Spring源码浅析之bean实例的创建过程(二)
在上一篇内容中,介绍了doGetBean方法的源码内容,知道了bean在创建的过程中,有三个范围,单例、多例、Scope,里面都使用到了createBean。下面本篇文章的主要内容,就是围绕createBean来进行展开。
createBean方法
/**
* Create a bean instance for the given merged bean definition (and arguments).
* The bean definition will already have been merged with the parent definition
* in case of a child definition.
* <p>All bean retrieval methods delegate to this method for actual bean creation.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
*/
protected abstract Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException;
在AbstractBeanFactory类中,有createBean接口,具体的创建过程交给了子类进行实现:AbstractAutowireCapableBeanFactory
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* 创建bean实例、填充bean实例,以及进行一些后置处理
* @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
//将bean类名解析为class引用
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
//如果resolvedClass不为空,且bean定义中没有beanClass,且bean定义拥有beanClassName
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//准备方法覆盖
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//给BeanPostProcessors一个创建代理对象的机会
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
//创建原生的bean实例
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
mbdToUse.prepareMethodOverrides()方法的作用,就是验证当前方法是不是被重载了,如果这个方法只重载了一次,那么就设置overloaded为false,来避免参数类型的检查。因为如果这个方法被重载多次,那么在实例化bean实例的时候,就会根据参数类型进行匹配,这一步消耗的时间比较多。
在Spring里面,支持两种方法的覆盖:lookup-method和replace-method,下面简单看下lookup-method的代码示例:
public class User {
public void showUser() {
System.out.println("用户。。。。。。");
}
}
public class Student extends User {
@Override
public void showUser() {
System.out.println("学生。。。。。。");
}
}
public abstract class DemoTest {
public void showUser() {
getBean().showUser();
}
public abstract User getBean();
public abstract User getBean(String name);
}
<bean id="demoTest" class="edu.demo.spring.instantiate.DemoTest" >
<lookup-method name="getBean" bean="user"></lookup-method>
</bean>
<bean id="student" class="edu.demo.spring.instantiate.Student" />
<bean id="user" class="edu.dongnao.courseware.spring.instantiate.User" />
resolveBeforeInstantiation方法就是通过调用InstantiationAwareBeanPostProcessor里面的方法,在bean实例化的前后进行一些处理,这里是一个扩展点,它会返回bean实例的代理对象,来干涉bean的实例化。
//包可见字段,表示bean实例化前后的处理器已经启动
@Nullable
volatile Boolean beforeInstantiationResolved;
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
//如果bean实例化前后的处理器已经启动,就执行下面的代码
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
//在这里要确保bean类已经被实际解析了
//如果bean不是Spring容器自己定义的,并且持有InstantiationAwareBeanPostProcessors
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
//确定给定bean定义的目标类型
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
//bean实例化前的处理
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
//bean实例化后的处理
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
doCreateBean方法
/** Cache of unfinished FactoryBean instances: FactoryBean name to BeanWrapper. */
private final ConcurrentMap<String, BeanWrapper> factoryBeanInstanceCache = new ConcurrentHashMap<>();
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
//实例化bean定义
BeanWrapper instanceWrapper = null;
//如果bean是单例的,那就先从缓存中获取,然后再进行移除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//这一步就是创建bean实例的主要步骤,里面使用了一些简单的策略,来实例化bean
//工厂方法、构造函数,简单初始化
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//获取包装之后的实例对象
Object bean = instanceWrapper.getWrappedInstance();
//获取包装之后的实例对象的类型
Class<?> beanType = instanceWrapper.getWrappedClass();
//如果类型不是空bean,就进行赋值
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
}
在bean实例化的时候,把bean封装成了BeanWrapper,BeanWrapper主要有下面的作用:
- Bean的包装
- 属性编辑器
- 属性编辑器注册表
- 类型转换器
createBeanInstance方法
/** Common lock for the four constructor fields below. */
final Object constructorArgumentLock = new Object();
/** Package-visible field for caching the resolved constructor or factory method. */
//包可见字段,用来缓存解析的构造函数和工厂方法
@Nullable
Executable resolvedConstructorOrFactoryMethod;
/** Package-visible field that marks the constructor arguments as resolved. */
//包可见字段,用来标识构造函数的参数已经解析了
boolean constructorArgumentsResolved = false;
/**
* 使用实例化策略,为指定的bean创建一个实例
* 工厂方法、构造器的自动装配、简单实例化
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
//解析获得bean对应的class
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//如果beanClass不为空,并且beanClass类的修饰符不是public
//而且不允许访问非公共的构造函数和方法,那么就抛出异常
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());
}
//如果存在Supplier实例化回调接口,那么就使用给定的回调方法来创建一个实例对象
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//如果存在工厂方法,即配置了'factory-method',那么就调用该方法来创建一个实例对象
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
//在这里,主要就是判断bean定义的构造方法是不是已经解析了
//因为找到匹配的构造方法是一个比较繁琐的过程,所以这里在找到后,会设置到bean定义中
//避免重复的去寻找匹配的构造方法
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
//加锁
synchronized (mbd.constructorArgumentLock) {
//构造方法已经解析出来了
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
//方法有参数的话,这里进行设置一下
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//如果构造方法或者工厂方法已经解析出来了
if (resolved) {
//如果有参数,就使用下面的方法进行实例化bean
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
//没有参数就使用下面的方法进行实例化bean
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
//如果上面都没有实例化bean,则意味着构造方法或者工厂方法还没有被解析
//通过SmartInstantiationAwareBeanPostProcessor来获取一些构造方法
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
//如果构造方法不为空,是通过构造器注入的,构造方法持有构造参数,或者定义了一些参数
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
//获取优先的构造函数
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
//如果上面都没有实例化,那么就使用默认的构造函数进行实例化,即无参的构造函数
return instantiateBean(beanName, mbd);
}
总结一下步骤:
- 首先解析获取到beanName对应的beanClass
- 然后进行了判断,beanClass不是空的,修饰符不是public,且不允许访问非公共的方法,就抛出异常
- 如果存在Supplier实例化回调接口,那么就使用给定的回调方法来创建一个实例对象,里面调用了方法obtainFromSupplier
- 如果配置了factory-method,那么就使用该工厂方法来实例化bean,调用了方法instantiateUsingFactoryMethod
- 接下来就是一波判断,判断该bean定义的构造方法是不是已经解析出来了,是不是有参数,参数是不是已经解析出来了
- 如果构造方法已经解析出来,且有参数的话,就调用autowireConstructor方法来实例化bean,如果没有参数,就调用instantiateBean方法来实例化bean
- 如果上面都没有实例化bean,就获取bean定义的一些构造方法,如果获取到的构造方法不是空的,并且是通过构造器注入的,且构造方法定义了一些参数,或者通过getBean外部传进来了一些参数,就调用autowireConstructor方法来实例化bean
- 如果还是没有实例化,就获取优先的构造方法,如果获取到了,就调用autowireConstructor方法来实例化bean
- 最后,上面都没有实例化bean,就使用默认的构造方法,即无参构造函数来进行实例化bean,调用了instantiateBean方法
obtainFromSupplier方法
/**
* The name of the currently created bean, for implicit dependency registration
* on getBean etc invocations triggered from a user-specified Supplier callback.
*/
private final NamedThreadLocal<String> currentlyCreatedBean = new NamedThreadLocal<>("Currently created bean");
/**
* 从给定的供应商来获取一个bean实例
* @param instanceSupplier the configured supplier
* @param beanName the corresponding bean name
* @return a BeanWrapper for the new instance
* @since 5.0
* @see #getObjectForBeanInstance
*/
protected BeanWrapper obtainFromSupplier(Supplier<?> instanceSupplier, String beanName) {
Object instance;
//获取当前线程创建的bean的名称
String outerBean = this.currentlyCreatedBean.get();
//设置当前线程创建的bean的名称
this.currentlyCreatedBean.set(beanName);
try {
//通过调用Supplier的get方法,返回一个bean实例
instance = instanceSupplier.get();
}
finally {
if (outerBean != null) {
//设置当前线程创建的bean的名称
this.currentlyCreatedBean.set(outerBean);
}
else {
//移除
this.currentlyCreatedBean.remove();
}
}
//如果instance为空,就创建NullBean 空对象
if (instance == null) {
instance = new NullBean();
}
//把实例化的bean封装成BeanWrapper
BeanWrapper bw = new BeanWrapperImpl(instance);
//初始化BeanWrapper对象
initBeanWrapper(bw);
//最后返回出去
return bw;
}
总结步骤:
- 调用Supplier的get方法返回一个bean实例对象
- 使用BeanWrapper对bean实例对象进行包装
- 初始化BeanWrapper对象
Supplier接口也是用来创建对象的,这里可以替代bean工厂,简单的使用例子如下:
public class SupplierBean {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
GenericBeanDefinition definition = new GenericBeanDefinition();
definition.setBeanClass(People.class);
definition.setInstanceSupplier(SupplierBean::getPeople);
context.registerBeanDefinition("user2", definition);
context.refresh();
}
private static People getPeople() {
return new People("翠花");
}
static class People {
private String name;
public People(String name) {
this.name = name;
}
}
}
instantiateUsingFactoryMethod方法
protected BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}
创建ConstructorResolver对象,然后调用了instantiateUsingFactoryMethod方法:
public BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
}
上面的方法比较长,下面按照顺序分段进行。
//创建BeanWrapperImpl对象
BeanWrapperImpl bw = new BeanWrapperImpl();
//初始化BeanWrapperImpl,设置ConversionService类型转换器
//并且注册了自定义的属性编辑器
this.beanFactory.initBeanWrapper(bw);
//工厂bean
Object factoryBean;
//工厂方法所在的类
Class<?> factoryClass;
//是不是静态工厂
boolean isStatic;
//获取工厂bean的名称
String factoryBeanName = mbd.getFactoryBeanName();
//如果工厂bean的名称不为空,即没有配置factory-bean,
//意味着这是一个非静态工厂
if (factoryBeanName != null) {
//如果bean定义里面获取的bean匹配上正在创建的bean,则抛异常
//工厂bean引用指向了相同的bean定义
if (factoryBeanName.equals(beanName)) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"factory-bean reference points back to the same bean definition");
}
//获取工厂bean,即工厂方法所在类的bean,不然的话没办法调用工厂方法
factoryBean = this.beanFactory.getBean(factoryBeanName);
//如果bean定义是单例的,并且bean工厂中存在了这个bean,则抛异常,重复创建
if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
throw new ImplicitlyAppearedSingletonException();
}
//获取工厂的类
factoryClass = factoryBean.getClass();
//标记为非静态工厂
isStatic = false;
}
else {
// It's a static factory method on the bean class.
//这是一个静态工厂,如果找不到对应的beanClass,那么就无法调用方法,抛出异常
if (!mbd.hasBeanClass()) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"bean definition declares neither a bean class nor a factory-bean reference");
}
//静态工厂不需要工厂bean,这里设置为null
factoryBean = null;
//获取到工厂类beanClass
factoryClass = mbd.getBeanClass();
//标记为静态工厂
isStatic = true;
}
上面一部分代码的作用,主要就是用来获取工厂方法相关的信息,继续往下查看:
//包可见字段,标志着构造函数已经被解析
boolean constructorArgumentsResolved = false;
//包可见字段,缓存中已经完全解析的参数字段
@Nullable
Object[] resolvedConstructorArguments;
//包可见字段,缓存中准备解析的参数字段
@Nullable
Object[] preparedConstructorArguments;
//工厂方法对象
Method factoryMethodToUse = null;
ArgumentsHolder argsHolderToUse = null;
//相关的参数
Object[] argsToUse = null;
//如果是通过getBean方法指定了参数,那么就直接使用
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
//否则,就通过bean定义来获取工厂方法和参数
else {
Object[] argsToResolve = null;
//加锁
synchronized (mbd.constructorArgumentLock) {
factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
//如果工厂方法已经被解析过了,并且参数也被解析了
if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached factory method...
//解析到的参数
argsToUse = mbd.resolvedConstructorArguments;
//如果为空,那就尝试去获取未被解析过的参数
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
//如果获取到了未被解析的参数,那就调用下面的方法进行解析
if (argsToResolve != null) {
//处理参数值,进行一些类型转换,比如把配置的String类型转为Int类型:A("1")转为A(1)
argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
}
}
上面的源码内容,是尝试从缓存中获取工厂方法和参数,获取不到就走下面的代码:
//如果上面没有获取到工厂方法和对应的参数,就走下面的代码
if (factoryMethodToUse == null || argsToUse == null) {
// Need to determine the factory method...
// Try all methods with this name to see if they match the given arguments.
//获取工厂方法所在类的实例class,因为它可能是cglib包装过的类
factoryClass = ClassUtils.getUserClass(factoryClass);
List<Method> candidates = null;
//
if (mbd.isFactoryMethodUnique) {
//获取工厂方法
if (factoryMethodToUse == null) {
factoryMethodToUse = mbd.getResolvedFactoryMethod();
}
//获取所有候选的工厂方法
if (factoryMethodToUse != null) {
candidates = Collections.singletonList(factoryMethodToUse);
}
}
//如果候选的工厂方法为空
if (candidates == null) {
candidates = new ArrayList<>();
//获取工厂方法所在的类中所有的方法
Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
//遍历进行过滤
for (Method candidate : rawCandidates) {
//是不是和上面的isStatic进行匹配
//是不是和定义的工厂方法名称一样,是的话就加入到候选方法的集合
if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
candidates.add(candidate);
}
}
}
//如果只找到一个匹配的方法,并且getBean里面传进来的参数explicitArgs是空的,
//并且bean定义里面也没有参数,就直接调用这个方法进行实例化,然后返回
if (candidates.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
Method uniqueCandidate = candidates.get(0);
if (uniqueCandidate.getParameterCount() == 0) {
mbd.factoryMethodToIntrospect = uniqueCandidate;
synchronized (mbd.constructorArgumentLock) {
mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
mbd.constructorArgumentsResolved = true;
mbd.resolvedConstructorArguments = EMPTY_ARGS;
}
bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, uniqueCandidate, EMPTY_ARGS));
return bw;
}
}
通过上面的代码,找到了所有匹配的工厂方法,那么到底哪个方法是真正匹配上的呢,继续往下看:
//如果找到的工厂方法大于1,先进行排序
if (candidates.size() > 1) { // explicitly skip immutable singletonList
//public修饰的构造函数优先,然后根据参数数量降序
//非public的构造函数根据参数数量降序
candidates.sort(AutowireUtils.EXECUTABLE_COMPARATOR);
}
//用来存放解析后的方法的参数值
ConstructorArgumentValues resolvedValues = null;
//是否是构造器注入的
boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
int minTypeDiffWeight = Integer.MAX_VALUE;
//匹配方法的集合
Set<Method> ambiguousFactoryMethods = null;
//确定方法参数的入参数量,匹配的方法的参数要等于或者多余它
//方法的参数数量的最小值
int minNrOfArgs;
//如果getBean里面指定了参数,那直接使用它作为参数数量的最小值
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
//否则,从bean定义中获取参数的最小值
else {
// We don't have arguments passed in programmatically, so we need to resolve the
// arguments specified in the constructor arguments held in the bean definition.
//如果bean定义中有参数值
if (mbd.hasConstructorArgumentValues()) {
//获取到方法的参数
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
//解析定义的参数值,并返回参数数量
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
else {
//无参,参数最小值为0
minNrOfArgs = 0;
}
}
//记录UnsatisfiedDependencyException异常的集合
LinkedList<UnsatisfiedDependencyException> causes = null;
上面一段代码,首先对找到的工厂方法进行了排序,然后确定方法参数的入参数量,后面要找匹配的方法,就是根据参数数量及其类型进行匹配了。
//遍历候选的方法
for (Method candidate : candidates) {
//获取到方法参数的数量
int parameterCount = candidate.getParameterCount();
//方法参数的数量必须要大于或者等于最小参数值
if (parameterCount >= minNrOfArgs) {
//保存参数的对象
ArgumentsHolder argsHolder;
//获取方法参数的类型
Class<?>[] paramTypes = candidate.getParameterTypes();
//如果通过getBean指定了参数,直接使用
if (explicitArgs != null) {
// Explicit arguments given -> arguments length must match exactly.
//指定的参数,参数长度必须完全匹配
if (paramTypes.length != explicitArgs.length) {
continue;
}
//创建ArgumentsHolder对象
argsHolder = new ArgumentsHolder(explicitArgs);
}
else {
//否则使用下面的代码解析参数
// Resolved constructor arguments: type conversion and/or autowiring necessary.
try {
String[] paramNames = null;
//获取到参数名称探测器
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
//获取到方法的参数名称
paramNames = pnd.getParameterNames(candidate);
}
//在给定解析参数的情况下,创建一个ArgumentsHolder对象
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
paramTypes, paramNames, candidate, autowiring, candidates.size() == 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;
}
}
//根据权重来获取最匹配的方法
//判断是在宽松模式还是在严格模式下进行解析
//宽松模式:使用具有“最接近的模式”来进行匹配
//严格模式:解析构造函数时,必须所有的都要进行匹配,否则抛出异常
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this factory method if it represents the closest match.
//如果该工厂方法作为接近,那就使用该工厂方法
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).
//如果具有相同参数数量的方法具有相同类型的差异权重,那么就把它加入到ambiguousFactoryMethods中
//但是,只能在非宽容的构造函数解析模式下执行该检查
//并显式忽略被覆盖的方法(具有相同的参数签名)
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);
}
}
}
上面一段代码很长,首先遍历了所有的候选方法,然后解析出方法的入参,最后再获取最佳的匹配方法。
if (factoryMethodToUse == null || argsToUse == null) {
if (causes != null) {
UnsatisfiedDependencyException ex = causes.removeLast();
for (Exception cause : causes) {
this.beanFactory.onSuppressedException(cause);
}
throw ex;
}
//。。。。。。省略的代码
//把解析出来的工厂方法和参数进行缓存,防止下次使用时再次解析
if (explicitArgs == null && argsHolderToUse != null) {
mbd.factoryMethodToIntrospect = factoryMethodToUse;
argsHolderToUse.storeCache(mbd, factoryMethodToUse);
}
}
最后执行下面的代码:
//调用工厂方法创建实例,并设置到bw中,然后返回
bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
return bw;
最后的调用在SimpleInstantiationStrategy类中的instantiate方法:
Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
try {
currentlyInvokedFactoryMethod.set(factoryMethod);
//调用工厂方法
Object result = factoryMethod.invoke(factoryBean, args);
if (result == null) {
result = new NullBean();
}
return result;
}
finally {
if (priorInvokedFactoryMethod != null) {
currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
}
else {
currentlyInvokedFactoryMethod.remove();
}
}
可以看到,上面使用到了invoke来进行工厂方法的调用。上面整个流程特别长,下面来总结一下步骤。
总结步骤:
- 创建了BeanWrapperImpl对象,然后进行了初始化,设置ConversionService类型转换器,并且注册了自定义的属性编辑器
- 然后根据factoryBeanName来判断,这个工厂方法是不是静态工厂
- 尝试获取工厂方法和对应的参数,这一步是从缓存中获取
- 上面一步获取不到,就去找所有匹配的工厂方法,然后根据方法的参数数量进行匹配
- 最后使用反射调用工厂方法进行实例化bean
总而言之,就是要获取到最匹配的工厂方法,然后获取到相关的参数,最后调用该工厂方法进行实例化bean。
autowireConstructor方法
autowireConstructor方法本质上和instantiateUsingFactoryMethod方法类似,一个是找工厂方法,一个是找构造函数,代码里面有很多相似的地方,接下来看一下代码:
protected BeanWrapper autowireConstructor(
String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {
return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}
创建ConstructorResolver对象,然后调用了autowireConstructor方法:
public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd,
@Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {}
这里的代码也比较长,下面进行分段。
//创建BeanWrapperImpl对象
BeanWrapperImpl bw = new BeanWrapperImpl();
//初始化BeanWrapperImpl,设置ConversionService类型转换器
//并且注册了自定义的属性编辑器
this.beanFactory.initBeanWrapper(bw);
//构造方法
Constructor<?> constructorToUse = null;
ArgumentsHolder argsHolderToUse = null;
//构造方法的参数
Object[] argsToUse = null;
//如果getBean中设置了参数,就直接使用
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
//否则,从bean定义中尝试获取已经解析的构造方法和参数
else {
//这里是为了防止再次进行解析,因为前面可能已经解析过了
Object[] argsToResolve = null;
//加锁
synchronized (mbd.constructorArgumentLock) {
//获取已经解析的构造方法
constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
//如果解析后的构造方法不为空,并且参数也被解析过了
if (constructorToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached constructor...
argsToUse = mbd.resolvedConstructorArguments;
//如果获取到的解析过的构造参数是空的,那么就尝试从bean定义中获取未被解析的构造参数
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
//如果获取到了未被解析的构造参数,那就调用下面的方法进行解析
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve, true);
}
}
上面的代码,首先是尝试获取构造方法和参数,如果获取不到就走下面的代码:
//如果上一步没有找到对应的构造方法和参数,就开始寻找匹配的构造方法
if (constructorToUse == null || argsToUse == null) {
// Take specified constructors, if any.
//获取所有的构造方法,如果指定了构造方法的集合,就使用这个集合chosenCtors
Constructor<?>[] candidates = chosenCtors;
if (candidates == null) {
//获取到beanClass
Class<?> beanClass = mbd.getBeanClass();
try {
candidates = (mbd.isNonPublicAccessAllowed() ?
beanClass.getDeclaredConstructors() : beanClass.getConstructors());
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Resolution of declared constructors on bean Class [" + beanClass.getName() +
"] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
}
}
//如果只找到了一个构造方法,并且getBean传过来的参数是空的
//并且bean定义也没有参数,那么就直接调用这个构造方法来返回一个bean实例
if (candidates.length == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
Constructor<?> uniqueCandidate = candidates[0];
if (uniqueCandidate.getParameterCount() == 0) {
synchronized (mbd.constructorArgumentLock) {
mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
mbd.constructorArgumentsResolved = true;
mbd.resolvedConstructorArguments = EMPTY_ARGS;
}
bw.setBeanInstance(instantiate(beanName, mbd, uniqueCandidate, EMPTY_ARGS));
return bw;
}
}
}
上面一部分代码,是开始获取构造函数和方法了。
// Need to resolve the constructor.
//是否是构造器注入
boolean autowiring = (chosenCtors != null ||
mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
//用来存放解析后的方法的参数值
ConstructorArgumentValues resolvedValues = null;
//方法参数值的最小数量
int minNrOfArgs;
//如果getBean传来的参数不为空,就直接使用它的长度作为参数值的最小数量
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
//否则,从bean定义中获取
else {
//获取构造参数
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
//解析构造参数,并返回参数的数量
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
//进行排序,public优先,参数个数多的优先
AutowireUtils.sortConstructors(candidates);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Constructor<?>> ambiguousConstructors = null;
LinkedList<UnsatisfiedDependencyException> causes = null;
上面的内容,是获取构造方法的入参数量,下面会根据这个参数的数量来进行匹配:
//遍历所有的构造函数
for (Constructor<?> candidate : candidates) {
//获取构造参数的数量
int parameterCount = candidate.getParameterCount();
//如果已经存在匹配的构造函数和参数,则跳出循环
if (constructorToUse != null && argsToUse != null && argsToUse.length > parameterCount) {
// Already found greedy constructor that can be satisfied ->
// do not look any further, there are only less greedy constructors left.
break;
}
//如果这个构造函数的参数小于最小参数值,则不符合
if (parameterCount < minNrOfArgs) {
continue;
}
//用来保存参数的对象
ArgumentsHolder argsHolder;
Class<?>[] paramTypes = candidate.getParameterTypes();
if (resolvedValues != null) {
try {
//获取构造方法的参数名称
String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, parameterCount);
if (paramNames == null) {
//如果没有获取到,再使用ParameterNameDiscoverer来获取
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
}
//在给定解析参数的情况下,创建一个ArgumentsHolder对象
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames,
getUserDeclaredConstructor(candidate), autowiring, candidates.length == 1);
}
catch (UnsatisfiedDependencyException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Ignoring constructor [" + candidate + "] of bean '" + beanName + "': " + ex);
}
// Swallow and try next constructor.
if (causes == null) {
causes = new LinkedList<>();
}
causes.add(ex);
continue;
}
}
else {
// Explicit arguments given -> arguments length must match exactly.
//给出的显式参数->参数长度必须完全匹配
if (parameterCount != explicitArgs.length) {
continue;
}
//创建ArgumentsHolder对象
argsHolder = new ArgumentsHolder对象(explicitArgs);
}
//根据权重来获取最匹配的方法
//判断是在宽松模式还是在严格模式下进行解析
//宽松模式:使用具有“最接近的模式”来进行匹配
//严格模式:解析构造函数时,必须所有的都要进行匹配,否则抛出异常
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this constructor if it represents the closest match.
if (typeDiffWeight < minTypeDiffWeight) {
constructorToUse = candidate;
argsHolderToUse = argsHolder;
argsToUse = argsHolder.arguments;
minTypeDiffWeight = typeDiffWeight;
ambiguousConstructors = null;
}
else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
if (ambiguousConstructors == null) {
ambiguousConstructors = new LinkedHashSet<>();
ambiguousConstructors.add(constructorToUse);
}
ambiguousConstructors.add(candidate);
}
}
上面这么长的代码,就是根据参数的数量和类型,来获取最为匹配的构造方法
//把解析出来的构造方法和参数进行缓存,防止下次使用时再次解析
if (explicitArgs == null && argsHolderToUse != null) {
argsHolderToUse.storeCache(mbd, constructorToUse);
}
//调用构造方法创建实例,并设置到bw中,然后返回
Assert.state(argsToUse != null, "Unresolved constructor arguments");
bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse));
return bw;
然后进入到InstantiationStrategy类中,查看调用的接口:
//通过指定的构造函数实例化bean对象
Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
Constructor<?> ctor, Object... args) throws BeansException;
看下具体的实现:
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
final Constructor<?> ctor, Object... args) {
//bean是否存在方法重写,如果不存在就使用newInstance实例化
//否则使用cglib实例化
if (!bd.hasMethodOverrides()) {
if (System.getSecurityManager() != null) {
// use own privileged to change accessibility (when security is on)
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
ReflectionUtils.makeAccessible(ctor);
return null;
});
}
return BeanUtils.instantiateClass(ctor, args);
}
else {
return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
}
}
来看下BeanUtils.instantiateClass方法:
try {
ReflectionUtils.makeAccessible(ctor);
if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
return KotlinDelegate.instantiateClass(ctor, args);
}
else {
Class<?>[] parameterTypes = ctor.getParameterTypes();
Assert.isTrue(args.length <= parameterTypes.length, "Can't specify more arguments than constructor parameters");
Object[] argsWithDefaultValues = new Object[args.length];
for (int i = 0 ; i < args.length; i++) {
if (args[i] == null) {
Class<?> parameterType = parameterTypes[i];
argsWithDefaultValues[i] = (parameterType.isPrimitive() ? DEFAULT_TYPE_VALUES.get(parameterType) : null);
}
else {
argsWithDefaultValues[i] = args[i];
}
}
return ctor.newInstance(argsWithDefaultValues);
}
}
如果使用cglib会进入CglibSubclassingInstantiationStrategy类中:
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor == null) {
instance = BeanUtils.instantiateClass(subclass);
}
else {
try {
Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
instance = enhancedSubclassConstructor.newInstance(args);
}
catch (Exception ex) {
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
}
}
// SPR-10785: set callbacks directly on the instance instead of in the
// enhanced class (via the Enhancer) in order to avoid memory leaks.
Factory factory = (Factory) instance;
factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
return instance;
总结步骤:
- 创建了BeanWrapperImpl对象,然后进行了初始化,设置ConversionService类型转换器,并且注册了自定义的属性编辑器
- 尝试获取构造方法和参数,如果getBean指定了参数就直接使用,否则从bean定义中取获取。从bean定义首先获取已经解析的构造方法和参数,如果获取到了尚未被解析的参数,那么就进行解析
- 获取所有匹配的构造方法,如果直接指定了构造方法的集合chosenCtors,就直接使用。如果当前获取到的构造方法只有一个,并且getBean没有指定参数,而且从bean定义中也获取不到,那么就调用这个构造方法进行实例的创建
- 上面一步如果没有实例化,就开始确定参数的最小数量,要找的构造方法的参数的数量要大于等于它,然后使用权重找到最匹配的构造方法
- 最后使用newInstance或者cglib实例化出一个bean实例
总而言之,就是要找到匹配的构造方法,如果有参数,就要进行注入,然后调用这个构造函数来实例化一个bean
instantiateBean方法
使用默认的无参构造函数进行实例化,来看下代码:
/**
* 使用默认的无参构造函数实例化bean.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @return a BeanWrapper for the new instance
*/
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
//权限验证
if (System.getSecurityManager() != null) {
//获取InstantiationStrategy对象,调用instantiate方法来创建实例对象
beanInstance = AccessController.doPrivileged(
(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
}
else {
//获取InstantiationStrategy对象,调用instantiate方法来创建实例对象
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
//保存实例化的bean
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
//初始化BeanWrapper
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
看下SimpleInstantiationStrategy类中instantiate方法:
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
//如果没有方法覆盖,就使用反射来进行实例化
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
//加锁
synchronized (bd.constructorArgumentLock) {
//尝试从bean定义中获取已经解析的构造函数
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
//如果为空,就使用默认的构造函数
if (constructorToUse == null) {
//获取class
final Class<?> clazz = bd.getBeanClass();
//如果这个类是接口,抛出异常
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
//从clazz中获取构造方法
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
//获取默认的构造方法
constructorToUse = clazz.getDeclaredConstructor();
}
//设置resolvedConstructorOrFactoryMethod,即这个构造方法已经被解析了
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//通过反射实例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
//通过CGLIB生成一个子类对象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
总结步骤:
- 首先看是否有方法覆盖,如果没有就使用反射进行实例化
- 如果没有找到已经解析出来的构造函数,就使用默认的构造函数
- 通过这个默认的构造函数实例化bean对象
- 如果存在方法覆盖,就使用CGLIB生成一个子类对象
好了,到此整个bean的创建过程源码,就已经看的差不多了,如有错误请指正,多谢!