Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.
Get started with mocking and improve your application tests using our Mockito guide:
Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.
Get started with understanding multi-threaded applications with our Java Concurrency guide:
Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:
Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.
But these can also be overused and fall into some common pitfalls.
To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:
Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:
Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.
I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.
You can explore the course here:
Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.
Get started with Spring Data JPA through the guided reference course:
Refactor Java code safely — and automatically — with OpenRewrite.
Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.
Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.
Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.
Distributed systems often come with complex challenges such as service-to-service communication, state management, asynchronous messaging, security, and more.
Dapr (Distributed Application Runtime) provides a set of APIs and building blocks to address these challenges, abstracting away infrastructure so we can focus on business logic.
In this tutorial, we'll focus on Dapr's pub/sub API for message brokering. Using its Spring Boot integration, we'll simplify the creation of a loosely coupled, portable, and easily testable pub/sub messaging system:
1. Overview
In our previous article, we discussed how we could read the values of private fields from a different class in Java. However, there can be scenarios when we need to set the values of fields, such as in some libraries where we don’t have access to the fields.
In this quick tutorial, we’ll discuss how can we set the values of fields from a different class in Java by using the Reflection API.
Note that we’ll be using the same Person class for the examples here as we used in our previous article.
2. Setting Primitive Fields
We can set the fields that are primitives by using the Field#setXxx methods.
2.1. Setting Integer Fields
We can use the setByte, setShort, setInt, and setLong methods to set the byte, short, int, and long fields, respectively:
@Test
public void whenSetIntegerFields_thenSuccess()
throws Exception {
Person person = new Person();
Field ageField = person.getClass()
.getDeclaredField("age");
ageField.setAccessible(true);
byte age = 26;
ageField.setByte(person, age);
Assertions.assertEquals(age, person.getAge());
Field uidNumberField = person.getClass()
.getDeclaredField("uidNumber");
uidNumberField.setAccessible(true);
short uidNumber = 5555;
uidNumberField.setShort(person, uidNumber);
Assertions.assertEquals(uidNumber, person.getUidNumber());
Field pinCodeField = person.getClass()
.getDeclaredField("pinCode");
pinCodeField.setAccessible(true);
int pinCode = 411057;
pinCodeField.setInt(person, pinCode);
Assertions.assertEquals(pinCode, person.getPinCode());
Field contactNumberField = person.getClass()
.getDeclaredField("contactNumber");
contactNumberField.setAccessible(true);
long contactNumber = 123456789L;
contactNumberField.setLong(person, contactNumber);
Assertions.assertEquals(contactNumber, person.getContactNumber());
}It’s also possible to perform unboxing with primitive types:
@Test
public void whenDoUnboxing_thenSuccess()
throws Exception {
Person person = new Person();
Field pinCodeField = person.getClass()
.getDeclaredField("pinCode");
pinCodeField.setAccessible(true);
Integer pinCode = 411057;
pinCodeField.setInt(person, pinCode);
Assertions.assertEquals(pinCode, person.getPinCode());
}The setXxx methods for primitive data types also support narrowing:
@Test
public void whenDoNarrowing_thenSuccess()
throws Exception {
Person person = new Person();
Field pinCodeField = person.getClass()
.getDeclaredField("pinCode");
pinCodeField.setAccessible(true);
short pinCode = 4110;
pinCodeField.setInt(person, pinCode);
Assertions.assertEquals(pinCode, person.getPinCode());
}2.2. Setting Floating Type Fields
To set float and double fields, we need to use the setFloat and setDouble methods, respectively:
@Test
public void whenSetFloatingTypeFields_thenSuccess()
throws Exception {
Person person = new Person();
Field heightField = person.getClass()
.getDeclaredField("height");
heightField.setAccessible(true);
float height = 6.1242f;
heightField.setFloat(person, height);
Assertions.assertEquals(height, person.getHeight());
Field weightField = person.getClass()
.getDeclaredField("weight");
weightField.setAccessible(true);
double weight = 75.2564;
weightField.setDouble(person, weight);
Assertions.assertEquals(weight, person.getWeight());
}2.3. Setting Character Fields
To set the char fields, we can use the setChar method:
@Test
public void whenSetCharacterFields_thenSuccess()
throws Exception {
Person person = new Person();
Field genderField = person.getClass()
.getDeclaredField("gender");
genderField.setAccessible(true);
char gender = 'M';
genderField.setChar(person, gender);
Assertions.assertEquals(gender, person.getGender());
}2.4. Setting Boolean Fields
Similarly, we can use the setBoolean method to set the boolean field:
@Test
public void whenSetBooleanFields_thenSuccess()
throws Exception {
Person person = new Person();
Field activeField = person.getClass()
.getDeclaredField("active");
activeField.setAccessible(true);
activeField.setBoolean(person, true);
Assertions.assertTrue(person.isActive());
}3. Setting Fields That Are Objects
We can set the fields that are objects by using the Field#set method:
@Test
public void whenSetObjectFields_thenSuccess()
throws Exception {
Person person = new Person();
Field nameField = person.getClass()
.getDeclaredField("name");
nameField.setAccessible(true);
String name = "Umang Budhwar";
nameField.set(person, name);
Assertions.assertEquals(name, person.getName());
}4. Exceptions
Now, let’s discuss the exceptions that the JVM can throw while setting the fields.
4.1. IllegalArgumentException
The JVM will throw IllegalArgumentException if we use a setXxx mutator that is incompatible with the target field’s type. In our example, if we write nameField.setInt(person, 26), the JVM throws this exception since the field is of type String and not int or Integer:
@Test
public void givenInt_whenSetStringField_thenIllegalArgumentException()
throws Exception {
Person person = new Person();
Field nameField = person.getClass()
.getDeclaredField("name");
nameField.setAccessible(true);
Assertions.assertThrows(IllegalArgumentException.class, () -> nameField.setInt(person, 26));
}As we’ve already seen, the setXxx methods support narrowing for the primitive types. It’s important to note that we need to provide the correct target for narrowing to be successful. Otherwise, the JVM throws an IllegalArgumentException:
@Test
public void givenInt_whenSetLongField_thenIllegalArgumentException()
throws Exception {
Person person = new Person();
Field pinCodeField = person.getClass()
.getDeclaredField("pinCode");
pinCodeField.setAccessible(true);
long pinCode = 411057L;
Assertions.assertThrows(IllegalArgumentException.class, () -> pinCodeField.setLong(person, pinCode));
}4.2. IllegalAccessException
If we’re trying to set a private field that doesn’t have access rights, then the JVM will throw an IllegalAccessException. In the above example, if we don’t write the statement nameField.setAccessible(true), then the JVM throws the exception:
@Test
public void whenFieldNotSetAccessible_thenIllegalAccessException()
throws Exception {
Person person = new Person();
Field nameField = person.getClass()
.getDeclaredField("name");
Assertions.assertThrows(IllegalAccessException.class, () -> nameField.set(person, "Umang Budhwar"));
}5. Conclusion
In this tutorial, we’ve seen how we can modify or set the values of private fields of a class from another class in Java. We’ve also seen the exceptions that the JVM can throw and what causes them.
