Why TestInstance lifecycle per class in JUnit? - Purpose & Use Cases

The Big Idea

What if you could stop wasting time creating new test objects for every single test method?

The Scenario

Imagine running multiple tests on a class where each test needs to create a new object every time. You have to manually create and clean up these objects before and after each test.

The Problem

This manual approach is slow and repetitive. You might forget to reset the object state, causing tests to interfere with each other. It's easy to make mistakes and waste time rewriting setup code.

The Solution

Using the TestInstance lifecycle per class in JUnit lets you create one test object for all tests in a class. This means setup happens once, tests share the same instance, and you avoid repeated object creation and cleanup.

Before vs After

✗ Before

class TestExample {
  MyClass obj;
  @BeforeEach
  void setup() {
    obj = new MyClass();
  }
  @Test
  void test1() { ... }
  @Test
  void test2() { ... }
}

✓ After

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class TestExample {
  MyClass obj = new MyClass();
  @Test
  void test1() { ... }
  @Test
  void test2() { ... }
}

What It Enables

You can write cleaner, faster tests by sharing one test instance across all tests in a class, reducing setup time and avoiding repeated code.

Real Life Example

When testing a database connection class, you open the connection once per test class instead of opening and closing it before every test method, saving time and resources.

Key Takeaways

Manual setup for each test is slow and error-prone.

TestInstance lifecycle per class creates one shared test object.

This reduces repeated setup and speeds up testing.