tools.jar is a system dependency in checkstyle (defined in weld-parent)and weld-common-build-config. In Jdk 9 most classes included in tools.jar are now part of the JDK in dedicated modules.
Due to dependencies multiple Weld projects must be upgraded to correct this issues:

• weld-common-build-config
• weld-api which depend on the previous one
• weld-parent also depend on weld-common-build-config and define version for checkstyle maven plugin and checkstyle runtime
• weld-core using weld-parent
• Others ?