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Google App Engine Java Debugging Guide

Jetty Debug Logging

To enable debug logging in your web application, add a JUL logging.properties file under your application's WEB-INF directory.

In your appengine-web.xml file, specify the location of the logging file as a system property:

<system-properties>
  <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
</system-properties>

For runtimes using the Jetty 9.4 code path (Java 8 - Java 17), there is a known issue where debug logging will not be output unless full debug logging is enabled with .level=ALL.

In the Java 21 runtime with Jetty 12, more granular debug logging is available. For example:

.level=INFO
org.eclipse.jetty.session.level=ALL
org.eclipse.jetty.server.level=ALL

Jetty Server Dump

A Jetty Server Dump is useful for debugging issues by providing details about Jetty components and their configuration, including thread pools, connectors, contexts, classloaders, servlets, and more.

To enable a Jetty Server Dump after the AppEngineWebAppContext is started (which may occur after the first request in RPC mode), use the following system property:

<system-properties>
  <property name="jetty.server.dumpAfterStart" value="true"/>
</system-properties>