Singleton vs Singleton beans design pattern in Spring container

Question

As we all know, we have beans as a singleton by default in the Spring container, and if we have a web application based on the Spring structure, then in this case we really need to implement the Singleton design pattern to store global ones and not just create a bean via spring .

Please carry me if I cannot explain what I really wanted to ask.

Answer 1

The syntax is bean in Spring, and the singleton pattern is completely different. The Singleton template says that one instance of a particular class will be created for each class loader.

The scope of the Spring singlet is described as "for each container per bean." This is the bean definition for an instance of a single object in an IoC container. By default, Spring uses Singleton.

Although the default scope is single, you can change the scope of the bean by specifying the attribute of the <bean ../> scope.

 <bean id=".." class=".." scope="prototype" /> 

Answer 2

Singleton scope in spring means one instance in the context of spring.
The Spring container returns the same instance over and over for subsequent calls to obtain a bean.


And spring doesn't bother if the bean class is encoded as single or not, in fact, if the class is encoded as singleton, whose constructor is private, spring uses BeanUtils.instantiateClass (javadoc here ) to set the constructor to an accessible one and call it.

Alternatively, we can use the factory method attribute in a bean definition similar to this

  <bean id="exampleBean" class="example.Singleton" factory-method="getInstance"/> 

Answer 3

Let's look at a simple example: you have an application, and you just use the default class loader. You have a class that, for some reason, you decide that there should not be more than one instance in the application. (Think of a scenario where several people are working on parts of an application).

If you do not use the Spring framework, the Singleton pattern ensures that your application will not have more than one instance of the class. This is because you cannot create instances of the class by doing the “new” because the constructor is private. The only way to get an instance of a class is to call some static class method (usually called "getInstance"), which always returns the same instance.

Saying that you use the Spring framework in your application, simply means that in addition to the usual methods of obtaining an instance of the class (new or static methods that return an instance of the class), you can also ask Spring to provide you with an instance of this class, and Spring will ensure that everyone the time you request an instance of this class, it always returns the same instance, even if you are not writing the class using the Singleton template, in other words, even if the class has an open constructor, if you always request Spring for An instance of this class, Spring will only call this constructor once during the lifetime of your application.

Normally, if you use Spring, you should use Spring to create instances, and you may have an open constructor for the class. But if your constructor is not private, you do not forbid anyone to create new instances of the class directly, bypassing Spring.

If you really need a single instance of the class, even if you use Spring in your application and define the class in Spring as a singleton, the only way to make sure that the class also implements using Singleton. This ensures that there will be one instance if people use Spring to get an instance or bypass Spring.

Answer 4

Spring's Singleton scope means that this bean will be created only once by Spring. Unlike the prototype area (a new instance each time), the request area (once per request), the session area (once for an HTTP session).

The Singleton area is technically linked to the singleton design pattern. You do not need to implement your beans as singletones to place them in the Singleton area.

Answer 5

I find the "on the bean container" difficult to understand. I would say "one bean per bean identifier". Let there be an example to understand this. We have a sample bean class. I defined two bean components from this class in a bean definition, for example:

 <bean id="id1" class="com.example.Sample" scope="singleton"> <property name="name" value="James Bond 001"/> </bean> <bean id="id7" class="com.example.Sample" scope="singleton"> <property name="name" value="James Bond 007"/> </bean> 

Therefore, whenever I try to get the bean with id "id1", the Spring container will create one bean, cache it and return the same bean where id1 was ever used. If I try to get it using id7, another component will be created from the Sample class, which will be cached and returned every time you refer to it using id7.

This is unlikely in the case of the singleton pattern. In the Singlton template, a single object is always created per classloader. But spring, many objects are created for one class. However, Spring creates a scope like in Singleton, returning the same object for the same identifier. Link

Answer 6

Singleton beans in Spring and the classes based on the Singleton design pattern are completely different.

The syntax pattern ensures that the same instance of a particular class is ever created for each class loader, where the Spring singleton bean scope is described as "for every container on the bean." Spring's Singleton scope means that this bean will be created only once by Spring. The Spring container simply returns the same instance over and over for subsequent calls to get a bean.

Answer 7

"singleton" in spring uses a bean factory instance of get, and then caches it; which has a strict design pattern, an instance can only be retrieved from the static get method, and the object can never be publicly created.

Answer 8

EX: "per container per bean."

  <bean id="myBean" class="com.spring4hibernate4.TestBean"> <constructor-arg name="i" value="1"></constructor-arg> <property name="name" value="1-name"></property> </bean> <bean id="testBean" class="com.spring4hibernate4.TestBean"> <constructor-arg name="i" value="10"></constructor-arg> <property name="name" value="10-name"></property> </bean> </beans> public class Test { @SuppressWarnings("resource") public static void main(String[] args) { ApplicationContext ac = new ClassPathXmlApplicationContext("ws.xml"); TestBean teatBean = (TestBean) ac.getBean("testBean"); TestBean myBean1 = (TestBean) ac.getBean("myBean"); System.out.println("a : " + teatBean.test + " : " + teatBean.getName()); teatBean.setName("a TEST BEAN 1"); System.out.println("uPdate : " + teatBean.test + " : " + teatBean.getName()); System.out.println("a1 : " + myBean1.test + " : " + myBean1.getName()); myBean1.setName(" a1 TEST BEAN 10"); System.out.println("a1 update : " + teatBean.test + " : " + myBean1.getName()); } } public class TestBean { public int test = 0; public String getName() { return name; } public void setName(String name) { this.name = name; } private String name = "default"; public TestBean(int i) { test += i; } } 

JAVA SINGLETON:

 public class Singleton { private static Singleton singleton = new Singleton(); private int i = 0; private Singleton() { } public static Singleton returnSingleton() { return singleton; } public void increment() { i++; } public int getInt() { return i; } } public static void main(String[] args) { System.out.println("Test"); Singleton sin1 = Singleton.returnSingleton(); sin1.increment(); System.out.println(sin1.getInt()); Singleton sin2 = Singleton.returnSingleton(); System.out.println("Test"); sin1.increment(); System.out.println(sin1.getInt()); } 

Answer 9

Spring singleton bean is described as "for every container on the bean." The Singleton scope in Spring means that the same object in the same memory location will be returned to the same bean id. If you create several beans of different identifiers of the same class, then the container will return different objects in different identifiers. This is like matching key values, where the key is the bean id, and the value is a bean in a single Spring container. Where, as a Singleton template, it is guaranteed that one instance of a particular class will be created for each class loader.

Answer 10

There is a very fundamental difference between the two. In the case of the Singleton development template, only one instance of the class will be created for a single classLoader, while in the case of the Spring singleton, this is not the case as a single instance of the shared component is subsequently created for this identifier for the IoC container.

For example, if I have a class called "SpringTest" and my XML file looks something like this:

 <bean id="test1" class="com.SpringTest" scope="singleton"> --some properties here </bean> <bean id="test2" class="com.SpringTest" scope="singleton"> --some properties here </bean> 

So now in the main class, if you check the link to the above two, it will return false, as indicated in the Spring documentation:

When a component is singleton, only one common instance of the component will be managed, and all requests for components with an identifier or identifiers that match the definition of the component will cause one specific instance of the component to be returned by the Spring container.

So, as in our case, the classes are the same, but the identifiers we provided are different, which leads to the creation of two different instances.