Sometimes, I find some class names including Aware such as ApplicationContextAware and MessageSourceAware (spring). Does this Aware have any special meanings or is it a famous rule?
Those are not classes, are interfaces. The name is just a convention on Spring meaning that some special framework object is going to be injected to that class if it is managed by the framework.
Directly from the docs of ApplicationContextAware:
Interface to be implemented by any object that wishes to be notified of the ApplicationContext that it runs in.
In this case, the bean that implements this interface will get a reference to the ApplicationContext managing the application.
Appending adjectives like "aware" to the end is a naming scheme often used for Java interfaces. This can then be implemented by classes, and the resulting is code which is more fluent to read for human beings, like
class Broker implements ApplicationContextAware { /* ... */ }
where it's quite easy to see that this class is a broker for something, and it knows how to deal with application contexts. Besides that, the "Aware" suffix has no special meaning from the Java (compiler) perspective.
The interfaces you cite seem to be a Spring-specific convention for allowing objects to interact with the dependency injection container that created them. By implementing the interface, a class signals that it wants certain information from the container and at the same time provides a method through which to pass that information.
I'd see it simply as an attempt to find a generic name for an interface that offers such functionality, not necessarily a strong convention with a specific technical meaning.
The concept of aware interfaces:
If I want the reference of objects of spring classes like XmlBeanFactory,ApplicationContext... In 2 or more classes, then there are 3 possible ways are there.
creating 2 BeanFactories in two classes.
creating at one class and sharing to all required classes .
In the 1st case ,we are unnecessarely creating 2 BeanFactories.
In the 2nd case, classes are tightly coupled with each other.
If our class implements BeanFactoryAware interface and overrides the contractual method called public BeanFactory setBeanFactory(BeanFactory factory) then IOC container see that special interface and calls setBeanFactory method by setting BeanFactory reference to that.
In 3. case above two problems are not there.
Related
I have two data source class, LocalDataSource and RemoteDataSource. In DataRepository, I need both the classes but on need basis. I don't want to inject both the classes in DataRepository constructor. Instead want to overload constructor with single data source class. How to implement this with dagger?
To directly answer the question: Dagger supports one #Inject constructor at most. You can use a #Provides method to call any constructor or factory method you'd like, but if you want Dagger to manage your dependencies it will only read the single constructor you specify with #Inject. That said, that's not quite the right solution here in any case.
If your class needs a LocalDataSource or a RemoteDataSource, and you don't know which one until runtime, inject a Provider<LocalDataSource> and Provider<RemoteDataSource> (or Lazy<LocalDataSource> and Lazy<RemoteDataSource>). These objects are inexpensive Dagger-implemented abstractions over your LocalDataSource and RemoteDataSource constructors, which Dagger automatically allows you to inject: For any T available in your graph, you can inject Provider<T> and Lazy<T> (or even Provider<Lazy<T>>, as described in the documentation).
In this way, Provider and Lazy can help you avoid creating either DataSource instance until you know which one you need.
The difference between them is that Lazy<T> will locally cache the object instance, acting like a field in your class. Provider<T> will always consult your Dagger Component, which will in most cases will return you a newly-allocated object unless you've applied a scope like #Singleton. If your objects are expensive to construct, Lazy is a good choice, since you'll only request one instance across the lifetime of your consumer; if they are cheap to construct, then you might even choose to use a Provider within a method and letting your object be garbage-collected as soon as it is no longer needed.
Of course, as Steven and AlphaOne point out in the comments, you might also consider a design where your DataRepository consumer accepts an arbitrary DataSource that you or Dagger can provide. This generally makes sense, as DataSource is likely a useful interface and DataRepository might not need to know implementation details of the source it consults. However, if you need to handle an arbitrary source specification at runtime, at some point you'll still need to inject both a #Local DataRepository and #Remote DataRepository (assuming you've created #Qualifier annotations #Local and #Remote), and at that point it makes even more sense to use a Provider/Lazy instead of creating both DataRepository objects and their respective DataSources.
I'm newly started reading about Java Beans and I had a question which was exactly same as this Topic's question. So I repeat The question:
in definition it is said "java bean encapsulates many objects into one object(Bean)."
1.What does "many objects" here mean?
and
2.How they are encapsulated into one object by java beans?
Edit:
From Java Beans Wikipedia:
in computing based on the Java Platform, JavaBeans are classes that encapsulate many objects into a single object (the bean).
Edit2:
all of classes have ability of having multiple attributes and fields.
If encapsulating of many objects means having multiple attributes and fields, I don't understand why they mentioned to this ability as a advantage of java bean class.
First to make it clear, every Class in Java extends the type Object. Something like String is also an Object.
The "many objects" is referring to how we can use different objects as fields within the bean. This creates a has-a relationship with the bean to your Objects.
For example, say we have this Bean:
public class YourBean implements java.io.Serializable {
private String s;
private ArrayList<String> list;
//Omitted rest of bean boilerplate
}
This example will contain two different Objects inside of it, the String s and the ArrayList<String> named list. You can add as many different Objects and primitives to your bean as you want.
To create the bean with a no-args constructor, you would then use:
YourBean bean = new YourBean();
And you can set and get the values of the Objects encapsulated within with:
ArrayList<String> yourList = new ArrayList<>();
bean.setList(yourList);
System.out.println(bean.getList());
You will be able to refer to all the Objects inside the bean this way by referencing the bean Object I named bean.
Additionally, you can create multiple of the same type of bean as well, so every time you make a new YourBean(), you will also be able to use all the Objects contained within.
This functionality is not unique to a Bean, you can do this in any Class, rather a Bean is a term used to describe a specific way you write some classes.
I recommend looking into Java Composition to learn when you should use a has-a relationship, rather than inheritance which is an is-a relationship.
We usually talk about Spring beans, but I think that's not what you're talking about. It seems to me that those JavaBeans are nothing but classes with multiple attributes and only getters/setters but whose constructor has zero arguments (and therefore it is mutable). As simple as that. The fact of encapsulating many objects is due to it having multiple attributes.
However, I have never referred to them as JavaBeans and I think the most similar concept I have ever worked with are the POJOs. I'm not sure if there is any difference, but the purpose looks the same.
If you ever talk about a bean in Java, I think anyone will think of a Spring bean. I suggest you not to use it in another context.
This is just my guessing. If I've said anything wrong please tell me.
JAVA Beans
The concept of JavaBeans was originally devised for Swing to facilitate the development of standalone GUI components, but the pattern has been repurposed for the land of Spring beans and back-end persistence with Hibernate
On the other hand, POJOs are simple java classes.
Another View:
Any POJO on having interaction with some third party become JAVA BEAN :)
Java Classes interacting with any ORM(say Hibernate)
Java Classes being used as Session Objects in EJB
As they say, "With Great Powers Comes Great Responsibilities" [excerpt from spiderman]
So our normal Pojos become JAVA BEANS :)
An article worth going through: https://mossgreen.github.io/Java-Bean-VS-Spring-Bean/
I'm having a hard time understanding why javabeans are necessary and why they didn't just place the javabean features directly into the object class(root class) in java?
My understanding is you turn an object(instance) into a java bean and that way you get all the benefits like serializable and so on for all the objects in the bean. But if that is the case, why even have a separate bean class for this, why not just have built into the root object class?
Or am I not understand this?
You are not understanding it correctly.
There is no actual Java class or interface that is a bean. It is merely a pattern, a convention.
The bean convention is basically that a class will publicly expose some or all of its properties via public getXxx and setXxx methods, where XXX is the name of the property.
Beans generally should be serializable, but any class can be serializable, and does not need to follow the bean convention.
Besides that answer by Grey: even if there would be a Bean interface or method - not every fruit is an apple!
Meaning: there are zillions of classes that are just fine NOT being beans.
Beyond that: serialization as java does it was often found to be not that helpful. There are good reasons why we mostly serialize into JSON nowadays. In other words: we are pretty happy now that not all classes implement Serializeable by default; because Object being a Bean.
Just to talk about the no-argument constructor part of the bean pattern: A class does have a no argument constructor by default, but as soon as you create another constructor, that effectively removes it, as you probably know. But if you were forced to always have a no-argument constructor, this would be really annoying if you had a class which had a final field which is assigned by an argument in the constructor. You would then need to have some no argument constructor which either assigns everything to null, or throws an exception to tell other developers not to use the no argument so, which is just ugly.
I started learning MVC with spring. I have heard lot of time Bean, that contains setter and getter. Model is basically what data flows around, and Pojo which is same as Bean. But I am really confused in all this term and all this look same to me can you please explain the exact difference among all of them.
JAVABEAN
POJO
MODEL
If you're using the MVC architecture then the Model represents your domain: means your entities and it's not a java related term.
Your Models are represented in Java as Java Beans (best practice in Java EE).
A Java Bean is a normal Java class which implements the Serializable interface and have a parameterless constructor and have getters and setters for each field.
However POJO is just a denomination for objects not bound by any restriction other than those forced by the Java Language Specification (Wikipeadia). This is just for conventions sake and it's not strictly related to the MVC architecture.
Note that Java beans are POJOs implementing the Serializable interface.
Only difference is bean can be serialized.
From Java docs - http://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html
Serializability of a class is enabled by the class implementing the java.io.Serializable interface. Classes that do not implement this interface will not have any of their state serialized or deserialized. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.
While model is different thing which is dealing with your business logic.
you can refere below link
Programming difference between POJO and Bean
As a supplement, it's necessary to describe the intention of each item.
As defined from wiki,
The term "POJO" initially denoted a Java object which does not follow
any of the major Java object models, conventions, or frameworks
Ideally speaking, a POJO is a Java object not bound by any restriction other than those forced by the Java Language Specification
Generally, a POJO doesn't dependent on any library, interface or annotation. Therefore, a POJO is more likely to be reused by different system.
Ok, so what is Java Bean and why we create this item?
The description from this link clarified it clear enough I think.
JavaBeans are classes that encapsulate many objects into a single
object (the bean). They are serializable, have a zero-argument
constructor, and allow access to properties using getter and setter
methods.
Why we want Jave beans to behave like this?
The class must have a public default constructor (with no arguments).
This allows easy instantiation within editing and activation
frameworks.
The class properties must be accessible using get, set, is (can be used for boolean properties instead of get), to and other methods (so-called accessor methods and mutator methods) according to a standard naming convention.
This allows easy automated inspection and updating of bean state
within frameworks, many of which include custom editors for various
types of properties. Setters can have one or more than one argument.
The class should be serializable.
This allows applications and frameworks to reliably save, store, and
restore the bean's state in a manner independent of the VM and of the
platform.
Generally, the model isn't compared with POJO or JaveBean cause it's quite a different item. Like what has mentioned by other answer, the model is generally a concept from MVC.
The model is the central component of the pattern. It is the
application's dynamic data structure, independent of the user
interface.[6] It directly manages the data, logic and rules of the
application.
As you can see, POJO or JavaBean can be at model layer in MVC pattern but model layer but there are a lot more stuff in model layer, for example, the logic and rules of the application.
I want to create a DAO class named BaseDAO that should have the JPA and JDBC capabilities in Spring. I mean, I want to extend JPADAOSupport and JDBCDAOSupport classes of spring in to my BaseDAO class. I am aware that multiple inheritance is not an option in Java.
I have created two separate Base classes like BaseJPADao and BaseJdbcDao extending the respective classes. Is it possible to have a single class to extend both? Is there any design pattern solving this issue. Please advise.
Why don't you have a DaoGateway bean having injected the actual JPA DAO and the JDBC DAO beans.
This gateway can then decide which DAO to delegate a given request (to JPA or to JDBC).
You should always favour composition vs inheritance when reusing functionalities.
no it is not. if it was possible, you would still have the same result as in
one class extending JPADAOSupport and JDBCDAOSupport, which you yourself say you know is not possible because multiple inheritance is impossible.
you can write to an interface, and provide two implementations, though.
This would be easy to do with delegation if they both had interface level access you want:
public class MyUberClass implements WhateverJPADAOSupportDoes, WhateverJDBCDAOSupportDoes {
private JPADAOSuport jpa;
private JDBCDAOSupport jdbc;
// now implement all methods specified by the interfaces on the class signature and delegate to their respective member
}
But it seems you want access to all of their public methods. As there is no interface for both you can do the same as above but it can't be of both types simultaneously. The language expressly denies you this.
Your only other option is to create an adapter interface that your code can rely on and then use the combination delegation. If you're hoping to have one class that you can just drop in as a substitution for both then the answer is you can't.