class MyService {
    private UserDao userDao;
}

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.MockitoAnnotations;
import org.mockito.runners.MockitoJUnitRunner;

public class MyServiceTest {

    @InjectMocks
    private MyService myService;

    @Mock
    private UserDao userDao;
    
    @Before
    public void setUp() {
        myService = new MyService();
    }
}
Cannot mock/spy class java.lang.String Mockito cannot mock/spy following: - final classes - anonymous classes - primitive types

org.mockito.exceptions.base.MockitoException:
Cannot mock/spy class java.lang.String
Mockito cannot mock/spy following:
- final classes
- anonymous classes
- primitive types
at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl$1.withBefores(JUnit45AndHigherRunnerImpl.java:27)
at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)

In case you have to mock String, perhaps you can add a function only visible for testing via
import com.google.common.annotations.VisibleForTesting;
And in this function, mock the String type's value.
For Example:

class MyService {
    
    private String name = "myservice";

    @VisibleForTesting
    void forTest(String mockName) {
        this.name = mockName;
    }
}

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.MockitoAnnotations;
import org.mockito.runners.MockitoJUnitRunner;

public class MyServiceTest {

    private MyService myService;

    @Before
    public void setUp() {
        myService = new MyService();
        myService.forTest("mockName");
    }
}