1. Overview
record has been introduced to represent immutable data since Java 14. Records contain fields with various values, and sometimes, we need to extract all those fields along with their corresponding values programmatically.
In this tutorial, we’ll explore how to retrieve all the fields and their values within a record class using Java’s Reflection API.
2. Introduction to the Problem
An example can explain the problem quickly. Let’s say we have a Player record:
record Player(String name, int age, Long score) {}
As the code above shows, the Player record has three fields with different types: String, primitive int, and Long. Also, we’ve created a Player instance:
Player ERIC = new Player("Eric", 28, 4242L);
Now, we’ll find all fields declared in the Player record, and take this ERIC player instance, extract their corresponding values programmatically.
For simplicity, we’ll leverage unit test assertions to verify whether each approach produces the expected result.
Next, let’s dive in.
3. Using RecordComponent
We’ve mentioned that the record class was introduced in Java 14. Together with record, a new member came to the java.lang.reflect package: RecordComponent. It’s a preview feature in Java 14 and 15. But, it “upgraded” to a permanent feature in Java 16. The RecordComponent class provides information about a component of a record class.
Moreover, the Class class provides the getRecordComponents() method to return all components of a record class if the class object is a Record instance. It’s worth noting that the components in the returned array are in the same order declared in the record.
Next, let’s see how to use RecordComponent together with the reflection API to get all fields from our Player record class and the corresponding values of the ERIC instance:
var fields = new ArrayList<Field>();
RecordComponent[] components = Player.class.getRecordComponents();
for (var comp : components) {
try {
Field field = ERIC.getClass()
.getDeclaredField(comp.getName());
field.setAccessible(true);
fields.add(field);
} catch (NoSuchFieldException e) {
// for simplicity, error handling is skipped
}
}
First, we created an empty field list to hold fields we extract later. We know we can retrieve a declared field in a Java class using the class.getDeclaredField(fieldName) method. Therefore, fieldName becomes the key to solving the problem.
RecordComponent carries various information about a record’s field, including type, name, etc. That is to say, if we have RecordComponent objects of Player, we can have its Field objects. Player.class.getRecordComponents() returns all components in the Player record class as an array. Therefore, we can get all Field objects by names from the components array.
Since we want to extract these fields’ values later, setting setAccessible(true) on each field is required before we add it to our result field list.
So next, let’s verify the fields we obtained from the above loop are expected:
assertEquals(3, fields.size());
var nameField = fields.get(0);
var ageField = fields.get(1);
var scoreField = fields.get(2);
try {
assertEquals("name", nameField.getName());
assertEquals(String.class, nameField.getType());
assertEquals("Eric", nameField.get(ERIC));
assertEquals("age", ageField.getName());
assertEquals(int.class, ageField.getType());
assertEquals(28, ageField.get(ERIC));
assertEquals("score", scoreField.getName());
assertEquals(Long.class, scoreField.getType());
assertEquals(4242L, scoreField.get(ERIC));
} catch (IllegalAccessException exception) {
// for simplicity, error handling is skipped
}
As the assertion code shows, we can get the ERIC instance’s value by calling field.get(ERIC). Also, we must catch the IllegalAccessException checked exception when calling this method.
4. Using Class.getDeclaredFields()
The new RecordComponent allows us to get record components’ attributes easily. However, the problem can be solved without using the new RecordComponent class.
Java reflection API provides the Class.getDeclaredFields() method to get all declared fields in the class. Therefore, we can get a record class’s fields using this method.
It’s worth noting that we shouldn’t use the Class.getFields() method to get a record class’s fields. This is because getFields() only returns public fields declared in the class. However, all fields in a record class are private. Therefore, if we call Class.getFields() on a record class, we won’t get any field:
// record has no public fields
assertEquals(0, Player.class.getFields().length);
Similarly, we apply setAccessible(true) on each field before we add it to the result list:
var fields = new ArrayList<Field>();
for (var field : Player.class.getDeclaredFields()) {
field.setAccessible(true);
fields.add(field);
}
Next, let’s check if the fields in the result list match the Player class and whether we can get the expected values of the ERIC object through these fields:
assertEquals(3, fields.size());
var nameField = fields.get(0);
var ageField = fields.get(1);
var scoreField = fields.get(2);
try {
assertEquals("name", nameField.getName());
assertEquals(String.class, nameField.getType());
assertEquals("Eric", nameField.get(ERIC));
assertEquals("age", ageField.getName());
assertEquals(int.class, ageField.getType());
assertEquals(28, ageField.get(ERIC));
assertEquals("score", scoreField.getName());
assertEquals(Long.class, scoreField.getType());
assertEquals(4242L, scoreField.get(ERIC));
} catch (IllegalAccessException ex) {
// for simplicity, error handling is skipped
}
When we give the test a run, it pass. So, this approach solves the problem, too.
5. Conclusion
In this article, we’ve explored two approaches to extracting fields from a record class using reflection.
In the first solution, we obtained the fields’ names from the new RecordComponent class. Then, we can get the field objects by calling Class.getDeclaredField(Field_Name).
A record class is also a Java class. Therefore, alternatively, we can also get all fields of a record class from the Class.getDeclaredFields() method.
As always, the complete source code for the examples is available over on GitHub.
