File.toURI（）。toURL（）如何抛出异常？

Question

I noticed that File.toURL() is deprecated in favour of File.toURI().toURL() . (I am using Java 8.)

I see URI.toURL() can throw MalformedURLException (extends IOException ).

Under what conditions can File.toURI().toURL() throw an exception?

Answer 1

Technically it's possible. For example, consider this program:

import java.io.File;
import java.net.MalformedURLException;
import java.net.URL;

public class Test {
    public static void main(String[] args) throws MalformedURLException {
        URL.setURLStreamHandlerFactory(protocol -> {
            throw new UnsupportedOperationException();
        });

        System.out.println(new File("/etc/passwd").toURI().toURL());
    }
}

Here toURL call actually fails with MalformedURLException :

Exception in thread "main" java.net.MalformedURLException
    at java.net.URL.<init>(URL.java:627)
    at java.net.URL.<init>(URL.java:490)
    at java.net.URL.<init>(URL.java:439)
    at java.net.URI.toURL(URI.java:1089)
    at com.example.Test.main(Test.java:20)
Caused by: java.lang.UnsupportedOperationException
    at com.example.Test.lambda$main$0(Test.java:17)
    at java.net.URL.getURLStreamHandler(URL.java:1142)
    at java.net.URL.<init>(URL.java:599)
    ... 4 more

However I doubt that your program will function correctly anyways if you set custom URLStreamHandlerFactory which does not support even file scheme.

With default URLStreamHandlerFactory I cannot think up or remember cases when toURL may fail, so if you don't mess with URLStreamHandlerFactory , you can use it safely.

If you need to produce URL objects often, you may consider creating some utility method like this:

public class FileUtils {
    public static URL toURL(File file) {
        try {
            return file.toURI().toURL();
        } catch (MalformedURLException e) {
            throw new InternalError(e);
        }
    }
}

And use it everywhere. It just asserts that MalformedURLException is impossible in your program, so if it actually occurs, it will be considered as internal program error.

Another possibility is not to use URL class at all. For many purposes the URI class can be used instead.