[logback-dev] [JIRA] (LOGBACK-1287) Breaking change? Custom appender can not be assigned to OutputStreamAppender

[QOS.CH (JIRA)]

Jack created LOGBACK-1287:
-----------------------------

             Summary: Breaking change? Custom appender can not be assigned to OutputStreamAppender
                 Key: LOGBACK-1287
                 URL: https://jira.qos.ch/browse/LOGBACK-1287
             Project: logback
          Issue Type: Bug
          Components: logback-core
    Affects Versions: 1.2.2, 1.2.1
            Reporter: Jack
            Assignee: Logback dev list
            Priority: Blocker


With the release of logback 1.2.1 our custom appender does not work anymore and Joran fails with the error below:
{noformat}
INFO in ch.qos.logback.core.joran.action.NestedComplexPropertyIA - Assuming default type [ch.qos.logback.classic.encoder.PatternLayoutEncoder] for [encoder] property

ERROR in ch.qos.logback.core.joran.util.PropertySetter at 434f3900 - A "com.example.logging.logback.extensions.DebugFileDumper" object is not assignable to a "ch.qos.logback.core.OutputStreamAppender" variable. The class "ch.qos.logback.core.OutputStreamAppender" was loaded by [startJarLoader at 2d95ebec] whereas object of type "com.example.logging.logback.extensions.DebugFileDumper" was loaded by [startJarLoader at 2d95ebec].
{noformat}
I think the issue has been introduced by commit:

[https://github.com/qos-ch/logback/commit/d3d704c12e7b8e6503187744ac2200c244088c63]

which makes {{LayoutWrappingEncoder}} assume that it is always owned by an OutputstreamAppender:

[https://github.com/qos-ch/logback/commit/d3d704c12e7b8e6503187744ac2200c244088c63#diff-3b83fcf26e3af7434aec2f08b2461be6R35])

However our custom Appender extends AppenderBase and uses composition to use two FileAppenders internally. The encoder of one of those FileAppenders can be configured in logback.xml while the other has a fixed encoding. Basically the Java code delegates setEncoder() to a FileAppender. Code roughly looks like:
{code:java}
public class DebugFileDumper extends AppenderBase<ILoggingEvent> {

  // writes a debug-timestamp.log from a cyclic buffer
  private FileAppender<ILoggingEvent> debugBufferFileDumper;
  // writes a debug-timestamp.meta containing corresponding meta infos stored in MDC in an easy to parse key=value format for further processing by external tools.
  private FileAppender<ILoggingEvent> mdcFileDumper;
  
  // ... additional properties

  public DebugFileDumper() {
    mdcFileDumper.setEncoder(new MdcOnlyEncoder());
  }

  public void setEncoder(Encoder<ILoggingEvent> encoder) {
    debugBufferFileDumper.setEncoder(encoder);
  }

  public Encoder<ILoggingEvent> getEncoder() {
    return debugBufferFileDumper.getEncoder();
  }

  // .. additional methods
}
{code}
With configuration like:
{code:xml}
<appender name="debugDumper" class="com.example.logging.logback.extensions.DebugFileDumper">

  <discriminator class="ch.qos.logback.classic.sift.MDCBasedDiscriminator">
    <key>SESSION_ID</key>
    <defaultValue>server</defaultValue>
  </discriminator>

  <evaluator class="ch.qos.logback.classic.boolex.OnErrorEvaluator" />

  <cyclicBufferTracker class="ch.qos.logback.core.spi.CyclicBufferTracker">
    <bufferSize>128</bufferSize>
    <maxComponents>128</maxComponents>
    <timeout>1800000</timeout>
  </cyclicBufferTracker>

  <folder>${logback.logs}/dumps</folder>

  <encoder>
    <pattern>[SOME PATTERN FOR DEBUG LOG FILE]</pattern>
  </encoder>

</appender>
{code}


I think our implementation strategy is valid and the assumption made in the mentioned commit, that an encoder configuration block always belongs to an OutputStreamAppender, is a bit too strict. Basically composition/delegation pattern hasn't been taken into account.

Currently we would need to rewrite our appender in order to make it work with Logback 1.2.1+, so I thought I should let you know about that implicit breaking change. Maybe it was not intended on your side and you can relax the assumption to fix our issue.



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