CODEWITHSUNDEEP
javareflectionannotations
nwaltham
nwaltham

Reputation: 2084

How to find which java bean property has a known annontation

I have a series of classes, which are unrelated. Each classs has one property with and @PrimaryKey (with getter and setter) that could be of any type. How can I use reflection to find which property of an instance of any class has the @PrimaryKey annotation - so I can get its value as a string.

The code doesn't know which type of class its being passed - it will just be of type "Object"

Upvotes: 0

Views: 1186

Answers (3)

Igorry
Igorry

Reputation: 177

First of all you need to find all classes that may have the annotation in their members. This can be accomplished using Spring Framework ClassUtils: 

    public static void traverse(String classSearchPattern, TypeFilter typeFilter) {
    ClassLoader classLoader = ClassUtils.getDefaultClassLoader();
    ResourcePatternResolver resourceResolver = new    PathMatchingResourcePatternResolver(classLoader);

    Resource[] resources = null;
    try {
        resources = resourceResolver.getResources(classSearchPattern);
    } catch (IOException e) {
        throw new FindException(
                "An I/O problem occurs when trying to resolve resources matching the pattern: "
                        + classSearchPattern, e);
    }

    MetadataReaderFactory metadataReaderFactory = new SimpleMetadataReaderFactory();

    for (Resource resource : resources) {
        try {
            MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(resource);

            if (typeFilter.match(metadataReader, metadataReaderFactory)) {
                String className = metadataReader.getClassMetadata().getClassName();
                Class<?> annotatedClass = classLoader.loadClass(className);

        // CHECK IF THE CLASS HAS PROPERLY ANNOTATED FIELDS AND 
        // DO SOMETHING WITH THE CLASS FOUND... E.G., PUT IT IN SOME REGISTRY 

            }
        } catch (Exception e) {
            throw new FindException("Failed to analyze annotation for resource: " + resource, e);
        }
    }
}

Upvotes: -1

Adam Arold
Adam Arold

Reputation: 30568

You can do something like this:

for (Field field : YourClass.class.getDeclaredFields()) {
    try {
        Annotation annotation = field.getAnnotation(PrimaryKey.class);
        // what you want to do with the field
    } catch (NoSuchFieldException e) {
        // ...
    }
}

If you are working with the instance of your class then you can do this to get its class object:

Class<?> clazz = instance.getClass();

so the first line becomes something like this:

instance.getClass().getDeclaredFields()

If you are in trouble you can always check out the official documentation. I believe it is quite good.

Upvotes: 2

Mikita Belahlazau
Mikita Belahlazau

Reputation: 15434

You can get all fields of a class and then iterate and find which field has your annotation:

Field[] fields = YourClass.class.getDeclaredFields();
for (Field field : fields) {
    Annotation annot = field.getAnnotation(PrimaryKey.class);  
    if (annot != null) {
        System.out.println("Found! " + field);
    }
}

Upvotes: 2

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