| ## Why is `-Djava.ext.dirs` not supported? |
| |
| `-Djava.ext.dirs` provides the extension class loader mechanism which was introduced in JDK 1.2, which was released in 1998. |
| According to [JEP 220: Modular Run-Time Images](http://openjdk.java.net/jeps/220), it ends in JDK 9, |
| to simplify both the Java SE Platform and the JDK we have removed the extension mechanism, |
| including the java.ext.dirs system property and the lib/ext directory. |
| |
| This JEP has been applied since JDK11, which is the most active LTS JDK version. When use `-Djava.ext.dirs` in JDK11+, |
| the JVM would not be able to boot with following error. |
| ```shell |
| <JAVA_HOME>/lib/ext exists, extensions mechanism no longer supported; Use -classpath instead. |
| .Error: Could not create the Java Virtual Machine. |
| Error: A fatal exception has occurred. Program will exit. |
| ``` |
| |
| So, SkyWalking agent would not support the extension class loader mechanism. |
| |
| ### How to resolve this issue? |
| If you are using JDK8 and `-Djava.ext.dirs`, follow the JRE recommendations, **Use -classpath instead**. |
| This should be a transparent change, which only affects your booting script. |
| |
| Also, if you insist on keeping using `-Djava.ext.dirs`, the community had [a pull request](https://github.com/apache/skywalking-java/pull/19), |
| which leverages the bootstrap instrumentation core of the agent to support the extension class loader. |
| |
| In theory, this should work, but the SkyWalking doesn't officially verify it before noticing the above JEP. |
| You could take it as a reference. |
| |
| The official recommendation still keeps as **Use -classpath instead**. |
