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Spring Bean Scope

In Spring, bean scope is used to decide which type of bean instance should be return from Spring container back to the caller.

5 types of bean scopes supported :

1. singleton – Return a single bean instance per Spring IoC container

2. prototype – Return a new bean instance each time when requested

3. request – Return a single bean instance per HTTP request. *

4. session – Return a single bean instance per HTTP session. *

5. globalSession – Return a single bean instance per global HTTP session. *

In most cases, you may only deal with the Spring’s core scope – singleton and prototype, and the default scope is singleton.

P.S * means only valid in the context of a web-aware Spring ApplicationContext

Singleton vs Prototype

Here’s an example to show you what’s the different between bean scope : singleton and prototype.

package usoft;

public class CustomerService {
    String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

1. Singleton example

If no bean scope is specified in bean configuration file, default to singleton.

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
       <bean id="customerService" 
            class="com.mkyong.customer.services.CustomerService" />
 
</beans>

Run it

package com.mkyong.common;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
package usoft;

public class App {
    public static void main(String[] args) {
        ApplicationContext context =
                new ClassPathXmlApplicationContext(new String[]{"Spring-Customer.xml"});

        CustomerService custA = (CustomerService) context.getBean("customerService");
        custA.setMessage("Message by custA");
        System.out.println("Message : " + custA.getMessage());

        //retrieve it again
        CustomerService custB = (CustomerService) context.getBean("customerService");
        System.out.println("Message : " + custB.getMessage());
    }
}

Output

Message : Message by custA

Message : Message by custA

Since the bean ‘customerService’ is in singleton scope, the second retrieval by ‘custB’ will display the message set by ‘custA’ also, even it’s retrieve by a new getBean() method. In singleton, only a single instance per Spring IoC container, no matter how many time you retrieve it with getBean(), it will always return the same instance.

2. Prototype example

If you want a new ‘customerService’ bean instance, every time you call it, use prototype instead.

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
   <bean id="customerService" class="com.mkyong.customer.services.CustomerService" 
         scope="prototype"/>
 
</beans>

Run it again

Message : Message by custA

Message : null

In prototype scope, you will have a new instance for each getBean() method called.

3. Bean scopes annotation

You can also use annotation to define your bean scope.

package usoft;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Service;

@Service
@Scope("prototype")
public class CustomerService {
    String message;

    public String getMessage() {
        return message;
    }


    public void setMessage(String message) {
        this.message = message;
    }

}

Enable auto component scanning

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-2.5.xsd">
 
       <context:component-scan base-package="com.mkyong.customer" />
 
</beans>

Spring Bean Scope

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原文地址：http://my.oschina.net/xinxingegeya/blog/397513