how to do unit test for single pattern using jmockit

There is a java class, it is based on single pattern. how to do unit test for this class? the following is my related code:

public class ConfigFromFile implements ConfigStrategy {
private String endpoint;
private final static String CONFIG_FILE = "/conf/config.properties";

private InputStream getInpustStream(String configFilePath) throws FileNotFoundException{
    return new FileInputStream(configFilePath);
}

private void initFromFile(){
    Properties prop = new Properties();
    InputStream input = null;
    try {
        input = getInpustStream(CONFIG_FILE);
        prop.load(input);
        endpoint = prop.getProperty("endpoint");
    }catch(Exception e){
        e.printStackTrace();
    }finally {
        if (input != null) {
            try {
                input.close();
            } catch (IOException e) {
            }
        }
    }

}

private ConfigFromFile() {
    initFromFile();
}

private static class ConfigFromFileHolder {
    private static ConfigFromFile instance = new ConfigFromFile();
}

public static ConfigFromFile getInstance() {
    return ConfigFromFileHolder.instance;
}

@Override
public String getEndpoint() {
    return endpoint;
}

}

I need to write unit test for this class.

• the unit test can't call the external resource, so we need to mock call "/conf/config.properties" file. we can use jmockit.
• this class is based on single pattern. We hope that the interaction between the two cases can not be affected.

The following is my case:

1. Case1, it is normal case, the file content is "endpoint=www.baidu.com"
2. Case2, it is an abnormal case, we can mock this file does not exist.

how to implement these cases? Thanks!