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Spring实例化的策略有几种 ，可以看一下 InstantiationStrategy 相关的类。

UML 结构图

InstantiationStrategy的实现类有 SimpleInstantiationStrategy。 CglibSubclassingInstantiationStrategy 又继承了SimpleInstantiationStrategy。

InstantiationStrategy

InstantiationStrategy . 实例化的策略， 可以根据 给定名称及默认的构造函数、 或者是指定的构造函数、 工厂方法 实例化。

InstantiationStrategy 源码如下：

/*** 负责创建与基础bean定义相对应的实例。*/
public interface InstantiationStrategy {/***  根据给定名称，返回该工厂中的bean实例。使用默认的构造函数。*/Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner)throws BeansException;/*** 返回该工厂中具有给定名称的bean实例，并通过给定构造函数创建它。*/Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,Constructor<?> ctor, @Nullable Object... args) throws BeansException;/*** 返回该工厂中具有给定名称的bean实例，并通过给定的工厂方法创建它。*/Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,@Nullable Object factoryBean, Method factoryMethod, @Nullable Object... args)throws BeansException;}

SimpleInstantiationStrategy

InstantiationStrategy 实现了 SimpleInstantiationStrategy.

SimpleInstantiationStrategy 调用的方法栈如下：

	  at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:155)at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:653)at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:486)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1352)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1195)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)at org.springframework.beans.factory.support.AbstractBeanFactory$$Lambda$195.404947798.getObject(Unknown Source:-1)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:213)at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:171)at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:746)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:564)at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145)at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:745)at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:420)at org.springframework.boot.SpringApplication.run(SpringApplication.java:307)at org.springframework.boot.SpringApplication.run(SpringApplication.java:1317)at org.springframework.boot.SpringApplication.run(SpringApplication.java:1306)

• ConstructorResolver.instantiate 实例化：
  ConstructorResolver.instantiate 会先获取对应的策略，然后再实例化。

方法具体路径：
org.springframework.beans.factory.support.ConstructorResolver#instantiate(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object, java.lang.reflect.Method, java.lang.Object[])

代码如下：

	private Object instantiate(String beanName, RootBeanDefinition mbd,@Nullable Object factoryBean, Method factoryMethod, Object[] args) {try {if (System.getSecurityManager() != null) {return AccessController.doPrivileged((PrivilegedAction<Object>) () ->this.beanFactory.getInstantiationStrategy().instantiate(mbd, beanName, this.beanFactory, factoryBean, factoryMethod, args),this.beanFactory.getAccessControlContext());}else {// 关键看这一句， 先获取对应的策略，然后再实例化。return this.beanFactory.getInstantiationStrategy().instantiate(mbd, beanName, this.beanFactory, factoryBean, factoryMethod, args);}}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Bean instantiation via factory method failed", ex);}}

接下来会调用 SimpleInstantiationStrategy#instantiate 。

org.springframework.beans.factory.support.SimpleInstantiationStrategy#instantiate(org.springframework.beans.factory.support.RootBeanDefinition, java.lang.String, org.springframework.beans.factory.BeanFactory, java.lang.Object, java.lang.reflect.Method, java.lang.Object...)

通过反射, 设置 bean 的属性值, 进行 实例化.

如下:

	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,@Nullable Object factoryBean, final Method factoryMethod, Object... args) {try {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {ReflectionUtils.makeAccessible(factoryMethod);return null;});}else {ReflectionUtils.makeAccessible(factoryMethod);}// 重点看这里, 反射,进行实例化.// currentlyInvokedFactoryMethod 的类型是 ThreadLocal<Method>  ,以此保证线程安全.Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();try {currentlyInvokedFactoryMethod.set(factoryMethod);// 通过反射, 设置 bean 的属性值Object result = factoryMethod.invoke(factoryBean, args);if (result == null) {result = new NullBean();}// 返回实例化的 bean.return result;}finally {if (priorInvokedFactoryMethod != null) {currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);}else {currentlyInvokedFactoryMethod.remove();}}}catch (IllegalArgumentException ex) {throw new BeanInstantiationException(factoryMethod,"Illegal arguments to factory method '" + factoryMethod.getName() + "'; " +"args: " + StringUtils.arrayToCommaDelimitedString(args), ex);}catch (IllegalAccessException ex) {throw new BeanInstantiationException(factoryMethod,"Cannot access factory method '" + factoryMethod.getName() + "'; is it public?", ex);}catch (InvocationTargetException ex) {String msg = "Factory method '" + factoryMethod.getName() + "' threw exception";if (bd.getFactoryBeanName() != null && owner instanceof ConfigurableBeanFactory &&((ConfigurableBeanFactory) owner).isCurrentlyInCreation(bd.getFactoryBeanName())) {msg = "Circular reference involving containing bean '" + bd.getFactoryBeanName() + "' - consider " +"declaring the factory method as static for independence from its containing instance. " + msg;}throw new BeanInstantiationException(factoryMethod, msg, ex.getTargetException());}}

CglibSubclassingInstantiationStrategy

CglibSubclassingInstantiationStrategy 实现了 SimpleInstantiationStrategy.

CglibSubclassingInstantiationStrategy 实例化 bean 的代码如下:

				/***     实例化, 动态生成*/public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {Class<?> subclass = createEnhancedSubclass(this.beanDefinition);Object instance;if (ctor == null) {instance = BeanUtils.instantiateClass(subclass);}else {try {// 实例化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);}}Factory factory = (Factory) instance;factory.setCallbacks(new Callback[] {NoOp.INSTANCE,new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});return instance;}