bean的加載
對於加載bean的功能,在Spring中的調用方式爲:
MyTestBean bean = (MyTestBean)bf.getBean("myTestBean")
這行代碼在Spring中是這樣實現的
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
return this.doGetBean(name, requiredType, args, false);
}
protected <T> T doGetBean(String name, Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {
//提取對應的beanName
final String beanName = this.transformedBeanName(name);
/*
*檢查緩存中或者實例工廠中是否有對應的實例
*爲什麼首先會使用這段代碼呢,因爲在創建單例bean的時候會存在依賴注入的情況,而在創建依賴的時候爲了避免循環依賴,
*Spring創建bean的原則是不等bean創建完成就會將創建bean的ObjectFactory提早曝光,
*也就是將ObjectFactory加入到緩存中,一旦下個bean創建時候需要依賴上個bean則直接使用ObjectFactory
*/
//直接嘗試從緩存獲取或者singletonFactories中的ObjectFactory中獲取
Object sharedInstance = this.getSingleton(beanName);
Object bean;
if(sharedInstance != null && args == null) {
if(this.logger.isDebugEnabled()) {
if(this.isSingletonCurrentlyInCreation(beanName)) {
this.logger.debug("Returning eagerly cached instance of singleton bean \'" + beanName + "\' that is not fully initialized yet - a consequence of a circular reference");
} else {
this.logger.debug("Returning cached instance of singleton bean \'" + beanName + "\'");
}
}
//返回對應的實例,有時候存在諸如BeanFactory的情況並不是直接返回實例本身而是返回指定方法返回的實例
bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null);
} else {
//只有在單例情況下才會嘗試解決循環依賴,原型模式情況下,如果存在
//A中有B的屬性,B中有A的屬性,那麼當依賴注入的時候,就會產生當A還未創建完的時候因爲
//對於B的創建再次返回創建A,造成循環依賴,也就是下面的情況
//isPrototypeCurrentlyInCreation(beanName)爲true
if(this.isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
BeanFactory parentBeanFactory = this.getParentBeanFactory();
//如果BeanDefinitionMap中也就是在所有已經加載的類中不包括beanName則嘗試從parentBeanFactory中檢測
if(parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
String var19 = this.originalBeanName(name);
//遞歸到BeanFactory中尋找
if(args != null) {
return parentBeanFactory.getBean(var19, args);
}
return parentBeanFactory.getBean(var19, requiredType);
}
//如果不是僅僅做類型檢查則是創建bean,這裏要進行記錄
if(!typeCheckOnly) {
this.markBeanAsCreated(beanName);
}
//將存儲XML配置文件的GernericBeanDefinition轉換爲RootBeanDefinition,如果指定BeanName是子Bean的話同時會合並父類的相關屬性
final RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
this.checkMergedBeanDefinition(mbd, beanName, args);
String[] dependsOn = mbd.getDependsOn();
String scopeName;
//若存在依賴則需要遞歸實例化依賴的bean
if(dependsOn != null) {
String[] var14 = dependsOn;
int ex = dependsOn.length;
for(int scope = 0; scope < ex; ++scope) {
scopeName = var14[scope];
this.getBean(scopeName);
//緩存依賴調用
this.registerDependentBean(scopeName, beanName);
}
}
//實例化依賴的bean後便可以實例化mbd本身了
//singleton模式的創建
if(mbd.isSingleton()) {
sharedInstance = this.getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
return AbstractBeanFactory.this.createBean(beanName, mbd, args);
} catch (BeansException var2) {
AbstractBeanFactory.this.destroySingleton(beanName);
throw var2;
}
}
});
bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} else if(mbd.isPrototype()) {
//prototype模式的創建(new)
scopeName = null;
Object var20;
try {
this.beforePrototypeCreation(beanName);
var20 = this.createBean(beanName, mbd, args);
} finally {
this.afterPrototypeCreation(beanName);
}
bean = this.getObjectForBeanInstance(var20, name, beanName, mbd);
} else {
//指定的scope上實例化bean
scopeName = mbd.getScope();
Scope var21 = (Scope)this.scopes.get(scopeName);
if(var21 == null) {
throw new IllegalStateException("No Scope registered for scope \'" + scopeName + "\'");
}
try {
Object var22 = var21.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
AbstractBeanFactory.this.beforePrototypeCreation(beanName);
Object var2;
try {
var2 = AbstractBeanFactory.this.createBean(beanName, mbd, args);
} finally {
AbstractBeanFactory.this.afterPrototypeCreation(beanName);
}
return var2;
}
});
bean = this.getObjectForBeanInstance(var22, name, beanName, mbd);
} catch (IllegalStateException var18) {
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", var18);
}
}
}
//檢查需要的類型是否符合bean的實際類型
if(requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
} else {
return bean;
}
}
對於Spring加載bean的過程,大致分爲以下幾步:
轉換對應beanName
這裏傳入的參數name不一定就是beanName,有可能是別名或FactoryBean,所以需要進行一系列的解析,這些解析內容包括如下內容
去除FactoryBean的修飾符,也就是如果name=”&aa”,那麼會首先去除&而使name=”aa”
取指定alias所表示的最終beanName,例如別名A指向名稱爲B的bean則返回B;若別名A指向別名B,別名B又指向名稱爲C的bean則返回C
嘗試從緩存中加載單例
單例在Spring的同一個容器內只會被創建一次,後續再獲取bean,就直接從單例緩存中獲取了。這裏只是嘗試加載,首先嚐試從緩存中加載,如果加載不成功則再次嘗試從singletonFactories中加載,因爲在創建單例bean的時候會存在依賴注入的情況,而在創建依賴的時候爲了避免循環依賴,在Spring中創建bean的原則是不等bean創建完成就會將創建bean的ObjectFactory提早曝光加入到緩存中,一旦下一個bean創建時候需要依賴上一個bean則直接使用ObjectFactory
bean的實例化
如果從緩存中得到了bean的原始狀態,則需要對bean進行實例化,這裏有必要強調一下,在緩存中記錄的只是最原始的bean狀態,並不一定是我們最終想要的bean
原型模式的依賴檢查
只有在單例情況下才會嘗試解決循環依賴,如果存在A中有B的屬性,B中有A的屬性,那麼當依賴注入的時候,就會產生當A還未創建完的時候因爲對於B的創建再次返回創建A,造成循環依賴,也就是情況:isPrototypeCurrentlyInCreation(beanName)判斷true
檢測parentBeanFactory
從代碼上來看,如果緩存沒有數據的話直接轉到父類工廠上去加載,!this.containsBeanDefinition(beanName檢測如果當前加載的XML配置文件中不包含beanName所對應的配置,就只能到parentBeanFactory去嘗試,然後再去遞歸的調用getBean方法
將存儲XML配置文件的GernericBeanDefinition轉換爲RootBeanDefinition
因爲從XML配置文件中讀取到的Bean信息是存儲在GernericBeanDefinition中的,但是所有的Bean後續處理都是針對於RootBeanDefinition的,所以這裏需要進行一個轉換,轉換的同時如果父類bean不爲空的話,則會一併合併父類屬性
尋找依賴
因爲bean的初始化過程很可能會用到某些屬性,而某些屬性很可能是動態配置的,並且配置成依賴於其他的bean,那麼這個時候就有必要先加載依賴的bean,所以,在Spring的加載順尋中,在初始化某一個bean的時候首先會初始化這個bean所對應的依賴
針對不同的scope進行bean的創建
在Spring中存在着不同的scope,其中默認的是singleton,但是還有些其他的配置諸如prototype、request之類的,在這個步驟中,Spring會根據不同的配置進行不同的初始化策略
類型轉換
程序到這裏返回bean後已經基本結束了,通常對該方法的調用參數requiredType是爲空的,但是可能會存在這樣的情況,返回的bean其實是個Spring,但是requiredType卻傳入Integer類型,那麼這時候本步驟就會起作用了,它的功能是將返回的bean轉換爲requiredType所指定的類型,當然,Spring轉換爲Integer是最簡單的一種轉換,在Spring中提供了各種各樣的轉換器,用戶也可以自己擴展轉換器來滿足需求
FactoryBean的使用
一般情況下, Spring通過反射機制利用bean的class屬性指定實現類來實例化bean,在某些情況下,實例化bean過程比較複雜,如果按照傳統的方式,則需要在<bean>中提供大量的配置信息,配置方式的靈活性是受限的,這時採用編碼的方式可能會得到一個簡單的方案
Spring爲此提供了一個org.Springframework.bean.factory.FactoryBean的工廠類接口,用戶可以通過實現該接口定製實例化bean的邏輯
FactoryBean接口對於Spring框架來說佔有重要的地位,Spring自身就提供了70多個FactoryBean的實現,它們隱藏了實例化一些複雜bean的細節,給上層應用帶來了便利,從Spring 3.0開始,FactoryBean開始支持泛型,即接口聲明改爲FactoryBean<T>的形式:
package org.springframework.beans.factory;
public interface FactoryBean<T> {
T getObject() throws Exception;
Class<?> getObjectType();
boolean isSingleton();
}
在該接口中還定義了以下3個方法:
- T getObject():返回由FactoryBean創建的bean實例,如果isSingleton()返回true,則該實例會放到Spring容器中單實例緩存池中
- boolean isSingleton():返回由FactoryBean創建的bean實例的作用域是singleton還是prototype
- Class<T> getObjectType():返回FactoryBean創建的bean類型
當配置文件中<bean>的class屬性配置的實現類是FactoryBean時,通過getBean()方法返回的不是FactoryBean本身,而是FactoryBean#getObject()方法所返回的對象,相當於FactoryBean#getObject()代理了getBean()方法。例如:如果使用傳統方式配置下面Car的<bean>時,Car的每個屬性分別對應一個<propery>元素標籤
public class Car{
private int maxSpeed;
private String brand;
private double price;
//get/set方法
}
如果用FactoryBean的方式實現就會靈活一些,下例通過逗號分隔符的方式一次性地爲Car的所有屬性指定配置值
public class CarFactoryBean implements FactoryBean<Car>{
private String carInfo;
public Car getObject() throws Exceptino {
Car car = new Car();
String[] infos = carInfo.split(",");
car.setBrand(infos[0]);
car.setMaxSpeed(Integer.valueOf(infos[1]));
car.setPrice(Double.valueOf(infos[2]));
return car;
}
public class<Car> getObjectType(){
return Car.class;
}
public boolean isSingleton(){
return false;
}
public String getCarInfo(){
return this.carInfo;
}
//接受逗號分割符設置屬性信息
public void setCarInfo(String carInfo){
this.carInfo = carInfo;
}
}
有了這個CarFactoryBean後,就可以在配置文件中使用下面這種自定義的配置方式配置CarBean了
<bean id="car" class="com.test.factorybean.CarFactoryBean" carInfo="超級跑車,400,2000000" />
當調用getBean(“car”)時,Spring通過反射機制發現CarFactoryBean實現了FactoryBean的接口, 這時Spring容器就調用接口方法CarFactoryBean#getObject()方法返回,如果希望獲取CarFactoryBean的實例,則需要在使用getBean(beanName)方法時在beanName前顯示的加上”&”前綴,例如getBean(“&car”)
緩存中獲取單例bean
public Object getSingleton(String beanName) {
//參數true設置標識允許早期依賴
return this.getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//檢查緩存中是否存在實例
Object singletonObject = this.singletonObjects.get(beanName);
if(singletonObject == null) {
//如果爲空,則鎖定全局變量並進行處理
Map var4 = this.singletonObjects;
synchronized(this.singletonObjects) {
//如果此bean正在加載則不處理
singletonObject = this.earlySingletonObjects.get(beanName);
if(singletonObject == null && allowEarlyReference) {
//當某些方法需要提前初始化的時候則會調用addSingletonFactory方法將對應的ObjectFactory初始化策略存儲在singletonFactories
ObjectFactory singletonFactory = (ObjectFactory)this.singletonFactories.get(beanName);
if(singletonFactory != null) {
//調用預先設定的getObject方法
singletonObject = singletonFactory.getObject();
//記錄在緩存中,earlySingletonObjects和singletonFactories互斥
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject != NULL_OBJECT?singletonObject:null;
}
這個方法首先嚐試從singletonObjects裏面獲取實例,如果獲取不到再從earlySingletonObjects裏面獲取,如果還獲取不到,再嘗試從singletonFactories裏面獲取beanName對應的ObjectFactory,然後調用這個ObjectFactory的getObject來創建bean,並放到earlySingletonObjects裏面去,並且從singletonFactories裏面remove掉這個ObjectFactory,而對於後續的所有內存操作都只爲了循環依賴檢測時候使用,也就是在allowEarlyReference爲true的情況下才會使用
這裏涉及用於存在bean的不同map,說明如下:
- singletonObjects:用於保存BeanName和創建bean實例之間的關係,bean name->bean instance
- singletonFactories:用於保存BeanName和創建bean的工廠之間的關係,bean name->ObjectFactory
- earlySingletonObjects:也是保存BeanName和創建bean實例之間的關係,與singletonObjects的不同之處在於,當一個單例bean被放到這裏面後,那麼當bean還在創建過程中,就可以通過getBean方法獲取到了,其目的是用來檢測循環引用
- registeredSingletons:用來保存當前所有已註冊的bean
從bean的實例中獲取對象
在getBean方法中,getObjectForBeanInstance是個高頻率使用的方法,無論是從緩存中獲得bean還是根據不同的scope策略加載bean。總之,得到bean的實例後要做的第一步就是調用這個方法來檢測一下正確性,其實就是用於檢測當前bean是否是FactoryBean類型的bean,如果是,那麼需要調用該bean對應的FactoryBean實例中的getObject()作爲返回值
無論是從緩存中獲取到的bean還是通過不同的scope策略加載的bean都只是最原始的bean狀態,並不一定是最終想要的bean,如需要對工廠bean進行處理,那麼這裏得到的其實是工廠bean的初始狀態, 但是我們真正需要的是工廠bean中定義的factory-method方法中返回的bean,而getObjectForBeanInstance方法就是完成這個工作的
protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
//如果指定的name是工廠相關(以&爲前綴)且beanInstance又不是FactoryBean類型則驗證不通過
if(BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(this.transformedBeanName(name), beanInstance.getClass());
} else if(beanInstance instanceof FactoryBean && !BeanFactoryUtils.isFactoryDereference(name)) {
//加載FactoryBean
Object object = null;
if(mbd == null) {
//嘗試從緩存中加載bean
object = this.getCachedObjectForFactoryBean(beanName);
}
//到這裏已經明確知道beanInstance一定是FactoryBean類型
if(object == null) {
FactoryBean factory = (FactoryBean)beanInstance;
//containsBeanDefinition檢測BeanDefinitionMap中也就是在所有已經加載的類中檢測是否定義beanName
if(mbd == null && this.containsBeanDefinition(beanName)) {
//將存儲XML配置文件的GernericBeanDefinition轉換爲RootBeanDefinition,如果指定BeanName是子Bean的話同時會合並父類的相關屬性
mbd = this.getMergedLocalBeanDefinition(beanName);
}
//是否是用戶定義的而不是應用程序本身定義的
boolean synthetic = mbd != null && mbd.isSynthetic();
object = this.getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
} else {
return beanInstance;
}
}
getObjectForBeanInstance中所做的工作如下:
- 對FactoryBean正確性的驗證
- 對非FactoryBean不做任何處理
- 對bean進行轉換
將從Factory中解析bean的工作委託給getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) { if(factory.isSingleton() && this.containsSingleton(beanName)) { //如果是單例模式 synchronized(this.getSingletonMutex()) { Object object = this.factoryBeanObjectCache.get(beanName); if(object == null) { object = this.doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess); this.factoryBeanObjectCache.put(beanName, object != null?object:NULL_OBJECT); } return object != NULL_OBJECT?object:null; } } else { return this.doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess); } }
這個方法中只做了一件事情,就是返回的bean如果是單例的,那就必須保證全局唯一,同時,也因爲是單例的,所以不必重複創建,可以使用緩存來提高性能,也就是說已經加載過就要記錄下來以便於下次複用, 否則的話就直接獲取了
doGetObjectFromFactoryBean方法的代碼如下:
private Object doGetObjectFromFactoryBean(final FactoryBean factory, String beanName, boolean shouldPostProcess) throws BeanCreationException {
Object object;
try {
//需要權限驗證
if(System.getSecurityManager() != null) {
AccessControlContext ex = this.getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction() {
public Object run() throws Exception {
return factory.getObject();
}
}, ex);
} catch (PrivilegedActionException var8) {
throw var8.getException();
}
} else {
//直接調用getObject方法
object = factory.getObject();
}
} catch (FactoryBeanNotInitializedException var9) {
throw new BeanCurrentlyInCreationException(beanName, var9.toString());
} catch (Throwable var10) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", var10);
}
if(object == null && this.isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");
} else {
if(object != null && shouldPostProcess) {
try {
//調用ObjectFactory的後處理器
object = this.postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable var7) {
throw new BeanCreationException(beanName, "Post-processing of the FactoryBean\'s object failed", var7);
}
}
return object;
}
}
doGetObjectFromFactoryBean實現了從FactoryBean中對應的getObject方法得到bean,但是得到後並沒有立即返回,而是做了些後處理的操作
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return this.applyBeanPostProcessorsAfterInitialization(object, beanName);
}
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {
Object result = existingBean;
Iterator var5 = this.getBeanPostProcessors().iterator();
while(var5.hasNext()) {
BeanPostProcessor beanProcessor = (BeanPostProcessor)var5.next();
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if(result == null) {
return result;
}
}
return result;
}
在Spring獲取bean的規則中有這樣一條:儘可能保證所有bean初始化後都會調用註冊的BeanPostProcessor的postProcessAfterInitialization方法進行處理,在實際開發過程中大可以針對此特性設計自己的業務邏輯
獲取單例
如果緩存中不存在已經加載的單例bean就需要從頭開始bean的加載過程了,而Spring中使用getSingleton的重載方法實現bean的加載過程
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
Assert.notNull(beanName, "\'beanName\' must not be null");
Map var3 = this.singletonObjects;
//全局變量需要同步
synchronized(this.singletonObjects) {
//首先檢查對應的bean是否已經加載過,因爲singleton模式其實就是複用以前創建的bean,所以這一步是必須的
Object singletonObject = this.singletonObjects.get(beanName);
//如果爲null纔可以進行singleton的bean的初始化
if(singletonObject == null) {
if(this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName, "Singleton bean creation not allowed while the singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if(this.logger.isDebugEnabled()) {
this.logger.debug("Creating shared instance of singleton bean \'" + beanName + "\'");
}
this.beforeSingletonCreation(beanName);
boolean recordSuppressedExceptions = this.suppressedExceptions == null;
if(recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet();
}
try {
//初始化bean
singletonObject = singletonFactory.getObject();
} catch (BeanCreationException var13) {
BeanCreationException ex = var13;
if(recordSuppressedExceptions) {
Iterator var8 = this.suppressedExceptions.iterator();
while(var8.hasNext()) {
Exception suppressedException = (Exception)var8.next();
ex.addRelatedCause(suppressedException);
}
}
throw ex;
} finally {
if(recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
this.afterSingletonCreation(beanName);
}
//加入緩存
this.addSingleton(beanName, singletonObject);
}
return singletonObject != NULL_OBJECT?singletonObject:null;
}
}
上述代碼中其實是使用了回調方法,使得程序可以在單例創建的前後做一些準備及處理操作,而真正獲取單例bean的方法其實並不是再此方法中實現的,其實現邏輯是在ObjectFactory類型的實例singletonFactory中實現的,而這些準備及處理操作包括如下內容
- 檢查緩存是否已經加載過
- 若沒有加載,則記錄beanName的正在加載狀態
加載單例前記錄加載狀態
beforeSingletonCreation方法是個空實現,其目的是:記錄加載狀態,也就是通過this.singletonsCurrentlyInCreation.add(beanName)將當前正在創建的bean記錄在緩存中,這樣便可以對循環依賴進行檢測
protected void beforeSingletonCreation(String beanName) { if(!this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } }
通過調用參數傳入的ObjectFactory的個體Object方法實例化bean
加載單例後的處理方法調用
當bean加載結束後需要移除緩存中對該bean的正在加載狀態的記錄
protected void afterSingletonCreation(String beanName) { if(!this.singletonsCurrentlyInCreation.remove(beanName)) { throw new IllegalStateException("Singleton \'" + beanName + "\' isn\'t currently in creation"); } }
將結果記錄至緩存並刪除加載bean過程中所記錄的各種輔助狀態
protected void addSingleton(String beanName, Object singletonObject) { Map var3 = this.singletonObjects; synchronized(this.singletonObjects) { this.singletonObjects.put(beanName, singletonObject != null?singletonObject:NULL_OBJECT); this.singletonFactories.remove(beanName); this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName); } }
返回處理結果
bean的加載邏輯其實是在傳入的ObjectFactory類型的參數singletonFactory中定義的,反推參數的獲取,得到如下代碼:
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>(){ public Object getObject() throws BeansException { try { return createBean(beanName, mbd, args); } catch(BeansException ex) { destroySingleton(beanName); throw ex; } } });
ObjectFactory的核心部分其實只是調用了createBean的方法