Spring 源碼學習 – 單例bean的實例化過程
本文作者：geek，一個聰明好學的同事
1. 簡介
開發中我們常用@Commpont，@Service，@Resource等註解或者配置xml去聲明一個類，使其成為spring容器中的bean，以下我將用從源碼角度看以AnnotationConfigApplicationContext為例看spring如何把帶有註解的類生成spring中bean。
2. 示例代碼
public class TestContext {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
SingleBean singleBean = context.getBean(SingleBean.class);
System.out.println("<=====>"+singleBean.getTestStr());
}
}
@ComponentScan("com.geek")
public class AppConfig {
}
@Component
public class SingleBean {
private String testStr = "testStr";
public String getTestStr() {
return testStr;
}
}
注意：以上代碼僅需要引入spring-context依賴即可。
3. 源碼分析
上面的demo在調用AnnotationConfigApplicationContext構造函數的時候，AppConfig類會被註冊到AnnotatedBeanDefinitionReader，由這個reader把AppConfig解釋為beanDefination，從而被spring獲取到要實例化的類信息，以下為bean生產的源碼及其注釋。（源碼基於springFramework 5.1.X）
3.1 創建入口
單例bean的創建的入口為DefaultListableBeanFactory.java#preInstantiateSingletons，下面源碼可見創建的bean條件為非抽象，非@LazyInit註解，Scope為singleTon（默認為singleTon）。
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
//所有可能需要去實例化的class（lazy，scope）
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
/**
* 非抽象，非懶初始化，單例bean
*/
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
//非工廠bean實例化
else {
getBean(beanName);
}
}
}
/**
* 實例化完成後觸發實現了SmartInitializingSingleton方法的bean
* 的afterSingletonsInstantiated方法
*/
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
3.2 創建前doGetBean代碼邏輯
getBean方法進來後便是直接調用doGetBean，doGetBean執行的源碼解釋如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
/**
* 獲取beanName
* 1，去掉factortoryBean前綴&
* 2，帶有別名的bean轉換為原來名字
*/
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
/**
* 從DefaultSingletonBeanRegistry的singletonObjects
* （spring內部用來緩存單例bean的currentHashMap）檢查是否存在該bean，不存在則創建
* 涉及單例模式下的循環依賴解決
*/
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
/**
*獲取給定bean實例的對象，如果是Factory Bean，
* 則可以是bean實例本身或其創建的對象。
*/
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assembly within a circular reference.
/**
* 原型模式下的bean存在循環依賴則會拋異常
*/
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
/**
* 找不到則從父容器中查找
*/
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
/**
* 父容器中也找不到該bean，則需要重新實例化
* 1，獲取要實例化bean的beanDefinition,
* 2，檢查bean的實例化需要依賴的其他bean
*/
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
/**
* 若給定的依賴 bean 已經註冊為依賴給定的bean
*/
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
/**
* 遞歸調用getBean，優先創建依賴的bean
*/
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
/**
* 通過調用DefaultSingletonBeanRegistry的getSingleton，從而調用核心方法createBean創建
*/
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
/**
* 檢查需要的bean類型是否符合
*/
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
doGetBean的操作流程如下：
1，執行transformedBeanName方法轉換beanName
傳遞的參數可能是bean的alias或者為FactoryBean，transformedBeanName執行的操作:(1)若傳進來的FactoryBean（FactoryBean以&作為前綴標記），去掉&修飾符。（2）經過（1）的處理後，有alias的bean則從aliasMap中獲取bean的原始beanName。
2，從容器的緩存中獲取bean
getSingleton先從spring三級緩存中的第一級singletonObjects（Map結構）中獲取，若不存在，則檢查該bean是否正在創建isSingletonCurrentlyInCreation(beanName)) ，正在創建的bean會從二級緩存earlySingletonObjects（Map結構）獲取。獲取到緩存的bean後會調用AbstractBeanFactory#getObjectForBeanInstance轉換bean的實例本身返回。因為從緩存中拿到的可能是factoryBean，所以getObjectForBeanInstance需要把是通過從緩存factoryBeanObjectCache獲取或通過factory.getObject()獲得相應的bean返回。
3，bean實例化前檢查
（1）先檢查是否原型模式下的bean是否存在循環依賴，是則會拋異常。
（2）檢查父類工廠（parentBeanFactory）是否存在，存在則從parentBeanFactory中遞歸調用doGetBean。
（3）獲取改bean的beanDefinition，檢查該bean實例化過程中是否涉及依賴了其他的bean，若是則遞歸調用getBean，優先創建依賴的bean。（涉及單例下的循環以來解決，下篇文章詳細介紹）。
（4）對創建bean代碼加synchronized和執行beforeSingletonCreation(beanName)前置處理。
3.3 創建前createBean邏輯
經過前面的doGetBean的一輪檢查與準備後，便在AbstractAutowireCapableBeanFactory#createBean中開始bean的創建。
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* 解析指定 BeanDefinition 的 class
* 處理 override 屬性
* 實例化的前置處理
* 創建 bean
* @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;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
/**
* 解釋bean的class，看beanDefinition是否有class，否則load class
*/
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
/**
* 對bean不存在lookup-method 和 replace-method
* 標記其方法的overloaded為false
*/
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
/**
* 調用實現實現BeanPostProcessor的bean後置處理生成代理對象，
* 有代理對象則直接返回代理對象
*/
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
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 {
/**
* 無需代理的bean實例化
*/
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);
}
}
createBean的操作流程如下：
1，resolveBeanClass
解釋beanDefinition的class，並且保存在beanDefinition中。
2，prepareMethodOverrides
處理bean中的lookup-method （在單例bean用 @Lookup註解標記的方法，註解的方法返回的對象是原型）和 replace-method（
3，resolveBeforeInstantiation
調用實現實現BeanPostProcessor的bean後置處理生成代理對象，有代理對象則直接返回代理對象。（spring AOP則是基於此處實現）
3.4 bean的真正實例化createBeanInstance
在AbstractAutowireCapableBeanFactory#createBeanInstance中，真正創建bean，源碼及注釋如下：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
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());
}
/**
* 通過提供supplier回調方法創建
*/
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
/**
* 通過工廠方法創建 bean 實例，可以是靜態工廠方法或者實例工廠
*/
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) {
/**
* 查找已經bean已經緩存解析的構造函數或者工廠方法
*/
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
/**
* 已經有緩存的構造函數或者工廠方法，直接實例化
*/
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
/**
* 通過實現BeanPostProcessor的代理類autowired的構造函數實例化
*/
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
/**
* 通過本身帶有autowired的構造函數實例化，通過調用反射newInstance實現
*/
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
/**
* 無參構造函數實例化，通過調用反射newInstance實現
*/
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
由上述源碼可以看出，實例化bean操作流程如下：
1，如果存在 Supplier 回調，則通過提供supplier回調方法創建，如以下方式定義的bean：
@RunWith(SpringRunner.class)
@SpringBootTest(classes = Spring5Application.class)
public class BeanRegistrationTest {
@Autowired
private GenericWebApplicationContext context;
context.registerBean(A.class, () -> new A());
}
2，如果存在工廠方法，則通過工廠方法創建 bean 實例，可以是靜態工廠方法或者實例工廠，如以下方式定義的bean：
public class AFactory implements FactoryBean<A> {
@Override
public A getObject() throws Exception {
return new A();
}
@Override
public Class<?> getObjectType() {
return A.class;
}
}
//或：
@Configuration
public class BeanConfigration {
@Bean
public A a(){
return new A();
}
}
3，已經有緩存的構造函數或者工廠方法，直接實例化。
4，以上三點都不存在，則使用帶參構造函數與無參構造函數實例化。如以下方式定義的bean：
@Commponet
public class A{}
4.總結
spring單例bean的實例化流程大概就是這樣，很多細節地方，包括循環依賴處理，bean屬性填充等細節點下一章介紹。
參考
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