Spring 学习记录4 ResourceLoader

ResourceLoader

Spring的ApplicationContext继承了ResourceLoader接口.这个接口主要就是可以加载各种resource..

接口还是比较简单的:

/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.io;

import org.springframework.util.ResourceUtils;

/**
 * Strategy interface for loading resources (e.. class path or file system
 * resources). An {@link org.springframework.context.ApplicationContext}
 * is required to provide this functionality, plus extended
 * {@link org.springframework.core.io.support.ResourcePatternResolver} support.
 *
 * <p>{@link DefaultResourceLoader} is a standalone implementation that is
 * usable outside an ApplicationContext, also used by {@link ResourceEditor}.
 *
 * <p>Bean properties of type Resource and Resource array can be populated
 * from Strings when running in an ApplicationContext, using the particular
 * context's resource loading strategy.
 *
 * @author Juergen Hoeller
 * @since 10.03.2004
 * @see Resource
 * @see org.springframework.core.io.support.ResourcePatternResolver
 * @see org.springframework.context.ApplicationContext
 * @see org.springframework.context.ResourceLoaderAware
 */
public interface ResourceLoader {

    /** Pseudo URL prefix for loading from the class path: "classpath:" */
    String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;


    /**
     * Return a Resource handle for the specified resource.
     * The handle should always be a reusable resource descriptor,
     * allowing for multiple {@link Resource#getInputStream()} calls.
     * <p><ul>
     * <li>Must support fully qualified URLs, e.g. "file:C:/test.dat".
     * <li>Must support classpath pseudo-URLs, e.g. "classpath:test.dat".
     * <li>Should support relative file paths, e.g. "WEB-INF/test.dat".
     * (This will be implementation-specific, typically provided by an
     * ApplicationContext implementation.)
     * </ul>
     * <p>Note that a Resource handle does not imply an existing resource;
     * you need to invoke {@link Resource#exists} to check for existence.
     * @param location the resource location
     * @return a corresponding Resource handle
     * @see #CLASSPATH_URL_PREFIX
     * @see org.springframework.core.io.Resource#exists
     * @see org.springframework.core.io.Resource#getInputStream
     */
    Resource getResource(String location);

    /**
     * Expose the ClassLoader used by this ResourceLoader.
     * <p>Clients which need to access the ClassLoader directly can do so
     * in a uniform manner with the ResourceLoader, rather than relying
     * on the thread context ClassLoader.
     * @return the ClassLoader (only {@code null} if even the system
     * ClassLoader isn't accessible)
     * @see org.springframework.util.ClassUtils#getDefaultClassLoader()
     */
    ClassLoader getClassLoader();

}

我感觉主要可能就是getResource方法了.

具体使用

实验如下:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:test-application-context.xml")
public class ResourceLoaderTest implements ApplicationContextAware {
    ApplicationContext applicationContext;

    @Test
    public void testLoadResource() throws IOException {

        Resource resource = applicationContext.getResource("classpath:test.properties");
        System.out.println(resource.exists()); // true

        Resource resource2 = applicationContext.getResource("/test.properties");
        System.out.println(resource2.exists()); // true

        Resource resource3 = applicationContext.getResource("test.properties");
        System.out.println(resource3.exists()); // true

        Resource resource4 = applicationContext.getResource("classpath:1.html");
        System.out.println(FileUtils.readFileToString(resource4.getFile())); // 文件内容

        Resource resource5 = applicationContext.getResource("https://www.baidu.com/");
        System.out.println(IOUtils.toString(resource5.getInputStream())); // 网页内容

        Resource resource6 = applicationContext.getResource("/spring/Config.class");
        System.out.println(resource6.exists()); // true

        Resource resource7 = applicationContext.getResource("org/springframework/context/support/GenericApplicationContext.class");
        System.out.println(resource7.exists()); // true
    }

    @Test
    public void a() {
        System.out.println(1);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

实验中发现:

1.可以通过classpath:XXX下载classpath下的资源

2.可以通过/XXX也是加载classpath下的资源

3.直接XXX,可以根据不同的协议去加载资源(比如http),没有的话去加载classpath下的资源

4.不光可以加载classes下的资源,也可以加载lib里jar里面的资源.

我用的junit 测试,applicationcontext是GenericApplicationContext的实例,getResource方法调的是DefaultResourceLoader的实现

    @Override
    public Resource getResource(String location) {
        Assert.notNull(location, "Location must not be null");
        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 new UrlResource(url);
            }
            catch (MalformedURLException ex) {
                // No URL -> resolve as resource path.
                return getResourceByPath(location);
            }
        }
    }

从代码中我们可以发现,

1.如果是/开头的资源,会调用getResourceByPath方法,最后返回的其实也是ClassPathResource

    /**
     * Return a Resource handle for the resource at the given path.
     * <p>The default implementation supports class path locations. This should
     * be appropriate for standalone implementations but can be overridden,
     * e.g. for implementations targeted at a Servlet container.
     * @param path the path to the resource
     * @return the corresponding Resource handle
     * @see ClassPathResource
     * @see org.springframework.context.support.FileSystemXmlApplicationContext#getResourceByPath
     * @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath
     */
    protected Resource getResourceByPath(String path) {
        return new ClassPathContextResource(path, getClassLoader());
    }


    /**
     * ClassPathResource that explicitly expresses a context-relative path
     * through implementing the ContextResource interface.
     */
    protected static class ClassPathContextResource extends ClassPathResource implements ContextResource {

        public ClassPathContextResource(String path, 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());
        }
    }

2.如果是classpath:开头,也是ClassPathResource

3.如果是XXX.XX的话用URL去找资源失败的话,还是会返回ClassPathResource,成功的话就是返回UrlResource.

然后我想到了1个问题.就是我们在项目中加载文件的时候经常会用classpath*:.............这种形式在这里似乎没有出现,可能是Servlet环境下的ApplicationContext覆盖了getResource方法,也可能是其他方法加载资源..等我学习了其他的applicationcontext就明白了...可能会再做分享.

小结

我感觉使用resourceloader相比于getResource里面写classpath:xxxxxx比自己去getClass().getResource的好处在于:

1.更简单清晰...看过去就知道资源是相对于classpath的...

2.resourceloader产生的ClassPathResource对于你传入的路径字符串是会转化的...你传入的windows的也会被转化成/..而getClass那种并不会....所以getClass().getResource在传入的String拼接的时候如果用到了File.sperator可能会找不到资源,而resourceloader不会...不过更多的时候可能都不需要拼接...直接写1个完整的字符串用/分割路径就行了...