Spring Bean 的作用域

当定义一个Spring的<bean>,必须声明bean 作用域的选项。例如,要强制Spring需要产生一个新的bean实例,应该声明bean的scope属性为prototype。如果你希望Spring 每次都返回同一个bean实例,应该声明bean的作用域,方式类似属性是单例。

Spring框架支持以下五个作用域,其中三个只有当您使用Web感知的 ApplicationContext 可用。

范围 描述
singleton This scopes the bean definition to a single instance per Spring IoC container (default).
prototype This scopes a single bean definition to have any number of object instances.
request This scopes a bean definition to an HTTP request. Only valid in the context of a web-aware Spring ApplicationContext.
session This scopes a bean definition to an HTTP session. Only valid in the context of a web-aware Spring ApplicationContext.
global-session This scopes a bean definition to a global HTTP session. Only valid in the context of a web-aware Spring ApplicationContext.

本章将讨论前两个范围和其余三将讨论的时候,我们将讨论有关Web感知Spring的ApplicationContext。

 

singleton作用域:

如果范围设置为单例,Spring IoC容器创建了一个由该bean定义的对象只有一个实例。这个单一实例存储在这样的单例bean的高速缓存,以及所有后续请求和引用针对该bean返回缓存对象。

默认范围是始终单例,但是当你需要bean的一个实例,可以设置的范围属性单例在bean配置文件中,如下图所示:

<!-- A bean definition with singleton scope -->
<bean id="..." class="..." scope="singleton">
    <!-- collaborators and configuration for this bean go here -->
</bean>

示例:

让我们使用Eclipse IDE,然后按照下面的步骤来创建一个Spring应用程序:

步骤 描述
1 Create a project with a name SpringExample and create a package com.manongjc under the src folder in the created project.
2 Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3 Create Java classes HelloWorld and MainApp under the com.manongjc package.
4 Create Beans configuration file Beans.xml under the src folder.
5 The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

这里是HelloWorld.java 文件的内容:

package com.manongjc;

public class HelloWorld {
   private String message;

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

   public void getMessage(){
      System.out.println("Your Message : " + message);
   }
}

以下是MainApp.java文件的内容:

package com.manongjc;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = 
             new ClassPathXmlApplicationContext("Beans.xml");

      HelloWorld objA = (HelloWorld) context.getBean("helloWorld");

      objA.setMessage("I'm object A");
      objA.getMessage();

      HelloWorld objB = (HelloWorld) context.getBean("helloWorld");
      objB.getMessage();
   }
}

以下是需要singleton作用域配置文件beans.xml文件:

<?xml version="1.0" encoding="UTF-8"?>

<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-3.0.xsd">

   <bean id="helloWorld" class="com.manongjc.HelloWorld" 
      scope="singleton">
   </bean>

</beans>

一旦创建源代码和bean配置文件来完成,运行应用程序。如果一切顺利,这将打印以下信息:

Your Message : I'm object A
Your Message : I'm object A

 

原型作用域:

如果范围设置为原型,那么Spring IoC容器创建对象的新的bean实例为每个特定的bean发出请求时的时间。作为一项规则,使用prototype作用域为所有状态的bean类和singleton作用域为无状态的bean。

要定义一个原型作用域,可以设置的范围属性为原型的bean配置文件中,如下图所示:

<!-- A bean definition with singleton scope -->
<bean id="..." class="..." scope="prototype">
    <!-- collaborators and configuration for this bean go here -->
</bean>

示例:

让我们在地方工作的Eclipse IDE,然后按照下面的步骤来创建一个Spring应用程序:

步骤 描述
1 Create a project with a name SpringExample and create a package com.manongjc under the src folder in the created project.
2 Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3 Create Java classes HelloWorld and MainApp under the com.manongjc package.
4 Create Beans configuration file Beans.xml under the src folder.
5 The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

这里是HelloWorld.java 文件的内容:

package com.manongjc;

public class HelloWorld {
   private String message;

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

   public void getMessage(){
      System.out.println("Your Message : " + message);
   }
}

以下是MainApp.java文件的内容:

package com.manongjc;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = 
             new ClassPathXmlApplicationContext("Beans.xml");

      HelloWorld objA = (HelloWorld) context.getBean("helloWorld");

      objA.setMessage("I'm object A");
      objA.getMessage();

      HelloWorld objB = (HelloWorld) context.getBean("helloWorld");
      objB.getMessage();
   }
}

以下是必需的原型作用域的配置文件beans.xml:

<?xml version="1.0" encoding="UTF-8"?>

<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-3.0.xsd">

   <bean id="helloWorld" class="com.manongjc.HelloWorld" 
      scope="prototype">
   </bean>

</beans>

创建源代码和bean配置文件完成后,让我们运行应用程序。如果一切顺利,这将打印以下信息:

Your Message : I'm object A
Your Message : null