ThrowablePatternConverter deprecated constructors refer to a package private constructor

[snicoll]

Description

I am trying to upgrade to 2.25.0 in Spring Boot and address the deprecation that this release introduces. Both deprecated constructor refer to the canonical constructor but it is package private so I can't access it from our class that extends from it.

logging-log4j2/log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/ThrowablePatternConverter.java

Line 85 in 8ec5703

ThrowablePatternConverter(

Configuration

not relevant

Logs

not relevant

Reproduction

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[vy]

@snicoll, thanks for the report. I see that issue requires more than just exporting the canonical constructor – ThrowablePropertyRendererFactory and ThrowableStackTraceRendererFactory need to be exported too. I presume you're extending the class through inheritance; could you use composition instead? That is, extend from LogEventPatternConverter but delegate to a ThrowablePatternConverter created using ThrowablePatternConverter::newInstance.

[snicoll]

could you use composition instead?

I am not sure we can use composition considering we're overriding ThrowablePatternConverter#format. Perhaps you can have a look to our pattern converters?

• WhitespaceThrowablePatternConverter overrides format
• ExtendedWhitespaceThrowablePatternConverter overrides format

[vy]

could you use composition instead?

I am not sure we can use composition considering we're overriding ThrowablePatternConverter#format. Perhaps you can have a look to our pattern converters?

@snicoll, see the following:

Index: spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverterTests.java
===================================================================
diff --git a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverterTests.java b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverterTests.java
--- a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverterTests.java	(revision 04d071046d5bdd52c40d5f91c83fa6395762f8af)
+++ b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverterTests.java	(date 1752069087378)
@@ -19,7 +19,7 @@
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.config.DefaultConfiguration;
 import org.apache.logging.log4j.core.impl.Log4jLogEvent;
-import org.apache.logging.log4j.core.pattern.ThrowablePatternConverter;
+import org.apache.logging.log4j.core.pattern.LogEventPatternConverter;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -32,7 +32,7 @@
  */
 class ExtendedWhitespaceThrowablePatternConverterTests {
 
-	private final ThrowablePatternConverter converter = ExtendedWhitespaceThrowablePatternConverter
+	private final LogEventPatternConverter converter = ExtendedWhitespaceThrowablePatternConverter
 		.newInstance(new DefaultConfiguration(), new String[] {});
 
 	@Test
Index: spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverterTests.java
===================================================================
diff --git a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverterTests.java b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverterTests.java
--- a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverterTests.java	(revision 04d071046d5bdd52c40d5f91c83fa6395762f8af)
+++ b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverterTests.java	(date 1752069087386)
@@ -19,7 +19,7 @@
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.config.DefaultConfiguration;
 import org.apache.logging.log4j.core.impl.Log4jLogEvent;
-import org.apache.logging.log4j.core.pattern.ThrowablePatternConverter;
+import org.apache.logging.log4j.core.pattern.LogEventPatternConverter;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -31,7 +31,7 @@
  */
 class WhitespaceThrowablePatternConverterTests {
 
-	private final ThrowablePatternConverter converter = WhitespaceThrowablePatternConverter
+	private final LogEventPatternConverter converter = WhitespaceThrowablePatternConverter
 		.newInstance(new DefaultConfiguration(), new String[] {});
 
 	@Test
Index: spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java
===================================================================
diff --git a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java
--- a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java	(revision 04d071046d5bdd52c40d5f91c83fa6395762f8af)
+++ b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java	(date 1752069900762)
@@ -21,6 +21,7 @@
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.apache.logging.log4j.core.pattern.ConverterKeys;
 import org.apache.logging.log4j.core.pattern.ExtendedThrowablePatternConverter;
+import org.apache.logging.log4j.core.pattern.LogEventPatternConverter;
 import org.apache.logging.log4j.core.pattern.PatternConverter;
 import org.apache.logging.log4j.core.pattern.ThrowablePatternConverter;
 
@@ -34,21 +35,24 @@
  */
 @Plugin(name = "ExtendedWhitespaceThrowablePatternConverter", category = PatternConverter.CATEGORY)
 @ConverterKeys({ "xwEx", "xwThrowable", "xwException" })
-public final class ExtendedWhitespaceThrowablePatternConverter extends ThrowablePatternConverter {
+public final class ExtendedWhitespaceThrowablePatternConverter extends LogEventPatternConverter {
 
-	private final ExtendedThrowablePatternConverter delegate;
+	private final LogEventPatternConverter delegate;
+
+	private final String separator;
 
 	private ExtendedWhitespaceThrowablePatternConverter(Configuration configuration, String[] options) {
-		super("WhitespaceExtendedThrowable", "throwable", options, configuration);
+		super("WhitespaceExtendedThrowable", "throwable");
 		this.delegate = ExtendedThrowablePatternConverter.newInstance(configuration, options);
+		this.separator = WhitespaceThrowablePatternConverter.readSeparatorOption(options);
 	}
 
 	@Override
 	public void format(LogEvent event, StringBuilder buffer) {
 		if (event.getThrown() != null) {
-			buffer.append(this.options.getSeparator());
+			buffer.append(this.separator);
 			this.delegate.format(event, buffer);
-			buffer.append(this.options.getSeparator());
+			buffer.append(this.separator);
 		}
 	}
 
Index: spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverter.java
===================================================================
diff --git a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverter.java b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverter.java
--- a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverter.java	(revision 04d071046d5bdd52c40d5f91c83fa6395762f8af)
+++ b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/WhitespaceThrowablePatternConverter.java	(date 1752069900778)
@@ -20,6 +20,8 @@
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.apache.logging.log4j.core.pattern.ConverterKeys;
+import org.apache.logging.log4j.core.pattern.ExtendedThrowablePatternConverter;
+import org.apache.logging.log4j.core.pattern.LogEventPatternConverter;
 import org.apache.logging.log4j.core.pattern.PatternConverter;
 import org.apache.logging.log4j.core.pattern.ThrowablePatternConverter;
 
@@ -32,18 +34,24 @@
  */
 @Plugin(name = "WhitespaceThrowablePatternConverter", category = PatternConverter.CATEGORY)
 @ConverterKeys({ "wEx", "wThrowable", "wException" })
-public final class WhitespaceThrowablePatternConverter extends ThrowablePatternConverter {
+public final class WhitespaceThrowablePatternConverter extends LogEventPatternConverter {
+
+	private final LogEventPatternConverter delegate;
+
+	private final String separator;
 
 	private WhitespaceThrowablePatternConverter(Configuration configuration, String[] options) {
-		super("WhitespaceThrowable", "throwable", options, configuration);
+		super("WhitespaceThrowable", "throwable");
+		this.delegate = ExtendedThrowablePatternConverter.newInstance(configuration, options);
+		this.separator = readSeparatorOption(options);
 	}
 
 	@Override
 	public void format(LogEvent event, StringBuilder buffer) {
 		if (event.getThrown() != null) {
-			buffer.append(this.options.getSeparator());
-			super.format(event, buffer);
-			buffer.append(this.options.getSeparator());
+			buffer.append(this.separator);
+			this.delegate.format(event, buffer);
+			buffer.append(this.separator);
 		}
 	}
 
@@ -58,4 +66,15 @@
 		return new WhitespaceThrowablePatternConverter(configuration, options);
 	}
 
+	static String readSeparatorOption(String[] options) {
+		if (options != null) {
+			for (String option : options) {
+				if (option != null && option.startsWith("separator(") && option.endsWith(")")) {
+					return option.substring("separator(".length(), option.length() - 1);
+				}
+			}
+		}
+		return System.lineSeparator();
+	}
+
 }

@ppkarwasz, would you mind proof-reading the above patch, please?

[ppkarwasz]

Hi @vy,

The implementation looks good overall. However, you should also override LogEventPatternConverter#handlesThrowable() to ensure exceptions are handled correctly by the layout.

The motivation behind %wEx and %xwEx seems to be to support a logging format, where exceptions are visually separated from the log message by a blank line. For example:

2025-07-09T18:50:02,060 INFO  Main - Hello, world!

and when an exception occurs:

2025-07-09T18:50:02,059 ERROR Main - An exception occurred.

java.lang.RuntimeException: This is a test exception
	at eu.copernik.sandbox.Main.createException(Main.java:18)
	at eu.copernik.sandbox.Main.main(Main.java:13)
Caused by: java.lang.RuntimeException: Caused by another exception
	at eu.copernik.sandbox.Main.createNestedException(Main.java:22)
	... 2 more
Caused by: java.lang.RuntimeException: Root cause
	at eu.copernik.sandbox.Main.createRootCause(Main.java:26)
	... 3 more

While this formatting could theoretically be achieved using something like:

<PatternLayout pattern="%d %-5p %c{1.} - %m%n%notEmpty{%n%ex%n}"
               alwaysWriteExceptions="false"/>

there's a caveat: VariablesNotEmptyReplacementConverter does not implement handlesThrowable(), so Log4j will still append %ex unless alwaysWriteExceptions="false" is set explicitly on the layout.

Note

The %wEx and %xwEx converters may also have been introduced as Log4j 2 Core equivalents to similar converters provided by Spring Boot's Logback integration.