regular methods

Notice that CONSTRUCTOR is completely missing from its @Target list. Because of this definition, Java does not permit the annotation on constructors.

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Apply @Resource Annotation in constructor in Spring Autowiring | Spring - @Resource 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 javax.annotation.Resource;

public class Employee

{

    private String name;

    private Address address;

    @Resource

    public Employee(String name, Address address)

    {

        System.out.println("Employee(String name, Address address) constructor "

                + "is called by the Spring container");

        this.name = name;

        this.address = address;

    }

    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 23, 2018 9:23:02 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5a2e4553: startup date [Thu Aug 23 09:23:02 IST 2018]; root of context hierarchy

Aug 23, 2018 9:23:02 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

Aug 23, 2018 9:23:03 AM org.springframework.context.support.AbstractApplicationContext refresh

WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'employee' defined in class path resource [applicationContext.xml]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.ram.model.Employee]: Constructor threw exception; nested exception is java.lang.Error: Unresolved compilation problem:

    The annotation @Resource is disallowed for this location

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'employee' defined in class path resource [applicationContext.xml]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.ram.model.Employee]: Constructor threw exception; nested exception is java.lang.Error: Unresolved compilation problem:

    The annotation @Resource is disallowed for this location

    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:278)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1270)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1127)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502)

    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312)

    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)

    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310)

    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)

    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:758)

    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868)

    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)

    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)

    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)

    at com.ram.core.App.main(App.java:12)

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.ram.model.Employee]: Constructor threw exception; nested exception is java.lang.Error: Unresolved compilation problem:

    The annotation @Resource is disallowed for this location

    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:175)

    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:117)

    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:271)

    ... 14 more

Caused by: java.lang.Error: Unresolved compilation problem:

    The annotation @Resource is disallowed for this location

    at com.ram.model.Employee.<init>(Employee.java:10)

    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)

    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)

    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)

    at java.lang.reflect.Constructor.newInstance(Unknown Source)

    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:163)

    ... 16 more