部类|这个地方_[死磕Spring37/43]IOC之bean的实例化策略：InstantiationStrategy

篇首语：本文由编程笔记#小编为大家整理，主要介绍了[死磕 Spring 37/43] --- IOC 之 bean 的实例化策略： InstantiationStrategy相关的知识，希望对你有一定的参考价值。

引用原文&＃xff1a;
https://www.cmsblogs.com/article/1391375583832838144
[死磕 Spring 37/43] — IOC 之 bean 的实例化策略&＃xff1a; InstantiationStrategy

instantiation [ɪnstænʃɪ’eɪʃən] 实例化&＃xff0c;例示;

正文

在开始分析 InstantiationStrategy 之前&＃xff0c;我们先来简单回顾下 bean 的实例化过程&＃xff1a;

• bean 的创建&＃xff0c;主要是 AbstractAutowireCapableBeanFactory.doCreateBean() &＃xff0c;在这个方法中有 bean 的实例化、属性注入和初始化过程&＃xff0c;对于 bean 的实例化过程这是根据 bean 的类型来判断的&＃xff0c;如果是单例模式&＃xff0c;则直接从 factoryBeanInstanceCache 缓存中获取&＃xff0c;否则调用 createBeanInstance() 创建。
• 在 createBeanInstance() 中&＃xff0c;如果 Supplier 不为空&＃xff0c;则调用 obtainFromSupplier() 实例化 bean。如果 factory 不为空&＃xff0c;则调用 instantiateUsingFactoryMethod() 实例化 bean &＃xff0c;如果都不是则调用 instantiateBean() 实例化bean 。但是无论是 instantiateUsingFactoryMethod() 还是 instantiateBean() 最后都一定会调用到 InstantiationStrategy 接口的 instantiate()。

InstantiationStrategy

InstantiationStrategy 接口定义了 Spring Bean 实例化的策略&＃xff0c;根据创建对象情况的不同&＃xff0c;提供了三种策略&＃xff1a;无参构造方法、有参构造方法、工厂方法。如下&＃xff1a;

public interface InstantiationStrategy

/**
* 默认构造方法
*/
Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner)
throws BeansException;

/**
* 指定构造方法
*/
Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner,
Constructor<?> ctor, &＃64;Nullable Object... args) throws BeansException;

/**
* 工厂方法
*/
Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner,
&＃64;Nullable Object factoryBean, Method factoryMethod, &＃64;Nullable Object... args)
throws BeansException;




SimpleInstantiationStrategy

InstantiationStrategy 接口有两个实现类&＃xff1a;SimpleInstantiationStrategy 和 CglibSubclassingInstantiationStrategy。SimpleInstantiationStrategy 对以上三个方法都做了简单的实现。

如果是工厂方法实例化&＃xff0c;则直接使用反射创建对象&＃xff0c;如下&＃xff1a;

public Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner,
&＃64;Nullable Object factoryBean, final Method factoryMethod, &＃64;Nullable Object... args)

try
if (System.getSecurityManager() !&＃61; null)
AccessController.doPrivileged((PrivilegedAction<Object>) () ->
ReflectionUtils.makeAccessible(factoryMethod);
return null;
);

else
ReflectionUtils.makeAccessible(factoryMethod);


Method priorInvokedFactoryMethod &＃61; currentlyInvokedFactoryMethod.get();
try
currentlyInvokedFactoryMethod.set(factoryMethod);
Object result &＃61; factoryMethod.invoke(factoryBean, args);
if (result &＃61;&＃61; null)
result &＃61; new NullBean();

return result;

finally
if (priorInvokedFactoryMethod !&＃61; null)
currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);

else
currentlyInvokedFactoryMethod.remove();



// 省略 catch



如果是构造方法实例化&＃xff0c;则是先判断是否有 MethodOverrides&＃xff0c;如果没有则是直接使用反射&＃xff0c;如果有则就需要 CGLIB 实例化对象。如下&＃xff1a;

public Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner)
// Don&＃39;t override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides())
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock)
constructorToUse &＃61; (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse &＃61;&＃61; null)
final Class<?> clazz &＃61; bd.getBeanClass();
if (clazz.isInterface())
throw new BeanInstantiationException(clazz, "Specified class is an interface");

try
if (System.getSecurityManager() !&＃61; null)
constructorToUse &＃61; AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);

else
constructorToUse &＃61; clazz.getDeclaredConstructor();

bd.resolvedConstructorOrFactoryMethod &＃61; constructorToUse;

catch (Throwable ex)
throw new BeanInstantiationException(clazz, "No default constructor found", ex);



return BeanUtils.instantiateClass(constructorToUse);

else
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);



public Object instantiate(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner,
final Constructor<?> ctor, &＃64;Nullable Object... args)

if (!bd.hasMethodOverrides())
if (System.getSecurityManager() !&＃61; null)
// use own privileged to change accessibility (when security is on)
AccessController.doPrivileged((PrivilegedAction<Object>) () ->
ReflectionUtils.makeAccessible(ctor);
return null;
);

return (args !&＃61; null ? BeanUtils.instantiateClass(ctor, args) : BeanUtils.instantiateClass(ctor));

else
return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);




SimpleInstantiationStrategy 对 instantiateWithMethodInjection() 的实现任务交给了子类 CglibSubclassingInstantiationStrategy。

MethodOverrides

对于 MethodOverrides&＃xff0c;如果读者是跟着小编文章一路跟过来的话一定不会陌生&＃xff0c;在 BeanDefinitionParserDelegate 类解析 的时候是否还记得这两个方法&＃xff1a;parseLookupOverrideSubElements() 和 parseReplacedMethodSubElements() 这两个方法分别用于解析 lookup-method 和 replaced-method。parseLookupOverrideSubElements() 源码如下&＃xff1a;

更多关于 lookup-method 和 replaced-method 请看&＃xff1a;【死磕 Spring】----- IOC 之解析 bean 标签&＃xff1a;meta、lookup-method、replace-method

CGLIB 实例化策略

类 CglibSubclassingInstantiationStrategy 为 Spring 实例化 bean 的默认实例化策略&＃xff0c;其主要功能还是对父类功能进行补充&＃xff1a;其父类将 CGLIB 的实例化策略委托其实现。

--- SimpleInstantiationStrategy
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner)
throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");


--- CglibSubclassingInstantiationStrategy
&＃64;Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner)
return instantiateWithMethodInjection(bd, beanName, owner, null);



CglibSubclassingInstantiationStrategy 实例化 bean 策略是通过其内部类 CglibSubclassCreator 来实现的。

protected Object instantiateWithMethodInjection(RootBeanDefinition bd, &＃64;Nullable String beanName, BeanFactory owner,
&＃64;Nullable Constructor<?> ctor, &＃64;Nullable Object... args)
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);



创建 CglibSubclassCreator 实例然后调用其 instantiate()&＃xff0c;该方法用于动态创建子类实例&＃xff0c;同时实现所需要的 lookups&＃xff08;lookup-method、replace-method&＃xff09;。

public Object instantiate(&＃64;Nullable Constructor<?> ctor, &＃64;Nullable Object... args)
Class<?> subclass &＃61; createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor &＃61;&＃61; null)
instance &＃61; BeanUtils.instantiateClass(subclass);

else
try
Constructor<?> enhancedSubclassConstructor &＃61; subclass.getConstructor(ctor.getParameterTypes());
instance &＃61; enhancedSubclassConstructor.newInstance(args);

catch (Exception ex)
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" &＃43; subclass.getName() &＃43; "]", ex);



//这个地方解决一个bug&＃xff0c;bug提交报告https://jira.spring.io/browse/SPR-10785
// 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 &＃61; (Factory) instance;
factory.setCallbacks(new Callback[] NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner));
return instance;



调用 createEnhancedSubclass() 为提供的 BeanDefinition 创建 bean 类的增强子类。

private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition)
// cglib里面的用法&＃xff0c;对原始class进行增强&＃xff0c;并设置callback
Enhancer enhancer &＃61; new Enhancer();
enhancer.setSuperclass(beanDefinition.getBeanClass());
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
if (this.owner instanceof ConfigurableBeanFactory)
ClassLoader cl &＃61; ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();
enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));

// 过滤&＃xff0c;自定义逻辑来指定调用的callback下标
enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
enhancer.setCallbackTypes(CALLBACK_TYPES);
return enhancer.createClass();



获取子类增强 class 后&＃xff0c;如果 Constructor 实例 ctr 为空&＃xff0c;则调用默认构造函数&＃xff08;BeanUtils.instantiateClass()&＃xff09;来实例化类&＃xff0c;否则则根据构造函数类型获取具体的构造器&＃xff0c;调用 newInstance() 实例化类。在 createEnhancedSubclass() 我们注意两行代码&＃xff1a;

enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
enhancer.setCallbackTypes(CALLBACK_TYPES);


通过 MethodOverrideCallbackFilter 来定义调用 callback 类型&＃xff0c;MethodOverrideCallbackFilter 是用来定义 CGLIB 回调过滤方法的拦截器行为&＃xff0c;它继承 CglibIdentitySupport 实现 CallbackFilter 接口&＃xff0c; CallbackFilter 是 CGLIB 的一个回调过滤器&＃xff0c;CglibIdentitySupport 则为 CGLIB 提供 hashCode() 和 equals() 方法&＃xff0c;以确保 CGLIB 不会为每个 bean 生成不同的类。MethodOverrideCallbackFilter 实现 CallbackFilter accept()&＃xff1a;

public int accept(Method method)
MethodOverride methodOverride &＃61; getBeanDefinition().getMethodOverrides().getOverride(method);
if (logger.isTraceEnabled())
logger.trace("Override for &＃39;" &＃43; method.getName() &＃43; "&＃39; is [" &＃43; methodOverride &＃43; "]");

if (methodOverride &＃61;&＃61; null)
return PASSTHROUGH;

else if (methodOverride instanceof LookupOverride)
return LOOKUP_OVERRIDE;

else if (methodOverride instanceof ReplaceOverride)
return METHOD_REPLACER;

throw new UnsupportedOperationException("Unexpected MethodOverride subclass: " &＃43;
methodOverride.getClass().getName());



根据 BeanDefinition 中定义的 MethodOverride 不同&＃xff0c;返回不同的值&＃xff0c; 这里返回的 PASSTHROUGH 、LOOKUP_OVERRIDE、METHOD_REPLACER 都是 Callbak 数组的下标&＃xff0c;这里对应的数组为 CALLBACK_TYPES 数组&＃xff0c;如下&＃xff1a;

private static final Class<?>[] CALLBACK_TYPES &＃61; new Class<?>[]
NoOp.class, LookupOverrideMethodInterceptor.class, ReplaceOverrideMethodInterceptor.class;


这里又定义了两个熟悉的拦截器 &＃xff1a;LookupOverrideMethodInterceptor 和 ReplaceOverrideMethodInterceptor&＃xff0c;两个拦截器分别对应两个不同的 callback 业务&＃xff1a;

LookupOverrideMethodInterceptor

private static class LookupOverrideMethodInterceptor extends CglibIdentitySupport implements MethodInterceptor

private final BeanFactory owner;

public LookupOverrideMethodInterceptor(RootBeanDefinition beanDefinition, BeanFactory owner)
super(beanDefinition);
this.owner &＃61; owner;


&＃64;Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable
// Cast is safe, as CallbackFilter filters are used selectively.
LookupOverride lo &＃61; (LookupOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
Assert.state(lo !&＃61; null, "LookupOverride not found");
Object[] argsToUse &＃61; (args.length > 0 ? args : null); // if no-arg, don&＃39;t insist on args at all
if (StringUtils.hasText(lo.getBeanName()))
return (argsToUse !&＃61; null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
this.owner.getBean(lo.getBeanName()));

else
return (argsToUse !&＃61; null ? this.owner.getBean(method.getReturnType(), argsToUse) :
this.owner.getBean(method.getReturnType()));





ReplaceOverrideMethodInterceptor

private static class ReplaceOverrideMethodInterceptor extends CglibIdentitySupport implements MethodInterceptor

private final BeanFactory owner;

public ReplaceOverrideMethodInterceptor(RootBeanDefinition beanDefinition, BeanFactory owner)
super(beanDefinition);
this.owner &＃61; owner;


&＃64;Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable
ReplaceOverride ro &＃61; (ReplaceOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
Assert.state(ro !&＃61; null, "ReplaceOverride not found");
// TODO could cache if a singleton for minor performance optimization
MethodReplacer mr &＃61; this.owner.getBean(ro.getMethodReplacerBeanName(), MethodReplacer.class);
return mr.reimplement(obj, method, args);




通过这两个拦截器&＃xff0c;再加上这篇博客&＃xff1a;【死磕 Spring】----- IOC 之解析 bean 标签&＃xff1a;meta、lookup-method、replace-method&＃xff0c;是不是一道绝佳的美食。