class.getClassLoader getResourceasStream在jar中返回null

Question

I am facing this strange issue while building my project through Maven.

Here is my code for reading the text file, which I have put under resources folder in a Maven project. While Running from IDE everything is fine but once I create a jar and run from command line, it is returning me null .

public static void main(String[] args) throws IOException {
    URL jarUrl = FreeCalcTest.class.getProtectionDomain().getCodeSource().getLocation();
    System.out.println("JarUrl : "+jarUrl);
    String filePath = "UserDetails\\ReadFile.txt";
    URL url = FreeCalcTest.class.getClassLoader().getResource(filePath);
    System.out.println(url);
    InputStream inputStream = FreeCalcTest.class.getClassLoader().getResourceAsStream(filePath);
    UserDetailReader userDetailReader = new UserDetailReader(inputStream);
    List<UserDetailValueSet> userDetails = userDetailReader.read();
    Set<String> attrList =  UserDetailAttributes.getAttributes();
    Iterator<String> attrItr = attrList.iterator();
    Iterator<UserDetailValueSet> itr = userDetails.iterator();
    while (itr.hasNext()) {
        UserDetailValueSet userDetailValueSet = (UserDetailValueSet) itr.next();
        while (attrItr.hasNext()) {
            String attr = (String) attrItr.next();
            System.out.println(userDetailValueSet.getValueSet().get(attr));
        }

    }
}

Exception stacktrace:

JarUrl : file:/C:/Users/bnath/Desktop/FreeCalc-0.0.1-SNAPSHOT.jar null
Exception in thread "main" java.lang.NullPointerException
    at java.io.Reader.<init>(Unknown Source) at java.io.InputStreamReader.<init>(Unknown Source)
    at com.sec.io.reader.UserDetailReader.read(UserDetailReader.java:24)
    at com.sec.testFreeCalc.FreeCalcTest.main(FreeCalcTest.java:24)

Answer 1

You should use a forward slash as separator for resource paths, therefore try "UserDetails/ReadFile.txt"

The classloader is responsible to interpret the path and find the resource. When searching for a resource in the filesystem a path with a backslash seems to work in Windows, probably because it is simple interpreted as relative path.

Such a path does not work when resources inside a JAR file are requested since the format for JAR entries requires the forward slash as separator char.