Spring源码浅析之bean实例的创建过程(二)

2021/9/3 17:07:32

本文主要是介绍Spring源码浅析之bean实例的创建过程(二),对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!

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:

  1. 首先解析获取到beanName对应的beanClass
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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:

  1. 调用Supplier的get方法返回一个bean实例对象
  2. 使用BeanWrapper对bean实例对象进行包装
  3. 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:

  1. 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
  2. 然后根据factoryBeanName来判断,这个工厂方法是不是静态工厂
  3. 尝试获取工厂方法和对应的参数,这一步是从缓存中获取
  4. 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
  5. 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:

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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:

  1. 首先看是否有方法覆盖,如果没有就使用反射进行实例化
  2. 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
  3. 通过这个默认的构造函数实例化bean对象
  4. 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!



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