spring容器的懶加載

默認情況下，spring的IOC容器中lazy-init是false的，即沒有打開懶加載模式。 

如果你沒有看到這個lazy-init 的參數設置就說明是false啦。 

那麼什麼是懶加載？ 

懶加載---就是我們在spring容器啓動的是先不把所有的bean都加載到spring的容器中去，而是在當需要用的時候，才把這個對象實例化到容器中。 

例如我有如下的代碼： 

package com.luch.spring.demo;

import org.springframework.beans.factory.annotation.Autowired;

import com.luch.spring.bean.Person;

public class NewPerson {

    @Autowired
    private Person person;

    public NewPerson(){
        System.out.println("lazy loading...");
    }
    public void printMsg(){
        if(person !=null) {
            System.out.println(person.getName());
        } else {
            System.out.println("no person initialize!");
        }
    }

    public void setPerson(Person person) {
        //this.person = person;
    }


}

在無慘構造器裏輸出一句話，然後我先不設置懶加載模式：我有一個beans.xml的配置文件： 

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd">

           <context:annotation-config/>
           <bean id="person" class="com.luch.spring.bean.Person">
               <property name="id" value="22"></property>
               <property name="name" value="Jack"></property>
           </bean>

           <bean id="newPerson" class="com.luch.spring.demo.NewPerson">
               <property name="person" ref="person"></property>
           </bean>

</beans>

然後我用一個junit來做測試： 

package junit.test;

import static org.junit.Assert.*;

import org.junit.Test;

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

import com.luch.spring.demo.NewPerson;

public class JunitTest {

    @Test
    public void printMsg(){
        AbstractApplicationContext ctx = new ClassPathXmlApplicationContext("/beans.xml");
        //NewPerson test = (NewPerson) ctx.getBean("newPerson");
        //test.printMsg();
    }
}

這個時候輸出的結果爲： 
四月 17, 2014 9:26:41 下午 org.springframework.context.support.AbstractApplicationContext prepareRefresh 
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@762efe5d: startup date [Thu Apr 17 21:26:41 CST 2014]; root of context hierarchy 
四月 17, 2014 9:26:42 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions 
信息: Loading XML bean definitions from class path resource [beans.xml] 
四月 17, 2014 9:26:42 下午 org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons 
信息: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@77caeb3e: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,person,newPerson,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy 
lazy loading.. 
即對象被實例化了，也就是被加載到spring的容器中去了。 


然後我們設置一下懶加載模式：我們beans.xml的配置文件. lazy-init="true"即 

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd">

           <context:annotation-config/>
           <bean id="person" class="com.luch.spring.bean.Person">
               <property name="id" value="22"></property>
               <property name="name" value="Jack"></property>
           </bean>

           <bean id="newPerson" lazy-init="true" class="com.luch.spring.demo.NewPerson">
               <property name="person" ref="person"></property>
           </bean>

</beans>

再重新跑一次junit：結果爲： 
四月 17, 2014 9:33:54 下午 org.springframework.context.support.AbstractApplicationContext prepareRefresh信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@762efe5d: startup date [Thu Apr 17 21:33:54 CST 2014]; root of context hierarchy四月 17, 2014 9:33:54 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions信息: Loading XML bean definitions from class path resource [beans.xml]四月 17, 2014 9:33:54 下午 org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons信息: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@77caeb3e: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,person,newPerson,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy 
即沒有了實例化的過程，這個時候只有在你需要它的時候，它纔會出現。比如你執行了： 
NewPerson test = (NewPerson) ctx.getBean("newPerson"); 這個時候你的bean就實例化出來了。 
那麼是不是我如果很多的bean都不想在IOC容器啓動的時候就加載，而是要beans.xml的每個bean裏都加上lazy-init屬性呢。 
不用的，spring提供了default-lazy-init方法來實現這個業務。 
我們只要在beans的頭裏面加上這個就ok 了 

< beans  default-lazy-init ="true" > 

以上代碼本人親測，可用