How to Fix \'Invalid Byte Tag Exception\' in Tomcat 7 when Upgrading to Java 8?

Invalid Byte Tag Exception in Tomcat 7 with Java 8

When porting a web application from Tomcat 7 and Java 7 to another server running Tomcat 7 and Java 8, an error may occur during startup:

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

This exception stems from BCEL, the Java bytecode engineering library used by Tomcat. In particular, annotation scanning (with metadata-complete="true" in web.xml) can trigger issues due to BCEL's inability to process certain Java 8 byte codes.

Solutions:

1. Disable Annotation Scanning:

If annotation scanning is not essential, it can be disabled by setting metadata-complete="false" in web.xml.

2. Tomcat 7.0.53 or Later:

Tomcat 7.0.53 and subsequent releases include an updated compiler with improved Java 8 support. Upgrading to these versions should resolve the exception.

3. Skip Java 8 JARs for Annotation Scanning (Advanced):

If annotation scanning is required and the web application 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 line instructs Tomcat to ignore specific Java 8 JARs during annotation scanning.

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