Spring与设计模式---代理模式

背景

spring的配置文件一般有两种类型:DTD或者XSD类型．配置文件中配置了bean和bean依赖关系的定义，加载配置文件并解析文件中的bean定义是为容器注册和获取bean的准备工作．Spring将配置文件看成一个资源(Resource),首先会将改Resource进行编码，判断资源的验证模式validationMode(即是属于DTD验证模式还是XSD验证模式)，根据对应的模式加载和解析配置文件得到Document文件．对于xml的解析交给了SAX来负责．

在解析XML文件过程中，SAX首先会读取改文档上的声明,根据声明寻找相应的DTD定义，以便对文档进行验证．默认寻找方式是从网络上下载相应的DTD声明，并进行验证．下载过程比较漫长也受网络的影响，所以spring在项目中提供了DTD声明文件(如　spring-beans-2.0.dtd). EntityResolver类的作用就是项目本身提供一个如何寻找DTD声明的方法，即有程序来实现寻找DTD声明过程，将获取到的DTD文件交给SAX,避免网络的寻找．

Spring中的EntityResolver采用了代理模式来实现，下面我会详细的介绍源码中的该代理模式设计．

源码中的设计

结构

源码实现

在spring源码设计中，

//定义了一个公共的 EntityResolver 接口
public interface EntityResolver {
    public abstract InputSource resolveEntity (String publicId,
	                                       String systemId)
	throws SAXException, IOException;
}

具体实现类，即DTD和XSD的声明寻找方式：
DTD声明的寻找方式:

public class BeansDtdResolver implements EntityResolver {
	private static final String DTD_EXTENSION = ".dtd";
	private static final String DTD_FILENAME = "spring-beans-2.0";
	private static final String DTD_NAME = "spring-beans";
	private static final Log logger = LogFactory.getLog(BeansDtdResolver.class);
	@Override 
	public InputSource resolveEntity(String publicId, String systemId) throws IOException {
			//实现了寻找DTD声明，并返回该声明的InputSource
			...

	　　　　　　　　return null;
	}

	@Override
	public String toString() {
		return "EntityResolver for spring-beans DTD";
	}
}

XSD声明的寻找方式：

public class PluggableSchemaResolver implements EntityResolver {
	//省略属性声明
	...

	public PluggableSchemaResolver(ClassLoader classLoader) {
		this.classLoader = classLoader;
		this.schemaMappingsLocation = DEFAULT_SCHEMA_MAPPINGS_LOCATION;
	}
	public PluggableSchemaResolver(ClassLoader classLoader, String schemaMappingsLocation) {
		Assert.hasText(schemaMappingsLocation, "'schemaMappingsLocation' must not be empty");
		this.classLoader = classLoader;
		this.schemaMappingsLocation = schemaMappingsLocation;
	}

	@Override
	public InputSource resolveEntity(String publicId, String systemId) throws IOException {
		//实现了寻找DTD声明，并返回该声明的InputSource　具体代码省略
		...
		return null;
	}
	//被resolveEntity　方法所调用．
	private Map<String, String> getSchemaMappings() {
		//省略具体实现
		....
	}
	@Override
	public String toString() {
		return "EntityResolver using mappings " + getSchemaMappings();
	}

}

采用代理模式来进行解析，在代理内部根据验证模式选择具体的代理对象(EntityResolver)来完成具体的寻找过程．

public class DelegatingEntityResolver implements EntityResolver {
	/** Suffix for DTD files */
	public static final String DTD_SUFFIX = ".dtd";
	/** Suffix for schema definition files */
	public static final String XSD_SUFFIX = ".xsd";
	private final EntityResolver dtdResolver;
	private final EntityResolver schemaResolver;
	public DelegatingEntityResolver(ClassLoader classLoader) {
		this.dtdResolver = new BeansDtdResolver();//定义默认的dtdResolver
		this.schemaResolver = new PluggableSchemaResolver(classLoader);
	}
	public DelegatingEntityResolver(EntityResolver dtdResolver, EntityResolver schemaResolver) {
		Assert.notNull(dtdResolver, "'dtdResolver' is required");
		Assert.notNull(schemaResolver, "'schemaResolver' is required");
		this.dtdResolver = dtdResolver;
		this.schemaResolver = schemaResolver;
	}
	@Override
	public InputSource resolveEntity(String publicId, String systemId) throws SAXException, IOException {
		if (systemId != null) {
			if (systemId.endsWith(DTD_SUFFIX)) {
				//dtd解析方法
				return this.dtdResolver.resolveEntity(publicId, systemId);
			}
			else if (systemId.endsWith(XSD_SUFFIX)) {
				//调用META-INF/Spring.schemas解析
				return this.schemaResolver.resolveEntity(publicId, systemId);
			}
		}
		return null;
	}
	@Override
	public String toString() {
		return "EntityResolver delegating " + XSD_SUFFIX + " to " + this.schemaResolver +
				" and " + DTD_SUFFIX + " to " + this.dtdResolver;
	}
}