Java – Connecting Jconsole to a remote JVM over SSH

javajmx

I am trying to connect to a remote java app using jconsole as a JMX client over ssh

I have built the CustomAgent described on https://blogs.oracle.com/jmxetc/entry/connecting_through_firewall_using_jmx

The code for the agent is here

/**
* CustomAgent.java

* https://blogs.oracle.com/jmxetc/entry/connecting_through_firewall_using_jmx
*/

package example.rmi.agent;

import java.io.IOException;
import java.lang.management.ManagementFactory;
import java.net.InetAddress;
import java.rmi.registry.LocateRegistry;
import java.util.HashMap;
import javax.management.MBeanServer;
import javax.management.remote.JMXConnectorServer;
import javax.management.remote.JMXConnectorServerFactory;
import javax.management.remote.JMXServiceURL;

/**
 * This CustomAgent will start an RMI COnnector Server using only
 * port "example.rmi.agent.port".
 *
 * @author dfuchs
 */
public class CustomAgent {

    private CustomAgent() { }

    public static void premain(String agentArgs) 
    throws IOException {

        // Ensure cryptographically strong random number generator used
        // to choose the object number - see java.rmi.server.ObjID
        //
        System.setProperty("java.rmi.server.randomIDs", "true");

        // Start an RMI registry on port specified by example.rmi.agent.port
        // (default 3000).
        //
        final int port= Integer.parseInt(
                System.getProperty("example.rmi.agent.port","3000"));
        System.out.println("Create RMI registry on port "+port);
        LocateRegistry.createRegistry(port);


        // Retrieve the PlatformMBeanServer.
        //
        System.out.println("Get the platform's MBean server");
        MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();

        // Environment map.
        //
        System.out.println("Initialize the environment map");
        HashMap<String,Object> env = new HashMap<String,Object>();

        // This where we would enable security - left out of this
        // for the sake of the example....
        //

        // Create an RMI connector server.
        //
        // As specified in the JMXServiceURL the RMIServer stub will be
        // registered in the RMI registry running in the local host on
        // port 3000 with the name "jmxrmi". This is the same name the
        // out-of-the-box management agent uses to register the RMIServer
        // stub too.
        //
        // The port specified in "service:jmx:rmi://"+hostname+":"+port
        // is the second port, where RMI connection objects will be exported.
        // Here we use the same port as that we choose for the RMI registry. 
        // The port for the RMI registry is specified in the second part
        // of the URL, in "rmi://"+hostname+":"+port
        //
        System.out.println("Create an RMI connector server");
        final String hostname = InetAddress.getLocalHost().getHostName();
        //JMXServiceURL url =
        //    new JMXServiceURL("service:jmx:rmi://"+hostname+
        //    ":"+port+"/jndi/rmi://"+hostname+":"+port+"/jmxrmi");

        //Added to allow  connection through a firewall
        // We use (port+1) to export the RMI connection objects
        JMXServiceURL url =
            new JMXServiceURL("service:jmx:rmi://"+hostname+
            ":"+(port+1)+"/jndi/rmi://"+hostname+":"+port+"/jmxrmi");




        // Now create the server from the JMXServiceURL
        //
        JMXConnectorServer cs =
            JMXConnectorServerFactory.newJMXConnectorServer(url, env, mbs);

        // Start the RMI connector server.
        //
        System.out.println("Start the RMI connector server on port "+port);
        cs.start();
    }
}

The build script is here

<project name="Agent" basedir="." default="main">

    <property name="src.dir"     value="src"/>
    <property name="build.dir"   value="build"/>
    <property name="classes.dir" value="${build.dir}/classes"/>
    <property name="jar.dir"     value="${build.dir}/jar"/>
    <property name="main-class"  value="example.rmi.agent.CustomAgent"/>

    <target name="clean">
        <delete dir="${build.dir}"/>
    </target>

    <target name="compile">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}"/>
    </target>

    <target name="jar" depends="compile">
        <mkdir dir="${jar.dir}"/>
        <jar destfile="${jar.dir}/${ant.project.name}.jar" basedir="${classes.dir}">
            <manifest>
                <attribute name="Main-Class" value="${main-class}"/>
            </manifest>
        </jar>
    </target>    

    <target name="run" depends="jar">
        <java jar="${jar.dir}/${ant.project.name}.jar" fork="true"/>
    </target>    
    <target name="clean-build" depends="clean,jar"/>    
    <target name="main" depends="clean,run"/>

     <!-- Builds dist.agent.jar -->    
    <target name="build-agent-jar" description="build an agent jar that can be used with -javaagent"   depends="compile">
        <mkdir dir="${jar.dir}"/>
        <jar basedir="${classes.dir}"  destfile="${jar.dir}/${ant.project.name}.jar">
                <manifest>
                    <attribute name="Premain-Class" value="example.rmi.agent.CustomAgent"/>
                </manifest>
        </jar>
        <echo>To use this application with agent try:</echo>
        <echo>java -Dexample.rmi.port=3000 -javaagent:${dist.agent.jar} -classpath [application-classpath] [application-classpath] [application-main-class]</echo>
    </target>
</project>

I start the application on the remote machine using

java -javaagent:agents/build/jar/Agent.jar
-Dexample.rmi.agent.port=6004 -server -server -Dcom.sun.management.jmxremote.authenticate="false" -Dcom.sun.management.jmxremote="true" -Dcom.sun.management.jmxremote.ssl="false" -Dcom.sun.management.jmxremote.local.only="false" -jar path-to-my-app/app.jar

Two ports open on the host machine, 6004 and 6005 and I can connect to the application through jconsole if the jconsole is local to the application.

I then create 2 ssh tunnels

ssh tunnels ssh -L 6004:localhost:6004 foo@server ssh tunnels
ssh -L 6005:localhost:6005 foo@server

The tunnels are created successfully. I can telnet in on these ports.

Now the problem is that I can not connect remotely using Jconsole though

service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi

jconsole was started by:

jconsole -J-Djava.util.logging.config.file=logging.properties -debug

The error log is:

Dec 18, 2011 4:57:46 PM RMIConnector connect
FINER: [javax.management.remote.rmi.RMIConnector: jmxServiceURL=service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi]
connecting…
Dec 18, 2011 4:57:46 PM RMIConnector connect
FINER: [javax.management.remote.rmi.RMIConnector: jmxServiceURL=service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi]
finding stub…
Dec 18, 2011 4:58:49 PM RMIConnector connect
FINER: [javax.management.remote.rmi.RMIConnector: jmxServiceURL=service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi]
connecting stub…
Dec 18, 2011 4:58:49 PM RMIConnector connect
FINER: [javax.management.remote.rmi.RMIConnector: jmxServiceURL=service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi]
getting connection…
Dec 18, 2011 4:59:52 PM RMIConnector connect
FINER: [javax.management.remote.rmi.RMIConnector: jmxServiceURL=service:jmx:rmi://localhost:6005/jndi/rmi://localhost:6004/jmxrmi]
failed to connect: java.rmi.ConnectException: Connection refused to
host: 172.16.0.111; nested exception is:
java.net.ConnectException: Connection timed out
java.rmi.ConnectException: Connection refused to host: 172.16.0.111; nested exception is:
java.net.ConnectException: Connection timed out
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:619)
at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:216)
at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:202)
at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:128)
at javax.management.remote.rmi.RMIServerImpl_Stub.newClient(Unknown
Source)
at javax.management.remote.rmi.RMIConnector.getConnection(RMIConnector.java:2343)
at javax.management.remote.rmi.RMIConnector.connect(RMIConnector.java:296)
at javax.management.remote.JMXConnectorFactory.connect(JMXConnectorFactory.java:267)
at javax.management.remote.JMXConnectorFactory.connect(JMXConnectorFactory.java:226)
at sun.tools.jconsole.ProxyClient.tryConnect(ProxyClient.java:366)
at sun.tools.jconsole.ProxyClient.connect(ProxyClient.java:316)
at sun.tools.jconsole.VMPanel$2.run(VMPanel.java:298)
Caused by: java.net.ConnectException: Connection timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:327)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:193)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:180)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:384)
at java.net.Socket.connect(Socket.java:546)
at java.net.Socket.connect(Socket.java:495)
at java.net.Socket.(Socket.java:392)
at java.net.Socket.(Socket.java:206)
at sun.rmi.transport.proxy.RMIDirectSocketFactory.createSocket(RMIDirectSocketFactory.java:40)
at sun.rmi.transport.proxy.RMIMasterSocketFactory.createSocket(RMIMasterSocketFactory.java:146)
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:613)
… 11 more

Best Answer

I needed to add

-Djava.rmi.server.hostname=localhost

to the java -server parameters

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Java – When to use LinkedList over ArrayList in Java

Summary ArrayList with ArrayDeque are preferable in many more use-cases than LinkedList. If you're not sure — just start with ArrayList.

TLDR, in ArrayList accessing an element takes constant time [O(1)] and adding an element takes O(n) time [worst case]. In LinkedList adding an element takes O(n) time and accessing also takes O(n) time but LinkedList uses more memory than ArrayList.

LinkedList and ArrayList are two different implementations of the List interface. LinkedList implements it with a doubly-linked list. ArrayList implements it with a dynamically re-sizing array.

As with standard linked list and array operations, the various methods will have different algorithmic runtimes.

For LinkedList<E>

get(int index) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use getFirst() and getLast()). One of the main benefits of LinkedList<E>
add(int index, E element) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use addFirst() and addLast()/add()). One of the main benefits of LinkedList<E>
remove(int index) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use removeFirst() and removeLast()). One of the main benefits of LinkedList<E>
Iterator.remove() is O(1). One of the main benefits of LinkedList<E>
ListIterator.add(E element) is O(1). One of the main benefits of LinkedList<E>

^{Note: Many of the operations need n/4 steps on average, constant number of steps in the best case (e.g. index = 0), and n/2 steps in worst case (middle of list)}

For ArrayList<E>

get(int index) is O(1). Main benefit of ArrayList<E>
add(E element) is O(1) amortized, but O(n) worst-case since the array must be resized and copied
add(int index, E element) is O(n) (with n/2 steps on average)
remove(int index) is O(n) (with n/2 steps on average)
Iterator.remove() is O(n) (with n/2 steps on average)
ListIterator.add(E element) is O(n) (with n/2 steps on average)

^{Note: Many of the operations need n/2 steps on average, constant number of steps in the best case (end of list), n steps in the worst case (start of list)}

LinkedList<E> allows for constant-time insertions or removals using iterators, but only sequential access of elements. In other words, you can walk the list forwards or backwards, but finding a position in the list takes time proportional to the size of the list. Javadoc says "operations that index into the list will traverse the list from the beginning or the end, whichever is closer", so those methods are O(n) (n/4 steps) on average, though O(1) for index = 0.

ArrayList<E>, on the other hand, allow fast random read access, so you can grab any element in constant time. But adding or removing from anywhere but the end requires shifting all the latter elements over, either to make an opening or fill the gap. Also, if you add more elements than the capacity of the underlying array, a new array (1.5 times the size) is allocated, and the old array is copied to the new one, so adding to an ArrayList is O(n) in the worst case but constant on average.

So depending on the operations you intend to do, you should choose the implementations accordingly. Iterating over either kind of List is practically equally cheap. (Iterating over an ArrayList is technically faster, but unless you're doing something really performance-sensitive, you shouldn't worry about this -- they're both constants.)

The main benefits of using a LinkedList arise when you re-use existing iterators to insert and remove elements. These operations can then be done in O(1) by changing the list locally only. In an array list, the remainder of the array needs to be moved (i.e. copied). On the other side, seeking in a LinkedList means following the links in O(n) (n/2 steps) for worst case, whereas in an ArrayList the desired position can be computed mathematically and accessed in O(1).

Another benefit of using a LinkedList arises when you add or remove from the head of the list, since those operations are O(1), while they are O(n) for ArrayList. Note that ArrayDeque may be a good alternative to LinkedList for adding and removing from the head, but it is not a List.

Also, if you have large lists, keep in mind that memory usage is also different. Each element of a LinkedList has more overhead since pointers to the next and previous elements are also stored. ArrayLists don't have this overhead. However, ArrayLists take up as much memory as is allocated for the capacity, regardless of whether elements have actually been added.

The default initial capacity of an ArrayList is pretty small (10 from Java 1.4 - 1.8). But since the underlying implementation is an array, the array must be resized if you add a lot of elements. To avoid the high cost of resizing when you know you're going to add a lot of elements, construct the ArrayList with a higher initial capacity.

If the data structures perspective is used to understand the two structures, a LinkedList is basically a sequential data structure which contains a head Node. The Node is a wrapper for two components : a value of type T [accepted through generics] and another reference to the Node linked to it. So, we can assert it is a recursive data structure (a Node contains another Node which has another Node and so on...). Addition of elements takes linear time in LinkedList as stated above.

An ArrayList, is a growable array. It is just like a regular array. Under the hood, when an element is added at index i, it creates another array with a size which is 1 greater than previous size (So in general, when n elements are to be added to an ArrayList, a new array of size previous size plus n is created). The elements are then copied from previous array to new one and the elements that are to be added are also placed at the specified indices.

Best Answer

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