How does Junit Rule work

Question

I want to  write test cases for a bulk of code  I would like to know details of JUnit  Rule annotation feature  so that I can use it for writing test cases  Please provide some good answers or links  which give detailed description of its functionality through a simple example

User · Answer

Junit Rules work on the principle of AOP (aspect oriented programming). It intercepts the test method thus providing an opportunity to do some stuff before or after the execution of a particular test method.

Take the example of the below code:

public class JunitRuleTest {

  @Rule
  public TemporaryFolder tempFolder = new TemporaryFolder();

  @Test
  public void testRule() throws IOException {
    File newFolder = tempFolder.newFolder("Temp Folder");
    assertTrue(newFolder.exists());
  }
}

Every time the above test method is executed, a temporary folder is created and it gets deleted after the execution of the method. This is an example of an out-of-box rule provided by Junit.

Similar behaviour can also be achieved by creating our own rules. Junit provides the TestRule interface, which can be implemented to create our own Junit Rule.

Here is a useful link for reference:

http://www.codeaffine.com/2012/09/24/junit-rules/

User · Answer

The explanation for how it works  JUnit wraps your test method in a Statement object so statement and Execute   runs your test  Then instead of calling statement Execute   directly to run your test  JUnit passes the Statement to a TestRule with the  Rule annotation  The TestRule s  quot apply quot  function returns a new Statement given the Statement with your test  The new Statement s Execute   method can call the test Statement s execute method  or not  or call it multiple times   and do whatever it wants before and after  Now  JUnit has a new Statement that does more than just run the test  and it can again pass that to any more rules before finally calling Execute

User · Answer

Rules are used to add additional functionality which applies to all tests within a test class  but in a more generic way   For instance  ExternalResource executes code before and after a test method  without having to use  Before and  After  Using an ExternalResource rather than  Before and  After gives opportunities for better code reuse  the same rule can be used from two different test classes   The design was based upon  Interceptors in JUnit  For more information see JUnit wiki   Rules

User · Answer

Rules are used to enhance the behaviour of each test method in a generic way  Junit rule intercept the test method and allows us to do something before a test method starts execution and after a test method has been executed   For example  Using  Timeout rule we can set the timeout for all the tests   public class TestApp        Rule     public Timeout globalTimeout   new Timeout 20  TimeUnit MILLISECONDS                                 TemporaryFolder rule is used to create temporary folders  files  Every time the test method is executed  a temporary folder is created and it gets deleted after the execution of the method    public class TempFolderTest      Rule  public TemporaryFolder tempFolder  new TemporaryFolder       Test  public void testTempFolder   throws IOException     File folder   tempFolder newFolder  demos      File file   tempFolder newFile  Hello txt       assertEquals folder getName     demos      assertEquals file getName     Hello txt              You can see examples of some in-built rules provided by junit at this link

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