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;
}