Spring这么初始化Bean实例对象

发布时间：2022-03-19 10:22:41 来源：亿速云阅读：153 作者：iii 栏目：云计算

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代码入口

DefaultListableBeanFactory的preInstantiateSingletons方法

DefaultListableBeanFactory的preInstantiateSingletons方法，顾名思义，初始化所有的单例Bean，看一下方法的定义：

public void preInstantiateSingletons() throws BeansException {
if (this.logger.isInfoEnabled()) {
this.logger.info("Pre-instantiating singletons in " + this);
}
synchronized (this.beanDefinitionMap) {
// 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<String>(this.beanDefinitionNames);
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
public Boolean run() {
return ((SmartFactoryBean) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
getBean(beanName);
}
}
}
}
}

前面的代码比较简单，根据beanName拿到BeanDefinition（即Bean的定义）。由于此方法实例化的是所有非懒加载的单例Bean，因此要实例化Bean，必须满足11行的三个定义：

（1）不是抽象的

（2）必须是单例的

（3）必须是非懒加载的

接着简单看一下第12行~第29行的代码，这段代码主要做的是一件事情：首先判断一下Bean是否FactoryBean的实现，接着判断Bean是否SmartFactoryBean的实现，假如Bean是SmartFactoryBean的实现并且eagerInit（这个单词字面意思是渴望加载，找不到一个好的词语去翻译，意思就是定义了这个Bean需要立即加载的意思）的话，会立即实例化这个Bean。Java开发人员不需要关注这段代码，因为SmartFactoryBean基本不会用到，我翻译一下Spring官网对于SmartFactoryBean的定义描述：

FactoryBean接口的扩展接口。接口实现并不表示是否总是返回单独的实例对象，比如FactoryBean.isSingleton()实现返回false的情况并不清晰地表示每次返回的都是单独的实例对象
不实现这个扩展接口的简单FactoryBean的实现，FactoryBean.isSingleton()实现返回false总是简单地告诉我们每次返回的都是单独的实例对象，暴露出来的对象只能够通过命令访问
注意：这个接口是一个有特殊用途的接口，主要用于框架内部使用与Spring相关。通常，应用提供的FactoryBean接口实现应当只需要实现简单的FactoryBean接口即可，新方法应当加入到扩展接口中去

代码示例

为了后面的代码分析方便，事先我定义一个Bean：

package org.xrq.action;

import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.InitializingBean;

public class MultiFunctionBean implements InitializingBean, BeanNameAware, BeanClassLoaderAware {

private int propertyA;

private int propertyB;

public int getPropertyA() {
return propertyA;
}

public void setPropertyA(int propertyA) {
this.propertyA = propertyA;
}

public int getPropertyB() {
return propertyB;
}

public void setPropertyB(int propertyB) {
this.propertyB = propertyB;
}

public void initMethod() {
System.out.println("Enter MultiFunctionBean.initMethod()");
}

@Override
public void setBeanClassLoader(ClassLoader classLoader) {
System.out.println("Enter MultiFunctionBean.setBeanClassLoader(ClassLoader classLoader)");
}

@Override
public void setBeanName(String name) {
System.out.println("Enter MultiFunctionBean.setBeanName(String name)");
}

@Override
public void afterPropertiesSet() throws Exception {
System.out.println("Enter MultiFunctionBean.afterPropertiesSet()");
}

@Override
public String toString() {
return "MultiFunctionBean [propertyA=" + propertyA + ", propertyB=" + propertyB + "]";
}

}

定义对应的spring.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="multiFunctionBean" class="org.xrq.action.MultiFunctionBean" init-method="initMethod" />

</beans>

利用这个MultiFunctionBean，我们可以用来探究Spring加载Bean的多种机制。

doGetBean方法构造Bean流程

上面把getBean之外的代码都分析了一下，看代码就可以知道，获取Bean对象实例，都是通过getBean方法，getBean方法最终调用的是DefaultListableBeanFactory的父类AbstractBeanFactory类的doGetBean方法，因此这部分重点分析一下doGetBean方法是如何构造出一个单例的Bean的。

看一下doGetBean方法的代码实现，比较长：

protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {

final String beanName = transformedBeanName(name);
Object bean;

// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}

else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}

// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}

if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}

final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);

// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}

// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}

// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}

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