\"Invalid Byte Tag in Constant Pool: 15\" - Why Does Tomcat 7 Throw This Error When Running on Java 8?

Invalid Byte Tag in Constant Pool: A Tomcat Issue with Java 8

Encountering an "Invalid byte tag in constant pool: 15" error when attempting to port a webapp from Tomcat 7 to another server running Tomcat 7 with Java 8? This article dives into the issue and offers solutions.

Issue Description:

Upon starting Tomcat, the application may encounter an error as seen in the log catalina.out:

org.apache.tomcat.util.bcel.classfile.ClassFormatException: Invalid byte tag in constant pool: 15

Possible Cause:

The error arises due to Java 8 introducing new byte codes that BCEL, the class parsing library used by Tomcat 7, may not be able to process effectively.

Solution:

Official Solution:

According to Tomcat documentation, running Tomcat 7 on Java 8 should work fine.

Annotation Scanning Issue:

However, if annotation scanning is enabled in the web application's web.xml file, issues may surface due to BCEL's inability to handle Java 8 byte codes. In such cases, exceptions like the one below may appear:

SEVERE: Unable to process Jar entry [jdk/nashorn/internal/objects/NativeString.class] from Jar [jar:file:/usr/lib/jvm/jdk1.8.0_5/jre/lib/ext/nashorn.jar!/] for annotations
org.apache.tomcat.util.bcel.classfile.ClassFormatException: Invalid byte tag in constant pool: 15

Solutions for Annotation Scanning:

1. Disable Annotation Scanning: Avoid enabling annotation scanning (set metadata-complete="true" to false in web.xml). This allows Tomcat 7 to function normally. However, this solution disables the scanning of annotations in the webapp.
2. Update Tomcat: Upgrade to Tomcat 7.0.53 or later, which includes a compiler updated to support Java 8 better.
3. Exclude Problematic Jars: If annotation scanning must be used and your code is not Java 8 based, add the following line to /etc/tomcat7/catalina.properties:

junit.jar,junit-*.jar,ant-launcher.jar,\
jfxrt.jar,nashorn.jar

This modification ensures that Tomcat skips problematic jars during annotation scanning.

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