SpringBean生命周期之实例化篇
文章目录
-
- 注意事项
- 环境
- 源码解析
-
- AbstractAutowireCapableBeanFactory
-
- createBeanInstance
- 假如我们使用的是构造器注入
- AbstractBeanFactory
-
- determineConstructorsFromBeanPostProcessors
- AutowiredAnnotationBeanPostProcessor
-
- determineCandidateConstructors
- AbstractAutowireCapableBeanFactory
-
- autowireConstructor
- ConstructorResolver
-
- autowireConstructor
-
- resolveConstructorArguments
-
- BeanDefinitionValueResolver
- instantiate
- SimpleInstantiationStrategy
-
- instantiate
- 小总结
- 没使用构造注入
-
- AbstractAutowireCapableBeanFactory
-
- instantiateBean
- SimpleInstantiationStrategy
-
- instantiate
- BeanUtils
-
- instantiateClass
- 最后总结
注意事项
阅读本文需要有一定的基础，并且要跟着调用链走，不然会很容易懵圈。
环境
Spring Framework Version：5.3.x
Gradle Version：7.5.1
源码解析
AbstractAutowireCapableBeanFactory
createBeanInstance
- 使用反射加载class
- 推断需要实例化bean的构造器
- 判断是否使用构造器注入
- 如果使用，那么进行构造器注入完成后即返回结果
- 不使用构造器注入则跳过执行第三点
- 使用无参构造器实例化对象
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// 确保bean类在此时实际上已被解析,使用反射加载Class
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());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// 自动装配的候选构造函数
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//构造器注入
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
//无需构造器注入，使用空参构造器实例化对象
return instantiateBean(beanName, mbd);
}
假如我们使用的是构造器注入
AbstractBeanFactory
determineConstructorsFromBeanPostProcessors
@Nullable
protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)
throws BeansException {
if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
//遍历后置处理器
for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
//遍历到AutowiredAnnotationBeanPostProcessor后置处理器时，点进去看看
Constructor<?>[] ctors = bp.determineCandidateConstructors(beanClass, beanName);
if (ctors != null) {
return ctors;
}
}
}
return null;
}
我们可以看到上图中有两个后置处理器，第一个是关于代理的，所以我们主要看第二个
AutowiredAnnotationBeanPostProcessor
determineCandidateConstructors
- 检查方法上的Lookup注解，存在则构建LookupOverride然后记录到MethodOverrides中
- 记录检查过的bean，避免重复检查
- 先尝试从缓存中获取，如果获取不到则延迟再次重新获取
- 如果延迟获取，还获取不到，那么从通过反射从beanClass中获取
- 遍历候选构造器数组
- 从候选构造器上查找@Autowired、@Value注解
- 如果没找到，那么获取父类构造器，然后再继续查找
- 如果找到，检查@Autowired注解上的require属性状态，然后赋值给requiredConstructor并添加到候选者集合中
- 如果候选者集合不为空
- 先看看默认构造器是否为空，不为空则添加到候选者集合中、
- 把list候选者集合转为candidateConstructors集合
- 添加到候选构造器缓存
@Override
@Nullable
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
throws BeanCreationException {
//在进行构造器注入前，先检查当前需要注入的对象里的方法上有没有使用Lookup注解进行标识
// Let's check for lookup methods here...
if (!this.lookupMethodsChecked.contains(beanName)) {
if (AnnotationUtils.isCandidateClass(beanClass, Lookup.class)) {
try {
Class<?> targetClass = beanClass;
do {
//获取当前class方法集合，并回调doWith方法，下面代码就是具体实现
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Lookup lookup = method.getAnnotation(Lookup.class);
//如果存在注解标识，构建LookupOverride对象并记录到MethodOverrides中，方便后续进行统一的方法重写
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);
}
}
//记录检查过的bean，避免重复检查
this.lookupMethodsChecked.add(beanName);
}
// Quick check on the concurrent map first, with minimal locking.
//先尝试从缓存读取
Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
if (candidateConstructors == null) {
// Fully synchronized resolution now...
synchronized (this.candidateConstructorsCache) {
//延迟读取缓存
candidateConstructors = this.candidateConstructorsCache.get(beanClass);
if (candidateConstructors == null) {
Constructor<?>[] rawCandidates;
try {
//获得构造器数组
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);
}
//候选者
List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
//带有@Autowired注解 require属性为true的构造器
Constructor<?> requiredConstructor = null;
//默认构造器
Constructor<?> defaultConstructor = null;
//处理kotlin构造函数的，不用管，正常我们用不到
Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
int nonSyntheticConstructors = 0;
for (Constructor<?> candidate : rawCandidates) {
if (!candidate.isSynthetic()) {
nonSyntheticConstructors++;
}
else if (primaryConstructor != null) {
continue;
}
//查找@Autowired、@Value注解
MergedAnnotation<?> ann = findAutowiredAnnotation(candidate);
if (ann == null) {
Class<?> userClass = ClassUtils.getUserClass(beanClass);
if (userClass != beanClass) {
try {
//获得父类构造器
Constructor<?> superCtor =
userClass.getDeclaredConstructor(candidate.getParameterTypes());
//继续查找@Autowired、@Value注解
ann = findAutowiredAnnotation(superCtor);
}
catch (NoSuchMethodException ex) {
// Simply proceed, no equivalent superclass constructor found...
}
}
}
if (ann != null) {
if (requiredConstructor != null) {
throw new BeanCreationException(beanName,
"Invalid autowire-marked constructor: " + candidate +
". Found constructor with 'required' Autowired annotation already: " +
requiredConstructor);
}
//检查@Autowired注解上require属性的状态
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) {
defaultConstructor = candidate;
}
}
if (!candidates.isEmpty()) {
// Add default constructor to list of optional constructors, as fallback.
//添加默认构造器
if (requiredConstructor == null) {
if (defaultConstructor != null) {
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]);
}
else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) {
candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
}
else if (nonSyntheticConstructors == 2 && primaryConstructor != null &&
defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) {
candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor};
}
else if (nonSyntheticConstructors == 1 && primaryConstructor != null) {
candidateConstructors = new Constructor<?>[] {primaryConstructor};
}
else {
candidateConstructors = new Constructor<?>[0];
}
//添加到候选构造器缓存
this.candidateConstructorsCache.put(beanClass, candidateConstructors);
}
}
}
//如果构造器数组长度大于0，则返回当前候选构造器数组，否则返回null
return (candidateConstructors.length > 0 ? candidateConstructors : null);
}
AbstractAutowireCapableBeanFactory
autowireConstructor
protected BeanWrapper autowireConstructor(
String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {
//看到autowireConstructor方法
return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}
ConstructorResolver
autowireConstructor
- 获取构造器参数的一些信息
- 解析构造器参数（重点）
- 创建参数数组，最后重新赋值给argsToUse
- 通过反射进行实例化
public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd,
@Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {
BeanWrapperImpl bw = new BeanWrapperImpl();
this.beanFactory.initBeanWrapper(bw);
Constructor<?> constructorToUse = null;
ArgumentsHolder argsHolderToUse = null;
Object[] argsToUse = null;
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
else {
Object[] argsToResolve = null;
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;
}
}
}
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve);
}
}
if (constructorToUse == null || argsToUse == null) {
// Take specified constructors, if any.
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);
}
}
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.
boolean autowiring = (chosenCtors != null ||
mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
ConstructorArgumentValues resolvedValues = null;
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
}
else {
//获取构造器参数的一些信息
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
//解析构造器参数
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
AutowireUtils.sortConstructors(candidates);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Constructor<?>> ambiguousConstructors = null;
Deque<UnsatisfiedDependencyException> causes = null;
for (Constructor<?> candidate : candidates) {
//统计参数数量
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;
}
if (parameterCount < minNrOfArgs) {
continue;
}
ArgumentsHolder argsHolder;
Class<?>[] paramTypes = candidate.getParameterTypes();
if (resolvedValues != null) {
try {
//获取参数名
String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, parameterCount);
if (paramNames == null) {
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
}
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 ArrayDeque<>(1);
}
causes.add(ex);
continue;
}
}
else {
// Explicit arguments given -> arguments length must match exactly.
if (parameterCount != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
//计算权重
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this constructor if it represents the closest match.
//比较权重， minTypeDiffWeight为Integer.MAX
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);
}
}
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 on bean class [" + mbd.getBeanClassName() + "] " +
"(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 on bean class [" + mbd.getBeanClassName() + "] " +
"(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");
//设置bean实例并且实例化bean
bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse));
return bw;
}
resolveConstructorArguments
解析构造器参数
private int resolveConstructorArguments(String beanName, RootBeanDefinition mbd, BeanWrapper bw,
ConstructorArgumentValues cargs, ConstructorArgumentValues resolvedValues) {
...省略
//统计参数数量
int minNrOfArgs = cargs.getArgumentCount();
...省略
for (ConstructorArgumentValues.ValueHolder valueHolder : cargs.getGenericArgumentValues()) {
if (valueHolder.isConverted()) {
resolvedValues.addGenericArgumentValue(valueHolder);
}
else {
//解析参数值，重点，主要 看这块
Object resolvedValue =
valueResolver.resolveValueIfNecessary("constructor argument", valueHolder.getValue());
ConstructorArgumentValues.ValueHolder resolvedValueHolder = new ConstructorArgumentValues.ValueHolder(
resolvedValue, valueHolder.getType(), valueHolder.getName());
resolvedValueHolder.setSource(valueHolder);
resolvedValues.addGenericArgumentValue(resolvedValueHolder);
}
}
return minNrOfArgs;
}
BeanDefinitionValueResolver
resolveValueIfNecessary
public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
//我们必须检查每个值，看它是否需要对另一个bean的运行时引用来解析。
if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value;
//解析引用，点进去看看
return resolveReference(argName, ref);
}
...省略无关紧要的code
}
resolveReference
- 获取bean类型
- 是否使用 parent factory去getBean
- beanType不为null，则根据beanType去进行解析bean，否则根据名称去解析，最终还是通过beanFactory.getBean获取，走一遍生命周期流程
- 最后返回
@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
//获取类型
Class<?> beanType = ref.getBeanType();
if (ref.isToParent()) {
BeanFactory parent = this.beanFactory.getParentBeanFactory();
if (parent == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean " + ref +
" in parent factory: no parent factory available");
}
if (beanType != null) {
bean = parent.getBean(beanType);
}
else {
bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName())));
}
}
else {
String resolvedName;
if (beanType != null) {
NamedBeanHolder<?> namedBean = this.beanFactory.resolveNamedBean(beanType);
bean = namedBean.getBeanInstance();
resolvedName = namedBean.getBeanName();
}
else {
resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));
//通过BeanFactory getBean获取需要注入的值
bean = this.beanFactory.getBean(resolvedName);
}
this.beanFactory.registerDependentBean(resolvedName, this.beanName);
}
if (bean instanceof NullBean) {
bean = null;
}
//最后返回
return bean;
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
instantiate
private Object instantiate(String beanName, RootBeanDefinition mbd, Constructor<?> constructorToUse, Object[] argsToUse) {
try {
InstantiationStrategy strategy = this.beanFactory.getInstantiationStrategy();
if (System.getSecurityManager() != null) {
return AccessController.doPrivileged((PrivilegedAction<Object>) () ->
strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse),
this.beanFactory.getAccessControlContext());
}
else {
return strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse);
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean instantiation via constructor failed",ex);
}
}
SimpleInstantiationStrategy
instantiate
- 检查方法重写
- 使用反射实例化bean
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
final Constructor<?> ctor, Object... args) {
//该bean是否有方法重写
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;
});
}
//使用反射实例化bean
return BeanUtils.instantiateClass(ctor, args);
}
else {
return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
}
}
小总结
至此，使用构造器注入的源码分析就结束了，接下来进行一下总结，大概是怎么玩的
首先 先去寻找自动装配候选构造器，找到了之后进入构造器自动装配流程，怎么玩的？
- 获取构造器参数的一些信息
- 解析构造器参数（重点）
- 创建参数数组，最后重新赋值给argsToUse
- 通过反射进行实例化，再设置到BeanWapper中（重点）
没使用构造注入
不使用构造器注入的话那就简单了，省去了解析参数的繁琐步骤，直接一把梭反射实例化bean
源码如下：
AbstractAutowireCapableBeanFactory
instantiateBean
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
//判断安全管理器，一般没有
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(
(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
}
else {
//获取实例化策略并通过反射进行实例化，看到这里
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
//把实例化后的bean包装成BeanWrapper
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);
}
}
SimpleInstantiationStrategy
instantiate
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// 是否有方法重写
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//通过反射使用构造器去实例化bean
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
//生成代理类
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
BeanUtils
instantiateClass
可以看到最终是通过反射去实例化bean的
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
//检查访问权限
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];
}
}
//最终通过构造器反射实例化对象
return ctor.newInstance(argsWithDefaultValues);
}
}
...省略
}
最后总结
看完之后可能人是懵圈的，不过不要紧，自己尝试跟着去debug看看，加深印象，构造器注入还是蛮复杂的
看完源码后，得出最终结论：无论是使用构造器注入还是不使用构造器注入，最终实例化都是通过反射去实现的