How to Convert a Java Object to a Map Using Introspection

How do I convert a Java object into a Map<String, Object> using introspection?

Map<String, Object> result = toMap(myObject, "attr1", "attr2");

In Java, introspection allows us to inspect classes, interfaces, fields, and methods at runtime, even if they are private. This can be useful for dynamic operations such as converting an object's attributes into a Map. Below, we explore how to achieve this with a generic method.

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

public class ObjectMapConverter {
    public static Map<String, Object> toMap(Object obj, String... attributeNames) throws Exception {
        Map<String, Object> map = new HashMap<>();
        if (attributeNames.length == 0) {
            Field[] fields = obj.getClass().getDeclaredFields();
            for (Field field : fields) {
                field.setAccessible(true);
                map.put(field.getName(), field.get(obj));
            }
        } else {
            for (String attr : attributeNames) {
                Method method = obj.getClass().getMethod(getGetterMethodName(attr));
                map.put(attr, method.invoke(obj));
            }
        }
        return map;
    }

    private static String getGetterMethodName(String fieldName) {
        return "get" + Character.toUpperCase(fieldName.charAt(0)) + fieldName.substring(1);
    }
}

Causes

• Lack of boilerplate code for object-to-map conversion.
• Desire to access private or protected fields dynamically.

Solutions

• Utilize Java Reflection API to inspect object fields and methods.
• Create a generic `toMap()` method that can accept both public fields and getter methods.

Mistake: Not handling exceptions properly during reflection calls.

Solution: Implement proper exception handling using try-catch blocks.

Mistake: Forgetting to call `setAccessible(true)` on private fields.

Solution: Always set the field's accessible flag before accessing it.

Mistake: Assuming all attributes have corresponding getter methods.

Solution: Check if the method exists before invoking it.