Spring DefaultResourceLoader
Spring DefaultResourceLoader继承ResourceLoader接口，用来加载资源， 通过ResourceLoader,给定其可以接受的资源路径，我们可以获得对应资源的Resource对象，然后进行进行相应的资源访问 。
Spring提供了一个默认的实现类DefaultResourceLoader，可以用来加载classpath或者文件系统中的某个文件，也可以采用URL的形式加载某个网络的资源。
ResourceLoader接口定义如下：
public interface ResourceLoader {
/** Pseudo URL prefix for loading from the class path: "classpath:". */
String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;
/**
* 根据给定的资源路径,返回响应的Resource资源对象
* 支持如下三种形式：
*"file:C:/test.dat".
*"classpath:test.dat".
*"WEB-INF/test.dat".
*/
Resource getResource(String location);
/**
* 获取资源类加载器
*/
@Nullable
ClassLoader getClassLoader();
}
DefaultResourceLoader类中定义方法及属性如下：
public class DefaultResourceLoader implements ResourceLoader {
@Nullable
private ClassLoader classLoader;
private final Set<ProtocolResolver> protocolResolvers = new LinkedHashSet<>(4);
private final Map<Class<?>, Map<Resource, ?>> resourceCaches = new ConcurrentHashMap<>(4);
//构造方法中获取默认个类加载器
public DefaultResourceLoader() {
this.classLoader = ClassUtils.getDefaultClassLoader();
}
public DefaultResourceLoader(@Nullable ClassLoader classLoader) {
this.classLoader = classLoader;
}
public void setClassLoader(@Nullable ClassLoader classLoader) {
this.classLoader = classLoader;
}
//返回类加载器
@Override
@Nullable
public ClassLoader getClassLoader() {
return (this.classLoader != null ? this.classLoader : ClassUtils.getDefaultClassLoader());
}
/**
* 注册资源解析器,会覆盖默认的资源规则
*/
public void addProtocolResolver(ProtocolResolver resolver) {
Assert.notNull(resolver, "ProtocolResolver must not be null");
this.protocolResolvers.add(resolver);
}
/**
* Return the collection of currently registered protocol resolvers,
* allowing for introspection as well as modification.
* @since 4.3
*/
public Collection<ProtocolResolver> getProtocolResolvers() {
return this.protocolResolvers;
}
/**
* Obtain a cache for the given value type, keyed by {@link Resource}.
* @param valueType the value type, e.g. an ASM {@code MetadataReader}
* @return the cache {@link Map}, shared at the {@code ResourceLoader} level
* @since 5.0
*/
@SuppressWarnings("unchecked")
public <T> Map<Resource, T> getResourceCache(Class<T> valueType) {
return (Map<Resource, T>) this.resourceCaches.computeIfAbsent(valueType, key -> new ConcurrentHashMap<>());
}
/**
* 清空资源缓存
*/
public void clearResourceCaches() {
this.resourceCaches.clear();
}
//加载资源并返回Resource对象
@Override
public Resource getResource(String location) {
Assert.notNull(location, "Location must not be null");
//如果有ProtocolResolver,会优先调用
for (ProtocolResolver protocolResolver : getProtocolResolvers()) {
Resource resource = protocolResolver.resolve(location, this);
//根据返回值是否为null来判断是否解决 资源加载的问题
if (resource != null) {
return resource;
}
}
//资源路径是否以/开头
if (location.startsWith("/")) {
return getResourceByPath(location);
}//判断资源路径是否以classpath:开头
else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
}
else {
try {
// T尝试将资源路径转化为URL形式去加载
URL url = new URL(location);
return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
}
catch (MalformedURLException ex) {
// 当上面无法转换为 URL的形式事，尝试从相对路径中加载资源
return getResourceByPath(location);
}
}
}
//从相对路径中下载资源
protected Resource getResourceByPath(String path) {
return new ClassPathContextResource(path, getClassLoader());
}
/**
* DefaultResourceLoader内部类类，用来加载相对路径的资源，可以参看ClassPathResource
https://www.cnblogs.com/haizhilangzi/p/12717368.html
*/
protected static class ClassPathContextResource extends ClassPathResource implements ContextResource {
public ClassPathContextResource(String path, @Nullable ClassLoader classLoader) {
super(path, classLoader);
}
@Override
public String getPathWithinContext() {
return getPath();
}
@Override
public Resource createRelative(String relativePath) {
String pathToUse = StringUtils.applyRelativePath(getPath(), relativePath);
return new ClassPathContextResource(pathToUse, getClassLoader());
}
}
}
关于ProtocolResolver接口的详细使用，请参考 Spring ProtocolResolver
测试代码
ResourceLoader resourceLoader = new DefaultResourceLoader();
//获取classpath:上的资源
// Resource resource = resourceLoader.getResource(
// "classpath:resource/conf.txt");
//获取相对路径上的某个资源，相当于classpath:
// Resource resource = resourceLoader.getResource(
// "resource/conf.txt");
//同样是获取classpath:上的资源
// Resource resource = resourceLoader.getResource(
// "/resource/conf.txt");
//获取网络上的资源
Resource resource = resourceLoader.getResource(
"https://www.cnblogs.com/haizhilangzi/p/12717368.html");
System.out.println(resource.getFilename());
System.out.println(resource.contentLength());
System.out.println(resource.getURL());
System.out.println(resource.getURI());
InputStream inputStream = resource.getInputStream();
StringBuffer out = new StringBuffer();
byte[] b = new byte[4096];
for (int n; (n = inputStream.read(b)) != -1; ) {
out.append(new String(b, 0, n));
}
System.out.println(out);