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:




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@Component("fooFormatter")

public class FooFormatter {



    public String format() {

        return "foo";

    }

}









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@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:




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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:




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public class FooService {



    private FooFormatter fooFormatter;



    @Autowired

    public FooService(FooFormatter fooFormatter) {

        this.fooFormatter = fooFormatter;

    }

}