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Spring 之lazy-init

为什么想讨论spring这个属性了，起因是自己在思考，spring容器管理的bean，是在spring容器启动完成后，所有的bean的都被实例化了吗？

先回答上面的问题，spring容器管理的bean.在spring容器启动完成后，都被实例化了。认证过程如下：

有三个类，StudyClient,StudyService,StudyDao.

StudyClient

package com.spring.study;import org.springframework.context.support.ClassPathXmlApplicationContext;public class StudyClient {    public static void main(String[] args){        ClassPathXmlApplicationContext classPathXmlApplicationContext =                new ClassPathXmlApplicationContext("classpath:spring.xml");    }}

StudyService

package com.spring.study;public class StudyService {    public StudyService(){        System.out.println("initialize study service");    }    private StudyDao studyDao;    public void setStudyDao(StudyDao studyDao){        this.studyDao = studyDao;    }    public void invokeStudyDao(){        studyDao.invoke();    }}

StudyDao

package com.spring.study;public class StudyDao {    public StudyDao(){        System.out.println("initialized study dao");    }    public void invoke(){        System.out.println("StudyDao was invoked");    }}

spring.xml的配置文件

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

调用main方法，启动spring 容器，可以如下打印信息，如红色字体锁是，看到两个类被实例化了。因为两个类的构造函数被调用了。

14:26:26.139 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyDao’

initialized study dao 14:26:26.149 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyDao’ to allow for resolving potential circular references 14:26:26.150 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyDao’ 14:26:26.151 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyService’ 14:26:26.151 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyService’ initialize study service

接下来，我新增一个类，StudyLazyInit

package com.spring.study;

public class StudyLazyInit {

public StudyLazyInit(){    System.out.println("is StudyLazyInit init?");}public void init(){    System.out.println("StudyLazyInit is initialize");}

}

修改了spring.xml

启动main函数，启动spring容器，看打印日志

[org.springframework.context.support.DelegatingMessageSource@e320068]

14:46:08.108 [main] DEBUG org.springframework.context.support.ClassPathXmlApplicationContext - Unable to locate ApplicationEventMulticaster with name ‘applicationEventMulticaster’: using default [org.springframework.context.event.SimpleApplicationEventMulticaster@34cd072c] 14:46:08.109 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@a67c67e: defining beans [studyService,studyDao,studyLazyInit]; root of factory hierarchy

14:46:08.110 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyService’ 14:46:08.110 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyService’ initialize study service 14:46:08.119 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyService’ to allow for resolving potential circular references 14:46:08.120 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyDao’ 14:46:08.120 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyDao’ initialized study dao 14:46:08.121 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyDao’ to allow for resolving potential circular references 14:46:08.121 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyDao’ 14:46:08.161 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyService’ 14:46:08.161 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Returning cached instance of singleton bean ‘studyDao’ 14:46:08.162 [main] DEBUG org.springframework.context.support.ClassPathXmlApplicationContext - Unable to locate LifecycleProcessor with name ‘lifecycleProcessor’: using default [org.springframework.context.support.DefaultLifecycleProcessor@4d1b0d2a] 14:46:08.162 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Returning cached instance of singleton bean ‘lifecycleProcessor’ 14:46:08.165 [main] DEBUG org.springframework.core.env.PropertySourcesPropertyResolver - Could not find key ‘spring.liveBeansView.mbeanDomain’ in any property source

Process finished with exit code 0

从上面的日志信息里面可以看到，StudyLazyInit没有被实体化，而是被预初始化了。原因就在Lazy-init = “true”. 如下是在spring.xml里面增加的配置内容

将Lazy-init 设置为false ，就可以在日志里面看StudyLazyInit被初始化了。

14:58:13.816 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@a67c67e: defining beans [studyService,studyDao,studyLazyInit]; root of factory hierarchy

14:58:13.817 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyService’ 14:58:13.817 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyService’ initialize study service 14:58:13.827 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyService’ to allow for resolving potential circular references 14:58:13.828 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyDao’ 14:58:13.828 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyDao’ initialized study dao 14:58:13.829 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyDao’ to allow for resolving potential circular references 14:58:13.830 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyDao’ 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyService’ 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Returning cached instance of singleton bean ‘studyDao’ 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean ‘studyLazyInit’ 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating instance of bean ‘studyLazyInit’ is StudyLazyInit init? 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Eagerly caching bean ‘studyLazyInit’ to allow for resolving potential circular references 14:58:13.888 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Finished creating instance of bean ‘studyLazyInit’ 14:58:13.889 [main] DEBUG org.springframework.context.support.ClassPathXmlApplicationContext - Unable to locate LifecycleProcessor with name ‘lifecycleProcessor’: using default [org.springframework.context.support.DefaultLifecycleProcessor@4d1b0d2a] 14:58:13.889 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Returning cached instance of singleton bean ‘lifecycleProcessor’ 14:58:13.892 [main] DEBUG org.springframework.core.env.PropertySourcesPropertyResolver - Could not find key ‘spring.liveBeansView.mbeanDomain’ in any property source

Process finished with exit code 0

如果Lazy-init 被设置成true,那么bean何时实例化了，答案是使用bean，向spring 容器索要bean的时候，即getBean（“beanName”）.并且bean所以依赖的bean也被实例化（尽管其也是懒加载）和注入。

在验证，被依赖的类如果也是延迟加载的话，我们在getBean依赖类时候，也会被实例化。但是这是有条件的。条件就是必须是set注入，不是autowire注入。

稍微解释一下。假设A 是依赖类， B被A依赖，那么B就是被依赖类。

A 属性B ，引入属性B有三种方式，构造函数，set ,autowire.

其中构造函数，和 set注入，不管B是否延迟加载，当我在spring容器种getBean()获取A的时候，B就会被实例化，而 用autowire 就不会实例化。

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