Module java.management.rmi

Package javax.management.remote.rmi

The RMI connector is a connector for the JMX Remote API that uses RMI to transmit client requests to a remote MBean server. This package defines the classes that the user of an RMI connector needs to reference directly, for both the client and server sides. It also defines certain classes that the user will not usually reference directly, but that must be defined so that different implementations of the RMI connector can interoperate.

The RMI connector supports the JRMP transport for RMI.

Like most connectors in the JMX Remote API, an RMI connector usually has an address, which is a JMXServiceURL. The protocol part of this address is rmi for a connector that uses the default RMI transport (JRMP).

There are two forms for RMI connector addresses:

  • In the JNDI form, the URL indicates where to find an RMI stub for the connector. This RMI stub is a Java object of type RMIServer that gives remote access to the connector server. With this address form, the RMI stub is obtained from an external directory entry included in the URL. An external directory is any directory recognized by JNDI, typically the RMI registry, LDAP, or COS Naming.
  • In the encoded form, the URL directly includes the information needed to connect to the connector server. When using RMI/JRMP, the encoded form is the serialized RMI stub for the server object, encoded using BASE64 without embedded newlines.

Addresses are covered in more detail below.

Creating an RMI connector server

The usual way to create an RMI connector server is to supply an RMI connector address to the method JMXConnectorServerFactory.newJMXConnectorServer. The MBean server to which the connector server is attached can be specified as a parameter to that method. Alternatively, the connector server can be registered as an MBean in that MBean server.

An RMI connector server can also be created by constructing an instance of RMIConnectorServer, explicitly or through the MBean server's createMBean method.

Choosing the RMI transport

You can choose the RMI transport by specifying rmi in the protocol part of the serviceURL when creating the connector server. You can also create specialized connector servers by instantiating an appropriate subclass of RMIServerImpl and supplying it to the RMIConnectorServer constructor.

Connector addresses generated by the server

If the serviceURL you specify has an empty URL path (after the optional host and port), or if you do not specify a serviceURL, then the connector server will fabricate a new JMXServiceURL that clients can use to connect:

  • If the serviceURL looks like:

            service:jmx:rmi://host:port

    then the connector server will generate an RMIJRMPServerImpl and the returned JMXServiceURL looks like:

            service:jmx:rmi://host:port/stub/XXXX

    where XXXX is the serialized form of the stub for the generated object, encoded in BASE64 without newlines.

  • If there is no serviceURL, there must be a user-provided RMIServerImpl. The connector server will generate a JMXServiceURL using the rmi form.

The host in a user-provided serviceURL is optional. If present, it is copied into the generated JMXServiceURL but otherwise ignored. If absent, the generated JXMServiceURL will have the local host name.

The port in a user-provided serviceURL is also optional. If present, it is also copied into the generated JMXServiceURL; otherwise, the generated JMXServiceURL has no port. For an serviceURL using the rmi protocol, the port, if present, indicates what port the generated remote object should be exported on. It has no other effect.

If the user provides an RMIServerImpl rather than a JMXServiceURL, then the generated JMXServiceURL will have the local host name in its host part and no port.

Connector addresses based on directory entries

As an alternative to the generated addresses just described, the serviceURL address supplied when creating a connector server can specify a directory address in which to store the provided or generated RMIServer stub. This directory address is then used by both client and server.

In this case, the serviceURL has the following form:

    service:jmx:rmi://host:port/jndi/jndi-name

Here, jndi-name is a string that can be supplied to javax.naming.InitialContext.bind.

As usual, the host and :port can be omitted.

The connector server will generate an RMIServerImpl based on the protocol (rmi) and the port if any. When the connector server is started, it will derive a stub from this object using its toStub method and store the object using the given jndi-name. The properties defined by the JNDI API are consulted as usual.

For example, if the JMXServiceURL is: 

      service:jmx:rmi://ignoredhost/jndi/rmi://myhost/myname
then the connector server will generate an RMIJRMPServerImpl and store its stub using the JNDI name 
      rmi://myhost/myname
which means entry myname in the RMI registry running on the default port of host myhost. Note that the RMI registry only allows registration from the local host. So, in this case, myhost must be the name (or a name) of the host that the connector server is running on.

In this JMXServiceURL, the first rmi: specifies the RMI connector, while the second rmi: specifies the RMI registry.

As another example, if the JMXServiceURL is: 

      service:jmx:rmi://ignoredhost/jndi/ldap://dirhost:9999/cn=this,ou=that
then the connector server will generate an RMIJRMPServerImpl and store its stub using the JNDI name 
      ldap://dirhost:9999/cn=this,ou=that
which means entry cn=this,ou=that in the LDAP directory running on port 9999 of host dirhost.

If the JMXServiceURL is: 

      service:jmx:rmi://ignoredhost/jndi/cn=this,ou=that
then the connector server will generate an RMIJRMPServerImpl and store its stub using the JNDI name 
      cn=this,ou=that
For this case to work, the JNDI API must have been configured appropriately to supply the information about what directory to use.

In these examples, the host name ignoredhost is not used by the connector server or its clients. It can be omitted, for example:

      service:jmx:rmi:///jndi/cn=this,ou=that

However, it is good practice to use the name of the host where the connector server is running. This is often different from the name of the directory host.

Connector server attributes

When using the default JRMP transport, RMI socket factories can be specified using the attributes jmx.remote.rmi.client.socket.factory and jmx.remote.rmi.server.socket.factory in the environment given to the RMIConnectorServer constructor. The values of these attributes must be of type RMIClientSocketFactory and RMIServerSocketFactory, respectively. These factories are used when creating the RMI objects associated with the connector.

Creating an RMI connector client

An RMI connector client is usually constructed using JMXConnectorFactory, with a JMXServiceURL that has rmi as its protocol.

If the JMXServiceURL was generated by the server, as described above under "connector addresses generated by the server", then the client will need to obtain it directly or indirectly from the server. Typically, the server makes the JMXServiceURL available by storing it in a file or a lookup service.

If the JMXServiceURL uses the directory syntax, as described above under "connector addresses based on directory entries", then the client may obtain it as just explained, or client and server may both know the appropriate directory entry to use. For example, if the connector server for the Whatsit agent uses the entry whatsit-agent-connector in the RMI registry on host myhost, then client and server can both know that the appropriate JMXServiceURL is:

    service:jmx:rmi:///jndi/rmi://myhost/whatsit-agent-connector

If you have an RMI stub of type RMIServer, you can construct an RMI connection directly by using the appropriate constructor of RMIConnector.

Dynamic code downloading

If an RMI connector client or server receives from its peer an instance of a class that it does not know, and if dynamic code downloading is active for the RMI connection, then the class can be downloaded from a codebase specified by the peer. The article Dynamic code downloading using Java RMI explains this in more detail.

Since:
1.5
See Also:
Java™ Remote Method Invocation (RMI), Java Naming and Directory Interface™ (JNDI), RFC 2045, section 6.8, "Base64 Content-Transfer-Encoding"