Spring Bean生命周期之Bean的实例化详解
作者：码农的进阶之路
这篇文章主要为大家详细介绍了Spring Bean生命周期之Bean的实例化，文中示例代码介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们可以参考一下，希望能够给你带来帮助
前言
上一节说到了BeanDefinition的合并过程，这节该说Bean的实例化过程了。根据AbstractAutowireCapableBeanFactory#createBean源码逻辑 可将实例化过程分为实例化前阶段、实例化过程、实例化后阶段。
实例化前阶段
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
//省略无关代码
try {
// 这里就是我们分析的重点了 ⭐️
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
//⭐️ 注意这个逻辑：如果postProcessBeforeInstantiation方法返回非null 则将返回值作为创建的Bean。并中断正常的创建流程
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
//省略异常信息
}
try {
//真正创建Bean的逻辑 实例化Bean对象，为Bean属性赋值等，这里暂不展开
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
//省略日志输出
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {//省略异常信息
}
resolveBeforeInstantiation这个方法在BeanPostProcessor浅析 这一节分析过了 这里不再具体展开了。
如果InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation 方法返回null，那么将不会中断正常Bean创建过程。
下面就来到的Bean实例化部分了。
实例化阶段
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
//解析BeanClass，在BeanDefinition中类信息是以字符串形式展现，这里解析到字符串后 会将其加载为Class
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
//如果设置了 Supplier 回调，则使用给定的回调方法初始化策略，通过获取Supplier#get得到实例化对象
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//如果工厂方法不为空，则使用工厂方法初始化
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
//加锁
synchronized (mbd.constructorArgumentLock) {
//条件成立 说明构造函数或FactoryMethod已经被解析并被缓存，可直接利用构造函数解析
//与后面的SimpleInstantiationStrategy#instantiate呼应
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//如果已经被解析过
if (resolved) {
//条件成立 使用构造函数注入
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认构造函数解析
return instantiateBean(beanName, mbd);
}
}
// 使用SmartInstantiationAwareBeanPostProcessor 找到候选的构造函数 用于注入
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
// AutowireMode==AUTOWIRE_CONSTRUCTOR 条件成立 说明使用基于构造函数的注入方式 (默认是AUTOWIRE_NO,需要动态检测)
// mbd.hasConstructorArgumentValues() 条件成立 说明构造函数中拥有参数
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//基于构造函数自动注入
return autowireConstructor(beanName, mbd, ctors, args);
}
// 如果mbd中配置了构造函数 则使用它进行注入
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// 使用默认构造函数实例化
return instantiateBean(beanName, mbd);
}
上面将doCreateBean精简一下，只暴露出我们比较关系的部分。一目了然，Bean的实例化过程就藏在createBeanInstance方法中。
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
//解析BeanClass，在BeanDefinition中类信息是以字符串形式展现，这里解析到字符串后 会将其加载为Class
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
//如果设置了 Supplier 回调，则使用给定的回调方法初始化策略，通过获取Supplier#get得到实例化对象
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//如果工厂方法不为空，则使用工厂方法初始化
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
//加锁
synchronized (mbd.constructorArgumentLock) {
//条件成立 说明构造函数或FactoryMethod已经被解析并被缓存，可直接利用构造函数解析
//与后面的SimpleInstantiationStrategy#instantiate呼应
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//如果已经被解析过
if (resolved) {
//条件成立 使用构造函数注入
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认构造函数解析
return instantiateBean(beanName, mbd);
}
}
// 使用SmartInstantiationAwareBeanPostProcessor 找到候选的构造函数 用于注入
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
// AutowireMode==AUTOWIRE_CONSTRUCTOR 条件成立 说明使用基于构造函数的注入方式 (默认是AUTOWIRE_NO,需要动态检测)
// mbd.hasConstructorArgumentValues() 条件成立 说明构造函数中拥有参数
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//基于构造函数自动注入
return autowireConstructor(beanName, mbd, ctors, args);
}
// 如果mbd中配置了构造函数 则使用它进行注入
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// 使用默认构造函数实例化
return instantiateBean(beanName, mbd);
}
分析了上面的源码之后，我们试着总结一下上面代码主要完成的事情：
1、如果mbd配置了instanceSupplier回调，则使用instanceSupplier去初始化BeanDefinition
2、如果mbd配置了工厂方法，则使用工厂方法区初始化BeanDefinition
3、实例化BeanDefinition
- 如果mbd已经被解析过了，则根据缓存 选择使用有参构造函数注入还是默认构造函数注入
- 如果mbd没有被解析过，找到mbd中候选的构造函数(一个类可能有多个构造函数)，再根据一些限定条件 选择是基于有参构造函数初始化还是默认构造函数初始化
针对第1点，其实就是lambda8 supplier接口的使用，不再介绍。
针对第3点，其实就是通过反射机制 创建实例对象，最终调用了SimpleInstantiationStrategy#instantiate方法
针对第2点 举例说明下 工厂方法与静态工厂生成Bean的两种形式，再来展开说下instantiateUsingFactoryMethod源码。
配置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
https://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="peopleFactory" class="com.wojiushiwo.factorymethod.PeopleFactory"/>
<!--实例方法-->
<bean id="instanceMethod" factory-bean="peopleFactory" factory-method="createPeople"/>
<!--静态方法-->
<bean id="staticFactoryMethod" class="com.wojiushiwo.factorymethod.People" factory-method="createPeople"/>
</beans>
//实体对象
@Data
public class People implements Serializable {
private String name;
private Integer age;
public People() {
}
public static People createPeople() {
People people = new People();
people.setAge(18);
people.setName("我就是我");
return people;
}
}
//People工厂类
public class PeopleFactory {
public People createPeople() {
return People.createPeople();
}
}
public class FactoryMethodDemo {
public static void main(String[] args) {
ClassPathXmlApplicationContext context=new ClassPathXmlApplicationContext("META-INF/spring.xml");
context.refresh();
People people = (People) context.getBean("staticFactoryMethod");
System.out.println(people);
People people = (People) context.getBean("instanceMethod");
System.out.println(people);
context.close();
}
}
public BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
BeanWrapperImpl bw = new BeanWrapperImpl();
this.beanFactory.initBeanWrapper(bw);
Object factoryBean;
Class<?> factoryClass;
boolean isStatic;
//获取FactoryBeanName，实例方法与静态工厂方法的区别就在于有没有FactoryBeanName
String factoryBeanName = mbd.getFactoryBeanName();
if (factoryBeanName != null) {
//如果存在FactoryBeanName，则说明是实例方法
if (factoryBeanName.equals(beanName)) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"factory-bean reference points back to the same bean definition");
}
//获取当前factoryBeanName名称的Bean
factoryBean = this.beanFactory.getBean(factoryBeanName);
if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
throw new ImplicitlyAppearedSingletonException();
}
//获取工厂类Class
factoryClass = factoryBean.getClass();
//标记为非静态
isStatic = false;
}
else {
// 走到这里，说明是静态工厂方法
if (!mbd.hasBeanClass()) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"bean definition declares neither a bean class nor a factory-bean reference");
}
//factoryBean设置为null
factoryBean = null;
//获取工厂类Class，这里使用BeanDefinition作为其工厂类
factoryClass = mbd.getBeanClass();
//标记为非静态
isStatic = true;
}
Method factoryMethodToUse = null;
ArgumentsHolder argsHolderToUse = null;
Object[] argsToUse = null;
//explicitArgs 这个是getBean方法传递过来的，一般为null
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
else {
Object[] argsToResolve = null;
//加锁
synchronized (mbd.constructorArgumentLock) {
//获取被解析的工厂方法
factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
//条件成立 说明工厂方法已经被解析过了，并存到了mbd中缓存起来了
if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached factory method...
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
}
}
if (factoryMethodToUse == null || argsToUse == null) {
//获取工厂类
factoryClass = ClassUtils.getUserClass(factoryClass);
//获取类中的方法
Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
List<Method> candidateList = new ArrayList<>();
for (Method candidate : rawCandidates) {
//如果方法修饰符包含static，并且方法名称是配置的FactoryMethod，则添加到候选集合中
if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
candidateList.add(candidate);
}
}
//如果候选集合有1个元素 并且BeanDefinition中未设置构造参数 (explicitArgs一般都为null )
if (candidateList.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
//获取方法
Method uniqueCandidate = candidateList.get(0);
//如果方法参数为空
if (uniqueCandidate.getParameterCount() == 0) {
mbd.factoryMethodToIntrospect = uniqueCandidate;
synchronized (mbd.constructorArgumentLock) {
//将下面这些全缓存到mbd中，下次直接用(与createBeanInstance方法呼应上了)
mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
mbd.constructorArgumentsResolved = true;
mbd.resolvedConstructorArguments = EMPTY_ARGS;
}
//实例化bd，设置到BeanWrapper中
bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, uniqueCandidate, EMPTY_ARGS));
return bw;
}
}
//程序走到这里 大概率是BeanDefinition中设置了构造参数
Method[] candidates = candidateList.toArray(new Method[0]);
//按照修饰符及方法参数 进行排序
AutowireUtils.sortFactoryMethods(candidates);
ConstructorArgumentValues resolvedValues = null;
//是否构造函数注入
boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Method> ambiguousFactoryMethods = null;
//最小参数数量 默认是0
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
else {
//走到这里 说明explicitArgs未被设置参数
//如果bd设置了构造参数，则从bd中解析参数
if (mbd.hasConstructorArgumentValues()) {
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
//得到解析的最小参数数量
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
else {
minNrOfArgs = 0;
}
}
LinkedList<UnsatisfiedDependencyException> causes = null;
//下面主要是推断参数、FactoryMethod，代码比较长 就先不分析了
for (Method candidate : candidates) {
Class<?>[] paramTypes = candidate.getParameterTypes();
if (paramTypes.length >= minNrOfArgs) {
ArgumentsHolder argsHolder;
if (explicitArgs != null) {
// Explicit arguments given -> arguments length must match exactly.
if (paramTypes.length != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
else {
// Resolved constructor arguments: type conversion and/or autowiring necessary.
try {
String[] paramNames = null;
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
paramTypes, paramNames, candidate, autowiring, candidates.length == 1);
}
catch (UnsatisfiedDependencyException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Ignoring factory method [" + candidate + "] of bean '" + beanName + "': " + ex);
}
// Swallow and try next overloaded factory method.
if (causes == null) {
causes = new LinkedList<>();
}
causes.add(ex);
continue;
}
}
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this factory method if it represents the closest match.
if (typeDiffWeight < minTypeDiffWeight) {
factoryMethodToUse = candidate;
argsHolderToUse = argsHolder;
argsToUse = argsHolder.arguments;
minTypeDiffWeight = typeDiffWeight;
ambiguousFactoryMethods = null;
}
// Find out about ambiguity: In case of the same type difference weight
// for methods with the same number of parameters, collect such candidates
// and eventually raise an ambiguity exception.
// However, only perform that check in non-lenient constructor resolution mode,
// and explicitly ignore overridden methods (with the same parameter signature).
else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&
!mbd.isLenientConstructorResolution() &&
paramTypes.length == factoryMethodToUse.getParameterCount() &&
!Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {
if (ambiguousFactoryMethods == null) {
ambiguousFactoryMethods = new LinkedHashSet<>();
ambiguousFactoryMethods.add(factoryMethodToUse);
}
ambiguousFactoryMethods.add(candidate);
}
}
}
if (factoryMethodToUse == null) {
if (causes != null) {
UnsatisfiedDependencyException ex = causes.removeLast();
for (Exception cause : causes) {
this.beanFactory.onSuppressedException(cause);
}
throw ex;
}
List<String> argTypes = new ArrayList<>(minNrOfArgs);
if (explicitArgs != null) {
for (Object arg : explicitArgs) {
argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
}
}
else if (resolvedValues != null) {
Set<ValueHolder> valueHolders = new LinkedHashSet<>(resolvedValues.getArgumentCount());
valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
valueHolders.addAll(resolvedValues.getGenericArgumentValues());
for (ValueHolder value : valueHolders) {
String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
(value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
argTypes.add(argType);
}
}
String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
//抛出异常
}
else if (void.class == factoryMethodToUse.getReturnType()) {
//抛出异常
}
else if (ambiguousFactoryMethods != null) {
//抛出异常
}
if (explicitArgs == null && argsHolderToUse != null) {
mbd.factoryMethodToIntrospect = factoryMethodToUse;
argsHolderToUse.storeCache(mbd, factoryMethodToUse);
}
}
Assert.state(argsToUse != null, "Unresolved factory method arguments");
//实例化bd 设置到BeanWrapper中
bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
return bw;
}
至此经过createBeanInstance方法 就为我们创建了一个实例对象，但是现在这个对象属性还未被赋值。
实例化后阶段
实例对象创建之后，就来到了对象属性赋值过程了，我们大致看一下populateBean方法，观察下InstantiationAwareBeanPostProcessor对属性赋值过程的影响
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
//省略无关代码
boolean continueWithPropertyPopulation = true;
//条件一 synthetic默认值是false 一般都会成立
//条件二 成立的话 说明存在InstantiationAwareBeanPostProcessor
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//在BeanPostProcessor浅析中分析到此方法时说过，若Bean实例化后回调不返回true 则对属性赋值过程产生影响。以下代码就是说明
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
//ibp.postProcessAfterInstantiation=false时 属性赋值过程终止
if (!continueWithPropertyPopulation) {
return;
}
总结
本篇文章就到这里了，希望能够给你带来帮助，也希望您能够多多关注脚本之家的更多内容!