Category: Java

Spring Dependency Injection – Quick Guide ”; Previous Next Spring DI – Overview Spring is the most popular application development framework for enterprise Java. Millions of developers around the world use Spring Framework to create high performing, easily testable, and reusable code. Spring framework is an open source Java platform. It was initially written by Rod Johnson and was first released under the Apache 2.0 license in June 2003. Spring provides Ioc Containers which tend to be lightweight, especially when compared to EJB containers, for example. This is beneficial for developing and deploying applications on computers with limited memory and CPU resources. Dependency Injection (DI) The technology that Spring is most identified with is the Dependency Injection (DI) flavor of Inversion of Control. The Inversion of Control (IoC) is a general concept, and it can be expressed in many different ways. Dependency Injection is merely one concrete example of Inversion of Control. When writing a complex Java application, application classes should be as independent as possible of other Java classes to increase the possibility to reuse these classes and to test them independently of other classes while unit testing. Dependency Injection helps in gluing these classes together and at the same time keeping them independent. What is dependency injection exactly? Let”s look at these two words separately. Here the dependency part translates into an association between two classes. For example, class A is dependent of class B. Now, let”s look at the second part, injection. All this means is, class B will get injected into class A by the IoC. Dependency injection can happen in the way of passing parameters to the constructor or by post-construction using setter methods. As Dependency Injection is the heart of Spring Framework, we will explain this concept in a separate chapter with relevant example. Spring DI – Environment Setup This chapter will guide you on how to prepare a development environment to start your work with Spring Framework. It will also teach you how to set up JDK, Maven and Eclipse on your machine before you set up Spring Framework − Setup Java Development Kit (JDK) You can download the latest version of SDK from Oracle”s Java site − Java SE Downloads. You will find instructions for installing JDK in downloaded files, follow the given instructions to install and configure the setup. Finally set PATH and JAVA_HOME environment variables to refer to the directory that contains java and javac, typically java_install_dir/bin and java_install_dir respectively. If you are running Windows and have installed the JDK in C:jdk-11.0.11, you would have to put the following line in your C:autoexec.bat file. set PATH=C:jdk-11.0.11;%PATH% set JAVA_HOME=C:jdk-11.0.11 Alternatively, on Windows NT/2000/XP, you will have to right-click on My Computer, select Properties → Advanced → Environment Variables. Then, you will have to update the PATH value and click the OK button. On Unix (Solaris, Linux, etc.), if the SDK is installed in /usr/local/jdk-11.0.11 and you use the C shell, you will have to put the following into your .cshrc file. setenv PATH /usr/local/jdk-11.0.11/bin:$PATH setenv JAVA_HOME /usr/local/jdk-11.0.11 Alternatively, if you use an Integrated Development Environment (IDE) like Borland JBuilder, Eclipse, IntelliJ IDEA, or Sun ONE Studio, you will have to compile and run a simple program to confirm that the IDE knows where you have installed Java. Otherwise, you will have to carry out a proper setup as given in the document of the IDE. Setup Eclipse IDE All the examples in this tutorial have been written using Eclipse IDE. So we would suggest you should have the latest version of Eclipse installed on your machine. To install Eclipse IDE, download the latest Eclipse binaries from www.eclipse.org/downloads. Once you download the installation, unpack the binary distribution into a convenient location. For example, in C:eclipse on Windows, or /usr/local/eclipse on Linux/Unix and finally set PATH variable appropriately. Eclipse can be started by executing the following commands on Windows machine, or you can simply double-click on eclipse.exe %C:eclipseeclipse.exe Eclipse can be started by executing the following commands on Unix (Solaris, Linux, etc.) machine − $/usr/local/eclipse/eclipse After a successful startup, if everything is fine then it should display the following result − Set Maven In this tutorial, we are using maven to run and build the spring based examples. Follow the Maven – Environment Setup to install maven. Spring DI – IoC Containers The Spring container is at the core of the Spring Framework. The container will create the objects, wire them together, configure them, and manage their complete life cycle from creation till destruction. The Spring container uses DI to manage the components that make up an application. These objects are called Spring Beans, which we will discuss in the next chapter. The container gets its instructions on what objects to instantiate, configure, and assemble by reading the configuration metadata provided. The configuration metadata can be represented either by XML, Java annotations, or Java code. The following diagram represents a high-level view of how Spring works. The Spring IoC container makes use of Java POJO classes and configuration metadata to produce a fully configured and executable system or application. Spring provides the following two distinct types of containers. Sr.No. Container & Description 1 Spring BeanFactory Container This is the simplest container providing the basic support for DI and is defined by the org.springframework.beans.factory.BeanFactory interface. The BeanFactory and related interfaces, such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in Spring for the purpose of backward compatibility with a large number of third-party frameworks that integrate with Spring. 2 Spring ApplicationContext Container This container adds more enterprise-specific functionality such as the ability to resolve textual messages from a properties file and

Spring DI – Setter-Based ”; Previous Next Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean. Example The following example shows a class TextEditor that can only be dependency-injected using pure setter-based injection. Let”s update the project created in Spring DI – Create Project chapter. We”re adding following files − TextEditor.java − A class containing a SpellChecker as dependency. SpellChecker.java − A dependency class. MainApp.java − Main application to run and test. Here is the content of TextEditor.java file − package com.tutorialspoint; public class TextEditor { private SpellChecker spellChecker; // a setter method to inject the dependency. public void setSpellChecker(SpellChecker spellChecker) { System.out.println(“Inside setSpellChecker.” ); this.spellChecker = spellChecker; } // a getter method to return spellChecker public SpellChecker getSpellChecker() { return spellChecker; } public void spellCheck() { spellChecker.checkSpelling(); } } Here you need to check the naming convention of the setter methods. To set a variable spellChecker we are using setSpellChecker() method which is very similar to Java POJO classes. Let us create the content of another dependent class file SpellChecker.java − package com.tutorialspoint; public class SpellChecker { public SpellChecker(){ System.out.println(“Inside SpellChecker constructor.” ); } public void checkSpelling() { System.out.println(“Inside checkSpelling.” ); } } Following is the content of the MainApp.java file package com.tutorialspoint; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class MainApp { public static void main(String[] args) { ApplicationContext context = new ClassPathXmlApplicationContext(“applicationcontext.xml”); TextEditor te = (TextEditor) context.getBean(“textEditor”); te.spellCheck(); } } Following is the configuration file applicationcontext.xml which has configuration for the setter-based injection − <?xml version = “1.0” encoding = “UTF-8”?> <beans xmlns = “http://www.springframework.org/schema/beans” xmlns:xsi = “http://www.w3.org/2001/XMLSchema-instance” xsi:schemaLocation = “http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd”> <!– Definition for textEditor bean –> <bean id = “textEditor” class = “com.tutorialspoint.TextEditor”> <property name = “spellChecker” ref = “spellChecker”/> </bean> <!– Definition for spellChecker bean –> <bean id = “spellChecker” class = “com.tutorialspoint.SpellChecker”></bean> </beans> You should note the difference in applicationcontext.xml file defined in the constructor-based injection and the setter-based injection. The only difference is inside the <bean> element where we have used <constructor-arg> tags for constructor-based injection and <property> tags for setter-based injection. The second important point to note is that in case you are passing a reference to an object, you need to use ref attribute of <property> tag and if you are passing a value directly then you should use value attribute. Output Once you are done creating the source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message − Inside SpellChecker constructor. Inside setSpellChecker. Inside checkSpelling. Print Page Previous Next Advertisements ”;

Spring Dependency Injection ”; Previous Next Every Java-based application has a few objects that work together to present what the end-user sees as a working application. When writing a complex Java application, application classes should be as independent as possible of other Java classes to increase the possibility to reuse these classes and to test them independently of other classes while unit testing. Dependency Injection (or sometime called wiring) helps in gluing these classes together and at the same time keeping them independent. Consider you have an application which has a text editor component and you want to provide a spell check. Your standard code would look something like this − public class TextEditor { private SpellChecker spellChecker; public TextEditor() { spellChecker = new SpellChecker(); } } What we”ve done here is, create a dependency between the TextEditor and the SpellChecker. In an inversion of control scenario, we would instead do something like this − public class TextEditor { private SpellChecker spellChecker; public TextEditor(SpellChecker spellChecker) { this.spellChecker = spellChecker; } } Here, the TextEditor should not worry about SpellChecker implementation. The SpellChecker will be implemented independently and will be provided to the TextEditor at the time of TextEditor instantiation. This entire procedure is controlled by the Spring Framework. Here, we have removed total control from the TextEditor and kept it somewhere else (i.e. XML configuration file) and the dependency (i.e. class SpellChecker) is being injected into the class TextEditor through a Class Constructor. Thus the flow of control has been “inverted” by Dependency Injection (DI) because you have effectively delegated dependances to some external system. The second method of injecting dependency is through Setter Methods of the TextEditor class where we will create a SpellChecker instance. This instance will be used to call setter methods to initialize TextEditor”s properties. Thus, DI exists in two major variants and the following two sub-chapters will cover both of them with examples − Sr.No. Dependency Injection Type & Description 1 Constructor-based dependency injection Constructor-based DI is accomplished when the container invokes a class constructor with a number of arguments, each representing a dependency on the other class. 2 Setter-based dependency injection Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean. You can mix both, Constructor-based and Setter-based DI but it is a good rule of thumb to use constructor arguments for mandatory dependencies and setters for optional dependencies. The code is cleaner with the DI principle and decoupling is more effective when objects are provided with their dependencies. The object does not look up its dependencies and does not know the location or class of the dependencies, rather everything is taken care by the Spring Framework. Constructor Based V/s Setter Based Dependency Injection. There are few noteworthy differences between constructor based and setter based dependency injection. Granular control − In case of a constructor accepting multiple values, we need to pass all the values during bean configuration while in case of setters, we can pass value in a particular setter method and can leave other setter unconfigured. Setter Overrides Constructor − In case of both constructor and setter based injection are used then setter based injection takes preference. Flexibility − Setter methods doesn”t create a new bean instance like constructor and their values can be changed as well. Print Page Previous Next Advertisements ”;

Spring DI – Inner Beans Setter ”; Previous Next As you know Java inner classes are defined within the scope of other classes, similarly, inner beans are beans that are defined within the scope of another bean. Thus, a <bean/> element inside the <property/> or <constructor-arg/> elements is called inner bean and it is shown below. <?xml version = “1.0” encoding = “UTF-8”?> <beans xmlns = “http://www.springframework.org/schema/beans” xmlns:xsi = “http://www.w3.org/2001/XMLSchema-instance” xsi:schemaLocation = “http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd”> <bean id = “outerBean” class = “…”> <property name = “target”> <bean id = “innerBean” class = “…”/> </property> </bean> </beans> Example The following example shows a class TextEditor that can only be dependency-injected using pure setter-based injection. Let”s update the project created in Spring DI – Create Project chapter. We”re adding following files − TextEditor.java − A class containing a SpellChecker as dependency. SpellChecker.java − A dependency class. MainApp.java − Main application to run and test. Here is the content of TextEditor.java file − package com.tutorialspoint; public class TextEditor { private SpellChecker spellChecker; // a setter method to inject the dependency. public void setSpellChecker(SpellChecker spellChecker) { System.out.println(“Inside setSpellChecker.” ); this.spellChecker = spellChecker; } // a getter method to return spellChecker public SpellChecker getSpellChecker() { return spellChecker; } public void spellCheck() { spellChecker.checkSpelling(); } } Following is the content of another dependent class file SpellChecker.java − package com.tutorialspoint; public class SpellChecker { public SpellChecker(){ System.out.println(“Inside SpellChecker constructor.” ); } public void checkSpelling(){ System.out.println(“Inside checkSpelling.” ); } } Following is the content of the MainApp.java file − package com.tutorialspoint; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class MainApp { public static void main(String[] args) { ApplicationContext context = new ClassPathXmlApplicationContext(“applicationcontext.xml”); TextEditor te = (TextEditor) context.getBean(“textEditor”); te.spellCheck(); } } Following is the configuration file applicationcontext.xml which has configuration for the setter-based injection but using inner beans − <?xml version = “1.0” encoding = “UTF-8”?> <beans xmlns = “http://www.springframework.org/schema/beans” xmlns:xsi = “http://www.w3.org/2001/XMLSchema-instance” xsi:schemaLocation = “http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd”> <!– Definition for textEditor bean using inner bean –> <bean id = “textEditor” class = “com.tutorialspoint.TextEditor”> <property name = “spellChecker”> <bean id = “spellChecker” class = “com.tutorialspoint.SpellChecker”/> </property> </bean> </beans> Output Once you are done creating the source and bean configuration files, let us run the application. If everything is fine with your application, it will print the following message − Inside SpellChecker constructor. Inside setSpellChecker. Inside checkSpelling. Print Page Previous Next Advertisements ”;