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@Resource Annotation in Spring

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Spring’s @Resource Annotation

By Atul Rai | Last Updated: October 7, 2023 Previous       Next

In the world of Java development, the Spring Framework has established itself as a powerful and versatile framework for building enterprise applications. One of the essential features of Spring is its support for Dependency Injection , which allows developers to manage and wire up components effortlessly. In this blog, we will delve into one of the commonly used annotations in Spring, the @Resource annotation, and understand its purpose, usage, and benefits with practical examples.

1. What is the @Resource Annotation?

The @Resource annotation is a part of the Java EE (Enterprise Edition) specification and is supported by the Spring Framework. It is used to declare a dependency on a resource, typically a bean or other managed component. The primary purpose of this annotation is to inject resources into your Spring beans, enabling them to access those resources seamlessly.

2. Usage of @Resource Annotation

The @Resource annotation can be used in various scenarios, such as injecting dependencies, specifying the name of the resource to be injected, and dealing with ambiguous dependencies. Here’s how you can use it:

2.1 Basic Usage

In its simplest form, you can use the @Resource annotation to inject a Spring bean into another bean. For example:

In this example, the OrderService bean injects the ProductRepository bean using the @Resource annotation.

2.2 Specifying Resource Name

You can specify the name of the resource to be injected by providing the name attribute in the @Resource annotation. This is particularly useful when you have multiple beans of the same type, and you want to disambiguate them. For instance:

Here, we’ve explicitly mentioned that the productRepo bean should be injected with the bean named “productRepository” .

Similar Post:   Spring autowiring using @Resource and @Inject annotation example

2.3 Dealing with Ambiguity

If multiple beans of the same type exist and you want to disambiguate them based on their qualifiers, you can use the @Qualifier annotation in conjunction with @Resource . For example:

In this case, we’ve combined @Resource with @Qualifier to specify that the productRepo bean should be injected with the bean named “mysqlProductRepository.”

3. Using @Resource with Spring

Let’s explore a complete example to illustrate how the @Resource annotation works in a Spring application. Suppose we have a simple e-commerce application with a ProductRepository and an OrderService :

In this example, we have two implementations of ProductRepository for different database systems. The OrderService uses the @Resource annotation with a @Qualifier to inject the mysqlProductRepository .

4. Benefits of Using @Resource

The @Resource annotation offers several benefits in a Spring application:

  • Simplicity : It simplifies the process of injecting resources into beans, reducing the need for complex configuration.
  • Type Safety : It ensures type safety by injecting resources by their types, making it harder to inject the wrong resource.
  • Resource Naming : You can specify the name of the resource explicitly, helping to disambiguate resources when multiple beans of the same type exist.
  • Integration with Java EE : The @Resource annotation aligns with Java EE standards, making it suitable for Java EE environments.

The @Resource annotation is a valuable tool in the Spring framework, enabling easy resource injection and management. It simplifies dependency injection, improves code readability, and provides a way to handle resource ambiguity in a Spring application.

  • Injection with @Resource – SpringDoc
  • Annotation Type Resource – JavaDoc
  • Spring autowiring using @Resource and @Inject annotation example
  • Spring @Autowired Annotation Example

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The difference between @Resource and @Autowired in Spring

Table of contents, 1. analysis from the usage level.

First, we create an interface UserService and two implementation classes UserServiceImpl1 and UserServiceImpl2 .

We can then perform dependency injection where we need to use UserService .

Use the @Resource annotation.

Use the @Autowired annotation.

In the above code, the @Resource annotation injects the UserService object directly into the userService property, while the @Autowired annotation needs to be combined with the @Qualifier annotation to specify the specific bean name and the required attribute to set whether or not it must be injected into the object. Note that the @Qualifier annotation can also be omitted from the @Autowired annotation, which will match the corresponding bean by type.

2. Analysis from the conceptual level

The implementation principle of dependency injection in Spring Framework is based on the Java reflection mechanism and JavaBean specification.

Instantiate objects through the reflection mechanism

The Spring framework instantiates the bean object to be injected through the reflection mechanism and stores it in a Map, where Key is the bean name and Value is the bean instance object.

bean property injection via the JavaBean specification

Next, Spring will find the property to be injected based on the JavaBean specification (i.e. the property name and the setter method of the property) and call the corresponding setter method to inject the property through the reflection mechanism.

Search strategies for dependency injection

During dependency injection, the Spring Framework typically uses two search strategies:

Search by type: When the type of the property to be injected matches the type of multiple beans in the container, a different search strategy is chosen depending on the annotation. For example, when using the @Autowired annotation, the default is to find the corresponding bean by type matching.

Search by name: When the type of the property to be injected is not unique, or the name of the bean to be injected does not match the name of the property, you can use the @Qualifier annotation to specify the name of the bean to be injected.

using various annotations for dependency injection

The Spring framework provides a variety of annotations for dependency injection, including:

  • @Autowired : injection by type;
  • @Resource : injection by name;
  • @Value : injection of properties of simple types or string types;
  • @Inject : an annotation defined by the JSR-330 standard, similar in function to @Autowired .

The Spring Framework’s implementation of dependency injection is based on the Java reflection mechanism and the JavaBean specification, and is implemented in different ways through various annotations.

Both @Resource and @Autowired are Spring dependency injection annotations, but they differ as follows:

Different sources: @Resource is an annotation defined by the Java EE specification, whereas @Autowired is an annotation provided by Spring.

Attributes are different: The @Resource annotation does not have a required attribute, but a name attribute that indicates the name of the bean to be injected, while the @Autowired annotation has a required and a name attribute, where required indicates whether the object must be injected and defaults to true, and name indicates the name of the bean to be injected.

Different lookup method: @Resource annotation by default is based on byName, if not found then byType, while @Autowired by default is based on byType.

Compatibility differences: The @Resource annotation can be used with the @Inject annotation in JSR-330; the @Autowired annotation can only be used with Spring components.

Different application scenarios: The @Resource annotation is mainly used in Java EE environments, while the @Autowired annotation is one of the most widely used dependency injection annotations in the Spring Framework and can be applied to different application scenarios.

The advantages and disadvantages of the @Resource and @Autowired annotations are described separately below:

Advantages of @Resource :

  • simple to use and very easy to use in classes.
  • Can be used in conjunction with the @Inject annotation.
  • Supports specifying bean names for more precise dependency injection.

@Resource disadvantages:

  • the Spring framework for @Resource annotation support is not as rich as @Autowired .
  • only support byName and byType two injection methods, more limited than @Autowired support.

@Autowired Advantages:

  • support for complex dependency injection configuration , you can dependency injection in a variety of ways.
  • the Spring framework for @Autowired annotation support is very rich, is one of the most widely used annotations in Spring.
  • you can specify whether the property must be injected, or you can specify the bean name for injection.
  • In addition to using it in classes, you can also use the @Autowired annotation in constructors, Setter methods and any method annotated with @Bean .

Disadvantages of @Autowired :

  • more cumbersome to configure, need to specify required and name attributes etc.
  • need to follow Spring’s automatic scanning mechanism, only classes marked with annotations such as @Component , @Service , @Controller and @Repository will be managed by the Spring container.

The principle of dependency injection can be summarised in three simple steps:

  • At application startup, the Spring container is responsible for creating and managing all bean instance objects.
  • when a bean needs to be used, the Spring container injects the bean into the class or object that needs to be used through the reflection mechanism.
  • During the bean injection process, the Spring container will use different dependency injection methods depending on the annotation (e.g. @Resource , @Autowired ) to complete the dependency injection process.

Both @Resource and @Autowired are dependency injection annotations provided by Spring, each with its own advantages and disadvantages. In practice, you can choose the right annotation to use depending on the specific application scenario.

2. Analysis from the code level

At the Spring Framework source level, the fully qualified paths for the @Resource and @Autowired annotations are javax.annotation.Resource and org.springframework.beans.factory.annotation.Autowired . Let’s take a look at their implementation.

Implementation principles of @Resource annotation:

  • if the name attribute of the @Resource annotation is not empty, the Spring container looks for the bean instance object to be injected based on the value of the attribute; if the name attribute is empty, the default is to use the field name as the bean name to look for it.
  • The Spring container will first look for the bean instance based on the byName injection method, and if it does not find the corresponding bean instance, it will look for it based on the byType method.
  • If the corresponding bean instance is found, the bean instance is injected into the class or object that needs to use it using the reflection mechanism.
  • If the corresponding bean instance is not found, a NoSuchBeanDefinitionException is thrown.

How the @Autowired annotation is implemented:

  • The Spring container will process the AutowiredAnnotationBeanPostProcessor class and the derived classes of the AutowiredAnnotationBeanPostProcessor class according to their derivatives, injecting all the properties marked with the @Autowired annotation into the class that needs to be used or object to be used.
  • For the AuthorizedAnnotationBeanPostProcessor, which implements the BeanPostProcessor interface, the @Autowired annotation is intercepted during the instantiation and initialization of the bean.
  • When the Spring container encounters an @Autowired annotation, it will automatically call the postProcessPropertyValues method of the AutowiredAnnotationBeanPostProcessor class, which will perform different dependency injection processes depending on the value of the annotated property.
  • When handling @Autowired annotations, the Spring container by default uses the byType method to look up the bean and if there are multiple beans of matching types, they are matched by class name; if it is still not sure which bean to inject, a NoSuchBeanDefinitionException is thrown.

Both the @Resource annotation and the @ Autowired annotation are implemented in the Spring Framework source code through the reflection mechanism and the BeanPostProcessor interface for dependency injection. In practice, we can study the corresponding source implementations to get a deeper understanding of how they work, so that we can use these dependency injection annotations better.

Reference: https://juejin.cn/post/7223286420794966076

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Dependency Injection: @Autowired, @Resource and @Inject

Wiring in Spring

In Spring Framework, you can basically use any of the three annotations for Dependency Injection, namely @Autowired , @Resource and @Inject. The @Autowired annotation belongs to the core-spring, however, the other two belongs to the Java extension package @javax.annotation.Resource and @javax.inject.Inject .

We will look into the use of each of these annotations with a practical use case, to help you choose the one that best suits you.

Common example

I will use the same set of classes to understand the Injections using @Resource , @Inject and @Autowired . As you can see the abstract class FileReader is extended by 2 classes, PdfFileReader and WordFileReader . The below example is used to explore the wiring (Dependency Injection) in all the 3 cases.

The @Configuration annotation in ConfigWiring indicates that this is used as a source of bean definitions. As you rightfully observed, the FileReader is not annotated with @Component as it is an abstract class. There are two concrete classes that extend FileReader, PdfFileReader and WordFileReader . Spring provides stereotype @Component annotation which registers the class as spring managed component.

1. @Resource – Dependency Injection using JSR-250

The @Resource annotation is from JSR-250 specification, which means it is from javax.annotation.Resource . If you are using JDK8+, make sure to add the JSR-250 dependency jar in your maven/Gradle file.

The Resource annotation class definition looks as below. As you can see, it accepts two important parameters that are type and name . The @Target({TYPE, FIELD, METHOD}) indicates where the @Resource annotation can be used.

Remember the following when working with @Resource annotation

  • You can only use @Resource annotation on fields or bean property setters.
  • In other words, @Resource annotation can never be used to achieve constructor injections.
  • Match by Name
  • Match by Type
  • Match by @Qualifier
  • The @Resource annotation takes 2 important optional parameter name and type . If no name is explicitly specified, the default name is derived from the field name or setter method.
  • Similarly, if no type is specified explicitly, it will do a type match and try to resolve it.

What is Dependency Ambiguity

All of the below codes uses common examples from above, where FileReader is an abstract class and there are 2 classes PdfFileReader and WordFileReader that extends FileReader.

When we want to inject a dependency, we can just annotate the property or its setter method with the @Resource annotation. The problem occurs when we have ambiguity . The code below throws NoUniqueBeanDefinitionException as we have 2 classes extending the FileReader .

The above code is the reason why we need to write code in such a way that there is no dependency ambiguity.

1.1. Resolve dependency by Name

For simplicity of understanding, I am using the @Resource annotation on the property, you should use them on setter for performance reasons. In the first example below, the field name pdfFileReader is used as the bean name to resolve the dependency.

You can also specify the bean name explicitly as shown below.

1.2. Resolve dependency by Type

Spring will try to resolve the dependency by name . However, it will not find any bean with this name so next, it tries to resolve by type. As we have WordFileReader bean already registered, spring will autodetect this type and resolve this field.

In the below code, Spring can neither detect the dependency by name nor resolve by type. Because there is ambiguity, type detection will fail with Exception BeanCreationException . So we will specify the type explicitly, type = PdfFileReader.class to inject PdfFileReader.

1.3. Resolve dependency by @Qualifier

Spring has this special @Qualifier annotation in which we can pass the name of the bean to be resolved.

2. @Inject – Dependency Injection using JSR-330

The @Inject annotation is from JSR-330 specification, which means it is from javax.inject.Inject . If you are using JDK8+, make sure to add the JSR-330 dependency jar in your pom/Gradle file.

The Inject annotation looks as follow, which means it can be used on the field, setter or constructor based annotations.

Remember the following when working with @Inject annotation:

  • The @Inject can be used on Setter, Field, or Constructor to do the respective type injections.
  • Resolve by Type
  • Resolve by Qualifier – @Qualifier annotation
  • Resolve by Name – @Named annotation
  • The Inject annotation does not take any parameter.

1. Resolve dependency by Type

In the below examples, I have used the annotation on the property fields. But in a real project, you should use it on setters or constructors due to performance reasons.

The example below fileReader is of the type PdfFileReader , so the dependency injection is straight forward.

2. Resolve by @Qualifier annotation

There are 2 implementations of the base class FileReader and they are as PdfFileReader and WordFileReader . To get rid of this ambiguity, you need to specify the bean name in @Qualifier annotation. The code @Qualifier("wordFileReader") below represents the same.

If you don’t specify the @Qualifier , you will end up reproducing the exception as below.

3. Resolve dependency by Name

The @Inject annotation detects the bean to be injected in 2 ways. First, if @Named("beanName") annotation is specified. Otherwise, the field name is used as the bean name. The @Named annotation is from JSR-330 javax.inject.* package and it is used to specify the bean name whose instance will be injected.

The line-6 represents the bean name detection using @Named annotation. On the other hand, line-17 represents the auto-detection by field name.

3. @Autowired – Dependency Injection in Spring style

The @Autowired annotation is similar to @Inject annotation. The only difference is @Inject is from JSR-330 specification, and the @Autowired is purely from the Spring framework.

The @Autowired annotation looks as below. As you can observe, @Autowired can be used on the constructor, setter, field and in the parameters as well.

The examples discussed below uses field-based dependency injection, but you should always use the setter based or constructor based injections in your application due to performance reasons.

Remember the following when working with @Autowired annotation:

  • The @Autowired can be used on Setter, Field, Constructor or on parameters to do the respective type injections.
  • Resolve by Qualifier – using the @Qualifier
  • Resolve by Name
  • The @Autowired(required=true) annotation takes required as an optional parameter which is always true by default.

Spring first tries to detect the bean to be injected by its type and do the DI. In the code below, spring simply injects PdfFileReader instance.

2. Resolve dependency by @Qualifier

When you have ambiguity, use the @Qualifier annotation to specify the bean name to be injected.

Spring @Autowired annotation can resolve the dependency by field name as discussed before. In this example, spring inject the bean with name wordFileReader .

Conclusion:

This article only focuses on the auto wiring part. I prefer using @Autowired or @Inject. If you want to understand the dependency injection and scopes etc in Spring, check out the respective articles.

  • Inversion of Control and Dependency Injection in Spring
  • Spring Bean Scopes – @Scope annotation

The complete code example is available on GitHub, download the code and try it out. The test cases are located here and the common classes are here .

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Dinesh Rajput

Dinesh Rajput is the chief editor of a website Dineshonjava, a technical blog dedicated to the Spring and Java technologies. It has a series of articles related to Java technologies. Dinesh has been a Spring enthusiast since 2008 and is a Pivotal Certified Spring Professional, an author of a book Spring 5 Design Pattern, and a blogger. He has more than 10 years of experience with different aspects of Spring and Java design and development. His core expertise lies in the latest version of Spring Framework, Spring Boot, Spring Security, creating REST APIs, Microservice Architecture, Reactive Pattern, Spring AOP, Design Patterns, Struts, Hibernate, Web Services, Spring Batch, Cassandra, MongoDB, and Web Application Design and Architecture. He is currently working as a technology manager at a leading product and web development company. He worked as a developer and tech lead at the Bennett, Coleman & Co. Ltd and was the first developer in his previous company, Paytm. Dinesh is passionate about the latest Java technologies and loves to write technical blogs related to it. He is a very active member of the Java and Spring community on different forums. When it comes to the Spring Framework and Java, Dinesh tops the list!

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Annotation Type Resource

Even though this annotation is not marked Inherited , deployment tools are required to examine all superclasses of any component class to discover all uses of this annotation in all superclasses. All such annotation instances specify resources that are needed by the application component. Note that this annotation may appear on private fields and methods of superclasses; the container is required to perform injection in these cases as well.

Optional Element Summary

Element detail, authenticationtype.

Application servers are not required to support any particular form or type of mapped name, nor the ability to use mapped names. The mapped name is product-dependent and often installation-dependent. No use of a mapped name is portable.

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COMMENTS

  1. Wiring in Spring: @Autowired, @Resource and @Inject

    Overview In this Spring Framework tutorial, we'll demonstrate how to use annotations related to dependency injection, namely the @Resource, @Inject, and @Autowired annotations. These annotations provide classes with a declarative way to resolve dependencies: @Autowired ArbitraryClass arbObject;

  2. java

    54 First of all, to understand the point of @Resource you need to understand the Inversion of Control (IoC). Inversion of Control is a principle in software development which goes that the control of objects should be transferred to a container or a framework.

  3. Resource (Java Platform SE 8 )

    The Resource annotation marks a resource that is needed by the application. This annotation may be applied to an application component class, or to fields or methods of the component class.

  4. @Resource Annotation in Spring

    1. The @Resource annotation is JSR-250 javax.annotation.Resource API. Some other JSR-250 annotations are @PostConstruct, and @PreDestroy . 2. The @Resource annotation is used to declare a reference to a resource. The @Resource annotation resolves dependency injection. We can use it in place of @Autowired annotation. 3.

  5. Injection with @Resource :: Spring Framework

    @Resource takes a name attribute. By default, Spring interprets that value as the bean name to be injected. In other words, it follows by-name semantics, as demonstrated in the following example: Java Kotlin

  6. Spring @Resource Annotation Example

    The @Resource annotation in spring performs the autowiring functionality. This annotation follows the autowire=byName semantics in the XML based configuration i.e. it takes the name attribute for the injection. Below snippet shows how to use this annotation. Code Snippet This annotation takes an optional name argument.

  7. Spring's @Resource Annotation

    1. What is the @Resource Annotation? The @Resource annotation is a part of the Java EE (Enterprise Edition) specification and is supported by the Spring Framework. It is used to declare a dependency on a resource, typically a bean or other managed component.

  8. The difference between @Resource and @Autowired in Spring

    Different sources: @Resource is an annotation defined by the Java EE specification, whereas @Autowired is an annotation provided by Spring. Attributes are different: The @Resource annotation does not have a required attribute, but a name attribute that indicates the name of the bean to be injected, while the @Autowired annotation has a required ...

  9. Resources :: Spring Framework

    Core Technologies Resources Resources This chapter covers how Spring handles resources and how you can work with resources in Spring. It includes the following topics: Introduction The Resource Interface Built-in Resource Implementations The ResourceLoader Interface The ResourcePatternResolver Interface The ResourceLoaderAware Interface

  10. Resource Injection

    Field-Based Injection. To use field-based resource injection, declare a field and decorate it with the @Resource annotation. The container will infer the name and type of the resource if the name and type elements are not specified. If you do specify the type element, it must match the field's type declaration.

  11. Dependency Injection: @Autowired, @Resource and @Inject

    Remember the following when working with @Resource annotation. You can only use @Resource annotation on fields or bean property setters.; In other words, @Resource annotation can never be used to achieve constructor injections. The precedence that @Resource annotation takes to do dependency injection is:. Match by Name; Match by Type; Match by @Qualifier; The @Resource annotation takes 2 ...

  12. If there is @ Resource, why do we need @ Autowired?

    If "Resource", why "Autowired"? @Autowired is used for type injection, while @Resource is used for name injection. If a name is not found, type matching is used as a fallback option. In ...

  13. @Resource Annotation in JSR-250 with Spring Framework

    The @Resource annotation takes a ' name ' attribute which will be interpreted as the bean name to be injected. You can say, it follows by-name autowiring semantics as demonstrated in the below example: package com.dineshonjava.sdnext.jsr.tutorial; import javax.annotation.Resource; public class Circle { private Point center; @Resource (name ...

  14. Resource Injection

    With field-based and method-based injection, the container will inject the resource when the application is initialized. For class-based injection, the resource is looked up by the application at runtime. The @Resource annotation has the following elements: name: The JNDI name of the resource. type: The Java language type of the resource.

  15. Resource (Java EE 6 )

    Resource (Java EE 6 ) javax.annotation Annotation Type Resource Resource The Resource annotation marks a resource that is needed by the application. This annotation may be applied to an application component class, or to fields or methods of the component class.

  16. How to specify name for resource annotations in compile time?

    10 This is not the right way to do things. Resources should be added to the local jndi name of individual EJBs. This is to separate the jndi name used in the bean code from the global jndi bindings set by the bean deployer.

  17. @Resource Annotation in Spring Autowiring

    Spring @Resource annotation. @Resource annotation in Spring takes a name attribute, and by default Spring interprets that value as the bean name to be injected. In other words, it follows by-name semantics. That's where it differs from other annotations for autowiring like @Autowired and @Inject which are equivalent to autowiring="byType" in ...

  18. I don't understand @Resource mappedName element at all

    The mappedName element is a non-portable, implementation-specific name that the resource should be mapped to. Because the name element, when specified or defaulted, is local only to the application, many Java EE servers provide a way of referring to resources across the application server. This is done by setting the mappedName element.

  19. No bean named '...' is defined and Spring @Resource annotation

    3. I use @Resource to annotate bean classes, @Autowired to autowire dependencies, and in Spring configuration file these things: context:component-scan base-package="package1,package2" tx:annotation-driven. So, it works fine (tested). Spring scans package1, package2, classes with @Resource annotation and then I can get them using getBean () IF ...

  20. Resource (Java(TM) EE 8 Specification APIs)

    The Resource annotation marks a resource that is needed by the application. This annotation may be applied to an application component class, or to fields or methods of the component class. ... The mappedName element provides for mapping the resource reference to the name of a resource known to the applicaiton server. The mapped name could be ...

  21. How to use @Resource annotation in servlet (or in any other Java class

    1 I am experimenting with Java Dependency Injection. Many questions here on SO talk about jndi resources being wired. But I think, a java class can be wired using @Resource annotation. I have a simple servlet in which I have two properties to be wired using CDI. I am using Tomcat6, Servlet 2.5, and Weld configuration. The servlet code: