spring IOC之篇五 bean的加载--缓存中获取
我们终于结束了对XML配置文件的解析，接下来我们要分析bean的加载，即对代码getBean的分析：
People p = (People)ctx.getBean("cutesource");
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 根据名称获取对应的beanName。如果传入的别名A指向bean B则返回bean B的beanName，如果别名A指向别名B 然后别名B指向bean c 则返回bean C，如果传入的是FactoryBean，则则去掉修饰符 ，比如 name=”&aa” 则变为 name=”aa”
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 尝试从缓存中加载单例。首次从单例缓存中加载，如果不成功则从singletonFactories中加载。因为在创建单例的时候存在着循环依赖，所以不等bean创建完成就就将创建bean的ObjectFactory提早曝光加入到缓存中，一旦下一个bean依赖上一个bean 则直接使用ObjectFactory
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 返回对应的实例，缓存中记录的只是bean最初始化的状态，不一定是最终的bean，有时间需要返回时的指定方法实例化的实例，例如 FactoryBean的getObject的方法的实例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 只有在单例的情况下面才去解决循环依赖，如果原型的情况下面则直接抛出异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 如果parentBeanFactory 不为空，且当前的beanFactory不包含beanName则递归到他的上一层去加载beanName
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
// args为传入的构造函数的参数
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// 没有构造参数则使用标准的方法获取bean
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 将GenericBeanDefinition转换为 RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 如果父类bean不为空的话，合并一下父类的属性
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 获取bean的其他依赖，如果依赖其他的bean，则先获取其他的bean
for (String dependsOnBean : dependsOn) {
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
}
// 把该bean依赖的其他的bean加入到dependentBeanMap集合中去
registerDependentBean(dependsOnBean, beanName);
// 获取其依赖的其他bean
getBean(dependsOnBean);
}
}
// Create bean instance.
// 根据不同的scope进行初始化策略
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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;
}
}
// 根据传入的参数 requiredType和返回的bean类型不一致的时候，进行装换
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
进一步分析 FactoryBean的使用
FactoryBean接口对于Spring框架来说占有重要的地位，Spring自身就提供了70多种实现类。他们隐藏了一下实例化的复杂的细节，给上层应用带来便利。
public interface FactoryBean<T> { // 返回由FactoryBean创建的Bean实例，如果isSingleton（）方法返回的是true，则该实例会放入到Spring容器的单例缓存池当中 T getObject() throws Exception; // 返回FactoryBean创建的bean类型 Class<?> getObjectType(); // 返回由factoryBean创建实例的Bean是singleton还是prototype boolean isSingleton(); }
创建一个实体用Factory进行实例化
public class People { private String id; private String name; private Integer age; public String getId() { return id; }
创建一个自定义的FactoryBean方法
public class PeopleFactoryBean implements FactoryBean<People> { private String peopleInfo; public People getObject() throws Exception { People people = new People(); String[] infos = peopleInfo.split(","); people.setId(infos[0]); people.setName(infos[1]); people.setAge(Integer.parseInt(infos[2])); return people; } public Class<People> getObjectType() { return People.class; } public boolean isSingleton() { return false; } public String getPeopleInfo() { return peopleInfo; } public void setPeopleInfo(String peopleInfo) { this.peopleInfo = peopleInfo; } }
配置文件中配置：
<bean id="people" class="com.spring.test.factory.PeopleFactoryBean">
<property name="peopleInfo" value="gh2,address2,22"></property>
</bean>
即可使用自定义的逻辑进行实例化，当调用getBean的时候，Spring通过反射机制发现PeopleFactoryBean实现了FactoryBean，这时候Spring容器就调用接口方法PeopleFactoryBean#getObject()方法返回。如果希望获取PeopleFactoryBean实例，则在beanName前面显示的加&
解下来对从单列缓存池获取对象进行分析：
Object sharedInstance = getSingleton(beanName)
代码如下：
// 参数allowEarlyReference 设置为true，则表示允许早期依赖 protected Object getSingleton(String beanName, boolean allowEarlyReference) { // 先从spring单列缓存池获取对象 Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { // 如果单列缓存池为空，则锁定全局变量进行处理 synchronized (this.singletonObjects) { // earlySingletonObjects存放的是允许早期依赖的实例化的半成品，此时表示此bean正在加载，如果正在加载则不予处理 singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { // 当某些方法需要初始化的时候，需要调用addSingletonFactory方法将对应的ObejctFactory初始化策略存储在singletonFactories,获取存放的singletonFactory ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { // 如果singletonFactory不为空，使用singletonFactory的初始化方法进行初始化 singletonObject = singletonFactory.getObject(); // earlySingletonObjects和singletonFactories互斥，如果earlySingletonObjects包含此bean，则singletonFactories中就不会存在此bean的初始化方法 this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return (singletonObject != NULL_OBJECT ? singletonObject : null); }
在getBean（）方法中
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);是一个高频率使用的方法，其目的是用于检测当前bean是否是FactoryBean类型的bean，如果是那么需要调用bean对应的FactoryBean实例中的getObject（）作为返回值。
protected Object getObjectForBeanInstance( Object beanInstance, String name, String beanName, RootBeanDefinition mbd) { // Don't let calling code try to dereference the factory if the bean isn't a factory. // 如果指定的name是工厂相关(以&为前缀)且beanInstance又不是FactoryBean类型的则检测不通过 if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) { throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass()); } // Now we have the bean instance, which may be a normal bean or a FactoryBean. // If it's a FactoryBean, we use it to create a bean instance, unless the // caller actually wants a reference to the factory. // 如果传入了beanName加了前缀&,则直接返回工厂而不是返回工程对应的getObejct（）方法的返回值 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } Object object = null; if (mbd == null) { // 到了这里，可以肯定此时的bean肯定是FactoryBean，从缓存中获取bean object = getCachedObjectForFactoryBean(beanName); } if (object == null) { // Return bean instance from factory. // 到了这里，可以肯定此时的bean肯定是FactoryBean,所以强制转换 FactoryBean<?> factory = (FactoryBean<?>) beanInstance; // Caches object obtained from FactoryBean if it is a singleton. if (mbd == null && containsBeanDefinition(beanName)) { // 将GenericBeanDefinition转换为RootBeanDefinition,如果指定的BeanName是子Bean的话，则同时合并父类的属性 mbd = getMergedLocalBeanDefinition(beanName); } // 是否是用户自定义而不是程序本身定义，在AOP时候，是用户自定义，此时是程序本身定义的 boolean synthetic = (mbd != null && mbd.isSynthetic()); // 获取bean object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object; } 继续进一步进行代码的分析 protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) { if (factory.isSingleton() && containsSingleton(beanName)) { // 如果单例对象，则加入了缓存的处理 synchronized (getSingletonMutex()) { Object object = this.factoryBeanObjectCache.get(beanName); if (object == null) { object = doGetObjectFromFactoryBean(factory, beanName); // Only post-process and store if not put there already during getObject() call above // (e.g. because of circular reference processing triggered by custom getBean calls) Object alreadyThere = this.factoryBeanObjectCache.get(beanName); if (alreadyThere != null) { object = alreadyThere; } else { if (object != null && shouldPostProcess) { try { // 获取完单例对象以后，并没有直接返回，而是调用了后处理器，在所有的bean初始化后都调用BeanPostProcessor的postProcessAfterInitialization方法进行处理 object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", ex); } } this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT)); } } return (object != NULL_OBJECT ? object : null); } } else { // 如果是非单例对象，则不用缓存 Object object = doGetObjectFromFactoryBean(factory, beanName); if (object != null && shouldPostProcess) { try { object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex); } } return object; } }
进一步分析代码
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName) throws BeanCreationException { Object object; try { if (System.getSecurityManager() != null) { AccessControlContext acc = getAccessControlContext(); try { object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() { @Override public Object run() throws Exception { // 返回FactoryBean对应的实例 getObejct（）方法 return factory.getObject(); } }, acc); } catch (PrivilegedActionException pae) { throw pae.getException(); } } else { object = factory.getObject(); } }
