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BeanFactory容器的设计原理
BeanFactory接口提供了使用IOC容器的规范。在这个基础上，Spring还提供了符合这个IOC容器借口了的一系列容器的实现供开发人员使用。先以XmlBeanFactory的实现为例来
说明简单IOC容器的设计原理，。下图为XmlBeanFactory设计的类继承关系。
可以看到，作为一个简单IOC容器系列最底层实现的XmlBeanFactory，与我们在Spring应用中用到的上下文相比，有一个明显的特点：它只提供最基本IOC容器的功能。
我们可以认为直接的BeanFactory实现是IOC容器的基本形式，而各种ApplicationContext的实现是IOC容器的高级表现形式。下面是XmlBeanFactory源码：
1 public class XmlBeanFactory extends DefaultListableBeanFactory { 2 3 private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this); 4 5 6 /** 7 * Create a new XmlBeanFactory with the given resource, 8 * which must be parsable using DOM. 9 * @param resource XML resource to load bean definitions from10 * @throws BeansException in case of loading or parsing errors11 */12 public XmlBeanFactory(Resource resource) throws BeansException {13 this(resource, null);14 }15 16 /**17 * Create a new XmlBeanFactory with the given input stream,18 * which must be parsable using DOM.19 * @param resource XML resource to load bean definitions from20 * @param parentBeanFactory parent bean factory21 * @throws BeansException in case of loading or parsing errors22 */23 public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {24 super(parentBeanFactory);25 this.reader.loadBeanDefinitions(resource);26 }27 28 }
XmlBeanFactory继承自DefaultListableBeanFactory这个类，后者很重要，是我们经常要用到的IOC容器的实现，比如在设计应用上下文ApplicationContext时就会用到它。
在Spring中，实际上是把DefaultListableBeanFactory作为一个默认功能完整的IOC容器来使用的。XmlBeanFactory继承DefaultListableBeanFactory容器功能的同时，增加
了新的功能。从名字我们可以看出，它是一个与XML相关的BeanFactory，也就是说它是一个可以读取以XML文件方式定义的BeanDefinition的IOC容器。（关于BeanDefinition，我们可以理解为spring ioc的bean配置文件）
在XmlBeanFactory中，初始化了一个XmlBeanDefinitionReader对象，有了这个Reader对象，那些以XML方式定义的BeanDefinition就有了处理的地方。我们可以看到，对这
些XML形式的信息的处理实际上是由这个XmlBeanDefinitionReader来完成的。
构造XmlBeanFactory这个IOC容器时，需要指定BeanDefinition的信息来源，而这个信息来源需要封装成Spring中的Resource类，下面为Resource源码：
public interface Resource extends InputStreamSource { /** * Return whether this resource actually exists in physical form. * <p>This method performs a definitive existence check, whereas the * existence of a {@code Resource} handle only guarantees a * valid descriptor handle. */ boolean exists(); /** * Return whether the contents of this resource can be read, * e.g. via {@link #getInputStream()} or {@link #getFile()}. * <p>Will be {@code true} for typical resource descriptors; * note that actual content reading may still fail when attempted. * However, a value of {@code false} is a definitive indication * that the resource content cannot be read. * @see #getInputStream() */ boolean isReadable(); /** * Return whether this resource represents a handle with an open * stream. If true, the InputStream cannot be read multiple times, * and must be read and closed to avoid resource leaks. * <p>Will be {@code false} for typical resource descriptors. */ boolean isOpen(); /** * Return a URL handle for this resource. * @throws IOException if the resource cannot be resolved as URL, * i.e. if the resource is not available as descriptor */ URL getURL() throws IOException; /** * Return a URI handle for this resource. * @throws IOException if the resource cannot be resolved as URI, * i.e. if the resource is not available as descriptor */ URI getURI() throws IOException; /** * Return a File handle for this resource. * @throws IOException if the resource cannot be resolved as absolute * file path, i.e. if the resource is not available in a file system */ File getFile() throws IOException; /** * Determine the content length for this resource. * @throws IOException if the resource cannot be resolved * (in the file system or as some other known physical resource type) */ long contentLength() throws IOException; /** * Determine the last-modified timestamp for this resource. * @throws IOException if the resource cannot be resolved * (in the file system or as some other known physical resource type) */ long lastModified() throws IOException; /** * Create a resource relative to this resource. * @param relativePath the relative path (relative to this resource) * @return the resource handle for the relative resource * @throws IOException if the relative resource cannot be determined */ Resource createRelative(String relativePath) throws IOException; /** * Determine a filename for this resource, i.e. typically the last * part of the path: for example, "myfile.txt". * <p>Returns {@code null} if this type of resource does not * have a filename. */ String getFilename(); /** * Return a description for this resource, * to be used for error output when working with the resource. * <p>Implementations are also encouraged to return this value * from their {@code toString} method. * @see Object#toString() */ String getDescription();}
public interface InputStreamSource { /** * Return an {@link InputStream}. * <p>It is expected that each call creates a <i>fresh</i> stream. * <p>This requirement is particularly important when you consider an API such * as JavaMail, which needs to be able to read the stream multiple times when * creating mail attachments. For such a use case, it is <i>required</i> * that each {@code getInputStream()} call returns a fresh stream. * @return the input stream for the underlying resource (must not be {@code null}) * @throws IOException if the stream could not be opened * @see org.springframework.mail.javamail.MimeMessageHelper#addAttachment(String, InputStreamSource) */ InputStream getInputStream() throws IOException;}
Resource是Spring用来封装I/O操作的类。比如我们的BeanDefinition信息的以XML文件的形式存在的，那么可以使用像ClassPathResource res = new ClassPathResource
("beans.xml")这样具体的ClassPathResource来构造需要的Resource，然后将Resource作为构造参数传递给XmlBeanFactory构造函数。这样，IOC容器就可以方便地定位到
需要的BeanDefinition信息来对Bean完成容器的初始化和依赖注入过程。
XmlBeanFactory的功能是建立在DefaultListableBeanFactory这个基本容器基础上的，并在这个基本容器的基本上实现了诸如XML读取的附加功能。如代码清单所示，在
XmlBeanFactory构造方法中需要得到Resource对象。对XmlBeanDefinitionReader初始化，以及使用这个对象来完成loadBeanDefinitions的调用，就是这个调用启动从
Resource中载人BeanDefinition的过程，loadBeanDefinitions同时也是IOC初始化的重要组成部分。
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException { return loadBeanDefinitions(new EncodedResource(resource)); }
public EncodedResource(Resource resource) { Assert.notNull(resource, "Resource must not be null"); this.resource = resource; }
我们可以看到XmlBeanFactory使用了DefaultListableBeanFactory作为基类，DefaultListableBeanFactory是很重要的一个IOC实现，在其他IOC容器中，比如ApplicationContext，其实现的基本原理和XmlBeanFactory一样，不过ApplicationContext是继承了其他BeanFactory。
参考XmlBeanFactory的实现，我们以编程的方式使用DefaultListableBeanFactory。从中我们可以看到IOC容器使用的一些基本过程。尽管应用中我们很少会使用这样的原始
方式，但了解这个基本过程，对我们了解IOC容器的工作原理是有帮助的。因为这个编程式使用容器的过程，很清楚揭示了在IOC容器实现中的那些关键的类（比如Resource，
XmlBeanDefinitionReader，DefaultListableBeanFactory）之间的相互关系，例如它们是如何把IOC容器的功能解耦的，又是如何结合在一起为IOC容器服务的，等等。在代码
清单中我们可以看到编程式使用IOC容器的过程。
//指定文件 ClassPathResource res= new ClassPathResource("beans.xml"); //初始化DefaultListableBeanFactory DefaultListableBeanFactory factory = new DefaultListableBeanFactory(); //初始化构造XmlBeanDefinitionReader XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory); //加载BeanDefinition reader.loadBeanDefinitions(res)
这样，我们就可以通过factory对象来使用DefaultListableBeanFactory这个容器了。
BeanFactory容器的设计原理