[FIXED] Getting null when I @Autowired for the interface in Junit Test

Issue

I'm writing JUnit test for this method processData - I'm doing @Autowired Cache mycache in Junit, and mocking Cache mycache under my Test method. When I'm doing this mycache = mock(Cache.class) - this returns some object in Junit method but when I invoke my actual method from Junit Cache mycache going as null in Main class. Please find the code below:

What I'm missing here - how to resolve this NullPointer exception issue - why mycache is going as null when actual method triggered from Junit - though I've mocked this object. Any help would be really appreciated. Thank you!

Main.Java

import com.github.benmanes.caffeine.cache.Cache;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class Main {

    @Autowired
    Cache mycache;

    public Product processData(String productId) {

    System.out.println("productId value: " +productId);
    System.out.println("mycache value: " +mycache);

        Product product = (Product) mycache.getIfPresent(productId); //getting NullPointer exception here
        if (product != null) {
            return product;
        }
        // do some processing
        return product;
    }
}

MainTest.Java

public class MainTest {
    @Autowired
    Cache mycache;

    @Test
    public void processDataTest() throws Exception {
        Main main = new Main();
        mycache = mock(Cache.class);
        System.out.println("mycache: " + mycache.toString()); // Mock for Cache, hashCode: 3244883
        when(mycache.getIfPresent("9393939")).thenReturn(null);
        Product product = main.processData("9393939");
        assertNotNull(product);
    }
}

Exception Log trace:

mycache: Mock for Cache, hashCode: 3244883
        productId value: 9393939
        mycache value: null

        java.lang.NullPointerException
        at com.test.processData(Main.java:122)
        at com.test.MainTest.processDataTest(MainTest.java:161)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)

Solution

Instead of doing:

@Component
public class Main {

   @Autowired
   Cache mycache;
   ...

Inject it via a constructor:

  @Component
  public class Main {
      private final Cache mycache;

      public Main(Cache mycache) {
          this.mycache = mycache;
      }
  }

Now it your test you can create an instance of Main where you can provide a mock of Cache.

Answered By - Eugene
Answer Checked By - Robin (JavaFixing Admin)