[logback-dev] [JIRA] Updates for LOGBACK-1615: Simplify Testing with ListAppender

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logback / LOGBACK-1615 [Open]
Simplify Testing with ListAppender

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 Issue created
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Taylor S Marks created this issue on 11/Feb/22 4:28 PM
Summary:              Simplify Testing with ListAppender
Issue Type:           Improvement
Assignee:             Logback dev list
Components:           logback-core
Created:              11/Feb/22 4:28 PM
Priority:             Major
Reporter:             Taylor S Marks
Description:
  My understanding is the main usage of ListAppender is for writing tests that verify expected logs are produced.
  
  Currently, setting this up involves a lot of boilerplate:
  {code:java}
  package com.mypackage;
  
  import static org.junit.jupiter.api.Assertions.assertTrue;
  
  import ch.qos.logback.classic.Logger;
  import ch.qos.logback.classic.spi.ILoggingEvent;
  import ch.qos.logback.core.read.ListAppender;
  import org.junit.jupiter.api.Test;
  import org.slf4j.LoggerFactory;
  
  
  public class TestMyClass {
      static ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
  
      static {
          listAppender.start();
          ((Logger) LoggerFactory.getLogger(MyClass.class)).addAppender(listAppender);
          ((Logger) LoggerFactory.getLogger(AnotherClass.class)).addAppender(listAppender);
      }
  
      @Test
      public void myTest() {
          MyClass.doThingThatLogs();
          int lastLogIndex = listAppender.list.size() - 1;
  
          // Note: When it fails, it unhelpfully says "False is not True".
          assertTrue(
              listAppender
                  .list
                  .get(lastLogIndex)
                  .getFormattedMessage()
                  .contains("My expected log"));
      }
  }{code}
  I believe this is a common use case for ListAppender, and the class should be improved to facilitate doing this kind of thing better and with a lot less code.
  
  Off the top of my head, the following improvements could be made:
   # Give ListAppender a constructor that takes in a vararg of classes or Strings to list itself to, and have it start itself.
   # Give ListAppender a size method that skips the need to go to the underlying list.
   # Give ListAppender a get method that skips asking the underlying list, and also support negative indexes to easily facilitate getting logs from the end rather than the start.
   # Give ListAppender an assertContains(int index, String... substrings) method that facilitates easily asserting that specific substrings showed up in the logs, and generates a good error message of what log was actually found vs which expected substrings were missing.
  
  With those proposals, the initial example test could be reduced down to just:
  {code:java}
  package com.mypackage;
  
  import ch.qos.logback.classic.spi.ILoggingEvent;
  import ch.qos.logback.core.read.ListAppender;
  import org.junit.jupiter.api.Test;
  
  
  public class TestMyClass {
      static ListAppender<ILoggingEvent> listAppender = new ListAppender<>(MyClass.class, AnotherClass.class);
  
      @Test
      public void myTest() {
          MyClass.doThingThatLogs();
  
          // Note: If the log is missing, the failure message says what it actually found.
          listAppender.assertContains(-1, "My expected log");
      }
  
  {code}
  I wasn't sure if people maybe thought this should go someplace different (maybe make a subclass of ListAppender?) or if it doesn't belong in Logback. I'm guessing the value:cost ratio is quite good here. Do others have suggestions to improve the proposed method signatures or have additional suggestions of what would be valuable here?
  
  Getting somewhat off-topic, it could utilize of try-with-resources to easily facilitate start/stop and making sure logs aren't leaked between tests.


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