Spring源码解析之推断构造方法

@Component
public class OrderService {
 
	public OrderService() {
		System.out.println("无参构造方法");
	}
 
	@Autowired(required = false)
	public OrderService(UserService userService) {
		System.out.println("一个参数的构造方法");
	}
 
	@Autowired(required = false)
	public OrderService(String userName, String passWord) {
		System.out.println("两个参数的构造方法");
	}
}

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
 
		// 加载类
		// Make sure bean class is actually resolved at this point.
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
 
		// 确保class不为空，并且访问权限为public
		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());
		}
 
		// 配置的一种特殊的callback回调方法，通过这个callback创建bean
		// 检查BeanDefinition是否包含了一个Supplier
		Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			// 如果有就直接调用Supplier的get方法得到一个对象直接返回
			return obtainFromSupplier(instanceSupplier, beanName);
		}
 
		// 通过工厂方法创建
		if (mbd.getFactoryMethodName() != null) {
			// 如果BeanDefinition中存在FactoryMethodName，那么调用工厂方法得到一个bean对象并返回
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}
 
		// 一个类可能有多个构造器，所以Spring得根据参数个数、类型确定需要调用的构造器
		// 在使用构造器创建实例后，Spring会将解析过后确定下来的构造器或工厂方法保存在缓存中，避免再次创建相同bean时再次解析
		// Shortcut when re-creating the same bean...
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					// 已经解析过class的构造器
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
		if (resolved) {
			// 已经解析过class的构造器，使用已经解析好的构造器
			if (autowireNecessary) {
				// 如果BeanDefinition中已经构造过
				// 构造函数自动注入
				// 自动装配构造函数，俗称推断构造方法
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				// 使用默认构造器
				return instantiateBean(beanName, mbd);
			}
		}
 
		// TODO 推断构造方法
		// 需要根据参数解析、确定构造函数
		// 寻找当前实例化的bean中构造器是否有@Autowire注解
		// Candidate constructors for autowiring?
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
 
		// 解析的构造器不为空 || 注入类型为构造函数自动注入 || beanDefinition指定了构造方法参数值 || getBean时指定了构造方法参数
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
 
			// 到这里可能找到了多个构造方法，还要决定到底用哪个进行反射初始化
			return autowireConstructor(beanName, mbd, ctors, args);
		}
 
		// 默认构造的首选构造函数？
		// Preferred constructors for default construction?
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}
 
		// 无需特殊处理：只需使用默认的无参构造函数
		// No special handling: simply use no-arg constructor.
		return instantiateBean(beanName, mbd);
	}

protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)
			throws BeansException {
 
		// 装配beanPostProcessor的时候会判断其类型并设置 hasInstantiationAwareBeanPostProcessors 属性, 符合条件才去找构造函数
		if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
 
			// getBeanPostProcessors拿到beanFactory中的所有BeanPostProcessor接口，找到一个合格的构造函数
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
					SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
 
					// 获取有autowire注解的构造函数 找到合格的构造函数
					// AutowiredAnnotationBeanPostProcessor.determineCandidateConstructors
					Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
					if (ctors != null) {
						return ctors;
					}
				}
			}
		}
		return null;
	}

public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
			throws BeanCreationException {
 
		// @Lookup标识的属性每次调用都会被重新初始化，
		// 有些场景下原型类型的Bean就需要这样做，否则每个Bean只会在spring容器初始化的时候创建一次，
		// 但是如果在一个单例的Bean中注入了一个原型的Bean，这样的话原本原型的Bean就相当于变成了一个单例的Bean失去了原有的意义，
		// 这时就需要@Lookup来解决，或者是每次都从新从spring容器中通过getBean来获取Bean
		// Let's check for lookup methods here...
		if (!this.lookupMethodsChecked.contains(beanName)) {
			if (AnnotationUtils.isCandidateClass(beanClass, Lookup.class)) {
				try {
					Class<?> targetClass = beanClass;
					do {
						ReflectionUtils.doWithLocalMethods(targetClass, method -> {
							Lookup lookup = method.getAnnotation(Lookup.class);
							if (lookup != null) {
								Assert.state(this.beanFactory != null, "No BeanFactory available");
								LookupOverride override = new LookupOverride(method, lookup.value());
								try {
									RootBeanDefinition mbd = (RootBeanDefinition)this.beanFactory.getMergedBeanDefinition(beanName);
									mbd.getMethodOverrides().addOverride(override);
								}
								catch (NoSuchBeanDefinitionException ex) {
									throw new BeanCreationException(beanName,
											"Cannot apply @Lookup to beans without corresponding bean definition");
								}
							}
						});
						targetClass = targetClass.getSuperclass();
					}
					while (targetClass != null && targetClass != Object.class);
 
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
				}
			}
			this.lookupMethodsChecked.add(beanName);
		}
 
		// 一般只有原型的bean才会创建多次
		// Quick check on the concurrent map first, with minimal locking.
		Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
		if (candidateConstructors == null) {
			// Fully synchronized resolution now...
			synchronized (this.candidateConstructorsCache) {
				candidateConstructors = this.candidateConstructorsCache.get(beanClass);
				if (candidateConstructors == null) {
					Constructor<?>[] rawCandidates;
					try {
						// 获取所有构造方法
						rawCandidates = beanClass.getDeclaredConstructors();
					}
					catch (Throwable ex) {
						throw new BeanCreationException(beanName,
								"Resolution of declared constructors on bean Class [" + beanClass.getName() +
								"] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
					}
					// 定义要选举的构造方法集合
					List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
 
					// 加了@AutoWire()并且是require=true的构造方法
					Constructor<?> requiredConstructor = null;
 
					// 默认构造发给方法
					Constructor<?> defaultConstructor = null;
 
					// 返回与 Kotlin 主构造函数相对应的 Java 构造函数, 否则，特别是对于非 Kotlin 类，这只会返回 {@code null}。
					Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
 
					// 记录合成的构造方法数量，理解为可用的构造方法个数吧
					int nonSyntheticConstructors = 0;
 
					// 遍历所有的构造方法
					for (Constructor<?> candidate : rawCandidates) {
						if (!candidate.isSynthetic()) {
							nonSyntheticConstructors++;
						}
						else if (primaryConstructor != null) {
							continue;
						}
 
						// 加了@Autowired的构造方法
						MergedAnnotation<?> ann = findAutowiredAnnotation(candidate);
						if (ann == null) {
							Class<?> userClass = ClassUtils.getUserClass(beanClass);
							if (userClass != beanClass) {
								try {
									Constructor<?> superCtor = userClass.getDeclaredConstructor(candidate.getParameterTypes());
 
									// 在父类中找@Autowired的构造方法
									ann = findAutowiredAnnotation(superCtor);
								}
								catch (NoSuchMethodException ex) {
									// Simply proceed, no equivalent superclass constructor found...
								}
							}
						}
						if (ann != null) {
							// 如果找到加了@Autowired注解的构造方法，再判断required属性
 
							// 加了@AutoWire()并且是require=true的构造方法
							if (requiredConstructor != null) {
								throw new BeanCreationException(beanName,
										"Invalid autowire-marked constructor: " + candidate +
										". Found constructor with 'required' Autowired annotation already: " +
										requiredConstructor);
							}
							boolean required = determineRequiredStatus(ann);
							if (required) {
								if (!candidates.isEmpty()) {
									throw new BeanCreationException(beanName,
											"Invalid autowire-marked constructors: " + candidates +
											". Found constructor with 'required' Autowired annotation: " +
											candidate);
								}
								requiredConstructor = candidate;
							}
							candidates.add(candidate);
						}
						else if (candidate.getParameterCount() == 0) {
							// 否则如果构造函数参数个数为0，把它赋值给变量defaultConstructor
							defaultConstructor = candidate;
						}
					}
 
					// 处理上面遍历后的结果
					if (!candidates.isEmpty()) {
						// Add default constructor to list of optional constructors, as fallback.
						if (requiredConstructor == null) {
							// 如果加了@Autowired、并且没有指定required为true、并且存在默认的构造方法
							if (defaultConstructor != null) {
								// 把默认构造方法加到待筛选的集合中
								candidates.add(defaultConstructor);
							}
							else if (candidates.size() == 1 && logger.isInfoEnabled()) {
								logger.info("Inconsistent constructor declaration on bean with name '" + beanName +
										"': single autowire-marked constructor flagged as optional - " +
										"this constructor is effectively required since there is no " +
										"default constructor to fall back to: " + candidates.get(0));
							}
						}
						candidateConstructors = candidates.toArray(new Constructor<?>[0]);
					}
					// 如果只有一个构造方法，并且构造数的参数大于0
					else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) {
						candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
					}
					// primaryConstructor 做java开发一般都是null
					else if (nonSyntheticConstructors == 2 && primaryConstructor != null &&
							defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) {
						candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor};
					}
					// primaryConstructor 做java开发一般都是null
					else if (nonSyntheticConstructors == 1 && primaryConstructor != null) {
						candidateConstructors = new Constructor<?>[] {primaryConstructor};
					}
					else {
						candidateConstructors = new Constructor<?>[0];
					}
					// 把推断的构造方法数组放到缓存map中
					this.candidateConstructorsCache.put(beanClass, candidateConstructors);
				}
			}
		}
		return (candidateConstructors.length > 0 ? candidateConstructors : null);

protected BeanWrapper autowireConstructor(
			String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {
		// 带有参数情况的实例化
		return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
	}

public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd,
			@Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {
 
		//实例化BeanWrapper，是包装bean的容器
		BeanWrapperImpl bw = new BeanWrapperImpl();
		this.beanFactory.initBeanWrapper(bw);
 
		Constructor<?> constructorToUse = null;
		ArgumentsHolder argsHolderToUse = null;
		Object[] argsToUse = null;
 
		// 1、首先判断是否通过getBean方法指定了构造方法参数值
		// 如果getBean中传入的参数不为空，那么就使用传入的参数
		if (explicitArgs != null) {
			argsToUse = explicitArgs;
		}
		// 否则就需要解析配置文件中的参数
		else {
			Object[] argsToResolve = null;
			// 先尝试从缓存中获取
			synchronized (mbd.constructorArgumentLock) {
				// 缓存中的构造器
				constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
 
				// 2、针对当前BeanDefinition是否缓存了构造方法和构造方法参数值
				if (constructorToUse != null && mbd.constructorArgumentsResolved) {
					// 在缓存中找到了构造器，就继续从缓存中寻找缓存的构造器参数
					// Found a cached constructor...
					argsToUse = mbd.resolvedConstructorArguments;
					if (argsToUse == null) {
						// 没有缓存的参数，就需要获取配置文件中配置的参数
						argsToResolve = mbd.preparedConstructorArguments;
					}
				}
			}
			// 如果缓存中没有缓存的参数的话，即argsToResolve不为空，就需要解析配置的参数
			if (argsToResolve != null) {
				// 解析参数类型，比如将配置的String类型转换成int、boolean等类型
				argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve, true);
			}
		}
 
		// 3、如果两者任意一个为空，则继续进行下面的步骤
		// 如果没有缓存，就需要从构造函数开始解析
		if (constructorToUse == null || argsToUse == null) {
 
			// 如果传入的构造器数组不为空，就使用传入的构造器参数，否则通过反射获取class中定义的构造器
			// Take specified constructors, if any.
			Constructor<?>[] candidates = chosenCtors;
 
			// 3.1 如果没有传入构造方法，那么则获取当前BeanDefinition对应的BeanClass中所有的构造方法作为候选者
			if (candidates == null) {
				Class<?> beanClass = mbd.getBeanClass();
				try {
					// 使用public的构造器或者所有构造器
					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);
				}
			}
 
			// 3.2 判断候选者构造方法是不是只有一个，并且没有指定构造方法参数
			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;
					}
					// 初始化并设置构造器参数
					bw.setBeanInstance(instantiate(beanName, mbd, uniqueCandidate, EMPTY_ARGS));
					return bw;
				}
			}
 
			// 是否需要解析构造器，在配置文件中指定注入方式为构造器注入
			// Need to resolve the constructor.
			boolean autowiring = (chosenCtors != null ||
					mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
 
			// 存放解析后的构造方法参数值
			ConstructorArgumentValues resolvedValues = null;
 
			int minNrOfArgs;
			if (explicitArgs != null) {
				// getBean方法传入的参数
				minNrOfArgs = explicitArgs.length;
			}
			else {
				// 配置文件中的配置的参数
				ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
				// 用于承载解析后的构造函数参数的值
				resolvedValues = new ConstructorArgumentValues();
				// 解析配置文件中的参数，并且返回参数个数
				minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
			}
 
			// 3.3 对候选者构造函数排序，public构造函数优先、参数数量降序排序
			AutowireUtils.sortConstructors(candidates);
 
			// 计算构造方法参数个数最少个数
			// 意思是如果指定了构造方法参数个数，所以当前BeanDefinition对应的BeanClass中所有构造方法参数个数至少满足手动指定的参数值个数
			int minTypeDiffWeight = Integer.MAX_VALUE;
 
			Set<Constructor<?>> ambiguousConstructors = null;
			LinkedList<UnsatisfiedDependencyException> causes = null;
 
			// 3.4 遍历所有的构造方法
			for (Constructor<?> candidate : candidates) {
				int parameterCount = candidate.getParameterCount();
 
				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;
				}
 
				// 如果候选者参数个数 < minNrOfArgs，则不匹配，继续下一个
				if (parameterCount < minNrOfArgs) {
					continue;
				}
 
				// 封装解析到的参数信息
				ArgumentsHolder argsHolder;
 
				Class<?>[] paramTypes = candidate.getParameterTypes();
 
				// 解析配置文件得到的构造方法参数值
				if (resolvedValues != null) {
					try {
						// 3.5 判断通过getBean方法指定构造方法参数
						String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, parameterCount);
						if (paramNames == null) {
							ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
							if (pnd != null) {
								paramNames = pnd.getParameterNames(candidate);
							}
						}
						// 参数个数匹配的情况下把所有参数封装为一个ArgumentsHolder对象，不匹配就直接报错了
						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 {
					// 处理参数由getBean方法传入的情况
					// Explicit arguments given -> arguments length must match exactly.
					if (parameterCount != explicitArgs.length) {
						continue;
					}
					argsHolder = new ArgumentsHolder(explicitArgs);
				}
 
				// 3.7 计算得到的构造方法参数值和参数的匹配程度
				// 因为不同构造函数的参数个数相同，而且参数类型为父子关系，所以需要找出类型最符合的一个构造函数
				// Spring用一种权重的形式来表示类型差异程度，差异权重越小越优先
				// 如果是以宽松的方式解析，默认为true，所以执行getTypeDifferenceWeight
				int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
						argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
 
				// 当前构造函数最为匹配的话，清空先前ambiguousConstructors列表
				// Choose this constructor if it represents the closest match.
				if (typeDiffWeight < minTypeDiffWeight) {
					constructorToUse = candidate;
					argsHolderToUse = argsHolder;
					argsToUse = argsHolder.arguments;
					minTypeDiffWeight = typeDiffWeight;
					ambiguousConstructors = null;
				}
				// 存在相同权重的构造器，将构造器添加到一个ambiguousConstructors列表变量中
				// 注意,这时候constructorToUse 指向的仍是第一个匹配的构造函数
				else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
					if (ambiguousConstructors == null) {
						ambiguousConstructors = new LinkedHashSet<>();
						ambiguousConstructors.add(constructorToUse);
					}
					ambiguousConstructors.add(candidate);
				}
			}
 
			/*******************************************************************************************************/
 
			if (constructorToUse == null) {
				// 如果没有匹配的构造函数，抛出异常。略
				if (causes != null) {
					UnsatisfiedDependencyException ex = causes.removeLast();
					for (Exception cause : causes) {
						this.beanFactory.onSuppressedException(cause);
					}
					throw ex;
				}
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Could not resolve matching constructor " +
						"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities)");
			}
			else if (ambiguousConstructors != null && !mbd.isLenientConstructorResolution()) {
				// 如果存在多个构造函数匹配程度相同，并且BeanDefinition中设置isLenientConstructorResolution为false(默认值为true)，
				// 表示构造器创建为严格模式的话，会抛出异常。异常代码略
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Ambiguous constructor matches found in bean '" + beanName + "' " +
						"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +
						ambiguousConstructors);
			}
 
			if (explicitArgs == null && argsHolderToUse != null) {
				argsHolderToUse.storeCache(mbd, constructorToUse);
			}
		}
 
		Assert.state(argsToUse != null, "Unresolved constructor arguments");
 
		// 初始化
		bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse));
		return bw;
	}