CODEWITHSUNDEEP
javaspringgetter
Arun Raaj
Arun Raaj

Reputation: 1798

What does getBean() method do here?

What is the getBean() method doing here and how does it work in a program? 

ApplicationContext aplicntxt = new 
ClassPathXmlApplicationContext("springconfig.xml");  

Hello h = (Hello) aplicntxt.getBean("springconfig.xml");  
h.display();

Hello h2 = new Hello();  //if I write this
h2.display();

My question is why h2.display retrieves null value and h.display retrieves the stored values through springconfig.xml?

Please tell me what does 

ApplicationContext aplicntxt = new ClassPathXmlApplicationContext("springconfig.xml");

do first?

Are all the values of xml file stored to the pojo class setters at first step?

Then we are storing the values to an object h by doing 

Hello h = (Hello) aplicntxt.getBean("springconfig.xml");

Upvotes: 3

Views: 32392

Answers (4)

bananas
bananas

Reputation: 1196

From spring doc

The interface org.springframework.context.ApplicationContext represents the Spring IoC container and is responsible for instantiating, configuring, and assembling the aforementioned beans. The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java code. It allows you to express the objects that compose your application and the rich interdependencies between such objects.

Several implementations of the ApplicationContext interface are supplied out-of-the-box with Spring. In standalone applications it is common to create an instance of ClassPathXmlApplicationContext or FileSystemXmlApplicationContext. While XML has been the traditional format for defining configuration metadata you can instruct the container to use Java annotations or code as the metadata format by providing a small amount of XML configuration to declaratively enable support for these additional metadata formats.

Now Your Question and its simple ApplicationContext activates the object(it is eager container) and looks for the beans declared so the objects are loaded whenever it is called.

Now consider if you have two beans , and you need one of them you will find that bean by using ctx.getBean("beanId") to load instance and to provide data declared with this bean where beanId will tell which object to load.

consider following example

<bean id="a" class="package.Hello" /> //here it will look for bean name and then loads the class

Now when You call it like

ApplicationContext ctx=new ClassPathXMLApplicationContext("configuration.xml");

//will look for configuration.xml bean file if it is not in path then throw execption.

now 

Hello hello=ctx.getBean("a");

// if configuration.xml contains any bean named "a" and holds reference to class(hello) load it immediately and return object of that class

is same as 

Hello hello=new Hello();
hello.method();

and it is creating object of Hello class by using xml

Upvotes: 1

Adam
Adam

Reputation: 36703

Your question is essentially "How does spring work", this is covered extensively by the official documentation

The following creates all the beans defined by the springconfig.xml, that is it creates objects of the given types, and injects any properties you've defined, depending on your exact configuration it may also do things like package scanning, annotation processing etc.

ApplicationContext aplicntxt= new ClassPathXmlApplicationContext("springconfig.xml");

XML

<bean class="org.example.Hello" id="foo" />
<bean class="org.example.Hello" id="bar" />

This would create an objects of type Hello and tag them with the IDs "foo" and "bar"

All the beans are stored against their IDs for later retrieval via getBean(), note this takes the bean ID or name, not the XML file. 

Hello h = (Hello) aplicntxt.getBean("foo");

Upvotes: 4

Mohammadreza Khatami
Mohammadreza Khatami

Reputation: 1332

What you are doing in code is called Spring Dependency Injection which let you define application beans and inject them when you need. like getBean() method that you use in your code which inject specific bean from XML file.

Upvotes: 2

jencoston
jencoston

Reputation: 1342

From looking at this quickly, it appears that this code uses spring to initialize the Hello object with the values specified in the spring bean found in the xml file (I'm assuming that's what's in the filr, but I could be more specific if you post it).. When you create a second hello object you are using the constructor's default value for display, which is null.

Upvotes: 1

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