Chapter 5: Encoders

What is an encoder

Encoders are responsible for transforming an event into a byte array as well as writing out that byte array into an OutputStream. Encoders were introduced in logback version 0.9.19. In previous versions, most appenders relied on a layout to transform an event into a string and write it out using a java.io.Writer. In previous versions of logback, users would nest a PatternLayout within FileAppender. Since logback 0.9.19, FileAppender and subclasses expect an encoder and no longer take a layout.

Why the breaking change?

Layouts, as discussed in detail in the next chapter, are only able to transform an event into a String which restricts their scope to non-binary output.

Encoder interface

Encoders are responsible for transforming an incoming event into a byte array. Here is the Encoder interface:

package ch.qos.logback.core.encoder;
/**
 * Encoders are responsible for transform an incoming event into a byte array
 */
public interface Encoder<E> extends ContextAware, LifeCycle {

    /**
     * Get header bytes. This method is typically called upon opening of an output
     * stream.
     *
     * @return header bytes. Null values are allowed.
     */
    byte[] headerBytes();

    /**
     * Encode an event as bytes.
     *
     * @param event
     */
    byte[] encode(E event);

    /**
     * Get footer bytes. This method is typically called prior to the closing of the
     * stream where events are written.
     *
     * @return footer bytes. Null values are allowed.
     */
    byte[] footerBytes();
}

As you can see, the Encoder interface consists of few methods, but surprisingly many useful things can be accomplished with these methods.

LayoutWrappingEncoder

Until logback version 0.9.19, many appenders relied on the Layout instances to control the format of log output. As there exists substantial amount of code based on the layout interface, we needed a way for encoders to interoperate with layouts. LayoutWrappingEncoder bridges the gap between encoders and layouts. It implements the encoder interface and wraps a layout to which it delegates the work of transforming an event into string.

Below is an excerpt from the LayoutWrappingEncoder class illustrating how delegation to the wrapped layout instance is done.

package ch.qos.logback.core.encoder;

public class LayoutWrappingEncoder<E> extends EncoderBase<E> {

  protected Layout<E> layout;
  private Charset charset;
 
   // encode a given event as a byte[]
   public byte[] encode(E event) {
     String txt = layout.doLayout(event);
     return convertToBytes(txt);
  }

  private byte[] convertToBytes(String s) {
    if (charset == null) {
      return s.getBytes();
    } else {
      return s.getBytes(charset);
    }
  } 
}

The doEncode() method starts by having the wrapped layout convert the incoming event into string. The resulting text string is converted to bytes according to the charset encoding chosen by the user.

PatternLayoutEncoder

Given that PatternLayout is the most commonly used layout, logback caters for this common use-case with PatternLayoutEncoder, an extension of LayoutWrappingEncoder restricted to wrapping instances of PatternLayout.

As of logback version 0.9.19, whenever a FileAppender or one of its subclasses was configured with a PatternLayout, a PatternLayoutEncoder must be used instead. This is explained in the relevant entry in the logback error codes.

immediateFlush property

As of logback 1.2.0, the immediateFlush property is part of the enclosing Appender.

Output pattern string as header

In order to facilitate parsing of log files, logback can insert the pattern used for the log output at the top of log files. This feature is disabled by default. It can be enabled by setting the outputPatternAsHeader property to 'true' for relevant PatternLayoutEncoder. Here is an example:

<configuration>
  <!-- omitted lines ... -->
  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>foo.log</file>
    <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
      <pattern>%d %-5level [%thread] %logger{0}: %msg%n</pattern>
      <outputPatternAsHeader>true</outputPatternAsHeader>
    </encoder>
  </appender>
  <!-- omitted lines ... -->
</configuration>

This will result output akin to the following in the log file:

#logback.classic pattern: %d [%thread] %-5level %logger{36} - %msg%n
2012-04-26 14:54:38,461 [main] DEBUG com.foo.App - Hello world
2012-04-26 14:54:38,461 [main] DEBUG com.foo.App - Hi again
...

The line starting with "#logback.classic pattern" is newly inserted pattern line.

JsonEncoder

since versions 1.3.8/1.4.8

JsonEncoder follows the Newline delimited JSON (ndjson) standard. It transforms a logging event into a valid json text in conformance with [RFC-8259]. Each logging event in JSON text format is followed by a new line. More specifically, certain characters such as quotations marks, backslash, forward slash, form feed, line feed, carriage return characters are escaped according to the rules described in section 7 of RFC-8259.

Here is sample output post-formatted for readability:

{
   "sequenceNumber":0,
   "timestamp":1686403686358,
   "nanoseconds":358357100,
   "level":"INFO",
   "threadName":"main",
   "loggerName":"org.foo.Bar",
   "context":{
      "name":"default",
      "birthdate":1686403685679,
      "properties":{
         "moo":"299857071"
      }
   },
   "mdc":{
      "a1":"v1299857071"
   },
   "kvpList":[
      {
         "ik299857071":"iv299857071"
      },
      {
         "a":"b"
      }
   ],
   "message":"Hello \"Alice\"",
   "throwable":{
      "className":"java.lang.RuntimeException",
      "message":"an error",
      "stepArray":[
         {
            "className":"org.foo.Bar",
            "methodName":"httpCall",
            "fileName":"Bar.java",
            "lineNumber":293
         },
         {
            "className":"jdk.internal.reflect.DirectMethodHandleAccessor",
            "methodName":"invoke",
            "fileName":"DirectMethodHandleAccessor.java",
            "lineNumber":104
         },
         .... omitted lines
      ]
   }            
}

Note that actual output will be denser, with only one logging event per line.

Here is a sample configuration file using JsonEncoder.

<configuration>
  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>foo.json</file>
    <encoder class="ch.qos.logback.classic.encoder.JsonEncoder"/>
  </appender>


  <root>
    <level value="DEBUG"/>
    <appender-ref ref="FILE"/>
  </root>

</configuration>