How to get attributes of annotations in Java using reflection?

Question

I have several data classes in java of which I want to know - using reflection - which fields have a certain annotation with a certain attribute which is the following:

@Column(columnDefinition = "text") // Text with unbound length private String comment;

I figured how to get the annotations of the field and whether it is of type Column:

private boolean isStringFieldWithBoundLength(Field attr) { Annotation[] annotations = attr.getDeclaredAnnotations(); for (Annotation ann : annotations) { Class<? extends Annotation> aClass = ann.annotationType(); if (Column.class == aClass) { // ... } } }

Now in the debugger I can see that the aClass object has the information about the provided parameters. Unfortunately I don't know how to access it with code. Is it possible to access this information in java?

f1sh · Accepted Answer · 2022-05-19 08:49:36Z

You should be able to get the instance of that annotation (including your values) using

Column fieldAnnotation = attr.getAnnotation(Column.class);

If the field is not annotated with @Column, getAnnotation returns null. That means you don't need to iterate over attr.getDeclaredAnnotations();

You can then call fieldAnnotation.columnDefinition() or whatever custom field that your annotation might have.

Addition: your annotation needs to have @Retention(RetentionPolicy.RUNTIME) for that to work, otherwise your annotations will be removed from classes/fields/methods during compilation.

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