spring学习（五）——Bean的创建（实例化bean）
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2022-05-22 20:26:54
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参考文章：
实例化bean的代码逻辑
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
// 解析 bean ，将 bean 类名解析为 class 引用。
Class<?> beanClass = resolveBeanClass(mbd, beanName);
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 回调，则使用给定的回调方法初始化策略
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
// 使用 FactoryBean 的 factory-method 来创建，支持静态工厂和实例工厂
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
// when re-creating the same bean
// 当创建相同的bean时候
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
// constructorArgumentLock 构造函数的常用锁
synchronized (mbd.constructorArgumentLock) {
// 如果已缓存的解析的构造函数或者工厂方法不为空，则可以利用构造函数解析
// 因为需要根据参数确认到底使用哪个构造函数，该过程比较消耗性能，所有采用缓存机制
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
// 已经解析好直接注入
if (resolved) {
// autowire 自动注入，调用构造函数自动注入
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
// 使用默认构造函数构造
else {
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
// 确定解析的构造函数
// 主要是检查已经注册的 SmartInstantiationAwareBeanPostProcessor
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
// 默认构造的首选构造函数
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
// 有参数时，又没获取到构造方法，则只能调用无参构造方法来创建实例了(兜底方法)
return instantiateBean(beanName, mbd);
}根据代码可以看出实例化bean的主要方法
- obtainFromSupplier(instanceSupplier, beanName)：使用给定的回调方法初始化策略
- instantiateUsingFactoryMethod(beanName, mbd, args)：使用 FactoryBean 的 factory-method 来创建
- autowireConstructor(beanName, mbd, null, null)：autowire 自动注入，调用构造函数自动注入
- instantiateBean(beanName, mbd) ：使用默认构造函数构造
obtainFromSupplier
首先,从 BeanDefinition 中获取 Supplier 对象。如果不为空，则使用此方法
Supplier
Supplier是个接口，用于指定创建 bean 的回调
@FunctionalInterface
public interface Supplier<T> {
/**
* Gets a result.
*
* @return a result
*/
T get();
}设置Supplier的时机
public <T> RootBeanDefinition(@Nullable Class<T> beanClass, String scope, @Nullable Supplier<T> instanceSupplier) {
super();
setBeanClass(beanClass);
setScope(scope);
setInstanceSupplier(instanceSupplier);
}obtainFromSupplier
protected BeanWrapper obtainFromSupplier(Supplier<?> instanceSupplier, String beanName) {
Object instance;
// 获得bean的实例对象
String outerBean = this.currentlyCreatedBean.get();
// 设置新的 Bean 的对象名，到 currentlyCreatedBean 中
this.currentlyCreatedBean.set(beanName);
try {
// 调用 Supplier 的 get()，返回一个 Bean 对象
instance = instanceSupplier.get();
}
finally {
// 创建bean的名字到currentlyCreatedBean中
if (outerBean != null) {
this.currentlyCreatedBean.set(outerBean);
}
else {
this.currentlyCreatedBean.remove();
}
}
// 未创建 Bean 对象，则创建 NullBean 对象
if (instance == null) {
instance = new NullBean();
}
// 创建bean的包装类
BeanWrapper bw = new BeanWrapperImpl(instance);
// 初始化bean的包装
initBeanWrapper(bw);
return bw;
}- 首先，调用 Supplier 的
get()方法，获得一个 Bean 实例对象。 - 然后，根据该实例对象构造一个 BeanWrapper 对象
bw。 - 最后，初始化该对象。
instantiateUsingFactoryMethod
假如存在工厂方法，则使用工厂方法完成bean的初始化工作
protected BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}ConstructorResolver
工厂类的委托类，最终使用的是其instantiateUsingFactoryMethod的方法
public BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
//构造bean的包装类
BeanWrapperImpl bw = new BeanWrapperImpl();
//初始化包装类
this.beanFactory.initBeanWrapper(bw);
Object factoryBean;
Class<?> factoryClass;
boolean isStatic;
//获得bean的工厂名字
String factoryBeanName = mbd.getFactoryBeanName();
//假如不为空
if (factoryBeanName != null) {
//假如bean的工厂名和bean名字相同
if (factoryBeanName.equals(beanName)) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"factory-bean reference points back to the same bean definition");
}
//根据名字获得bean的工厂
factoryBean = this.beanFactory.getBean(factoryBeanName);
//假如bean是单例，且工厂已经存在
if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
throw new ImplicitlyAppearedSingletonException();
}
// 获得工厂字节码
factoryClass = factoryBean.getClass();
isStatic = false;
}
else {
// 工厂名为空，则其可能是一个静态工厂
// 静态工厂创建bean，必须要提供工厂的全类名
// It's a static factory method on the bean class.
if (!mbd.hasBeanClass()) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"bean definition declares neither a bean class nor a factory-bean reference");
}
factoryBean = null;
factoryClass = mbd.getBeanClass();
isStatic = true;
}
// 获得 factoryMethodToUse、argsHolderToUse、argsToUse 属性
Method factoryMethodToUse = null;
ArgumentsHolder argsHolderToUse = null;
Object[] argsToUse = null;
// 如果指定了构造方法的参数，则构造bean 的时候使用此构造方法
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
else { // 没有指定，则尝试从配置文件中解析
Object[] argsToResolve = null;
synchronized (mbd.constructorArgumentLock) {
// 尝试用构造工厂中获取
factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached factory method...
// 尝试从缓存中获取
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
// 获取缓存中的构造函数参数的包可见字段
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
// 缓存中存在,则解析存储在 BeanDefinition 中的参数
// 如给定方法的构造函数 A(int ,int )，则通过此方法后就会把配置文件中的("1","1")转换为 (1,1)
// 缓存中的值可能是原始值也有可能是最终值
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
}
}
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.
// 获取工厂方法的全名称
factoryClass = ClassUtils.getUserClass(factoryClass);
// 获取所有的待定方法
Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
List<Method> candidateList = new ArrayList<>();
// 检索所有方法，这里是对方法进行过滤
for (Method candidate : rawCandidates) {
// 如果有方法为静态方法，且为工厂方法则添加到candidateList中
if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
candidateList.add(candidate);
}
}
// 假如静态的工厂方法数量为1
if (candidateList.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
Method uniqueCandidate = candidateList.get(0);
// 使用静态的工厂方法创建bean
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;
}
}
// 排序构造函数
Method[] candidates = candidateList.toArray(new Method[0]);
// public 构造函数优先参数数量降序，非 public 构造函数参数数量降序
AutowireUtils.sortFactoryMethods(candidates);
// 用来承载解析后构造函数的值
ConstructorArgumentValues resolvedValues = null;
boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Method> ambiguousFactoryMethods = null;
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
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getBean没有传递参数，则需要解析保存在 BeanDefinition 构造函数中指定的参数
if (mbd.hasConstructorArgumentValues()) {
// 构造函数的参数
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
// 将构造函数的参数解析进resolvedValues中
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
else {
minNrOfArgs = 0;
}
}
LinkedList<UnsatisfiedDependencyException> causes = null;
// 遍历 candidates 数组
for (Method candidate : candidates) {
// 方法体的参数
Class<?>[] paramTypes = candidate.getParameterTypes();
if (paramTypes.length >= minNrOfArgs) {
// 保存参数的对象
ArgumentsHolder argsHolder;
// 传递了参数
if (explicitArgs != null) {
// Explicit arguments given -> arguments length must match exactly.
// 显示给定参数，参数长度必须完全匹配
if (paramTypes.length != explicitArgs.length) {
continue;
}
// 根据参数创建参数持有者 ArgumentsHolder 对象
argsHolder = new ArgumentsHolder(explicitArgs);
}
else {
// 未提供参数
// Resolved constructor arguments: type conversion and/or autowiring necessary.
try {
String[] paramNames = null;
// 获取 ParameterNameDiscoverer 对象
// ParameterNameDiscoverer 是用于解析方法和构造函数的参数名称的接口，为参数名称探测器
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
// 获取指定构造函数的参数名称
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
// 在已经解析的构造函数参数值的情况下，创建一个参数持有者 ArgumentsHolder 对象
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
paramTypes, paramNames, candidate, autowiring, candidates.length == 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;
}
}
// isLenientConstructorResolution 判断解析构造函数的时候是否以宽松模式还是严格模式
// 严格模式：解析构造函数时，必须所有的都需要匹配，否则抛出异常
// 宽松模式：使用具有"最接近的模式"进行匹配
// typeDiffWeight：类型差异权重
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this factory method if it represents the closest match.
// 代表最接近的类型匹配，则选择作为构造函数
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的类型差异权重，则收集此类型选项
// 但是，仅在非宽松构造函数解析模式下执行该检查，并显式忽略重写方法（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<>();
// 存在多个可选方法
ambiguousFactoryMethods.add(factoryMethodToUse);
}
ambiguousFactoryMethods.add(candidate);
}
}
}
// 没有可执行的工厂方法，抛出异常
if (factoryMethodToUse == null) {
if (causes != null) {
UnsatisfiedDependencyException ex = causes.removeLast();
for (Exception cause : causes) {
this.beanFactory.onSuppressedException(cause);
}
throw ex;
}
List<String> argTypes = new ArrayList<>(minNrOfArgs);
if (explicitArgs != null) {
for (Object arg : explicitArgs) {
argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
}
}
else if (resolvedValues != null) {
Set<ValueHolder> valueHolders = new LinkedHashSet<>(resolvedValues.getArgumentCount());
valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
valueHolders.addAll(resolvedValues.getGenericArgumentValues());
for (ValueHolder value : valueHolders) {
String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
(value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
argTypes.add(argType);
}
}
String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"No matching factory method found: " +
(mbd.getFactoryBeanName() != null ?
"factory bean '" + mbd.getFactoryBeanName() + "'; " : "") +
"factory method '" + mbd.getFactoryMethodName() + "(" + argDesc + ")'. " +
"Check that a method with the specified name " +
(minNrOfArgs > 0 ? "and arguments " : "") +
"exists and that it is " +
(isStatic ? "static" : "non-static") + ".");
}
else if (void.class == factoryMethodToUse.getReturnType()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Invalid factory method '" + mbd.getFactoryMethodName() +
"': needs to have a non-void return type!");
}
else if (ambiguousFactoryMethods != null) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Ambiguous factory method matches found in bean '" + beanName + "' " +
"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +
ambiguousFactoryMethods);
}
if (explicitArgs == null && argsHolderToUse != null) {
mbd.factoryMethodToIntrospect = factoryMethodToUse;
argsHolderToUse.storeCache(mbd, factoryMethodToUse);
}
}
// 验证
Assert.state(argsToUse != null, "Unresolved factory method arguments");
// 创建 Bean 对象，并设置到 bw 中
bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
return bw;
}里面有几个小方法
storeCache将构造函数、构造参数保存到缓存中
public void storeCache(RootBeanDefinition mbd, Executable constructorOrFactoryMethod) {
// 构造函数的缓存锁
synchronized (mbd.constructorArgumentLock) {
// 缓存已经解析的构造函数或者工厂方法
mbd.resolvedConstructorOrFactoryMethod = constructorOrFactoryMethod;
// 标记字段，标记构造函数、参数已经解析了。默认为 false
mbd.constructorArgumentsResolved = true;
if (this.resolveNecessary) {
// 缓存已经解析的构造函数参数，包可见字段
mbd.preparedConstructorArguments = this.preparedArguments;
}
else {
mbd.resolvedConstructorArguments = this.arguments;
}
}
}缓存的提取方法
synchronized (mbd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
if (constructorToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached constructor...
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}instantiate创建Bean
private Object instantiate(String beanName, RootBeanDefinition mbd,
@Nullable Object factoryBean, Method factoryMethod, Object[] args) {
try {
// 设置 Method 可访问
if (System.getSecurityManager() != null) {
return AccessController.doPrivileged((PrivilegedAction<Object>) () ->
this.beanFactory.getInstantiationStrategy().instantiate(
mbd, beanName, this.beanFactory, factoryBean, factoryMethod, args),
this.beanFactory.getAccessControlContext());
}
else {
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);
}
}public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
@Nullable Object factoryBean, final Method factoryMethod, Object... args) {
try {
// 设置 Method 可访问
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
ReflectionUtils.makeAccessible(factoryMethod);
return null;
});
}
else {
ReflectionUtils.makeAccessible(factoryMethod);
}
// 获得原 Method 对象
Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
try {
// 设置新的 Method 对象，到 currentlyInvokedFactoryMethod 中
currentlyInvokedFactoryMethod.set(factoryMethod);
// 创建bean
Object result = factoryMethod.invoke(factoryBean, args);
// bean不存在则创建nullbean
if (result == null) {
result = new NullBean();
}
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());
}
}autowireConstructor
带有参数的构造方法，来初始化 Bean 对象
protected BeanWrapper autowireConstructor(
String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {
return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}
public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd,
@Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {
// 封装BeanWrapper
BeanWrapperImpl bw = new BeanWrapperImpl();
// 初始化BeanWrapper
this.beanFactory.initBeanWrapper(bw);
//构造函数
Constructor<?> constructorToUse = null;
//构造参数
ArgumentsHolder argsHolderToUse = null;
//构造参数
Object[] argsToUse = null;
//假如有参数则，构造参数为传递的参数
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
else {
Object[] argsToResolve = null;
synchronized (mbd.constructorArgumentLock) {
// 尝试从缓存中获取构造函数
constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
if (constructorToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached constructor...
// 从缓存中获取构造参数
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
// 缓存中存在,则解析存储在 BeanDefinition 中的参数
// 如给定方法的构造函数 A(int ,int )，则通过此方法后就会把配置文件中的("1","1")转换为 (1,1)
// 缓存中的值可能是原始值也有可能是最终值
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve, true);
}
}
// 没有缓存，则尝试从配置文件中获取参数
if (constructorToUse == null || argsToUse == null) {
// Take specified constructors, if any.
// 如果 chosenCtors 未传入，则获取构造方法们
Constructor<?>[] candidates = chosenCtors;
if (candidates == null) {
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);
}
}
// 假如参数匹配，构造bean的包装
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;
}
}
// Need to resolve the constructor.
// 是否需要解析构造器
boolean autowiring = (chosenCtors != null ||
mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
// 用于承载解析后的构造函数参数的值
ConstructorArgumentValues resolvedValues = null;
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
else {
// 从 BeanDefinition 中获取构造参数，也就是从配置文件中提取构造参数
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
// 解析构造函数的参数
// 将该 bean 的构造函数参数解析为 resolvedValues 对象，其中会涉及到其他 bean
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
// 对构造函数进行排序处理
// public 构造函数优先参数数量降序，非public 构造函数参数数量降序
AutowireUtils.sortConstructors(candidates);
// 最小参数类型权重
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Constructor<?>> ambiguousConstructors = null;
LinkedList<UnsatisfiedDependencyException> causes = null;
// 迭代所有构造函数
for (Constructor<?> candidate : candidates) {
// 获取构造函数的参数的类型
Class<?>[] paramTypes = candidate.getParameterTypes();
if (constructorToUse != null && argsToUse != null && argsToUse.length > paramTypes.length) {
// Already found greedy constructor that can be satisfied ->
// do not look any further, there are only less greedy constructors left.
// 如果已经找到选用的构造函数或者需要的参数个数小于当前的构造函数参数个数，则终止。
// 因为，已经按照参数个数降序排列了
break;
}
// 参数数量不正确，继续
if (paramTypes.length < minNrOfArgs) {
continue;
}
// 参数持有者 ArgumentsHolder 对象
ArgumentsHolder argsHolder;
if (resolvedValues != null) {
try {
// 注释上获取参数名称
String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);
if (paramNames == null) {
// 获取构造函数、参数探测器
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
// 通过探测器获取构造函数的参数名称
paramNames = pnd.getParameterNames(candidate);
}
}
// 根据构造函数和构造参数，创建参数持有者 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 {
// continue 构造函数没有参数
// Explicit arguments given -> arguments length must match exactly.
if (paramTypes.length != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
// isLenientConstructorResolution 判断解析构造函数的时候是否以宽松模式还是严格模式
// 严格模式：解析构造函数时，必须所有的都需要匹配，否则抛出异常
// 宽松模式：使用具有"最接近的模式"进行匹配
// typeDiffWeight：类型差异权重
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this constructor if it represents the closest match.
// 如果它代表着当前最接近的匹配则选择其作为构造函数
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);
}
}
// 没有可执行的工厂方法，抛出异常
if (constructorToUse == null) {
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()) {
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) {
// 将解析的构造函数加入缓存
argsHolderToUse.storeCache(mbd, constructorToUse);
}
}
// 创建bean并包装
Assert.state(argsToUse != null, "Unresolved constructor arguments");
bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse));
return bw;
}
instantiateBean
不带有参数的构造方法，来初始化 Bean 对象
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
// 没有覆盖，直接使用反射实例化即可
if (!bd.hasMethodOverrides()) {
// 获得构造方法 constructorToUse
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
// 如果是接口，抛出 BeanInstantiationException 异常
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
// 从 clazz 中，获得构造方法
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
constructorToUse = clazz.getDeclaredConstructor();
}
// 标记 resolvedConstructorOrFactoryMethod 属性
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
// 通过 BeanUtils 直接使用构造器对象实例化 Bean 对象
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
// 生成 CGLIB 创建的子类对象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}总结
bean的实例化主要包含几个方式
工厂方法初始化和构造函数自动注入初始化两种方式最为复杂，主要是因为构造函数和构造参数的不确定性，Spring 需要花大量的精力来确定构造函数和构造参数
- Supplier 回调方式
- 工厂方法初始化
- 构造函数自动注入初始化
- 默认构造函数注入。
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