If you want to satisfy the test coverage of private methods in JUnit

At times like this

I really want to make 100% coverage including private methods in JUnit! (Basically, it is more natural to pass all through the public method test)

For example

Suppose there is a private method in the following class that cannot pass the test naturally in only one place. (It's cute that the contents are squishy)

private String getStatus(String param, TestEntity testEntity) {
  String result = null;
  switch (param) {
    case "1":
      result = "A" + testEntity.getCode();
      break;
    case "2":
      result = "B" + testEntity.getCode();
      break;
    case "3":
      result = "C" + testEntity.getCode();
      break;
    default :
 result = "Z" + testEntity.getCode (); // ← The coverage here does not pass!
  }
  return result;
}

How to respond

Use Mockito https://site.mockito.org/

Preparation

In my case, Spring Boot and JUnit have already been installed, so Add the following to dependency to get Mockito library.

When using Maven

<!-- https://mvnrepository.com/artifact/org.mockito/mockito-all -->
<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-all</artifactId>
    <version>1.10.19</version>
    <scope>test</scope>
</dependency>

If you use Gradle

dependencies {
  // https://mvnrepository.com/artifact/org.mockito/mockito-all
  testCompile group: 'org.mockito', name: 'mockito-all', version: '1.10.19'
}

How to use

Just write the following in the test class

  private SampleUtil sampleUtil;

 private static int testId = 1; // Variables used in later validation

  @Test
  public void getStatus() {

 // Set to be able to execute SampleUtil # getStatus
    Method method = SampleUtil.class.getDeclaredMethod("getStatus", String.class, TestEntity.class);
    method.setAccessible(true);

 // Parameter settings for testing
    String param = "4"; 
    TestEntity testEntity = new TestEntity();
    testEntity.setCode("001");

 // Test run
    String status = (String) method.invoke(sampleUtil, param, testEntity);
    assertThat(status, is("Z001"));
  }

By setting accessible = true for getStatus () method of SampleUtil, it is possible to call it ignoring the accessor level.

Advanced version

If you want to get or set a private variable in your test

The following testId is a variable defined private in SampleUtil.java. Let's get it here.

  @Test
  public void getParams() {
    SampleUtil sampleUtil = new SampleUtil();
    assertThat(Whitebox.getInternalState(sampleUtil, "testId"), is(1));
  }

By implementing in this way, it is possible to call while ignoring the accessor level.

Finally

In addition to the functions introduced here, it has many functions such as rewriting static methods and expanding the range of JUnit tests. If you are interested, you may want to dig in and check it out.