Cannot get custom annotations from Java class

Question

I want to get class level annotation from Java class:

    class FixAnnotation {

        public String[] author;

    }

    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE)
    public @interface Fix {

        public String[] author() default "";

    }

I tried this example of compiled java class:

@Component("test")
@Fix(
    author = {"Example author 1", "Example author 2"}
)
public class Order implements Action {
..
}

But when I try:

public List listLocalFilesAndDirsAllLevels(File baseDir) {

        List<File>  collectedFilesAndDirs = new ArrayList<>();
        Deque<File> remainingDirs = new ArrayDeque<>();

        if(baseDir.exists()) {
            remainingDirs.add(baseDir);

            while(!remainingDirs.isEmpty()) {
                File dir = remainingDirs.removeLast();
                List<File> filesInDir = Arrays.asList(dir.listFiles());
                for(File fileOrDir : filesInDir)  {
                    // We need to process only .class files
                    if(fileOrDir.getName().endsWith(".class")){
                        collectedFilesAndDirs.add(fileOrDir);
                        if(fileOrDir.isDirectory()) {
                            remainingDirs.add(fileOrDir);
                        }
                    }
                }
            }
        }

        return collectedFilesAndDirs;
    }

      List<File> list;

      for(int i=0; i<list.size(); i++) {
            File item = list.get(i);
            System.out.println(item.getName());

            Fix name = item.getClass().getAnnotation(Fix.class);

            out.println("author: " + name.author());
       }

I get NPE. Do you know how I can get the annotation content?

EDIT: I tried this:

public static void main() throws Exception
        {    
            final File folder = new File("/opt/test");
            processAnnotatedFiles(listLocalFilesAndDirsAllLevels(folder));

        }

    public void processAnnotatedFiles(List<File> list) throws IOException, ClassNotFoundException {
        out.println("Directory files size " + list.size());

        for(int i=0; i<list.size(); i++) {
            out.println("File " + list.get(i).getName());

            File file = list.get(i);

            String path = file.getPath();

            String[] authors = getFixFromClassFile(Paths.get(path));
            System.out.println(Arrays.toString(authors));
        }

    }

    public List<File> listLocalFilesAndDirsAllLevels(File baseDir) {

        List<File>  collectedFilesAndDirs = new ArrayList<>();
        Deque<File> remainingDirs = new ArrayDeque<>();

        if(baseDir.exists()) {
            remainingDirs.add(baseDir);

            while(!remainingDirs.isEmpty()) {
                File dir = remainingDirs.removeLast();
                List<File> filesInDir = Arrays.asList(dir.listFiles());
                for(File fileOrDir : filesInDir)  {
                    // We need to process only .class files
                    if(fileOrDir.getName().endsWith(".class")){
                        collectedFilesAndDirs.add(fileOrDir);
                        if(fileOrDir.isDirectory()) {
                            remainingDirs.add(fileOrDir);
                        }
                    }
                }
            }
        }

        return collectedFilesAndDirs;
    }

    private String[] getFixFromClassFile(Path pathToClass) throws MalformedURLException, ClassNotFoundException {
        // Create class loader based on path
        URLClassLoader loader = new URLClassLoader(new URL[]{pathToClass.toUri().toURL()});

        // convert path to class with package
        String classWithPackage = getClassWithPackageFromPath(pathToClass);

        // Load class dynamically
        Class<?> clazz = loader.loadClass(classWithPackage);
        Fix fix = clazz.getAnnotation(Fix.class);
        if (fix == null) {
            return new String[0];
        }

        return fix.author();
    }

    private String getClassWithPackageFromPath(Path pathToClass) {
        final String packageStartsFrom = "com.";
        final String classFileExtension = ".class";
        final String pathWithDots = pathToClass.toString().replace(File.separator, ".");
        return pathWithDots.substring(pathWithDots.indexOf(packageStartsFrom)).replace(classFileExtension, "");
    }

I get java.lang.StringIndexOutOfBoundsException: String index out of range: -1 at java.lang.String.substring(String.java:1927)

You try to get the annotation from the `File` class, not from an actual instance of the `Order` class. You would need something like `Class` so you would need an instance or something where you can call `getClass()` on — CodeMatrix, Apr 29 '20 at 10:47

score 1 · Answer 1 · answered Apr 29 '20 at 11:50

When you invoke getClass method on File object it will return java.io.File Class instance. This method does not load class from given file.

If you want to load a class from given *.class file you need to use java.lang.ClassLoader implementation. For example, java.net.URLClassLoader. Below you can find example how to load class and check annotation:

import java.io.File;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;

@Fix(author = "Test author")
public class ReflectionApp {

    public static void main(String[] args) throws Exception {
        String path = "path/to/com/so/ReflectionApp.class";
        String[] authors = getFixFromClassFile(Paths.get(path));
        System.out.println(Arrays.toString(authors));
    }

    private static String[] getFixFromClassFile(Path pathToClass) throws MalformedURLException, ClassNotFoundException {
        // Create class loader based on path
        URLClassLoader loader = new URLClassLoader(new URL[]{pathToClass.toUri().toURL()});

        // convert path to class with package
        String classWithPackage = getClassWithPackageFromPath(pathToClass);

        // Load class dynamically
        Class<?> clazz = loader.loadClass(classWithPackage);
        Fix fix = clazz.getAnnotation(Fix.class);
        if (fix == null) {
            return new String[0];
        }

        return fix.author();
    }

    private static String getClassWithPackageFromPath(Path pathToClass) {
        final String packageStartsFrom = "com.";
        final String classFileExtension = ".class";
        final String pathWithDots = pathToClass.toString().replace(File.separator, ".");
        return pathWithDots.substring(pathWithDots.indexOf(packageStartsFrom)).replace(classFileExtension, "");
    }
}

Above code prints:

[Test author]

Cannot get custom annotations from Java class

1 Answers1