Cannot read annotations from .class

Question

I want to read class level annotations using core java. I tried this:

Annotation:

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

    public String[] author() default "";
}

Class:

@Fix(author="John Doe")
public class TestClass {

    public void test(){

    }
}

Test method to read the class:

ResourcePatternResolver resolversec = new PathMatchingResourcePatternResolver();
            Resource[] resour = resolversec.getResources("classpath*:/com/validation/*.class");
            Class<?> cl = resolversec.getClassLoader().loadClass("com.validation.ValidateCharacteristicsProcessor");

            if(cl != null){
                out.println("Class is not null " + cl.getSimpleName());
            }

            Fix fix = cl.getAnnotation(Fix.class);
            if (fix != null) {
                out.println("!!!! " + fix.author());
            }

But the annotation @Fix is coming empty. Do you know what is the proper way to read this annotation?

Fill example: https://pastebin.com/KbBAZVfB

Answer 1

You are calling getAnnotation(Fix.class) on com.validation.ValidateCharacteristicsProcessor, not on TestClass. The annotation is present as expected on TestClass:

jshell> TestClass.class.getAnnotations()
$4 ==> Annotation[1] { @Fix(author={"John Doe"}) }

jshell> TestClass.class.getAnnotation(Fix.class)
$5 ==> @Fix(author={"John Doe"})

jshell> TestClass.class.getAnnotation(Fix.class).author()[0]
$6 ==> "John Doe"

Looking at your full example, I would guess that either predictedName is not the right string, or that the class you're inspecting is actually not annotated properly.

It would likely help to create a small MCVE of the desired behavior and then reincorporate that back into your larger project once you have a basic approach working.

Answer 2

First of all, I don't think com.validation.ValidateCharacteristicsProcessor exist in your project. Still if you are trying to load that class (if present), then check whether this class is annotated with @Fix or not.

---

I'm showing with TestClass that you have provided.

I tried a simple approach.

First, I created a simple maven project to demonstrate two approaches.

1. Using Java Reflections.

2. Using Spring Core as you are using it in the Test method.

Project Structure :

Fix.java

package com.example;

import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

@Retention(RUNTIME)
@Target(TYPE)
public @interface Fix {

    public String[] author() default "";

}

TestClass.java

package com.example;

@Fix(author = "John Doe")
public class TestClass {

    public void test() {

    }

}

Main.java

package com.example;

    public class Main {

        public static void main(String[] args) throws ClassNotFoundException {

            // create an instance of TestClass
            TestClass t = new TestClass();

            // use reflection to extract annotation from TestClass.class 
            Fix fix = t.getClass().getAnnotation(Fix.class); // or TestClass.class.getAnnotation(Fix.class);


            // print the value   
            System.out.println(fix.author()[0]);

        }

    }

Output :

---

If you want to carry on with the code that you have provided, then I have a fix given below.

I have researched about ResourcePatternResolver and I think you are using spring-core to read classes.

So, I have updated your code.

package com.example;


import java.io.IOException;

import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
import org.springframework.core.io.support.ResourcePatternResolver;

public class Main {

    public static void main(String[] args) throws ClassNotFoundException {

        ResourcePatternResolver resolversec = new PathMatchingResourcePatternResolver();
        Class<?> cl = resolversec.getClassLoader().loadClass("com.example.TestClass");

        if (cl != null) {
            System.out.println("Class is not null " + cl.getSimpleName());
        }

        Fix fix = cl.getAnnotation(Fix.class);
        if (fix != null) {
            System.out.println("!!!! " + fix.author()[0]);
        }

    }

}

Output :

pom.xml :

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>read-annotations</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>5.2.5.RELEASE</version>
        </dependency>
    </dependencies>

    <build>
        <sourceDirectory>src</sourceDirectory>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.7.0</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Answer 3

Iterate class with native java or with library in package look this link: Can you find all classes in a package using reflection?

After it iterate and show with this code:

    Arrays.stream(getClasses("package.name.asdf")).forEach(aClass -> {
        Fix annotation = (Fix) aClass.getAnnotation(Fix.class);
        if (annotation != null) System.out.println(Arrays.asList(annotation.author()));
    });

Answer 4

You are loading in-correct class in your code. You should load TestClass as the Fix annotation is on TestClass, not on ValidateCharacteristicsProcessor class