Wednesday, August 14, 2019

Dependency Injection in Java

1.1. What is dependency injection?

Dependency injection is a concept valid for any programming language. The general concept behind dependency injection is called Inversion of Control. According to this concept a class should not configure its dependencies statically but should be configured from the outside.
A Java class has a dependency on another class, if it uses an instance of this class. We call this a _class dependency. For example, a class which accesses a logger service has a dependency on this service class.
Ideally Java classes should be as independent as possible from other Java classes. This increases the possibility of reusing these classes and to be able to test them independently from other classes.
If the Java class creates an instance of another class via the new operator, it cannot be used (and tested) independently from this class and this is called a hard dependency. The following example shows a class which has no hard dependencies.
package com.vogella.tasks.ui.parts;

import java.util.logging.Logger;

public class MyClass {

    private Logger logger;

    public MyClass(Logger logger) {
        this.logger = logger;
        // write an info log message
        logger.info("This is a log message.")
    }
}
Please note that this class is just a normal Java class, there is nothing special about it, except that it avoids direct object creation.
A framework class, usually called the dependency container, could analyze the dependencies of this class. With this analysis it is able to create an instance of the class and inject the objects into the defined dependencies, via Java reflection.
This way the Java class has no hard dependencies, which means it does not rely on an instance of a certain class. This allows you to testyour class in isolation, for example by using mock objects.
Mock objects (mocks) are objects which behave similar as the real object. But these mocks are not programmed; they are configured to behave in a certain predefined way. Mock is an English word which means to mimic or to imitate.
If dependency injection is used, a Java class can be tested in isolation.

1.2. Using annotations to describe class dependencies

Different approaches exist to describe the dependencies of a class. The most common approach is to use Java annotations to describe the dependencies directly in the class.
The standard Java annotations for describing the dependencies of a class are defined in the Java Specification Request 330 (JSR330). This specification describes the @Inject and @Named annotations.
The following listing shows a class which uses annotations to describe its dependencies.
// import statements left out

public class MyPart {

    @Inject private Logger logger;

    // inject class for database access
    @Inject private DatabaseAccessClass dao;

    @Inject
    public void createControls(Composite parent) {
        logger.info("UI will start to build");
        Label label = new Label(parent, SWT.NONE);
        label.setText("Eclipse 4");
        Text text = new Text(parent, SWT.NONE);
        text.setText(dao.getNumber());
    }

}
Please note that this class uses the new operator for the user interface components. This implies that this part of the code is nothing you plan to replace via your tests. In this case you made the decision to have a hard coupling to the corresponding user interface toolkit.

1.3. Where can objects be injected into a class according to JSR330?

Dependency injection can be performed on:
  • the constructor of the class (construction injection)
  • a field (field injection)
  • the parameters of a method (method injection)
It is possible to use dependency injection on static and on non-static fields and methods. Avoiding dependency injection on static fields and methods is a good practice, as it has the following restrictions and can be hard to debug.
  • Static fields will be injected after the first object of the class was created via DI, which means no access to the static field in the constructor
  • Static fields can not be marked as final, otherwise the compiler or the application complains at runtime about them
  • Static methods are called only once after the first instance of the class was created

1.4. Order in which dependency injection is performed on a class

According to JSR330 the injection is done in the following order:
  • constructor injection
  • field injection
  • method injection
The order in which the methods or fields annotated with @Inject are called is not defined by JSR330. You cannot assume that the methods or fields are called in the order of their declaration in the class.
As fields and method parameters are injected after the constructor is called, you cannot use injected member variables in the constructor.

2. Java and dependency injection frameworks

You can use dependency injection without any additional framework by providing classes with sufficient constructors or getter and setter methods.
A dependency injection framework simplifies the initialization of the classes with the correct objects.
Two popular dependency injection frameworks are Spring and Google Guice.
The usage of the Spring framework for dependency injection is described in Dependency Injection with the Spring Framework - Tutorial.
Also Eclipse 4 is using dependency injection.

2. Spring Overview

The Spring Framework is a very comprehensive framework. The fundamental functionality provided by the Spring Container is dependency injection. Spring provides a light-weight container, e.g. the Spring core container, for dependency injection (DI). This container lets you inject required objects into other objects. This results in a design in which the Java class are not hard-coupled. The injection in Spring is either done via setter injection of via construction injection. These classes which are managed by Spring must conform to the JavaBean standard. In the context of Spring classes are also referred to as beans or as Spring beans.
The Spring core container:
  • handles the configuration, generally based on annotations or on an XML file (XMLBeanFactory)
  • manages the selected Java classes via the BeanFactory
The core container uses the so-called bean factory to create new objects. New objects are generally created as Singletons if not specified differently.

3. Spring Installation

Download Spring from http://www.springframework.org/download. Select the -with-dependencies.zip to get also all required plugins. At the time of writing I downloaded the version Spring Framework 2.5.5.
The folder "dist" contains the Spring container "spring.jar". The folder lib contains additional require libraries. A minimal Spring application requires the spring.jar, commons-logging.jar (from \lib\jakarta-commons) and log4j*.jar (from \lib\log4j).

4. Datamodel

We will later use the following datamodel for the example.
Create a Java project "de.vogella.spring.di.model" and create the following packages and classes.
package writer;

public interface IWriter {
    public void writer(String s);
}
package writer;

public class Writer implements IWriter {
    public void writer (String s){
        System.out.println(s);
    }
}
package writer;

public class NiceWriter implements IWriter {
    public void writer (String s){
        System.out.println("The string is " + s);
    }
}
package testbean;

import writer.IWriter;

public class MySpringBeanWithDependency {
    private IWriter writer;

    public void setWriter(IWriter writer) {
        this.writer = writer;
    }

    public void run() {
        String s = "This is my test";
        writer.writer(s);
    }
}
The class "MySpringBeanWithDependency.java" contains a setter for the actual writer. We will use the Spring Framework to inject the correct writer into this class.

5. Using dependency injection with annotations

As of Spring 2.5 it is possible to configure the dependency injection via annotations. I recommend to use this way of configuring your Spring beans. The next chapter will also describe the way to configure this via XML. Create a new Java project "de.vogella.spring.di.annotations.first" and include the minimal required spring jars into your classpath. Copy your model class from the de.vogella.spring.di.model project into this project. You need now to add annotations to your model to tell Spring which beans should be managed by Spring and how they should be connected. Add the @Service annotation the MySpringBeanWithDependency.java and NiceWriter.java. Also define with @Autowired on the setWriter method that the property "writer" will be autowired by Spring.
@Autowired will tell Spring to search for a Spring bean which implements the required interface and place it automatically into the setter.
package testbean;

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

import writer.IWriter;

@Service
public class MySpringBeanWithDependency {
    private IWriter writer;

    @Autowired
    public void setWriter(IWriter writer) {
        this.writer = writer;
    }

    public void run() {
        String s = "This is my test";
        writer.writer(s);
    }
}
package writer;

import org.springframework.stereotype.Service;

@Service
public class NiceWriter implements IWriter {
    public void writer(String s) {
        System.out.println("The string is " + s);
    }
}
Under the src folder create a folder META-INF and create the following file in this folder. This is the Spring configuration file.
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:component-scan base-package="testbean" />
    <context:component-scan base-package="writer" />

</beans>
You can also configure the log4j logger (this is optional) by copying the following file into the source folder.
log4j.rootLogger=FATAL, first
log4j.appender.first=org.apache.log4j.ConsoleAppender
log4j.appender.first.layout=org.apache.log4j.PatternLayout
log4j.appender.first.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n
Afer this setup you can wire the application together. Create a main class which reads the configuration file and starts the application.
package main;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import testbean.MySpringBeanWithDependency;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "META-INF/beans.xml");
        BeanFactory factory = context;
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) factory
                .getBean("mySpringBeanWithDependency");
        test.run();
    }
}
If you run the application then the class for the IWriterInterface will be inserted into the Test class. By applying the dependency injecting I can later replace this writer with a more sophisticated writer. As a result the class Test does not depend on the concrete Writer class, is extensible and can be easily tested.

6. Using dependency injection with XML

The following example will demonstrate the usage of the dependency injection via xml. The example will inject a writer into another class.
I think annotations rock in general, therefore I recommend not to use the XML configuration but the annotation one. If you have good reason to use the XML configuration please feel free to do so.
Create a new Java project "de.vogella.spring.di.xml.first" and include the minimal required spring jars into your classpath. Copy your model class from the de.vogella.spring.di.model project into this project. Under the src folder create a folder META-INF and create the following file in this folder. This is the Spring configuration file.
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">


<bean id="writer" class="writer.NiceWriter" />

<bean id="mySpringBeanWithDependency" class="testbean.MySpringBeanWithDependency">
<property name="writer" ref="writer" />
</bean>

</beans>
Again, you can now wire the application together. Create a main class which reads the configuration file and starts the application.
package main;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import testbean.MySpringBeanWithDependency;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "META-INF/beans.xml");
        BeanFactory factory = context;
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) factory
                .getBean("mySpringBeanWithDependency");
        test.run();
    }
}

3.1. @Autowired on Properties

The annotation can be used directly on properties, therefore eliminating the need for getters and setters:
1
2
3
4
5
6
7
@Component("fooFormatter")
public class FooFormatter {
    public String format() {
        return "foo";
    }
}
1
2
3
4
5
6
7
@Component
public class FooService {
     
    @Autowired
    private FooFormatter fooFormatter;
}
In the above example, Spring looks for and injects fooFormatter when FooService is created.

3.2. @Autowired on Setters

The @Autowired annotation can be used on setter methods. In the below example, when the annotation is used on the setter method, the setter method is called with the instance of FooFormatter when FooService is created:
1
2
3
4
5
6
7
8
9
public class FooService {
    private FooFormatter fooFormatter;
    @Autowired
    public void setFooFormatter(FooFormatter fooFormatter) {
            this.fooFormatter = fooFormatter;
    }
}

3.3. @Autowired on Constructors

The @Autowired annotation can also be used on constructors. In the below example, when the annotation is used on a constructor, an instance of FooFormatter is injected as an argument to the constructor when FooService is created:
1
2
3
4
5
6
7
8
9
public class FooService {
    private FooFormatter fooFormatter;
    @Autowired
    public FooService(FooFormatter fooFormatter) {
        this.fooFormatter = fooFormatter;
    }
}

28 comments:

Priyanka said...

Attend The Data Analytics Courses in Bangalore with Placement From ExcelR. Practical Data Analytics Courses in Bangalore with Placement Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Data Analytics Courses in Bangalore with Placement.
ExcelR Data Analytics Courses in Bangalore with Placement

seoexpert said...

Nice Post...I have learn some new information.thanks for sharing.
Data Analytics Courses in Mumbai

Priyanka said...

Attend The Machine Learning courses in Bangalore From ExcelR. Practical Machine Learning courses in Bangalore Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Machine Learning courses in Bangalore.
ExcelR Machine Learning courses in Bangalore

rajeev said...

Oracle Apps R12 and Fusion Cloud Self Paced Training Videos by Industry Experts. Please Check https://www.oracleappstechnical.com

Priyanka said...

Attend The PMP Certification From ExcelR. Practical PMP Certification Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The PMP Certification.
ExcelR PMP Certification

Priyanka said...

Attend The Data Analytics Course in Bangalore From ExcelR. Practical Data Analytics Course in Bangalore Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Data Analytics Course in Bangalore.
ExcelR Data Analytics Course in Bangalore

Neha Khan said...

BA Exam Result - BA 1st Year, 2nd Year and 3rd Year Result

Bsc Exam Result - Bsc 1st Year, 2nd Year and 3rd Year Result

Priyanka said...

Attend The Data Analytics Course in Bangalore with Placement From ExcelR. Practical Data Analytics Course in Bangalore with Placement Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Data Analytics Course in Bangalore with Placement.
ExcelR Data Analytics Course in Bangalore with Placement

digitaltucr said...


I am a new user of this site so here i saw multiple articles and posts posted by this site,I curious more interest in some of them hope you will give more information on this topics in your next articles.
ExcelR business analytics courses

Priyanka said...

Attend The PMP Certification From ExcelR. Practical PMP Certification Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The PMP Certification.
ExcelR PMP Certification

priyanka said...

Attend The Analytics Training Institute From ExcelR. Practical Analytics Training Institute Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Analytics Training Institute.
ExcelR Analytics Training Institute

ravali said...

Cool stuff you have and you keep overhaul every one of us
ExcelR data science course in mumbai

ravali said...

This is a wonderful article, Given so much info in it, These type of articles keeps the users interest in the website, and keep on sharing more ... good luck.
data science course in mumbai

ravali said...

Great post i must say and thanks for the information. Education is definitely a sticky subject. However, is still among the leading topics of our time. I appreciate your post and look forward to more.
data science course in mumbai

Jagna Co Kalani said...

Great Article
Data Mining Projects


Python Training in Chennai

Project Centers in Chennai

Python Training in Chennai

ravali said...

Excellent Blog! I would like to thank for the efforts you have made in writing this post. I am hoping the same best work from you in the future as well. I wanted to thank you for this websites! Thanks for sharing. Great websites!
data analytics courses

ravali said...

Such a very useful article. Very interesting to read this article.I would like to thank you for the efforts you had made for writing this awesome article.
data analytics courses

ravali said...

Great post i must say and thanks for the information. Education is definitely a sticky subject. However, is still among the leading topics of our time. I appreciate your post and look forward to more.
data analytics courses

ravali said...

Great post i must say and thanks for the information. Education is definitely a sticky subject. However, is still among the leading topics of our time. I appreciate your post and look forward to more.
data analytics course mumbai

ravali said...

I have to search sites with relevant information on given topic and provide them to teacher our opinion and the article.
data analytics course mumbai

shakeel1995 said...

Thanks for the post...
best salesforce training in hyderabad
devops training
full stack network engineer training

AI Course in Malaysia said...

I am really enjoying reading your well written articles. It looks like you spend a lot of effort and time on your blog. I have bookmarked it and I am looking forward to reading new articles. Keep up the good work.



ai course in malaysia

tableau course said...

If your looking for Online Illinois license plate sticker renewals then you have need to come to the right place.We offer the fastest Illinois license plate sticker renewals in the state.


https://360digitmg.com/course/data-visualization-using-tableau

Shanthi Cabs said...

Nice blog, it’s so knowledgeable, informative, and good looking site. I appreciate your hard work. Good job. Thank you for this wonderful sharing with us. Keep Sharing.
Tours and Travels in Madurai | Best tour operators in Madurai
Best travel agency in Madurai | Best Travels in Madurai



Unknown said...

Enjoyed reading this article throughout.Nice post! IoT is the trendy course right now and is going to be in
a great demand in near future as jobs for this domain will be sky rocketted.To be on par with the current trend we have to
gain complete knowledge about the subject. For the complete course online
360Digitmg Iot Certification Training

sureshshetty said...

Thanks for this blog are more informative contents step by step. I here attached my site would you see this blog.

7 tips to start a career in digital marketing

“Digital marketing is the marketing of product or service using digital technologies, mainly on the Internet, but also including mobile phones, display advertising, and any other digital medium”. This is the definition that you would get when you search for the term “Digital marketing” in google. Let’s give out a simpler explanation by saying, “the form of marketing, using the internet and technologies like phones, computer etc”.

we have offered to the advanced syllabus course digital marketing for available join now.

more details click the link now.

https://www.webdschool.com/digital-marketing-course-in-chennai.html

sureshshetty said...

Amazing article useful information.

Web designing trends in 2020

When we look into the trends, everything which is ruling today’s world was once a start up and slowly begun getting into. But Now they have literally transformed our lives on a tremendous note. To name a few, Facebook, Whats App, Twitter can be a promising proof for such a transformation and have a true impact on the digital world.

we have offered to the advanced syllabus course web design and development for available join now.

more details click the link now.

https://www.webdschool.com/web-development-course-in-chennai.html

Banu Ravi said...


I really enjoyed while reading your article and it is good to know the latest updates. Do post more.
Salesforce Training in Chennai
salesforce developer training in chennai
salesforce certification in chennai
Angular 5 Training in Chennai
ccna course in Chennai
React JS Training in Chennai
Tally Training in Chennai
Salesforce Training in Anna Nagar
Salesforce Training in Tambaram
Salesforce Training in Velachery