I'm implementing caching in my Spring Boot (v2.5.2) app using GemFire.
The class which needs to be cached -
import org.springframework.data.annotation.Id;
import org.springframework.data.gemfire.mapping.annotation.Region;
import com.auth0.jwk.Jwk;
#Region("jwks")
public class Key {
#Id
private String id;
private Jwk jwk;
#PersistenceConstructor
public Key(String id, Jwk jwk){
this.id = id;
this.jwk = jwk;
}
// Getters and setters for variables
}
I get the following error while fetching the stored entity from cache -
org.apache.geode.SerializationException : While deserializing query result with root cause
java.lang.NoSuchMethodException: com.auth0.jwk.Jwk.<init>()
at java.lang.Class.getConstructors0(Class.java:3082)
at java.lang.Class.getDeclaredConstructor(Class.java:2187)
...
How can I deserialize this object when Jwk doesn't have a no-arg constructor?
Edit -
build.gradle
implementation 'org.springframework.geode:spring-geode-starter'
implementation 'org.springframework.geode:spring-data-geode'
Also, updated the constructor as per suggested answer which continues to give the same error.
Error causing code -
Optional<Key> key = this.keyRepository.findById(id);
First, please be precise when you ask questions.
Your Key class is not even valid Java. The Key class by itself would not even compile as is.
The constructors (JsonWebKey) in this case are 1) not named after the class (Key) and 2) you cannot have 2 no arg constructors.
I am assuming the second constructor would be:
public Key(String id, Jwk jwk) {
this.id = id;
this.jwk = jwk;
}
I am guessing either the constructors are misnamed or perhaps are part of some [static] inner class??
Never-the-less, assuming you are using Spring Boot for Apache Geode (and VMware Tanzu (Pivotal) GemFire), a.k.a. SBDG (see here; if you are not using SBDG, you should be!), and since SBDG enables GemFire/Geode PDX serialization by default, then SBDG with the help of Spring Data for Apache Geode (and VMware Tanzu GemFire), a.k.a. SDG (upon which SBDG is built, along with the core Spring Framework, Spring Data and Spring Boot) will handle these serialization concerns for you, with no extra effort on your part. See the corresponding SDG documentation.
By way of example, I wrote this test class, which resides in this package, to demonstrate.
By default, the test class is using PDX serialization configured with Spring using the test configuration. The Spring-based configuration class is here. The other test configuration classes are only enabled with the appropriate test Spring profile configured.
The model class for this test is the CompositeValue class in the model sub-package.
As you can see, the class has 2 primary constructors. I also declared a commented-out, default, public no-arg constructor, which I will explain further below.
This CompositeValue model class was designed very deliberately. You will notice that it is (mostly) immutable.
I use a Spring Data CrudRepository (see here) to save (persist/store) an instance of CompositeValue in a GemFire/Geode Region ("Values") and then retrieve it (findBy..). The "Values" Region is necessarily a PARTITION Region (see here), since a PARTITION Region stores values in serialized form, and in our case, PDX serialized.
Using the Spring configuration, the test runs and passes successfully! Spring is doing all the heavy lifting!
If you are NOT using Spring to its fullest extent, then you are (most likely) going to have problems unless you know what you are doing with GemFire/Geode.
Out-of-the-box, GemFire/Geode PDX serialization has certain limitations. See the documentation. Specifically, see here.
For instance, if you are using GemFire/Geode's ReflectionBasedAutoSerializer class (Javadoc, documentation; and I suspect you are) and not Spring, then it requires your application domain objects (model classes / entities) to have a default, public no-arg constructor.
This flies in the face of immutable, effectively immutable and mostly immutable classes since then you cannot appropriately initialize classes using constructors, which is crucial in a highly concurrent, multi-Threaded context, like GemFire/Geode.
You can see the effects of trying to use GemFire/Geode's ReflectionBasedAutoSerializer by enabling the "gemfire" profile in the example test class I wrote, for which the configuration is here.
The test will NOT pass without the commented-out, default, public no-arg constructor.
When using Apache Geode 1.13.4, I get the following error:
2021-11-01 11:53:08,720 WARN ode.pdx.internal.AutoSerializableManager: 274 - Class
io.stackoverflow.questions.spring.geode.serialization.pdx.model.CompositeValue
matched with '.*' cannot be auto-serialized due to missing public no-arg constructor.
Will attempt using Java serialization.
However, even with Java serialization (the GemFire/Geode backup serialization strategy), the test results in a failure:
Caused by: java.io.NotSerializableException: io.stackoverflow.questions.spring.geode.serialization.pdx.model.CompositeValue
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
at org.apache.geode.internal.InternalDataSerializer.writeSerializableObject(InternalDataSerializer.java:2184)
at org.apache.geode.internal.InternalDataSerializer.basicWriteObject(InternalDataSerializer.java:2058)
at org.apache.geode.DataSerializer.writeObject(DataSerializer.java:2839)
at org.apache.geode.internal.cache.CachedDeserializableFactory.calcSerializedSize(CachedDeserializableFactory.java:245)
... 60 common frames omitted
Well, the java.io.NotSerializableException is thrown because the CompositeValue class (deliberately) does not implement the java.io.Serialiable interface.
Why deliberately? Because you cannot implement java.io.Serializable on classes you do not own, which is true when using 3rd party libraries and their classes. Even though we own the CompositeValue in my case, I am making a point, because in your case, you don't own Jwk.
So, not only can we not use (mostly/effectively) immutable classes, we also cannot rely on default serialization mechanisms baked into GemFire/Geode.
Of course, we can handle this by implementing a custom PdxSerializer, the second strategy in the documentation (under "Procedure", Step 1, "Serializing your Domain Object with a PdxSerializer").
If we again change the active Spring profile to "gemfire-custom-pdxserializer" in the example test I wrote, then the test will pass.
But, it comes at a high price! See the necessary configuration to make this arrangement work.
In our case, we have only 1 such model / entity class to build a custom PdxSerializer for. However, imagine if we had hundreds of classes to handle.
To make matters worse, GemFire/Geode only allows a single PdxSerializer to be registered with a Singleton GemFire/Geode cache, which means you can only have 1. Now you must rely on the Composite Software Design Pattern to compose multiple PdxSerializers necessary to handle all your application domain model types requiring serialization. While this is elegant, you must build a custom PdxSerializer per application model / entity type. Of course, you could bake all type handling into 1 PdxSerializer implementation, but that would get ugly rather quickly!
Finally, your application model / entity types could implement GemFire/Geode's PdxSerializable interface (Javadoc). This is no better than java.io.Serializable and (again) does not work for types you don't own. It also couples your application to GemFire/Geode and is why I did NOT demonstrate this approach, as it should be considered an anti-pattern.
With SDG's MappingPdxSerializer, which SBDG's auto-configures for you (by default), you do not need to do any of the above. SBDG auto-configures PDX by default (making the SDG #EnablePdx annotation unnecessary) and there is no special requirement for your application domain object (model) / entity classes.
However, if you have more than 1 constructor in your entity class, then you will need to designate 1 constructor as the primary persistence constructor. In the CompositeValue class, this constructor was designated as the primary, persistence constructor using Spring Data's #PersistenceConstructor annotation, which SDG's MappingPdxSerializer takes into account when deserializing and constructing your application domain object model types.
If you only have 1 constructor in your class, then you do not even need to declare the #PersistenceConstructor annotation on the only constructor. That is if the other constructor in CompositeValue did not exist, then the #PersistenceConstructor annotation on this constructor would not be necessary. SD[G] can figure it out.
Feel free to play around with my example test for you learning purposes.
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 JavaBean 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?
A JavaBean is just a standard. It is a regular Java class, except it follows certain conventions:
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).
There's a term for it to make it sound special. The reality is nowhere near so mysterious.
Basically, a "Bean":
is a serializable object (that is, it implements java.io.Serializable, and does so correctly), that
has "properties" whose getters and setters are just methods with certain names (like, say, getFoo() is the getter for the "Foo" 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 "marker interface" (an interface that doesn't declare any functions) that tells Java that the implementing class consents to (and implies that it is capable of) "serialization" -- 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.)
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).
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;
}
}
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;
}
}
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.
You will find serialization useful when deploying your project across multiple servers since beans will be persisted and transferred across them.
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 "don't ask an object for its values, ask an object to do something for you"). 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)
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.
For a Java class to be usable as a Java bean, its method names need to be as per the JavaBeans guidelines (also called design patterns) for properties, methods, and events. The class needs to be a public class to be accessible to any beanbox tool or container. The container must be able to instantiate it; with the class as public, the container should be able to do so even if no explicit, public, zero-args constructor is provided. (A Java public class with no explicit constructor has a default public zero-args constructor.) So, minimally, a Java public class, even with a property as the sole member (of course, accompanying public getter and setter required) or a public method as the sole member, is a Java bean. The property can either be a read-only property (it has a getter method but no setter) or write-only property (has a setter method only). A Java public class with a public event listener registration method as the sole member is also a Java bean. The JavaBeans specification doesn’t require that if such a Java class has an explicit public constructor, it should be a zero-args one. If one could provide a file (with an extension, say, .ser) containing a serialized instance, a beanbox tool may be able to use that file to instantiate a prototype bean. Otherwise, the class would need a constructor, either explicit or default, that is public as well as zero-args.
Once the bean is instantiated, the JavaBeans API ( java.beans.*) can introspect it and call methods on it. If no class implementing the interface BeanInfo or extending a BeanInfo implementation,such as the SimpleBeanInfo class, is available, the introspection involves using reflection (implicit introspection) to study the methods supported by a target bean and then applying 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 out on it. Thus, the requirement of a public zero-args constructor. But, of course, the Serializable or Externalizable interface isn’t necessary for it to be recognized. However, the JavaBeans 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, the 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 {}
The description so far is the Java SE version (JavaBeans). The beans, as described below, are the Java EE versions. These versions have been built on the underlying ideas as explained above. In particular, one main idea they consider is what if a bean constructor does have some parameters. These parameters could be either simple types, class/interface types or both. There should be a way to let the container know values that it can substitute for the parameters when instantiating the bean. The way to do so is that the programmer can configure (specify values) by say annotations or XML configuration files or a mix of both.
Spring Beans
Spring beans run in a Spring IoC container. The programmer can configure via XML configuration files, annotations or a mix of both.
In Spring, if a bean constructor has simple-type or class/interface type parameters, values can be assigned as strings (as the <value> attribute of a constructor argument element in the former case and as an <idref> element of a constructor argument in the latter case) in a type-safe manner. Making references to other Spring beans (called collaborators; via the <ref> element in a constructor argument element) is basically dependency injection and is also 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. This scenario should ultimately terminate at injected dependency(ies) that are prototype beans that the container can instantiate by constructing.
JSF Managed Beans
JSF managed beans run in a web container. They can be configured either with the #ManagedBean annotation or with an application configuration resource file managed-bean.xml. The JSF spec supports injection via resource injection (not typesafe) only. This injection is not fit for injection on constructors. In any case, the spec requires that a JSF managed bean must have a public zero-argument constructor. 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 should be used, which do offer 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 the web tier. Thus, the web container needs to implement the CDI specification.
Managed Beans
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, or 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.
CDI Beans
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 the 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.
Bean constructors can have simple-type parameters since simple-types can be injected with the #Inject annotation.
EJBs
EJBs run in an EJB container. The EJB 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. Furthermore, 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.”
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.
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.
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.
They are serializable, have a zero-argument constructor, and allow access to properties using getter and setter methods. The name "Bean" 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.
It was repeated 6 or 7 times above that there is a no-argument constructor requirement for JavaBeans.
This is WRONG, there is no such requirement, especially in the context of Java Spring.
There is also no mention of that requirement in version (1.01) of the specification that describes the JavaBeanns APIs (https://download.oracle.com/otndocs/jcp/7224-javabeans-1.01-fr-spec-oth-JSpec/). Even more - this specification mentions 'null constructor' only 2 times in the following contexts:
"Each customizer should have a null constructor."
"Each PropertyEditor should have a null constructor."
So, it does not seem like the authors of the spec don't know or are not willing to use the term "null constructor", still no mention of it for the JavaBeans proper.
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 "no-arg constructor").
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;
}}
If you are familiar with C/Golang, you never heard C bean or Go bean because they have struct keyword, that developers can easily define structure types without writing complicated OOP keywords.
type User struct {
Name string
Age int
}
var user User
user.Name = "name"
user.Age = 18
var bytes, err = json.Marshal(user)
It's Java's mistake that lack of struct types, and developers find this bad shortage.
Then Java Bean is invented as just another boring rule to make class pretending struct, peace your editor or compiler won't be crying or yelling about your unsafe access to class members.
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.
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 & setter methods.
Many frameworks - like Spring - use JavaBean objects.
If you want to understand Java-Beans, you first have to understand software-components.
Software components
A software-component is a part of an application that runs a specific operation. A software component can also be part of a service.
A component is:
Coupled (has dependencies)
Statefull (it saves the states of instance variables)
Not standarised, it is designed for a specific use case (main difference between Java-EE Beans)
Runs in client machine
Java Beans (Enterprise Beans)
Standarised components that run in a Java EE-server
Including different business logics to complete a specific service
Simplify development of complex multilayer distributed systems
Java Beans are more of a concept to manage big systems. Thats why they need standarization.
Source
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.
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.
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