Why am I Getting \'Invalid Byte Tag in Constant Pool: 15\' Exception in Tomcat 7 with Java 8?

"Invalid Byte Tag in Constant Pool: 15" Exception in Tomcat 7 with Java 8

Tomcat 7 is officially compatible with Java 8. However, when migrating a webapp from Tomcat 7 with Java 6 to another Tomcat 7 server with Java 8, users may encounter the following exception:

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

Possible Causes

This exception occurs due to BCEL (Byte Code Engineering Library) issues in Tomcat 7 with Java 8. BCEL encounters difficulties processing the new Java 8 byte codes, especially when annotation scanning is enabled (metadata-complete="true" in web.xml).

Solutions

No Annotation Scanning:

If annotation scanning is not utilized, the issue can be resolved by updating Tomcat 7 to release 7.0.53 or later, which includes an updated compiler with improved Java 8 support.

Annotation Scanning:

When annotation scanning is enabled, there are two alternatives:

• Exclude Problem Libraries: Add the following line to /etc/tomcat7/catalina.properties:

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

This will instruct Tomcat to skip scanning the indicated libraries.

• Disable Annotation Scanning: Remove "metadata-complete="true" from web.xml to disable annotation scanning.

Additional Notes:

• This particular issue arose with Tomcat 7.0.28 on Debian 7.6 with Oracle JDK 8_25.
• The "official answer" that Tomcat 7 is compatible with Java 8 remains valid. However, BCEL issues may arise when annotation scanning is enabled.

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