How do I mock an autowired @Value field in Spring with Mockito?

You can use the magic of Spring's ReflectionTestUtils.setField in order to avoid making any modifications whatsoever to your code.

The comment from Michał Stochmal provides an example:

use ReflectionTestUtils.setField(bean, "fieldName", "value"); before invoking your bean method during test.

Check out this tutorial for even more information, although you probably won't need it since the method is very easy to use

UPDATE

Since the introduction of Spring 4.2.RC1 it is now possible to set a static field without having to supply an instance of the class. See this part of the documentation and this commit.

It was now the third time I googled myself to this SO post as I always forget how to mock an @Value field. Though the accepted answer is correct, I always need some time to get the "setField" call right, so at least for myself I paste an example snippet here:

Production class:

@Value("#{myProps[‘some.default.url']}")
private String defaultUrl;

Test class:

import org.springframework.test.util.ReflectionTestUtils;

ReflectionTestUtils.setField(instanceUnderTest, "defaultUrl", "http://foo");
// Note: Don't use MyClassUnderTest.class, use the instance you are testing itself
// Note: Don't use the referenced string "#{myProps[‘some.default.url']}", 
//       but simply the FIELDs name ("defaultUrl")

You can use this magic Spring Test annotation :

@TestPropertySource(properties = { "my.spring.property=20" }) 

see org.springframework.test.context.TestPropertySource

For example, this is the test class :

@ContextConfiguration(classes = { MyTestClass.Config.class })
@TestPropertySource(properties = { "my.spring.property=20" })
public class MyTestClass {

  public static class Config {
    @Bean
    MyClass getMyClass() {
      return new MyClass ();
    }
  }

  @Resource
  private MyClass myClass ;

  @Test
  public void myTest() {
   ...

And this is the class with the property :

@Component
public class MyClass {

  @Value("${my.spring.property}")
  private int mySpringProperty;
   ...

You can also mock your property configuration into your test class

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration({ "classpath:test-context.xml" })
public class MyTest 
{ 
   @Configuration
   public static class MockConfig{
       @Bean
       public Properties myProps(){
             Properties properties = new Properties();
             properties.setProperty("default.url", "myUrl");
             properties.setProperty("property.value2", "value2");
             return properties;
        }
   }
   @Value("#{myProps['default.url']}")
   private String defaultUrl;

   @Test
   public void testValue(){
       Assert.assertEquals("myUrl", defaultUrl);
   }
}

I'd like to suggest a related solution, which is to pass the @Value-annotated fields as parameters to the constructor, instead of using the ReflectionTestUtils class.

Instead of this:

public class Foo {

    @Value("${foo}")
    private String foo;
}

and

public class FooTest {

    @InjectMocks
    private Foo foo;

    @Before
    public void setUp() {
        ReflectionTestUtils.setField(Foo.class, "foo", "foo");
    }

    @Test
    public void testFoo() {
        // stuff
    }
}

Do this:

public class Foo {

    private String foo;

    public Foo(@Value("${foo}") String foo) {
        this.foo = foo;
    }
}

and

public class FooTest {

    private Foo foo;

    @Before
    public void setUp() {
        foo = new Foo("foo");
    }

    @Test
    public void testFoo() {
        // stuff
    }
}

Benefits of this approach: 1) we can instantiate the Foo class without a dependency container (it's just a constructor), and 2) we're not coupling our test to our implementation details (reflection ties us to the field name using a string, which could cause a problem if we change the field name).

I used the below code and it worked for me:

@InjectMocks
private ClassABC classABC;

@Before
public void setUp() {
    ReflectionTestUtils.setField(classABC, "constantFromConfigFile", 3);
}

Reference: https://www.jeejava.com/mock-an-autowired-value-field-in-spring-with-junit-mockito/

Also note that I have no explicit "setter" methods (e.g. setDefaultUrl) in my class and I don't want to create any just for the purposes of testing.

One way to resolve this is change your class to use Constructor Injection, that can be used for testing and Spring injection. No more reflection :)

So, you can pass any String using the constructor:

class MySpringClass {

    private final String defaultUrl;
    private final String defaultrPassword;

    public MySpringClass (
         @Value("#{myProps['default.url']}") String defaultUrl, 
         @Value("#{myProps['default.password']}") String defaultrPassword) {
        this.defaultUrl = defaultUrl;
        this.defaultrPassword= defaultrPassword;
    }

}

And in your test, just use it:

MySpringClass MySpringClass  = new MySpringClass("anyUrl", "anyPassword");

Whenever possible, I set the field visibility as package-protected so it can be accessed from the test class. I document that using Guava's @VisibleForTesting annotation (in case the next guy wonders why it's not private). This way I don't have to rely on the string name of the field and everything stays type-safe.

I know it goes against standard encapsulation practices we were taught in school. But as soon as there is some agreement in the team to go this way, I found it the most pragmatic solution.