Chapter 5: Encoders
和訳 (Japanese translation)ACTION THIS DAY Make sure they have all they want on extreme priority and report to me that this has been done.
—CHURCHILL on October 1941 to General Hastings Ismay in response to a request for more resources signed by Alan Turing and his cryptanalyst colleagues at Bletchley Park
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 sub-classes 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. Moreover, given that a layout has no control over when events get written out, layouts cannot aggregate events into batches. Contrast this with encoders which not only have total control over the format of the bytes written out, but also control when (and if) those bytes get written out.
At the present time, PatternLayoutEncoder is the
    only really useful encoder. It merely wraps a
    PatternLayout which does most of the work. Thus, it
    may seem that encoders do not bring much to the table except
    needless complexity. However, we hope that with the advent of new
    and powerful encoders this impression will change.
Encoder interface
Encoders are responsible for transforming an incoming event
    into a byte array and writing out the resulting byte array
    onto the appropriate OutputStream.  Thus, encoders
    have total control of what and when bytes gets written to the
    OutputStream maintained by the owning appender. Here
    is the Encoder
    interface:
   
    
package ch.qos.logback.core.encoder;
public interface Encoder<E> extends ContextAware, LifeCycle {
   /**
   * This method is called when the owning appender starts or whenever output
   * needs to be directed to a new OutputStream, for instance as a result of a
   * rollover.
   */
  void init(OutputStream os) throws IOException;
  /**
   * Encode and write an event to the appropriate {@link OutputStream}.
   * Implementations are free to defer writing out of the encoded event and
   * instead write in batches.
   */
  void doEncode(E event) throws IOException;
  /**
   * This method is called prior to the closing of the underling
   * {@link OutputStream}. Implementations MUST not close the underlying
   * {@link OutputStream} which is the responsibility of the owning appender.
   */
  void close() throws IOException;
}
    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 inter-operate 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 sub-classes 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:
<appender name="FILE" class="ch.qos.logback.core.FileAppender"> 
  <file>foo.log</file>
  <encoder>
    <pattern>%d %-5level [%thread] %logger{0}: %msg%n</pattern>
    <outputPatternAsHeader>true</outputPatternAsHeader>
  </encoder> 
</appender>
    
    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.