Can't get annotations from classes loaded by custom classloader in tomcat

问题

Given the class org.popper.example.pages.Login

@Page(name="Login")
public interface Login {
}

exported to c:\pos\example.jar and the following servlet

public class PopperServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) throws MalformedURLException, ClassNotFoundException {
        URLClassLoader ucl = new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()});
        System.out.println(Arrays.asList(ucl.loadClass("org.popper.example.pages.Login").getAnnotations()));
    }

    public PopperServlet() throws MalformedURLException, ClassNotFoundException {
        URLClassLoader ucl = new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()});
        System.out.println(Arrays.asList(ucl.loadClass("org.popper.example.pages.Login").getAnnotations()));
   }
}

Running the code as main shows the expected result

[@org.popper.fw.annotations.Page(name=Login)]

Running the code as servlet in tomcat doesn't find the annotations

[]

Can anybody tell me why?

回答1:

It's the same as always: Regard the classloader-hierarchy!

new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()}, PopperServlet.class.getClassloader());

did the trick. But it's surprising that annotations aren't found instead a ClassNotFoundException or NoClassDefError, thats what I expected when annotations are not found when loading a class...

回答2:

Yea sure you'll not find your annotation, you have to annotate your annotation to keep alive for the run time, add :

@Retention(RetentionPolicy.RUNTIME)

Retention Java doc : Indicates how long annotations with the annotated type are to be retained. If no Retention annotation is present on an annotation type declaration, the retention policy defaults to RetentionPolicy.CLASS.

for more details : here

回答3:

I have encountered the same problem. And I resolve it by my custome class loader as followed, Maybe you can have a try.

ClassLoader Code:

import java.util.HashMap;
import java.util.Map;
/**
 * Load class from byte[] which is compiled in memory.
 *
 * @author David
 */
class CustomClassLoader extends ClassLoader {

    // class name to class bytes:
    private Map<String, byte[]> classBytes = new HashMap<String, byte[]>();

    public CustomClassLoader(Map<String, byte[]> classBytes) {
        super(CustomClassLoader.class.getClassLoader());
        Thread.currentThread().setContextClassLoader(this);
        this.classBytes.putAll(classBytes);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] buf = classBytes.get(name);
        if (buf == null) {
            return super.findClass(name);
        }
        classBytes.remove(name);
        return defineClass(name, buf, 0, buf.length);
    }

}

Client Code:

byte[] code = this.createEntity(logicalTable, keyCount, relationMap);
Map<String, byte[]> results = Maps.newHashMap();
results.put(this.entityPackage + "." + logicalTable.getTableName(), code);
CustomClassLoader classLoader = new CustomClassLoader(results);
Class<?> clazz = classLoader.findClass(this.entityPackage + "." + logicalTable.getTableName());

来源：https://stackoverflow.com/questions/20451498/cant-get-annotations-from-classes-loaded-by-custom-classloader-in-tomcat

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java

Tomcat

annotations

classloader