How to implement custom pattern layout in log4j2

Question

I am migrating my application from log4j to log4j2 API. While migration, I found custom patternlayouts, patternparsers and patternconverters are used. I am not aware of how to implement these changes using log4j2 plugins. Can anyone help me on how to convert this custom layout TestPatternLayout to log4j2. Many thanks.

PFB the complete details on how custom pattern layout is implemented using log4j.

TestPatternLayout:

public class TestPatternLayout extends PatternLayout {

@Override 
protected PatternParser createPatternParser(String pattern) { 
    return new TestPatternParser(pattern); 
 } 
}

TestPatternParser:

public class TestPatternParser extends PatternParser {

private static final char Test_CHAR = 'e';
private static final char DATETIME_CHAR = 'd';

public TestPatternParser(String pattern) {
    super(pattern);
}

@Override
protected void finalizeConverter(char c) {
    switch (c) {
        case Test_CHAR:
            currentLiteral.setLength(0);
            addConverter(new TestPatternConverter());
            break;            
        default:
            super.finalizeConverter(c);
    }
 }
}

TestPatternConverter:

public class TestPatternConverter extends PatternConverter {

@Override
protected String convert(LoggingEvent event) {
    String testID = ObjectUtils.EMPTY_STRING;
    if(TestLogHandler.isTestLogEnabled()) {
        TestContextHolder contextHolder = TestLogHandler.getLatestContextHolderFromStack(event.getThreadName());
        if(contextHolder != null) {
            testID = contextHolder.getTestIDForThread(event.getThreadName());
        }
        else{
            testID = TestContextHolder.getTestIDForThread(event.getThreadName());
        }
    }
    return testID;
 }
}

Layout definition in log4j.xml:

<appender name="TEST_LOG_FILE" class="org.apache.log4j.RollingFileAppender">
...
   <layout class="com.test.it.logging.TestPatternLayout">
      <param name="ConversionPattern" value="%d %-5p [%c{1}] [TestId: %e] [%t] %m%n"/>
</layout>
...
</appender>

Answer 1

You just need to create a new Plugin and extend LogEventPatternConverter:

@Plugin(name = "TestPatternConverter", category = PatternConverter.CATEGORY)
@ConverterKeys({"e"})
public final class TestPatternConverter extends LogEventPatternConverter {

    /**
     * Private constructor.
     * @param options options, may be null.
     */
    private TestPatternConverter(final String[] options) {
        super("TestId", "testId");
    }

    /**
     * Obtains an instance of pattern converter.
     *
     * @param options options, may be null.
     * @return instance of pattern converter.
     */
    public static TestPatternConverter newInstance(final String[] options) {
        return new TestPatternConverter(options);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void format(final LogEvent event, final StringBuilder toAppendTo) {
        String testID = ObjectUtils.EMPTY_STRING;
        if(TestLogHandler.isTestLogEnabled()) {
            TestContextHolder contextHolder = TestLogHandler.getLatestContextHolderFromStack(event.getThreadName());
            if(contextHolder != null) {
                testID = contextHolder.getTestIDForThread(event.getThreadName());
            }
            else{
                testID = TestContextHolder.getTestIDForThread(event.getThreadName());
            }
        }
        toAppendTo.append(testID);
    }
}

and update your configuration:

<Appender type="RollingFile" name="TEST_LOG_FILE" fileName="${filename}">
      <Layout type="PatternLayout">
        <Pattern>%d %-5p [%c{1}] [TestId: %e] [%t] %m%n</Pattern>
      </Layout>
</Appender>

That should be all. Please see Extending log4j2 for more details.