单刀直入@ComponentScan之 资源加载

欢迎大家入坑，所谓师傅领进坑爬出去靠个人，首先我要说的是这个是上一篇《单刀直入@ComponentScan》的姊妹篇哈，接着把没聊透的事说明白，咱不是虎头蛇尾的人。

资源加载是啥意思

scan ，都认识吧，小学词汇连我都认识，扫到的是啥，扫到的是资源啊，如何让资源为我所用，就需要把资源搞进来，这就是资源加载。

spring如何加载资源的

首先不得不承认spring本身是很专一的，她把所有的资源都用一个统一的接口来表示，Resource,我们不妨看看她的真容。

```java
/*** Interface for a resource descriptor that abstracts from the actual* type of underlying resource, such as a file or class path resource.** <p>An InputStream can be opened for every resource if it exists in* physical form, but a URL or File handle can just be returned for* certain resources. The actual behavior is implementation-specific.** @author Juergen Hoeller* @since 28.12.2003* @see #getInputStream()* @see #getURL()* @see #getURI()* @see #getFile()* @see WritableResource* @see ContextResource* @see UrlResource* @see FileUrlResource* @see FileSystemResource* @see ClassPathResource* @see ByteArrayResource* @see InputStreamResource*/
public interface Resource extends InputStreamSource {/*** Determine 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();/*** Indicate whether non-empty contents of this resource can be read via* {@link #getInputStream()}.* <p>Will be {@code true} for typical resource descriptors that exist* since it strictly implies {@link #exists()} semantics as of 5.1.* 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()* @see #exists()*/default boolean isReadable() {return exists();}/*** Indicate whether this resource represents a handle with an open stream.* If {@code 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.*/default boolean isOpen() {return false;}/*** Determine whether this resource represents a file in a file system.* A value of {@code true} strongly suggests (but does not guarantee)* that a {@link #getFile()} call will succeed.* <p>This is conservatively {@code false} by default.* @since 5.0* @see #getFile()*/default boolean isFile() {return false;}/*** 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* @since 2.5*/URI getURI() throws IOException;/*** Return a File handle for this resource.* @throws java.io.FileNotFoundException if the resource cannot be resolved as* absolute file path, i.e. if the resource is not available in a file system* @throws IOException in case of general resolution/reading failures* @see #getInputStream()*/File getFile() throws IOException;/*** Return a {@link ReadableByteChannel}.* <p>It is expected that each call creates a <i>fresh</i> channel.* <p>The default implementation returns {@link Channels#newChannel(InputStream)}* with the result of {@link #getInputStream()}.* @return the byte channel for the underlying resource (must not be {@code null})* @throws java.io.FileNotFoundException if the underlying resource doesn't exist* @throws IOException if the content channel could not be opened* @since 5.0* @see #getInputStream()*/default ReadableByteChannel readableChannel() throws IOException {return Channels.newChannel(getInputStream());}/*** 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.*/@NullableString 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();}

请原谅我用代码占用了很大的篇幅，主要是为了让大家通过看文章不用去翻代码，能够连贯的读下来，同时没有把注释去掉的原因是，任何只看代码不看注释的行为都是耍流氓，多说一句，我觉得想要成为快乐的程序员，应该具备勇气和丰富的想象力。以上内容就需要你的想象力了，我啥都不说了。要用心，别走马观花，看热闹，觉得挺热闹然后给个赞，对自己啥收获也没有，那就是浪费时间，这是一个浪费了10余年老程序员的忠告，虽然现在他依然在浪费着时间，写到这里感觉自己有点飘了呢，开始好为人师了。
接下来着重说一下，Resource的神兵利器，ResourceLoader ，她算一个重头戏，资源加载器，拿来加载资源，为什么要有这么个东东呢，要根据她在spring中唯一的实现 DefaultResourceLoader 说起。

public class DefaultResourceLoader implements ResourceLoader {@Nullableprivate ClassLoader classLoader;private final Set<ProtocolResolver> protocolResolvers = new LinkedHashSet<>(4);private final Map<Class<?>, Map<Resource, ?>> resourceCaches = new ConcurrentHashMap<>(4);~~~~~~~~~~~~~~/*** Register the given resolver with this resource loader, allowing for* additional protocols to be handled.* <p>Any such resolver will be invoked ahead of this loader's standard* resolution rules. It may therefore also override any default rules.* @since 4.3* @see #getProtocolResolvers()*/public void addProtocolResolver(ProtocolResolver resolver) {Assert.notNull(resolver, "ProtocolResolver must not be null");this.protocolResolvers.add(resolver);}@Overridepublic Resource getResource(String location) {Assert.notNull(location, "Location must not be null");for (ProtocolResolver protocolResolver : getProtocolResolvers()) {Resource resource = protocolResolver.resolve(location, this);if (resource != null) {return resource;}}if (location.startsWith("/")) {return getResourceByPath(location);}else if (location.startsWith(CLASSPATH_URL_PREFIX)) {return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());}else {try {// Try to parse the location as a URL...URL url = new URL(location);return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));}catch (MalformedURLException ex) {// No URL -> resolve as resource path.return getResourceByPath(location);}}}

不好意思，上面的类代码是不完整的，我删除了一些简单的构造函数，还有一些我没看懂的复杂函数，捡我认为重要的说，Resource getResource(String location)，这核心的方法，也揭秘了ResourceLoader 存在的价值，首先我们知道我们在加载自愿的时候资源的形态是多样的，这个通过上面的注释也能看出来，有 UrlResource FileUrlResource FileSystemResource ClassPathResource，ResourceLoader 的作用就是可以让多样的资源来源加载用统一的方法来加载，主要是根据资源加载路径的组成方式，比如d:\abc.txt,www.baidu.com/abc.txt,classpath:abc.class, getResouce方法其实使用简单工厂模式的方式。public void addProtocolResolver(ProtocolResolver resolver) 这个方法其实是留给我们使用者去进行扩展的，当你的资源加载方式比较复杂，比如你的资源路径是通过读取另一些资源来拼装的，那么你可以自定义ProtocolResolver 来处理，因为getRource方法的第一句就是

	for (ProtocolResolver protocolResolver : getProtocolResolvers()) {Resource resource = protocolResolver.resolve(location, this);if (resource != null) {return resource;}}

如何去注册自己的ProtocolResolver 呢，applicationContext.addProtocolResolver();奥秘就是因为public abstract class AbstractApplicationContext extends DefaultResourceLoader。
如果你耐心的读到这里，我必须上点干货了，报答你的不走之恩。
ComponentScan 是如何应用这个ResourceLoader 的呢，ConfigurationClassPostProcessor 这个重头类就必须要说一下了，她是干嘛地…! 不得不说她是一个牛逼的类，干的事特别的多。

1. 解析 @Configuration。
2. 解析 @Bean 方法。
3. 解析 @ComponentScan。
4. 解析 @Import 注解。
5. 解析 @ImportResource 注解。
6. 解析 @PropertySource 注解。

可以这样说，所有bean的加载都是他来干，她实现了BeanDefinitionRegistryPostProcessor，懂得都懂，然后有一个关键台词，类里有一个成员变量，private ResourceLoader resourceLoader = new DefaultResourceLoader(); 然后把这个成员变量向下传递，一直到上文说到的private Set scanCandidateComponents(String basePackage)方法中哈，找到潜在的组件，什么组件无非就是上文说的加了@Component @Service @Controller等注解的类呗。

	private Set<BeanDefinition> scanCandidateComponents(String basePackage) {Set<BeanDefinition> candidates = new LinkedHashSet<>();try {String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +resolveBasePackage(basePackage) + '/' + this.resourcePattern;**Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);**boolean traceEnabled = logger.isTraceEnabled();boolean debugEnabled = logger.isDebugEnabled();for (Resource resource : resources) {if (traceEnabled) {logger.trace("Scanning " + resource);}if (resource.isReadable()) {try {MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);if (isCandidateComponent(metadataReader)) {ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);sbd.setResource(resource);sbd.setSource(resource);if (isCandidateComponent(sbd)) {if (debugEnabled) {logger.debug("Identified candidate component class: " + resource);}candidates.add(sbd);}else {if (debugEnabled) {logger.debug("Ignored because not a concrete top-level class: " + resource);}}}else {if (traceEnabled) {logger.trace("Ignored because not matching any filter: " + resource);}}}catch (Throwable ex) {throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);}}else {if (traceEnabled) {logger.trace("Ignored because not readable: " + resource);}}}}catch (IOException ex) {throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);}return candidates;
}

两句关键台词
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + ‘/’ + this.resourcePattern;
Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);

细心的读者会发现我们一直说的ResourceLoader 接口里面就没有getResources方法，这里面就提到一个衍生的接口，ResourcePatternResolver。

public interface ResourcePatternResolver extends ResourceLoader {/*** Pseudo URL prefix for all matching resources from the class path: "classpath*:"* This differs from ResourceLoader's classpath URL prefix in that it* retrieves all matching resources for a given name (e.g. "/beans.xml"),* for example in the root of all deployed JAR files.* @see org.springframework.core.io.ResourceLoader#CLASSPATH_URL_PREFIX*/String CLASSPATH_ALL_URL_PREFIX = "classpath*:";/*** Resolve the given location pattern into Resource objects.* <p>Overlapping resource entries that point to the same physical* resource should be avoided, as far as possible. The result should* have set semantics.* @param locationPattern the location pattern to resolve* @return the corresponding Resource objects* @throws IOException in case of I/O errors*/Resource[] getResources(String locationPattern) throws IOException;
}

getResourcePatternResolver 获取到的是PathMatchingResourcePatternResolver处理器，然后通过循环调用defaultResourceLoader的getResource方法来加载资源。今天先说这么多吧，对于加载还有不少内容，不放在一起了，以防都看蒙圈了。下一篇会着重讲下这里面的内容。