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Is it possible to use @Resource on a constructor?
I was wondering if it possible to use the @Resource annotation on a constructor.
My use case is that I want to wire a final field called bar .
I get a message that the @Resource is not allowed on this location. Is there any other way I could wire the final field?
- spring-annotations
3 Answers 3
From the source of @Resource :
means that this annotation can only be placed on Classes, Fields and Methods. CONSTRUCTOR is missing.
- Damn... i have read it multiple times, but missed this.. thanks! So i will fallback to @Autowire and @Qualifier then. – Marco Apr 29, 2011 at 11:10
To complement Robert Munteanu's answer and for future reference, here is how the use of @Autowired and @Qualifier on constructor looks :
In this example, bar is just autowired (i.e. there is only one bean of that class in the context so Spring knows which to use), while baz has a qualifier to tell Spring which particular bean of that class we want injected.
Use @Autowired or @Inject . This limitation is covered in the Spring reference documentation: Fine-tuning annotation-based autowiring with qualifiers :
@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, stick with qualifiers if your injection target is a constructor or a multi-argument method.
- 1 I used @Autowired together with the @Qualifier annotation. This because i have multiple implementations that can be wired. To express which implementation i need i used the Qualifier. In the documentation it states that it is prefered to used @Resource in this case. So i am trying to do that. – Marco Apr 29, 2011 at 11:05
- Which documentation item says that @Resource is preferred? – Robert Munteanu Apr 29, 2011 at 11:13
- It is in the official documentation: static.springsource.org/spring/docs/2.5.x/reference/beans.html below chapter 3.11.3 as the Tip! – Marco Apr 29, 2011 at 11:21
- 2 Good point. In the Spring 3 documentation this was refined to mention the very issue you encountered, look for the tip in static.springsource.org/spring/docs/3.0.x/reference/… – Robert Munteanu Apr 29, 2011 at 11:58
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Why You Should Use Constructor Injection in Spring
- March 28, 2020
- Spring Boot
Dependency injection is an approach to implement loose coupling among the classes in an application.
There are different ways of injecting dependencies and this article explains why constructor injection should be the preferred way.
Example Code
What is dependency injection.
- Dependency : An object usually requires objects of other classes to perform its operations. We call these objects dependencies.
- Injection : The process of providing the required dependencies to an object.
Thus dependency injection helps in implementing inversion of control (IoC). This means that the responsibility of object creation and injecting the dependencies is given to the framework (i.e. Spring) instead of the class creating the dependency objects by itself.
We can implement dependency injection with:
- constructor-based injection,
- setter-based injection, or
- field-based injection.
Constructor Injection
In constructor-based injection, the dependencies required for the class are provided as arguments to the constructor:
Before Spring 4.3, we had to add an @Autowired annotation to the constructor. With newer versions, this is optional if the class has only one constructor.
In the Cake class above, since we have only one constructor, we don’t have to specify the @Autowired annotation. Consider the below example with two constructors:
When we have a class with multiple constructors, we need to explicitly add the @Autowired annotation to any one of the constructors so that Spring knows which constructor to use to inject the dependencies.
Setter Injection
In setter-based injection, we provide the required dependencies as field parameters to the class and the values are set using the setter methods of the properties. We have to annotate the setter method with the @Autowired annotation.
The Cake class requires an object of type Topping . The Topping object is provided as an argument in the setter method of that property:
Spring will find the @Autowired annotation and call the setter to inject the dependency.
Field Injection
With field-based injection, Spring assigns the required dependencies directly to the fields on annotating with @Autowired annotation.
In this example, we let Spring inject the Topping dependency via field injection:
Combining Field and Setter Injection
What will happen if we add @Autowired to both, a field and a setter? Which method will Spring use to inject the dependency?
In the above example, we have added the @Autowired annotation to both the setter and the field. In this case, Spring injects dependency using the setter injection method.
Note that it’s bad practice to mix injection types on a single class as it makes the code less readable.
Why Should I Use Constructor Injection?
Now that we have seen the different types of injection, let’s go through some of the advantages of using constructor injection.
All Required Dependencies Are Available at Initialization Time
We create an object by calling a constructor. If the constructor expects all required dependencies as parameters, then we can be 100% sure that the class will never be instantiated without its dependencies injected.
The IoC container makes sure that all the arguments provided in the constructor are available before passing them into the constructor . This helps in preventing the infamous NullPointerException .
Constructor injection is extremely useful since we do not have to write separate business logic everywhere to check if all the required dependencies are loaded, thus simplifying code complexity.
What About Optional Dependencies?
With setter injection, Spring allows us to specify optional dependencies by adding @Autowired(required = false) to a setter method. This is not possible with constructor injection since the required=false would be applied to all constructor arguments.
We can still provide optional dependencies with constructor injection using Java's Optional type.
Identifying Code Smells
Constructor injection helps us to identify if our bean is dependent on too many other objects. If our constructor has a large number of arguments this may be a sign that our class has too many responsibilities . We may want to think about refactoring our code to better address proper separation of concerns.
Preventing Errors in Tests
Constructor injection simplifies writing unit tests. The constructor forces us to provide valid objects for all dependencies. Using mocking libraries like Mockito, we can create mock objects that we can then pass into the constructor.
We can also pass mocks via setters, of course, but if we add a new dependency to a class, we may forget to call the setter in the test, potentially causing a NullPointerException in the test.
Constructor injection ensures that our test cases are executed only when all the dependencies are available. It’s not possible to have half created objects in unit tests (or anywhere else for that matter).
Immutability
Constructor injection helps in creating immutable objects because a constructor’s signature is the only possible way to create objects. Once we create a bean, we cannot alter its dependencies anymore. With setter injection, it’s possible to inject the dependency after creation, thus leading to mutable objects which, among other things, may not be thread-safe in a multi-threaded environment and are harder to debug due to their mutability.
Constructor injection makes code more robust. It allows us to create immutable objects, preventing NullPointerException s and other errors.
You can find the code example on GitHub .
Vasudha Venkatesan
A budding software engineer curious to learn new things and open to innovative ideas.
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17. Spring Beans and Dependency Injection
You are free to use any of the standard Spring Framework techniques to define your beans and their injected dependencies. For simplicity, we often find that using @ComponentScan (to find your beans) and using @Autowired (to do constructor injection) works well.
If you structure your code as suggested above (locating your application class in a root package), you can add @ComponentScan without any arguments. All of your application components ( @Component , @Service , @Repository , @Controller etc.) are automatically registered as Spring Beans.
The following example shows a @Service Bean that uses constructor injection to obtain a required RiskAssessor bean:
If a bean has one constructor, you can omit the @Autowired , as shown in the following example:
Dependency Injection: @Autowired, @Resource and @Inject
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.
- 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.
- 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.
- 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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Spring @Autowired Annotation
The @Autowired annotation provides more fine-grained control over where and how autowiring should be accomplished. The @Autowired annotation can be used to autowire bean on the setter method just like @Required annotation, constructor, a property or methods with arbitrary names and/or multiple arguments.
@Autowired on Setter Methods
You can use @Autowired annotation on setter methods to get rid of the <property> element in XML configuration file. When Spring finds an @Autowired annotation used with setter methods, it tries to perform byType autowiring on the method.
Let us have working Eclipse IDE in place and follow the following steps to create a Spring application −
Here is the content of TextEditor.java file −
Following is the content of another dependent class file SpellChecker.java :
Following is the content of the MainApp.java file −
Following is the configuration file Beans.xml −
Once you are done with creating source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message −
@Autowired on Properties
You can use @Autowired annotation on properties to get rid of the setter methods. When you will pass values of autowired properties using <property> Spring will automatically assign those properties with the passed values or references. So with the usage of @Autowired on properties your TextEditor.java file will become as follows −
Once you are done with the above two changes in source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message −
@Autowired on Constructors
You can apply @Autowired to constructors as well. A constructor @Autowired annotation indicates that the constructor should be autowired when creating the bean, even if no <constructor-arg> elements are used while configuring the bean in XML file. Let us check the following example.
Once you are done with the above two changes in source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message:
@Autowired with (required = false) option
By default, the @Autowired annotation implies the dependency is required similar to @Required annotation, however, you can turn off the default behavior by using (required=false) option with @Autowired.
The following example will work even if you do not pass any value for age property but still it will demand for name property. You can try this example yourself because this is similar to @Required annotation example except that only Student.java file has been changed.
// Tutorial //
Spring @autowired annotation.
While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.
Spring @Autowired annotation is used for automatic dependency injection. Spring framework is built on dependency injection and we inject the class dependencies through spring bean configuration file.
- autowire byName - For this type of autowiring, setter method is used for dependency injection. Also the variable name should be same in the class where we will inject the dependency and in the spring bean configuration file.
- autowire byType - For this type of autowiring, class type is used. So there should be only one bean configured for this type in the spring bean configuration file.
- autowire by constructor - This is almost similar to autowire byType, the only difference is that constructor is used to inject the dependency.
- autowire by autodetect - If you are on Spring 3.0 or older versions, this is one of the autowire options available. This option was used for autowire by constructor or byType, as determined by Spring container. Since we already have so many options, this option is deprecated. I will not cover this option in this tutorial.
- @Autowired annotation - We can use Spring @Autowired annotation for spring bean autowiring. @Autowired annotation can be applied on variables and methods for autowiring byType. We can also use @Autowired annotation on constructor for constructor based spring autowiring. For @Autowired annotation to work, we also need to enable annotation based configuration in spring bean configuration file. This can be done by context:annotation-config element or by defining a bean of type org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor .
- @Qualifier annotation - This annotation is used to avoid conflicts in bean mapping and we need to provide the bean name that will be used for autowiring. This way we can avoid issues where multiple beans are defined for same type. This annotation usually works with the @Autowired annotation. For constructors with multiple arguments, we can use this annotation with the argument names in the method.
Spring @Autowired Annotation - Maven Dependencies
For spring autowiring, we don’t need to add any additional dependencies. Our pom.xml file has spring framework core dependencies and looks like below.
Spring @Autowired Annotation - Model Bean
Let’s create a simple Java Bean, named Employee. This bean will have a single property with getter and setter methods. We will initialize this property value in the spring bean configuration file.
Spring @Autowired Annotation - Service Class
Let’s create our service class in which we will inject Employee bean through spring autowiring.
We will use the same service class for perform spring autowiring byName, byType and by constructor. The setter method will be used for spring autowiring byName and byType whereas constructor based injection will be used by constructor autowire attribute. When we use spring autowire byName or byType, default constructor is used. That’s why we have explicitly defined the default constructor for the EmployeeService bean.
Spring @Autowired Annotation - autowiring byType Example
Let’s create a separate class with Spring @Autowired annotation for autowiring byType.
Note that I have annotated both Employee variable and it’s setter method with Spring @Autowired annotation, however only one of these is sufficient for spring bean autowiring.
Spring @Autowired Annotation and @Qualifier Bean autowiring by constructor Example
Let’s create another service class where we will use @Autowired annotation for constructor based injection. We will also see @Qualifier annotation usage.
When this bean will be initialized by Spring framework, bean with name as “employee” will be used for autowiring. Spring @Autowired annotation excepts one argument “required” that is a boolean with default value as TRUE. We can define it to be “false” so that spring framework don’t throw any exception if no suitable bean is found for autowiring.
Spring @Autowired Annotation - Bean Configuration File
Spring bean configuration file is the main part of any spring application, let’s see how our spring bean configuration file looks and then we will look into each part of it.
Important points about spring bean configuration file are:
- beans element default-autowire is used to define the default autowiring method. Here I am defining the default autowiring method to be byName.
- beans element default-autowire-candidates is used to provide the pattern for bean names that can be used for autowiring. For simplicity I am allowing all the bean definitions to be eligible for autowiring, however if we can define some pattern for autowiring. For example, if we want only DAO bean definitions for autowiring, we can specify it as default-autowire-candidates="*DAO" .
- autowire-candidate="false" is used in a bean definition to make it ineligible for autowiring. It’s useful when we have multiple bean definitions for a single type and we want some of them not to be autowired. For example, in above spring bean configurations “employee1” bean will not be used for autowiring.
- autowire attribute byName, byType and constructor is self understood, nothing much to explain there.
- context:annotation-config is used to enable annotation based configuration support. Notice that employeeAutowiredByTypeService and employeeAutowiredByConstructorService beans don’t have autowire attributes.
Spring @Autowired Annotation - Test Program
Now that our spring application is ready with all types of spring autowiring, let’s write a simple test program to see if it works as expected or not.
The program is simple, we are just creating the spring application context and using it to get different beans and printing the employee name. When we run above application, we get following output.
As you can see that for autowire byName and byType, default no-args constructor is used to initialize the bean. For autowire by constructor, parameter based constructor is used. From the hashcode of all the variables, we have confirmed that all the spring beans are different objects and not referring to the same object. Since we removed “employee1” from the list of eligible beans for autowiring, there was no confusion in the bean mapping. If we remove autowire-candidate="false" from the “employee1” definition, we will get below error message when executing the above main method.
That’s all for the Spring @Autowired Annotation and Spring autowiring feature, please download the example project from below link and analyse it to learn more.
Download Spring Bean Autowiring Project
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Still looking for an answer?
Hi Pankaj, Thank you for the tutorial. I have little confusion i.e if I have 2 beans in .xml file with names “employee” and “employee1” and as per employeeAutowiredByTypeService bean, It would create a bean of EmployeeAutowiredByTypeService.class till this part I am clear. Once you are getting this employeeAutowiredByTypeService obj and calling for “autowiredByTypeService.getEmployee().getName()”, then in such case which object would come because both “employee” and “employee1” are of type “Employee” how to solve this kind of collision, should we use @Qualifier (“nameOfBean”) annotation or by default does it pick up anyone? Please help me with this query.
- Raghu Ram
Description Resource Location Path Type ClassPathXmlApplicationContext cannot be resolved to a type SpringMain.java line 13 /SpringBeanAutowiring/src/main/java/com/journaldev/spring/autowiring/main Java Problem I am not able to find issue . please check
I hope I can get answer. I’m trying to create multiple object in bean configuration but it throws me an error It throws an exception of a conflict. But it doesn’t give any error when I instantiate random named service objects autowiredByConstructorService = ctx.getBean(“employeeAutowiredByConstructorService”,EmployeeAutowiredByConstructorService.class); w
Hi when your page reload it ask for allow for show notification. I want to implement the same on my website. My website is built on spring and jsp. Please help me to do. Thankyou
Above statement is wrong. It actually call the setter method. So if bean name is employee, it will call setEmployee method. Even if you change the property name to emp, it will work. Usually we make sure property name and bean name is same. Then just follow the naming convention of setter method. So it works and give impression that property name and bean name should be same.
If you write the configure your beans via javaConfig.java, instead of using .xml file that would be better.
- SprinfDev
Its nice explanation.
clean explanation sir
Thanks, nice post
- Binh Nguyen
how can i integrate webserices with spring framework
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Is there any other way I could wire the final field? java · spring · autowired · spring-annotations · Share.
We can resolve dependencies by field injection by annotating an instance variable with the @Resource annotation. 2.1.1. Match by Name. We'll use
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In Spring Framework, you can basically use any of the three annotations for Dependency Injection, namely @Autowired, @Resource and @Inject.
Spring supports Java SE Common Annotations (JSR-250). That means, we can use @Resource instead of using the combination of @Autowire and
You can apply @Autowired to constructors as well. A constructor @Autowired annotation indicates that the constructor should be autowired when creating the bean
AnnotationConfigApplicationContext constructor takes Class as argument that will be used to get the bean implementation to inject in component
The setter method will be used for spring autowiring byName and byType whereas constructor based injection will be used by constructor autowire