Spring5源码解析四
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- instantiateBean(beanName, mbd); 使用默认构造方法实例化对象。
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
getInstantiationStrategy().instantiate(mbd, beanName, parent),
getAccessControlContext());
}
else {
//getInstantiationStrategy()得到类的实例化策略
//默认情况下是得到一个反射的实例化策略
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
# SimpleInstantiationStrategy
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
//检测 bean 配置中是否配置了 lookup-method 或 replace-method
//如果配置了就需使用 CGLIB 构建 bean 对象
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 {
//得到默认的无参构造方法
//constructorToUse：表示spring使用哪个构造方法实例化对象
constructorToUse = clazz.getDeclaredConstructor();
}
//记录resolvedConstructorOrFactoryMethod为默认的无参构造方法
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//调用反射技术，实例化对象
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
// 设置构造方法为可访问
ReflectionUtils.makeAccessible(ctor);
//反射创建对象 ctor.newInstance(args)
return (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
}
catch (InstantiationException ex) {
throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
}
catch (IllegalAccessException ex) {
throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
}
catch (IllegalArgumentException ex) {
throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
}
catch (InvocationTargetException ex) {
throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
}
}
- autowireConstructor(beanName, mbd, ctors, args); 使用有参构造方法实例化对象。
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) {
//实力一个BeanWrapperImpl 对象很好理解
//前面外部返回的BeanWrapper 其实就是这个BeanWrapperImpl
//因为BeanWrapper是个接口
BeanWrapperImpl bw = new BeanWrapperImpl();
this.beanFactory.initBeanWrapper(bw);
//决定要使用哪个构造方法实例化对象
Constructor<?> constructorToUse = null;
//构造方法的值，注意不是参数
//我们都知道构造方法通过反射来实例一个对象
//在调用反射来实例对象的时候，需要具体的值
//这个变量就是用来记录这些值的
//但是这里需要注意的是argsHolderToUse是一个数据结构
ArgumentsHolder argsHolderToUse = null;
//argsToUse[]才是真正的值
Object[] argsToUse = null;
//确定参数值列表
//argsToUse可以有两种办法设置
//第一种通过beanDefinition设置
//第二种通过xml设置
if (explicitArgs != null) {
argsToUse = explicitArgs;
}
else {
Object[] argsToResolve = null;
synchronized (mbd.constructorArgumentLock) {
//获取已解析的构造方法
//一般不会有，因为构造方法一般会提供一个
//除非有多个。那么才会存在已经解析完成的构造方法
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) {
//如果没有已经解析的构造方法
//则需要去解析构造方法
//判断构造方法是否为空，判断是否根据构造方法自动注入
boolean autowiring = (chosenCtors != null ||
mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
ConstructorArgumentValues resolvedValues = null;
//定义了最小参数个数 minNrOfArgs
//如果你给构造方法的参数列表给定了具体的值
//那么这些值得个数就是构造方法参数的个数
int minNrOfArgs;
if (explicitArgs != null) {
//如果传递过来的参数不为null，那就以传递过来的参数个数作为“最小参数个数”
minNrOfArgs = explicitArgs.length;
}
else {
//mybatis：mbd.getConstructorArgumentValues().addGenericArgumentValue("com.index.dao");
//实例一个对象，用来存放构造方法的参数值
//当中主要存放了参数值和参数值所对应的下表
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
/**
* 确定构造方法参数数量,假设有如下配置：
* <bean id="llsydn" class="com.llsydn.Luban">
* <constructor-arg index="0" value="str1"/>
* <constructor-arg index="1" value="1"/>
* <constructor-arg index="2" value="str2"/>
* </bean>
*
* 在通过spring内部给了一个值的情况，那么表示你的构造方法的“最小参数个数”是确定的
*
* minNrOfArgs = 3
*/
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
// 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);
}
}
//怎么排序的呢？
//1.根据构造方法的访问权限级别 public -- protected -- private
//2.根据构造方法的参数数量进行排序 从多到小
/**
* 有限反问权限，继而参数个数
* 这个自己可以写个测试去看看到底是不是和我说的一样
* 1. public Luban(Object o1, Object o2, Object o3)
* 2. public Luban(Object o1, Object o2)
* 3. public Luban(Object o1)
* 4. protected Luban(Integer i, Object o1, Object o2, Object o3)
* 5. protected Luban(Integer i, Object o1, Object o2)
* 6. protected Luban(Integer i, Object o1)
*/
AutowireUtils.sortConstructors(candidates);
//定义了一个差异变量，这个变量很有分量，后面有注释
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Constructor<?>> ambiguousConstructors = null; //记录异常的构造方法（构造方法差异值一样，spring不知怎么选择）
LinkedList<UnsatisfiedDependencyException> causes = null;
//循环所有的构造方法
for (Constructor<?> candidate : candidates) {
Class<?>[] paramTypes = candidate.getParameterTypes();
/**
* 这个判断别看只有一行代码理解起来很费劲
* 首先constructorToUse != null这个很好理解
* 前面已经说过首先constructorToUse主要是用来装已经解析过了并且在使用的构造方法
* 只有在他等于空的情况下，才有继续的意义，因为下面如果解析到了一个符合的构造方法
* 就会赋值给这个变量（下面注释有写）。故而如果这个变量不等于null就不需要再进行解析了，说明spring已经
* 找到一个合适的构造方法，直接使用便可以
* argsToUse.length > paramTypes.length这个代码就相当复杂了
* 首先假设 argsToUse = [1,"llsydn",obj]
* 那么回去匹配到上面的构造方法的1和5
* 由于构造方法1有更高的访问权限，所有选择1，尽管5看起来更加匹配
* 但是我们看2,直接参数个数就不对所以直接忽略
*
*/
if (constructorToUse != 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.
break;
}
//如果遍历的当前构造方法的参数类型的长度，不等于最小的参数格式：证明不能用该构造方法实例化对象
if (paramTypes.length < minNrOfArgs) {
continue;
}
ArgumentsHolder argsHolder;
if (resolvedValues != null) {
try {
//判断是否加了ConstructorProperties注解如果加了则把值取出来
//可以写个代码测试一下
//@ConstructorProperties(value = {"xxx", "111"})
String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);
if (paramNames == null) {
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
//获取构造方法参数名称列表
/**
* 假设你有一个（String llsydn, Object zilu）
* 则paramNames=[llsydn,zilu]
*/
paramNames = pnd.getParameterNames(candidate);
}
}
//获取构造方法参数值列表
/**
* 这个方法比较复杂
* 因为spring只能提供字符串的参数值
* 故而需要进行转换
* argsHolder所包含的值就是转换之后的
*
* 例如：在xml配置文件设置了“order”这是一个字符串，要转成order对象
*/
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames,
getUserDeclaredConstructor(candidate), autowiring);
}
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.
if (paramTypes.length != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
/**
* typeDiffWeight 差异量，何谓差异量呢？
* argsHolder.arguments和paramTypes之间的差异
* 每个参数值得类型与构造方法参数列表的类型直接的差异
* 通过这个差异量来衡量或者确定一个合适的构造方法
*
* 值得注意的是constructorToUse=candidate
*
* 第一次循环一定会typeDiffWeight < minTypeDiffWeight，因为minTypeDiffWeight的值非常大
* 然后每次循环会把typeDiffWeight赋值给minTypeDiffWeight（minTypeDiffWeight = typeDiffWeight）
* else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight)
* 第一次循环肯定不会进入这个
* 第二次如果进入了这个分支代表什么？
* 代表有两个构造方法都符合我们要求？那么spring有迷茫了（spring经常在迷茫）
* spring迷茫了怎么办？
* ambiguousConstructors.add(candidate);
* 顾名思义。。。。
* ambiguousConstructors=null 非常重要？
* 为什么重要，因为需要清空
* 这也解释了为什么他找到两个符合要求的方法不直接抛异常的原因
* 如果这个ambiguousConstructors一直存在，spring会在循环外面去exception
* 很牛逼呀！！！！
*/
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
// Choose this constructor if it represents the closest match.
if (typeDiffWeight < minTypeDiffWeight) {
//第一遍100%会进这里；当找到差异值更小的，就将异常清空。
constructorToUse = candidate;
argsHolderToUse = argsHolder;
argsToUse = argsHolder.arguments;
minTypeDiffWeight = typeDiffWeight;
//清空异常的构造器
ambiguousConstructors = null;
}
else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
//这里表示，找到了差异值一样的构造参数，spring不知道怎么选择，就先记录在ambiguousConstructors。
if (ambiguousConstructors == null) {
ambiguousConstructors = new LinkedHashSet<>();
ambiguousConstructors.add(constructorToUse);
}
ambiguousConstructors.add(candidate);
}
}
//循环结束
//没有找打合适的构造方法
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)");
}
//如果ambiguousConstructors还存在则异常？为什么会在上面方法中直接exception？
//上面注释当中有说明
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) {
/*
* 缓存相关信息，比如：
* 1. 已解析出的构造方法对象 resolvedConstructorOrFactoryMethod
* 2. 构造方法参数列表是否已解析标志 constructorArgumentsResolved
* 3. 参数值列表 resolvedConstructorArguments 或 preparedConstructorArguments
* 这些信息可用在其他地方，用于进行快捷判断
*/
argsHolderToUse.storeCache(mbd, constructorToUse);
}
}
try {
/*
* 使用反射创建实例 lookup-method 通过CGLIB增强bean实例
*/
final InstantiationStrategy strategy = beanFactory.getInstantiationStrategy();
Object beanInstance;
if (System.getSecurityManager() != null) {
final Constructor<?> ctorToUse = constructorToUse;
final Object[] argumentsToUse = argsToUse;
beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
strategy.instantiate(mbd, beanName, beanFactory, ctorToUse, argumentsToUse),
beanFactory.getAccessControlContext());
}
else {
//最后：通过找到constructorToUse构造方法，进行实例化对象
beanInstance = strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse);
}
bw.setBeanInstance(beanInstance);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean instantiation via constructor failed", ex);
}
}
autowireConstructor使用特殊构造方法实例化对象，主要做了几步操作： 1.判断要用哪个构造方法实例化对象：constructorToUse，那些参数：argsToUse 2.先决定构造方法的最小参数个数：minNrOfArgs。 3.对所有的构造方法进行排序，排序规则：访问权限级别(public--protected--private) 和 参数数量(从多到小) 4.遍历所有构造方法，根据“最小参数个数”minNrOfArgs去判断构造方法是否符合 5.计算构造方法的“typeDiffWeight 差异量”，找到最小的差异量，spring就用这个构造方法实例化对象。 6.如果spring找到了多个差异量一样的，也符合最小参数个数的构造方法，就记录在ambiguousConstructors。 7.如果这个ambiguousConstructors一直存在，spring会在循环外面去exception 8.如果这个ambiguousConstructors为null，spring就会缓存找到的constructorToUse的构造方法 9.最后通过找到constructorToUse构造方法，进行实例化对象。
1.instanceWrapper = createBeanInstance(beanName, mbd, args);
至此，使用构造方法实例化对象的方法就解析到这里。创建的对象，就执行下面这步
2.addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
这个很重要，主要将刚new的对象，存放在singletonFactories对象。主要是用来解决循环依赖。
3.populateBean(beanName, mbd, instanceWrapper); 但是这个时候，对象还没进行属性赋值，需要调用该方法进行属性赋值
- populateBean(beanName, mbd, instanceWrapper);
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
//是否需要属性赋值
boolean continueWithPropertyPopulation = true;
//实现InstantiationAwareBeanPostProcessor接口，就可以不进行属性赋值，返回一个寡对象
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
//属性值
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//判断属性自动装配模型（NO）
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
//是否需要处理InstantiationAwareBeanPostProcessors
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
//是否需要深度检查（循环引用）
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
//拿到所有get和set的方法
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
这里进行属性赋值，主要是调用了两个BeanPostProcessor进行属性赋值。 1.AutowiredAnnotationBeanPostProcessor（处理@Autowired注解） 2.CommonAnnotationBeanPostProcessor（处理@Resource、@PostConstruct和@PreDestroy注解）
- AutowiredAnnotationBeanPostProcessor源码解析
// @Autowired实现属性注入
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
//找出类中被@Autowired注解的属性和方法
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
//属性的注入
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
//拿到需要注入的属性
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
//遍历一个一个注入属性
for (InjectedElement element : elementsToIterate) {
if (logger.isDebugEnabled()) {
logger.debug("Processing injected element of bean '" + beanName + "': " + element);
}
//属性注入
element.inject(target, beanName, pvs);
}
}
}
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
//拿到要注入的属性
Field field = (Field) this.member;
Object value;
if (this.cached) {
//如果被缓存了，从缓存里面拿
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
//从BeanFactory中转换这个依赖（重要）
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
if (!this.cached) {
if (value != null || this.required) {
this.cachedFieldValue = desc;
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
//这里isTypeMatch，就是从earlySingletonObjects中拿出“自动装配的bean”的类型是否相同
//如果类型相同，就真正的set值field.set(bean, value);
if (beanFactory.containsBean(autowiredBeanName) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
this.cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
else {
this.cachedFieldValue = null;
}
this.cached = true;
}
}
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
//判断依赖的类型
if (Optional.class == descriptor.getDependencyType()) {
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
//真正干活的方法，从BeanFactory中转换这个依赖
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
Class<?> type = descriptor.getDependencyType();
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String) {
String strVal = resolveEmbeddedValue((String) value);
BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
value = evaluateBeanDefinitionString(strVal, bd);
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
if (matchingBeans.size() > 1) {
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(type, matchingBeans);
}
else {
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
//处理自动装配属性，进行转换。@Autowired 重要
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
throws BeansException {
//再次调用了getBean方法
return beanFactory.getBean(beanName);
}
进行属性赋值，就可能会出现循环依赖的情况： 例如：A类有一个B类的属性，B类有一个A类的属性。 那么在属性填充的时候，就会出现在创建A对象的时候，然后自动装配B，那么spring也会去创建B对象。那么这个时候，就会有一个循环依赖的情况。 那么spring是怎么解决循环依赖的？
- 在处理循环依赖的情况，spring主要用了3个map，1个list，最重要的就是singletonFactories
//这个时候bean已经被创建出来，但是还没进行属性装配
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
//singletonObjects是完整的对象
if (!this.singletonObjects.containsKey(beanName)) {
//已经被new出来，但是属性没有被填充，主要是解决循环依赖
this.singletonFactories.put(beanName, singletonFactory);
//已经被new出来，但是属性没有被填充，主要是作为类型校验
this.earlySingletonObjects.remove(beanName);
//在registeredSingletons记录一下
this.registeredSingletons.add(beanName);
}
}
}
//进行属性赋值的时候，调用该方法，从singletonFactories中拿到那些依赖的bean，进行属性赋值
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//从map中获取bean如果不为空直接返回，不再进行初始化工作
//讲道理一个程序员提供的对象这里一般都是为空的
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
至此，spring对bean的实例化过程，就解析到这里了。 下面上spring实例化过程的流程图：
