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.
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:
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