Error connecting to JMX endpoint: Unsupported protocol: rest

I got this error message when I was trying to use JMX into the WLP web server, when using the restConnector-2.0 interface in Liberty.

The documentation was not that helpful.  Oneproblem was I suffered from the curse of cut and paste, and used a Windows environment variable %JAVA_HOME%  instead of using the Linux $JAVA_HOME.  Another problem was caused documentation saying add the jar to the class path, and then the class path was ignored.

True documentation.

When you use the  -jar option, the JAR file is the source of all user classes, and other user class path settings are ignored.  See here.

Unhelpful documentation

Configuring secure JMX connection to Liberty   says

jconsole -J-Djava.class.path=...;%WLP_HOME%/clients/restConnector.jar

This was for Windows – on Linux it would be $WLP_HOME – except on my system $WLP_HOME was not set.

jconsole

Some of the jar files are in $JAVA_HOME, you can use the environment variable, or specify it yourself.  Note %JAVA_HOME% is windows, so be careful when you use cut and paste.

• jconsole -J-Djava.class.path=$JAVA_HOME/lib/jconsole.jar:$JAVA_HOME/lib/tools.jar:/opt/mqm/web/clients/restConnector.jar
• jconsole -J-Djava.class.path=/usr/lib/jvm/java-8-openjdk-amd64/lib/jconsole.jar:/usr/lib/jvm/java-8-openjdk-amd64/lib/tools.jar:/opt/mqm/web/clients/restConnector.jar

If the window displays “Secure connection failed” restart jconsole and use the -debug option.   For me this gave  “java.io.IOException jmx.remote.credentials not provided. Set to a String[2] {user,password}”.  I entered my userid and password, and this connected successfully.

I could not connect using my .jar file.

I was using JMXQuery to extract the data.  I was getting the Error connecting to JMX endpoint: Unsupported protocol: rest message.
Adding the jar file to my class path did not help, as the class path is ignored when using java the -jar parameter.

How to fix it

There are two ways of fixing this.

1. Put the required jar file in the extensions path, not the class path
2. Use the java -classpath…  instead of specifying java -jar

1. You need to have the jar for in the extensions path, not in the class path.

See How classes are found  in the Java documentation.  It says

• Bootstrap classes – Classes that comprise the Java platform, including the classes in rt.jar and several other important jar files.
• Extension classes – Classes that use the Java Extension mechanism. These are bundled as .jar files located in the extensions directory.
• User classes – Classes defined by developers and third parties that do not take advantage of the extension mechanism. You identify the location of these classes using the -classpath option on the command line (the preferred method) or by using the CLASSPATH environment variable.

There is a system property java.ext.dirs  which gives the location of the Extension classes. On my system this was

/usr/lib/jvm/java-8-openjdk-amd64/jre/lib/ext:/usr/java/packages/lib/ext

• The /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/ext: is java dependent. You should not put your files in this directory
• /usr/java/packages/lib/ext This is for “user” extensions.

The directory /usr/java/packages/lib/ext did not exist on my system, so I had to do, create it, copy the web connection jar file to it, and grant permissions on it.

• sudo mkdir -p /usr/java/packages/lib/ext
• sudo cp /opt/mqm/web/clients/restConnector.jar /usr/java/packages/lib/ext/
• sudo chmod 444 /usr/java/packages/lib/ext/restConnector.jar

2. Use a -classpath – not a -jar

In a .jar file there is a META-INF/MANIFEST.MF file which includes information on the entry point.

Manifest-Version: 1.0
Main-Class: src.Client

Instead of using the -jar option to point to the jar, you can use the -classpath to point to the jar and explicitly specify the entry point.  For example

java -cp ./Client.jar:/opt/mqm/web/clients/restConnector.jar  
src/Client
service:jmx:rest://localhost:9443/IBMJMXConnectorREST

• java -cp ./Client.jar:/opt/mqm/web/clients/restConnector.jar –  use the classpath option, and specify the needed jar files.  Client.jar is my program.  /opt/mqm/web/clients/restConnector.jar  is the Liberty provided jar.
• src/Client – the “entry point” class to use
• service:jmx:rest://localhost:9443/IBMJMXConnectorREST – the url to use.
• note the absence of a -jar option.

In the manifest of the Client.jar file it had Main-Class: src.Client.   This is for the the src/Client.java source file.   This ties up with the src/Client.class as seen when you use the command jar -tvf Client.jar .