子先 class 加载器和服务提供者接口（SPI）

Question

I found a custom class loader , which loads classes by child-first principle. And it works fine, but I faced with the following issue. When I try to load classes that use SPI I get the exception:

Exception in thread "main" java.util.ServiceConfigurationError: test.spi.SayMyNameProvider: test.spi.ImplProvider not a subtype

I created simple SPI project with modules: spi-api, spi-impl and spi-app.

And it works when I use URLClassLoader, however whenever I use ChildFirstClassLoader I get the exception mentioned above:

public class TestMain {
    public static void main(String[] args) throws MalformedURLException {

        //!!! comment ChildFirstClassLoader and uncomment URLClassLoader to get the correct behavior 
        ChildFirstClassLoader classLoader = getCustomClassLoader();
        //URLClassLoader classLoader = getUrlClassLoader();

        ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
        Thread.currentThread().setContextClassLoader(classLoader);

        try {
            List<SayMyNameProvider> providers = Speaker.providers();
            for (SayMyNameProvider provider : providers) {
                SayMyNameManager sayMyNameManager = provider.create();
                sayMyNameManager.sayIt("main");
            }
            System.out.println("done");

        } finally {
            Thread.currentThread().setContextClassLoader(contextClassLoader);
        }
    }

    private static ChildFirstClassLoader getCustomClassLoader() throws MalformedURLException {
        URL[] urls = getUrls();

        return new ChildFirstClassLoader(urls);
    }

    private static URLClassLoader getUrlClassLoader() throws MalformedURLException {
        URL[] urls = getUrls();

        return new URLClassLoader(urls);
    }

    private static URL[] getUrls() throws MalformedURLException {
        File spiImpl = Paths.get("spi-impl", "target", "spi-impl-1.0.0-SNAPSHOT.jar").toFile();
        File spiApi = Paths.get("spi-api", "target", "spi-api-1.0.0-SNAPSHOT.jar").toFile();

        URL[] urls = new URL[2];
        urls[0] = spiImpl.toURI().toURL();
        urls[1] = spiApi.toURI().toURL();
        return urls;
    }
}

Maybe someone has already faced this problem before and knows how to solve it. I would be grateful for any help or advice.

Answer 1

So, after 3 days I finally got an answer to my question. And it says that I am stupid:) because in the article at the very end the author provided the example with correct behavior and it works in my case. However, it doesn`t work in my other test with slf4j and logback dependencies. But to my surprise, the code without system class loader works. In the nutshell, I try to use different versions of slf4j and logback.

Pom.xml:

<dependencies>
    <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.2.3</version>
    </dependency>
</dependencies>

TestMain.class

public class TestMain {
    private static Logger log = LoggerFactory.getLogger(TestMain.class);

    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, MalformedURLException {
        log.info("Hello");

        ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();

        ChildFirstClassLoader2 classLoader = new ChildFirstClassLoader2(getUrls());

        Thread.currentThread().setContextClassLoader(classLoader);

        try {
            Class<?> testClass = classLoader.loadClass("petrovskyi.TestClass");
            Object o = testClass.getDeclaredConstructor().newInstance();
            Method test = testClass.getMethod("test");
            test.setAccessible(true);
            test.invoke(o);

        } finally {
            Thread.currentThread().setContextClassLoader(contextClassLoader);
        }
    }

    private static URL[] getUrls() throws MalformedURLException {
        File libDir = Paths.get("src","main", "resources", "testClasses").toFile();

        URL[] urls;

        List<URL> urlsList = new ArrayList<>();

        URL classUrl = libDir.toURI().toURL();
        urlsList.add(classUrl);

        try (Stream<Path> walk = Files.walk(libDir.toPath())) {
            List<File> result = walk.map(Path::toFile)
                    .filter(x -> x.getName().endsWith(".jar"))
                    .collect(Collectors.toList());

            for (File jarFile : result) {
                urlsList.add(jarFile.toURI().toURL());
            }
        } catch (IOException e) {
            throw new RuntimeException("Error while walking through " + libDir + " to find jar files", e);
        }

        urls = urlsList.toArray(new URL[0]);
        return urls;
    }
}

Above I try to get all jars from some directory. The jars are the next:

• logback-classic-1.3.0-alpha5.jar
• logback-core-1.3.0-alpha5.jar
• slf4j-api-2.0.0-alpha1.jar

TestClass.class

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.event.Level;
import org.slf4j.spi.DefaultLoggingEventBuilder;
import org.slf4j.spi.LoggingEventBuilder;

 public class TestClass {
        private static Logger log = LoggerFactory.getLogger(TestClass.class);

        public TestClass() {
        }

        public void test() {
            LoggingEventBuilder loggingEventBuilder = new DefaultLoggingEventBuilder(log, Level.ERROR);
            loggingEventBuilder.log(" =========== Hello, World! ===========");
            log.info("Test from test class");
        }
    }

The class above uses LoggingEventBuilder that is not present in slf4j version mentioned in pom.xml

ChildFirstClassLoader2.class

public class ChildFirstClassLoader2 extends URLClassLoader {

    public ChildFirstClassLoader2(URL[] urls) {
        super(URLs);
    }

    @Override
    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        Class<?> loadedClass = findLoadedClass(name);
        if (loadedClass == null) {

            try {
                if (loadedClass == null) {
                    loadedClass = findClass(name);
                }

            } catch (ClassNotFoundException e) {
                loadedClass = super.loadClass(name, resolve);
            }
        }

        if (resolve) {
            resolveClass(loadedClass);
        }
        return loadedClass;
    }


    @Override
    public Enumeration<URL> getResources(String name) throws IOException {
        List<URL> allRes = new LinkedList<>();

        Enumeration<URL> thisRes = findResources(name);
        if (thisRes != null) {
            while (thisRes.hasMoreElements()) {
                allRes.add(thisRes.nextElement());
            }
        }

        Enumeration<URL> parentRes = super.findResources(name);
        if (parentRes != null) {
            while (parentRes.hasMoreElements()) {
                allRes.add(parentRes.nextElement());
            }
        }

        return new Enumeration<URL>() {
            Iterator<URL> it = allRes.iterator();

            @Override
            public boolean hasMoreElements() {
                return it.hasNext();
            }

            @Override
            public URL nextElement() {
                return it.next();
            }
        };
    }

    @Override
    public URL getResource(String name) {
        URL res = null;

        if (res == null) {
            res = findResource(name);
        }
        if (res == null) {
            res = super.getResource(name);
        }
        return res;
    }
}

The output:

2020-04-10 12:01:39,928 [main] INFO TestMain - Hello
2020-04-10 12:01:40,095 [main] ERROR TestClass -  =========== Hello, World! ===========
2020-04-10 12:01:40,097 [main] INFO TestClass - Test from test class