What is a JavaBean exactly

Question

I understood  I think  that a  Bean  is a Java class with properties and getters setters  As much as I understand  it is the equivalent of a C struct  Is that true   Also  is there a real syntactic difference between a bean and a regular class  Is there any special definition or an interface   Basically  why is there a term for this   Also what does the Serializable interface mean

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To understand JavaBean you need to notice the following:

JavaBean is conceptual stuff and can not represent a class of specific things

JavaBean is a development tool can be visualized in the operation of reusable software components

JavaBean is based on the Sun JavaBeans specification and can be reusable components. Its biggest feature is the re-usability.

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They are serializable  have a zero-argument constructor  and allow access to properties using getter and setter methods  The name  quot Bean quot  was given to encompass this standard  which aims to create reusable software components for Java  According to Wikipedia  The objects that form the backbone of your application and that are managed by the Spring IoC container are called beans  A bean is an object that is instantiated  assembled  and otherwise managed by a Spring IoC container  Otherwise  a bean is simply one of many objects in your application  According to Spring IoC

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A Java Bean is a Java class  conceptual  that should follow the following conventions   It should have a no-argument constructor  It should be serializable  It should provide methods to set and get the values of the properties  known as getter and setter methods   It is a reusable software component  It can encapsulate many objects into one object so that same object can be accessed from multiples places and is a step towards easy maintenance of code

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Java Beans are used for a less code and more work approach    Java Beans are used throughout Java EE as a universal contract for runtime discovery and access  For example  JavaServer Pages  JSP  uses Java Beans as data transfer objects between pages or between servlets and JSPs  Java EE s JavaBeans Activation Framework uses Java Beans for integrating support for MIME data types into Java EE  The Java EE Management API uses JavaBeans as the foundation for the instrumentation of resources to be managed in a Java EE environment  About Serialization  In object serialization an object can be represented as a sequence of bytes that includes the object s data as well as information about the object s type and the types of data stored in the object  After a serialized object has been written into a file  it can be read from the file and deserialized that is  the type information and bytes that represent the object and its data can be used to recreate the object in memory

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JavaBeans are Java classes which adhere to an extremely simple coding convention  All you have to do is to  implement the java io Serializable interface - to save the state of an object use a public empty argument constructor - to instantiate the object provide public getter setter methods - to get and set the values of private variables  properties

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Properties of JavaBeans  A JavaBean is a Java object that satisfies certain programming conventions     The JavaBean class must implement either Serializable or Externalizable  The JavaBean class must have a no-arg constructor All JavaBean properties must have public setter and getter methods All JavaBean instance variables should be private   Example of JavaBeans   Entity public class Employee implements Serializable       Id    private int id     private String name        private int salary        public Employee          public Employee String name  int salary          this name   name        this salary   salary          public int getId           return id          public void setId  int id           this id   id          public String getName           return name          public void setName  String name           this name   name          public int getSalary           return salary          public void setSalary  int salary           this salary   salary

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As per the Wikipedia   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   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     For more information follow this link

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Just a little background update on the bean concept   Many other answers actually have the what but not so much why of them  They were invented early on in Java as part of building GUIs   They followed patterns that were easy for tools to pull apart letting them create a properties panel so you could edit the attributes of the Bean   In general  the Bean properties represented a control on the screen  Think x y width height text     You can also think of it as a strongly typed data structure  Over time these became useful for lots of tools that used the same type of access  For example  Hibernate to persist data structures to the database  As the tools evolved  they moved more towards annotations and away from pulling apart the setter getter names   Now most systems don t require beans  they can take any plain old Java object with annotated properties to tell them how to manipulate them  Now I see beans as annotated property balls--they are really only useful for the annotations they carry  Beans themselves are not a healthy pattern   They destroy encapsulation by their nature since they expose all their properties to external manipulation and as they are used there is a tendency  by no means a requirement  to create code to manipulate the bean externally instead of creating code inside the bean  violates  quot don t ask an object for its values  ask an object to do something for you quot     Using annotated POJOs with minimal getters and no setters is much more OO restoring encapsulation and with the possibility of immutability  By the way  as all this stuff was happening someone extended the concept to something called Enterprise Java Beans   These are    different   and they are complicated enough that many people felt they didn t understand the entire Bean concept and stopped using the term   This is  I think  why you generally hear beans referred to as POJOs  since every Java object is a POJO this is technically OK  but when you hear someone say POJO they are most often thinking about something that follows the bean pattern

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In practice  Beans are just objects which are handy to use  Serializing them means to be able easily to persist them  store in a form that is easily recovered    Typical uses of Beans in real world    simple reusable objects POJO  Plain Old Java Objects  visual objects Spring uses Beans for objects to handle  for instance  User object that needs to be serialized in session  EJB  Enterprise Java Beans   more complex objects  like JSF Beans  JSF is old quite outdated technology  or JSP Beans   So in fact  Beans are just a convention   standard to expect something from a Java object that it would behave  serialization  and give some ways to change it  setters for properties  in a certain way   How to use them  is just your invention  but most common cases I enlisted above

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Spring  Bean annotation indicates that a method produces a bean to be managed by the Spring container  More reference  https   www concretepage com spring-5 spring-bean-annotation

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There s a term for it to make it sound special   The reality is nowhere near so mysterious  Basically  a  quot Bean quot    is a serializable object  that is  it implements java io Serializable  and does so correctly   that has  quot properties quot  whose getters and setters are just methods with certain names  like  say  getFoo   is the getter for the  quot Foo quot  property   and has a public zero-argument constructor  so it can be created at will and configured by setting its properties    As for Serializable   That is nothing but a  quot marker interface quot   an interface that doesn t declare any functions  that tells Java that the implementing class consents to  and implies that it is capable of   quot serialization quot  -- a process that converts an instance into a stream of bytes   Those bytes can be stored in files  sent over a network connection  etc   and have enough information to allow a JVM  at least  one that knows about the object s type  to reconstruct the object later -- possibly in a different instance of the application  or even on a whole other machine  Of course  in order to do that  the class has to abide by certain limitations   Chief among them is that all instance fields must be either primitive types  int  bool  etc    instances of some class that is also serializable  or marked as transient so that Java won t try to include them    This of course means that transient fields will not survive the trip over a stream   A class that has transient fields should be prepared to reinitialize them if necessary   A class that can not abide by those limitations should not implement Serializable  and  IIRC  the Java compiler won t even let it do so

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A bean is a Java class with method names that follow the Java Bean guidelines  also called design patterns  for properties  methods  and events   Thus  any public method of the bean class that is not part of a property definition is a bean method  Minimally  a Java class even with either a property as the sole member  of course  accompanying public getter and setter required   a public method as the sole member or just one public event listener registration method is a Java bean  Furthermore  the property can either be read-only property  has a getter method but no setter  or write-only property  has a setter method only   The Java bean needs to be a public class to be visible to any beanbox tool or container  The container must be able to instantiate it  thus  it must have a public constructor too  The JavaBeans specification doesn   t  require a bean to have a public zero-args constructor  explicit or default  for a container to instantiate it  If you could provide a file  with extension  ser  containing a serialized instance  a beanbox tool could use that file to instantiate a prototype bean  Otherwise  the bean must have a public zero-args constructor  either explicit or default    Once the bean is instantiated  the Java Bean API   java beans    can introspect it and call methods on it   If no class implementing the interface BeanInfo or extending a BeanInfo implementation   SimpleBeanInfo class  is available  the introspection involves using reflection  implicit introspection   to study the methods supported by a target bean and then apply simple design patterns the guidelines  to deduce from those methods what properties  events  and public methods are supported  If a class implementing the interface BeanInfo  for a bean Foo  it must be named FooBeanInfo  is available  the API bypasses implicit introspection and uses public methods  getPropertyDescriptor    getMethodDescriptors    getEventSetDescriptors     of this class to get the information  If a class extending SimpleBeanInfo is available  depending on which of the SimpleBeanInfo public methods  getPropertyDescriptor    getMethodDescriptors    getEventSetDescriptors      are overridden  it will use those overridden methods s  to get information  for a method that is not overridden  it   ll  default to the corresponding implicit introspection  A bean needs to be instantiated anyway even if no implicit introspection is carried on it  Thus  the requirement  of a public zeri-args constructor  But  of course  the Serializable or Externalizable interface isn   t necessary for it to be recognized  However the Java Bean specification says     We   d also like it to be    trivial    for the common case of a tiny Bean that simply wants to have its internal state saved and doesn   t want to think about it     So  all beans must implement Serializable or Externalizable interface   Overall  JavaBeans specification isn   t hard and fast about what constitutes a bean   Writing JavaBeans components is surprisingly easy  You don t need a special tool and you don t have to implement any interfaces  Writing beans is simply a matter of following certain coding conventions  All you have to do is make your class look like a bean     tools that use beans will be able to recognize and use your bean   Trivially  even the following class is a Java Bean   public class Trivial implements java io Serializable     Say  a bean constructor has some parameters  Suppose some are simple types  The container might not know what values to assign to them  even if it does  the resulting instance might not be reusable  It may make sense only if the user can configure  specify values  by say annotations or xml configuration files as in Spring beans  And suppose some parameters are class or interface types   Again  the container might not know what values to assign to it  It may make sense only if the user can configure  specify specific objects  by say annotations or xml configuration files  However  even in Spring  via xml configuration files   assigning specific objects  with string names  to constructor arguments    attribute or  element of constructor arguments is not typesafe it is basically like resource injection   Making references to other Spring beans called collaborators  via  element in a constructor argument element  is basically dependency injection and thus typesafe   Obviously  a dependency collaborator bean  might have a constructor with injected parameters  those injected dependency ies  might have a constructor with parameters and so on  In this scenario  ultimately  you would need some bean classes  e g  MyBean class  that the container can instantiate by simply calling new MyBean   before it can construct the other collaborating beans via dependency injection on constructors   thus  the requirement for the beans to have public zero-args constructor   Suppose  if a container doesn   t support dependency injection and or doesn   t allow assigning simple-type values to constructor via some annotations or xml config files as in Spring  bean constructors shouldn   t have parameters  Even a Spring beans application would need some beans to have public zero-args constructor  e g   in a scenario where your Spring application has no bean with just simple types as constructor arguments      JSF managed beans run in a web container  They can be configured either with  ManagedBean annotation or  with an application configuration resource file managed-bean xml   However  it supports injection via resource injection  not typesafe  only  not fit for injection on constructors  The JSF spec requires that managed beans must have a public zero-argument constructors   Further it says     As of version 2 3 of this specification  use of the managed bean facility as specified in this section is strongly discouraged  A better and more cohesively integrated solution for solving the same problem is to use Contexts and Dependency Injection  CDI   as specified in JSR-365   In other words  CDI managed beans to be used  which offers typesafe dependency injection on constructors akin to Spring beans   The CDI specification adopts the Managed Beans specification  which applies to all containers of the JEE platform  not just web tier  Thus  the web container needs to implement CDI specification    Here is an extract from the Managed Bean specification     Managed Beans are container-managed objects with minimal requirements  otherwise known under the acronym    POJOs     Plain Old Java Objects    they can be seen as a Java EE platform-enhanced version of the JavaBeans component model found on the Java SE platform     It won   t be missed by the reader that Managed Beans have a precursor in the homonymous facility found in the JavaServer Faces  JSF  technology   Managed Beans as defined in this specification represent a generalization of those found in JSF  in particular  Managed Beans can be used anywhere in a Java EE application  not just in web modules  For example  in the basic component model  Managed Beans must provide a no-argument constructor  but a specification that builds on Managed Beans  such as CDI  JSR-299   can relax that requirement and allow Managed Beans to provide constructors with more complex signatures  as long as they follow some well-defined rules   A Managed Bean must not be  a final class  an abstract class  a non-static inner class  A Managed Bean may not be serializable unlike a regular JavaBean component     Thus  the specification for Managed Beans  otherwise known as POJOs or POJO beans  allows extension as in CDI   The CDI specification re-defines managed beans as  When running in Java EE  A top-level Java class is a managed bean if it meets requirements       It is not an inner class      It is a non-abstract class  or is annotated  Decorator      It does not implement javax enterprise inject spi Extension      It is not annotated  Vetoed or in a package annotated  Vetoed      It has an appropriate constructor  either  the class has a constructor with    no parameters  or the class declares a constructor annotated  Inject   All Java classes that meet these conditions are managed beans and thus no special declaration is required to define a managed bean  Or  if it is defined to be a managed bean by any other Java EE specification and if      It is not annotated with an EJB component-defining annotation or declared as an EJB bean class in ejb-jar xml   Unlike Spring beans it doesn   t support constructors with simple-types  which might be possible if it supported configuration with xml config files like in Spring or any annotations    EJBs run in an EJB container   Its specification says     A session bean component is a Managed Bean      The class must have a public constructor that takes no arguments     it says for both session bean and message-driven bean   Further  it says     The session bean class is not required to implement the SessionBean interface or the Serializable interface     For the same reason as JSF beans  that EJB3 dependency injection is basically resource injection  JSF beans do not support constructors with arguments  that is  via dependency injection   However  if the EJB container implements CDI      Optionally  The class may have an additional constructor annotated with the Inject annotation      it says for both session bean and message-driven bean because     An EJB packaged into a CDI bean archive and not annotated with javax enterprise inject Vetoed annotation  is considered a CDI-enabled bean

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Explanation with an example   1  import java io Serializable  As for the Serialization  see the documentation   2  private fields  Fields should be private for prevent outer classes to easily modify those fields  Instead of directly accesing to those fields  usuagly getter setter methods are used   3  Constructor  A public constructor without any argument   4  getter setter  Getter and setter methods for accessing and modifying private fields       1  import java io Serializable    public class User implements java io Serializable           2  private fields        private int id      private String name           3  Constructor        public User               public User int id  String name            this id   id          this name   name                 4  getter setter           getter     public int getId             return id            public String getName             return name               setter     public void setId int id            this id   id            public void setName String name            this name   name

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A Java Bean is a component or the basic building block in the JavaBeans architecture  The JavaBeans architecture is a component architecture that benefits from reusability and interoperability of a component-based approach  A valid component architecture should allow programs to be assembled from software building blocks  Beans in this case   perhaps provided by different vendors and also make it possible for an architect   developer to select a component  Bean   understand its capabilities  and incorporate it into an application  Since classes objects are the basic building blocks of an OOP language like Java  they are the natural contenders for being the Bean in the JavaBeans architecture  The process of converting a plain Java class to a Java bean is actually nothing more than making it a reusable and interoperable component  This would translate into a Java class having abilities like   controlling the properties  events  and methods of a class that are exposed to another application   You can have a BeanInfo class that reports only those properties  events and methods that the external application needs   persistence  being serialisable or externizable - this would also imply having no-argument constructors  using transient for fields  ability to register for events and also to generate events  e g   making use of bound and constraint properties  customizers  to customise the Bean via GUIs or by providing documentation   In order for a Java class to be termed a Java bean it is not necessary that they need to possess all the above abilities  Instead  it implies to implement a subset of the above relevant to the context  e g   a bean in a certain framework may not need customizers  some other bean may not need bound and constrained properties  etc   Almost all leading frameworks and libraries in Java adhere to the JavaBeans architecture implicitly  in order to reap the above benefits

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POJO  plain old Java object   POJOs are ordinary Java objects  with no restriction other than those forced by the Java Language  Serialization  It is used to save state of an object and send it across a network  It converts the state of an object into a byte stream  We can recreate a Java object from the byte stream by process called deserialization  Make your class implement java io Serializable interface  And use writeObject   method of ObjectOutputStream class to achive Serialization  JavaBean class  It is a special POJO which have some restriction  or convention    Implement serialization Have public no-arg constructor All properties private with public getters  amp  setter methods   Many frameworks - like Spring - use JavaBean objects

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You will find serialization useful when deploying your project across multiple servers since beans will be persisted and transferred across them

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JavaBeans is a standard  and its basic syntax requirements have been clearly explained by the other answers  However  IMO  it is more than a simple syntax standard  The real meaning or intended usage of JavaBeans is  together with various tool supports around the standard  to facilitate code reuse and component-based software engineering  i e  enable developers to build applications by assembling existing components  classes  and without having to write any code  or only have to write a little glue code   Unfortunately this technology is way under-estimated and under-utilized by the industry  which can be told from the answers in this thread  If you read Oracle s tutorial on JavaBeans  you can get a better understanding in that

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A JavaBean is just a standard  All properties are private  use getters setters  A public no-argument constructor Implements Serializable   That s it  It s just a convention   Lots of libraries depend on it though  With respect to Serializable  from the API documentation   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   In other words  serializable objects can be written to streams  and hence files  object databases  anything really  Also  there is no syntactic difference between a JavaBean and another class -- a class is a JavaBean if it follows the standards  There is a term for it  because the standard allows libraries to programmatically do things with class instances you define in a predefined way  For example  if a library wants to stream any object you pass into it  it knows it can because your object is serializable  assuming the library requires your objects be proper JavaBeans

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A Java Bean is any Java class that satisfies the following three criteria   It should implement the serializable interface  a Marker interface   The constructor should be public and have no arguments  what other people call a  quot no-arg constructor quot    It should have getter and setters   Good to note the serialVersionUID field is important for maintaining object state  The below code qualifies as a bean  public class DataDog implements java io Serializable    private static final long serialVersionUID   -3774654564564563L   private int id  private String nameOfDog      The constructor should NOT have arguments public DataDog             4  getter setter        Getter s  public int getId         return id     public String getNameOfDog         return nameOfDog         Setter s  public void setId int id        this id   id     public void setNameOfDog String nameOfDog        this nameOfDog   nameOfDog

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Regarding the second part of your question  serialization is a persistence mechanism used to store objects as a sequence of signed bytes  Put less formally  it stores the state of an object so you can retrieve it later  by deserialization

[java] What is a JavaBean exactly?

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