Why reflection api not working in Test cases

I am trying to access main class variable from my test class using java reflection api but i am getting exception like below can some one guide me please

Main class

package com.example.microservice.controller;

public class PrivateStatic {

    public void process() {
        String message = "data";
        if (message != null && message.equalsIgnoreCase("data")) {
            System.out.println("print message"+message);
        } else {
            System.out.println("empty message"+message);
        }
    }

}

Test class

package com.example.microservice.controller;
import org.junit.jupiter.api.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Spy;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;


@SpringBootTest
@RunWith(SpringJUnit4ClassRunner.class)
public class PrivateStaticTest {

    @InjectMocks
    PrivateStatic private_Static;

    @Test
    public void processTest() throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {

        final java.lang.reflect.Field field = private_Static.getClass().getDeclaredField("message");
        final boolean origionalValue = field.isAccessible();
        field.setAccessible(true);
        field.set(private_Static, "data");
        field.setAccessible(origionalValue);

    }

}

Exception

java.lang.NullPointerException
    at com.example.microservice.controller.PrivateStaticTest.processTest(PrivateStaticTest.java:22)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:675)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)