yvasyliev opened a new issue, #2026:
URL: https://github.com/apache/logging-log4j2/issues/2026
# Description
In `log4j2` (ver `2.21.1`), we can create custom appender plugins with
nested elements.
Let's say we have the following `log4j2.xml` configuration:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn">
<Appenders>
<MyAppender name="myAppender">
<MessageTemplate prefix="Hello, " suffix="!"/>
</MyAppender>
</Appenders>
<Loggers>
<Root level="ALL">
<AppenderRef ref="myAppender"/>
</Root>
</Loggers>
</Configuration>
```
`MyAppender` is a custom appender and `MessageTemplate` is a custom nested
element.
A source code of `MyAppender` class:
```java
@Plugin(name = "MyAppender", category = Core.CATEGORY_NAME, elementType =
Appender.ELEMENT_TYPE)
public class MyAppender extends AbstractAppender {
private final MessageTemplate messageTemplate;
public MyAppender(String name, MessageTemplate messageTemplate) {
super(name, null, PatternLayout.createDefaultLayout(), true,
Property.EMPTY_ARRAY);
this.messageTemplate = messageTemplate;
}
@PluginFactory
public static MyAppender createAppender(@PluginAttribute("name") String
name,
@PluginElement("MessageTemplate") MessageTemplate messageTemplate) {
return new MyAppender(name, messageTemplate);
}
@Override
public void append(LogEvent event) {
var prefix = messageTemplate.getPrefix();
var formattedMessage = event.getMessage().getFormattedMessage();
var suffix = messageTemplate.getSuffix();
var text = prefix + formattedMessage + suffix;
System.out.println(text);
}
}
```
A source code of `MessageTemplate` class:
```java
@Plugin(name = "MessageTemplate", category = Core.CATEGORY_NAME, elementType
= Appender.ELEMENT_TYPE)
public class MessageTemplate {
private final String prefix;
private final String suffix;
public MessageTemplate(String prefix, String suffix) {
this.prefix = prefix;
this.suffix = suffix;
}
@PluginFactory
public static MessageTemplate createAppender(@PluginAttribute("prefix")
String prefix,
@PluginAttribute("suffix")
String suffix) {
return new MessageTemplate(prefix, suffix);
}
public String getPrefix() {
return prefix;
}
public String getSuffix() {
return suffix;
}
}
```
Let's create a main class:
```java
public class Log4j2Demo {
private static final Logger LOGGER =
LoggerFactory.getLogger(Log4j2Demo.class);
public static void main(String[] args) {
LOGGER.info("World");
}
}
```
If we run the code above, we'll get the following output:
```
Hello, World!
```
# Problem
If the project is developed with Java 17, you may think of making
`MessageTemplate` a record:
```java
@Plugin(name = "MessageTemplate", category = Core.CATEGORY_NAME, elementType
= Appender.ELEMENT_TYPE)
public record MessageTemplate(String prefix, String suffix) {
@PluginFactory
public static MessageTemplate createAppender(@PluginAttribute("prefix")
String prefix,
@PluginAttribute("suffix")
String suffix) {
return new MessageTemplate(prefix, suffix);
}
}
```
Now the code extremely clean.
But if we run it now, we'll get the following error:
```
ERROR StatusConsoleListener MyAppender contains an invalid element or
attribute "MessageTemplate"
ERROR StatusConsoleListener An exception occurred processing Appender
myAppender
java.lang.NullPointerException: Cannot invoke
"com.github.yvasyliev.log4j2.appenders.MessageTemplate.prefix()" because
"this.messageTemplate" is null
at
com.github.yvasyliev.log4j2.appenders.MyAppender.append(MyAppender.java:31)
at
org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:161)
at
org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:134)
at
org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:125)
at
org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:89)
at
org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:686)
at
org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:644)
at
org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:620)
at
org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:556)
at
org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:81)
at org.apache.logging.log4j.core.Logger.log(Logger.java:163)
at
org.apache.logging.log4j.spi.AbstractLogger.tryLogMessage(AbstractLogger.java:2165)
at
org.apache.logging.log4j.spi.AbstractLogger.logMessageTrackRecursion(AbstractLogger.java:2119)
at
org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2102)
at
org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:1982)
at
org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1835)
at org.apache.logging.slf4j.Log4jLogger.info(Log4jLogger.java:174)
at com.github.yvasyliev.log4j2.Log4j2Demo.main(Log4j2Demo.java:10)
```
# Suggestion
My suggestion is to add support of Java records to Log4j 2 annotations so we
can create clean plugin code.
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