Spring-Lazy-init源码探讨

在Spring中，使用azy-init="true"可以延迟实例化bean，等待调用bean时再实例化bean。
本文记录Spring如何实现延迟实例化。
Spring版本：5.0.4
测试代码

package cn.wzx;

import cn.wzx.model.Student;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * @author Walker
 * @since 1.0 2018-10-16
 */
public class SpringTest {

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:beans.xml");
        System.out.println("容器已经启动");
        Student student = context.getBean(Student.class);
    }

}

实体类：

package cn.wzx.model;

/**
 * @author Walker
 * @since 1.0 2018-10-16
 */
public class Student {

    public Student() {
        System.out.println("Student init.");
    }

    private String name;

    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

使用ClassPathXmlApplicationContext创建Spring容器的主要流程是先解析XML文件，接着实例化bean。
可以查看之前的文章
Spring-ClassPathXmlApplicationContext源码探讨-解析XML文件
Spring-Bean初始化源码探讨
在解析XML文件过程中，会解析是否有配置lazy-init属性。
BeanDefinitionParserDelegate#parseBeanDefinitionAttributes()

public AbstractBeanDefinition parseBeanDefinitionAttributes(Element ele, String beanName,
			@Nullable BeanDefinition containingBean, AbstractBeanDefinition bd) {

	if (ele.hasAttribute(SINGLETON_ATTRIBUTE)) {
		error("Old 1.x 'singleton' attribute in use - upgrade to 'scope' declaration", ele);
	}
	else if (ele.hasAttribute(SCOPE_ATTRIBUTE)) {
		bd.setScope(ele.getAttribute(SCOPE_ATTRIBUTE));
	}
	else if (containingBean != null) {
		// Take default from containing bean in case of an inner bean definition.
		bd.setScope(containingBean.getScope());
	}

	if (ele.hasAttribute(ABSTRACT_ATTRIBUTE)) {
		bd.setAbstract(TRUE_VALUE.equals(ele.getAttribute(ABSTRACT_ATTRIBUTE)));
	}
	//处理lazy-init属性
	String lazyInit = ele.getAttribute(LAZY_INIT_ATTRIBUTE);
	if (DEFAULT_VALUE.equals(lazyInit)) {
		lazyInit = this.defaults.getLazyInit();
	}
	bd.setLazyInit(TRUE_VALUE.equals(lazyInit));

	String autowire = ele.getAttribute(AUTOWIRE_ATTRIBUTE);
	bd.setAutowireMode(getAutowireMode(autowire));

	if (ele.hasAttribute(DEPENDS_ON_ATTRIBUTE)) {
		String dependsOn = ele.getAttribute(DEPENDS_ON_ATTRIBUTE);
		bd.setDependsOn(StringUtils.tokenizeToStringArray(dependsOn, MULTI_VALUE_ATTRIBUTE_DELIMITERS));
	}

	String autowireCandidate = ele.getAttribute(AUTOWIRE_CANDIDATE_ATTRIBUTE);
	if ("".equals(autowireCandidate) || DEFAULT_VALUE.equals(autowireCandidate)) {
		String candidatePattern = this.defaults.getAutowireCandidates();
		if (candidatePattern != null) {
			String[] patterns = StringUtils.commaDelimitedListToStringArray(candidatePattern);
			bd.setAutowireCandidate(PatternMatchUtils.simpleMatch(patterns, beanName));
		}
	}
	else {
		bd.setAutowireCandidate(TRUE_VALUE.equals(autowireCandidate));
	}

	if (ele.hasAttribute(PRIMARY_ATTRIBUTE)) {
		bd.setPrimary(TRUE_VALUE.equals(ele.getAttribute(PRIMARY_ATTRIBUTE)));
	}

	if (ele.hasAttribute(INIT_METHOD_ATTRIBUTE)) {
		String initMethodName = ele.getAttribute(INIT_METHOD_ATTRIBUTE);
		bd.setInitMethodName(initMethodName);
	}
	else if (this.defaults.getInitMethod() != null) {
		bd.setInitMethodName(this.defaults.getInitMethod());
		bd.setEnforceInitMethod(false);
	}

	if (ele.hasAttribute(DESTROY_METHOD_ATTRIBUTE)) {
		String destroyMethodName = ele.getAttribute(DESTROY_METHOD_ATTRIBUTE);
		bd.setDestroyMethodName(destroyMethodName);
	}
	else if (this.defaults.getDestroyMethod() != null) {
		bd.setDestroyMethodName(this.defaults.getDestroyMethod());
		bd.setEnforceDestroyMethod(false);
	}

	if (ele.hasAttribute(FACTORY_METHOD_ATTRIBUTE)) {
		bd.setFactoryMethodName(ele.getAttribute(FACTORY_METHOD_ATTRIBUTE));
	}
	if (ele.hasAttribute(FACTORY_BEAN_ATTRIBUTE)) {
		bd.setFactoryBeanName(ele.getAttribute(FACTORY_BEAN_ATTRIBUTE));
	}

	return bd;
}

接着在实例化bean过程中会判断属性是否延迟实例化。
DefaultListableBeanFactory#preInstantiateSingletons()

public void preInstantiateSingletons() throws BeansException {
	if (this.logger.isDebugEnabled()) {
		this.logger.debug("Pre-instantiating singletons in " + this);
	}

	// Iterate over a copy to allow for init methods which in turn register new bean definitions.
	// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		//如果是延迟实例化,则绕过
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					final FactoryBean<?> factory = (FactoryBean<?>) bean;
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
										((SmartFactoryBean<?>) factory)::isEagerInit,
								getAccessControlContext());
					}
					else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean<?>) factory).isEagerInit());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
			}
			else {
				getBean(beanName);
			}
		}
	}

	// Trigger post-initialization callback for all applicable beans...
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					smartSingleton.afterSingletonsInstantiated();
					return null;
				}, getAccessControlContext());
			}
			else {
				smartSingleton.afterSingletonsInstantiated();
			}
		}
	}
}