java踩坑记-getResourceAsStream

本文主要是研究下面集中方法到底谁才能真正的load到文件,你能一眼看出来吗？

GetResourcesSample.class.getClassLoader.getResourceAsStream("main-resources-file.txt") 

GetResourcesSample.class.getClass().getResourceAsStream("main-resources-file.txt") 

GetResourcesSample.class.getResourceAsStream("main-resources-file.txt") 

 

GetResourcesSample.class.getClassLoader.getResourceAsStream("/main-resources-file.txt") 

GetResourcesSample.class.getClass().getResourceAsStream("/main-resources-file.txt") 

GetResourcesSample.class.getResourceAsStream("/main-resources-file.txt") 

 

很晕吧，反正我是挺晕的。

 

目录结构：

 

代码：

public class GetResourcesSample {
    private static final String MAIN_FILE = "main-resources-file.txt";
    private static final String MAIN_FILE_WITH_SLASH = "/main-resources-file.txt";
    private static final String TEST_FILE = "test-resources-file.txt";
    private static final String TEST_FILE_WITH_SLASH = "/test-resources-file.txt";
    private static final String ROOT_FILE = "project-root-file.txt";
    private static final String ROOT_FILE_WITH_SLASH = "./project-root-file.txt";

    public static void main(String[] args) {
        validateResult(MAIN_FILE);
        validateResult(MAIN_FILE_WITH_SLASH);

        validateResult(TEST_FILE);
        validateResult(TEST_FILE_WITH_SLASH);

        validateResult(ROOT_FILE);
        validateResult(ROOT_FILE_WITH_SLASH);

    }

    private static void validateResult(String file) {

        InputStream inputStream1 = GetResourcesSample.class.getClass().getResourceAsStream(file);
        if (inputStream1 != null) {
            System.out.println(MessageFormat
                    .format("GetResourcesSample.class.getClass().getResourceAsStream(\"{0}\") is not null!", file));
        }

        InputStream inputStream2 = GetResourcesSample.class.getResourceAsStream(file);
        if (inputStream2 != null) {
            System.out.println(
                    MessageFormat.format("GetResourcesSample.class.getResourceAsStream(\"{0}\") is not null!", file));
        }

        InputStream inputStream3 = GetResourcesSample.class.getClassLoader().getResourceAsStream(file);
        if (inputStream3 != null) {
            System.out.println(
                    MessageFormat.format("GetResourcesSample.class.getClassLoader().getResourceAsStream(\"{0}\") is not null!", file));
        }

    }
}

结果：

GetResourcesSample.class.getClassLoader().getResourceAsStream("main-resources-file.txt") is not null!

GetResourcesSample.class.getClass().getResourceAsStream("/main-resources-file.txt") is not null!

GetResourcesSample.class.getResourceAsStream("/main-resources-file.txt") is not null!

 

结论：

1. Class.getResourceAsStream 最终调用的是此Class的Class Loader的getResourceAsStream来进行读文件（有些绕，尽量理解）
2. GetResourcesSample.class.getClass() 得到的是Class这个类的实例，它的ClassLoader是JVM的bootstrap Class Loader
3. GetResourcesSample.class.getResourceAsStream 由于去resolve了一下name，虽然是当前class的ClassLoader，它也读不到这个文件；除非你的文件名以“／”开头。详见源代码：

       /**
        * Add a package name prefix if the name is not absolute Remove leading "/"
        * if name is absolute
        */
       private String resolveName(String name) {
           if (name == null) {
               return name;
           }
           if (!name.startsWith("/")) {
               Class<?> c = this;
               while (c.isArray()) {
                   c = c.getComponentType();
               }
               String baseName = c.getName();
               int index = baseName.lastIndexOf('.');
               if (index != -1) {
                   name = baseName.substring(0, index).replace('.', '/')
                       +"/"+name;
               }
           } else {
               name = name.substring(1);
           }
           return name;
       }

4. project-root-file.txt这种最后最终不被打在Jar包里的文件，是没有机会被读取的，除非hard code一个文件所在的绝对路径
5. 把相同代码放到UT里面去跑，会发现UT能同时load到main和test下的文件。因为JUnit有它自己的ClassLoader，它能同时读到main和test下的文件。