上一节介绍了封装bean对象的BeanDefinition接口。从前面小结对BeanFactory的介绍中，我们知道bean对象是存储在map中，通过调用getBean方法可以得到bean对象。在接口BeanFactory中定义了多个相同签名的getBean方法，如下所示：
Object getBean(String name) throws BeansException;
<T> T getBean(String name, Class<T> requiredType) throws BeansException;
<T> T getBean(Class<T> requiredType) throws BeansException;
Object getBean(String name, Object... args) throws BeansException;
<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;
DefaultListableBeanFactory
DefaultListableBeanFactory是Bean工厂的一个默认实现，我们可以对它进行拓展以便实现自定义的Bean工厂。作为默认的Bean工厂实现，肯定也实现了接口中定义的getBean方法。截取DefaultListableBeanFactory中与getBean方法相关的源码如下：
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
......
@Override
public <T> T getBean(Class<T> requiredType) throws BeansException {
return getBean(requiredType, (Object[]) null);
}
@Override
public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {
NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);
if (namedBean != null) {
return namedBean.getBeanInstance();
}
BeanFactory parent = getParentBeanFactory();
if (parent != null) {
return parent.getBean(requiredType, args);
}
throw new NoSuchBeanDefinitionException(requiredType);
}
......
}
发现DefaultListableBeanFactory中只实现了两个getBean接口，BeanFactory中总共有五个接口，那另外三个接口呢？在哪里实现的？尤其是最重要的getBean(String name)接口。
查看DefaultListableBeanFactory类UML类图关系发现有一个AbstractBeanFactory抽象类，DefaultListableBeanFactory继承了AbstractBeanFactory，如下图所示：
AbstractBeanFactory
通过查找AbstractBeanFactory源码，终于找到BeanFactory接口中另外三个getBean方法的实现。源码如下：
package org.springframework.beans.factory.support;
import java.beans.PropertyEditor;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import org.springframework.beans.BeanUtils;
import org.springframework.beans.BeanWrapper;
import org.springframework.beans.BeansException;
import org.springframework.beans.PropertyEditorRegistrar;
import org.springframework.beans.PropertyEditorRegistry;
import org.springframework.beans.PropertyEditorRegistrySupport;
import org.springframework.beans.SimpleTypeConverter;
import org.springframework.beans.TypeConverter;
import org.springframework.beans.TypeMismatchException;
import org.springframework.beans.factory.BeanCreationException;
import org.springframework.beans.factory.BeanCurrentlyInCreationException;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanIsAbstractException;
import org.springframework.beans.factory.BeanIsNotAFactoryException;
import org.springframework.beans.factory.BeanNotOfRequiredTypeException;
import org.springframework.beans.factory.CannotLoadBeanClassException;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.beans.factory.SmartFactoryBean;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.config.BeanExpressionContext;
import org.springframework.beans.factory.config.BeanExpressionResolver;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.beans.factory.config.DestructionAwareBeanPostProcessor;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor;
import org.springframework.beans.factory.config.Scope;
import org.springframework.core.DecoratingClassLoader;
import org.springframework.core.NamedThreadLocal;
import org.springframework.core.ResolvableType;
import org.springframework.core.convert.ConversionService;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;
import org.springframework.util.StringValueResolver;
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
......
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
return doGetBean(name, requiredType, null, false);
}
@Override
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
return doGetBean(name, requiredType, args, false);
}
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
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.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
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) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
// Create bean instance.
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;
}
}
// 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;
}
......
}
在三个相同签名getBean方法中，均调用doGetBean方法。doGetBean方法负责从Bean工厂中获取bean对象的具体实现，下面来看看该方法的具体实现：
- 检查手动注册的单例集合缓存中是否含有该bean对象，若有，则取出返回，否则继续执行；
- 检查该bean是否已经创建，从而判断是否属于循环引用，若是，抛出异常返回，否则继续执行；
- 判断bean工厂中是否存在该bean definition，若存在，则取出返回，否则继续执行；
- 初始化该bean所依赖的bean对象；
- 判断该bean是否是单例模式（singleton），若是，创建单例对象，否则继续执行；
- 判断该bean是否是原型模式（prototype），若是，创建原型对象，否则继续执行；
- 创建自定义类型（scope）bean对象。
从上面对doGetBean方法分析，可看出创建并获取bean对象是一个非常复杂的过程，并不是简简单单的放入Map中再从其中取出。
小结
本文对IOC容器的核心getBean()方法作了简单的分析，初步了解了IOC容器中getBean()方法的执行流程原理，这对于帮助我们理解IOC容器是十分有帮助的。目前对IOC机制的各个模块都有了一定的了解，下面可以综合各个模块，串联起来研究IOC依赖注入的完整流程。
下文将以ClassPathXmlApplicationContext作为xml文件加载器，实现Spring IOC的类加载，通过断点跟踪来了解IOC对象注入完整流程。