The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The server and client programs run with a security manager installed. When you run either program, you need to specify a security policy file so that the code is granted the security permissions it needs to run. Here is an example policy file to use with the server program:
grant codeBase "file:/home/ann/src/" { permission java.security.AllPermission; };
Here is an example policy file to use with the client program:
grant codeBase "file:/home/jones/src/" { permission java.security.AllPermission; };
For both example policy files, all permissions are granted to the classes in the program's local class path, because the local application code is trusted, but no permissions are granted to code downloaded from other locations. Therefore, the compute engine server restricts the tasks that it executes (whose code is not known to be trusted and might be hostile) from performing any operations that require security permissions. The example client's Pi
task does not require any permissions to execute.
In this example, the policy file for the server program is named server.policy
, and the policy file for the client program is named client.policy
.
Before starting the compute engine, you need to start the RMI registry. The RMI registry is a simple server-side bootstrap naming facility that enables remote clients to obtain a reference to an initial remote object. It can be started with the rmiregistry
command. Before you execute rmiregistry
, you must make sure that the shell or window in which you will run rmiregistry
either has no CLASSPATH
environment variable set or has a CLASSPATH
environment variable that does not include the path to any classes that you want downloaded to clients of your remote objects.
To start the registry on the server, execute the rmiregistry
command. This command produces no output and is typically run in the background. For this example, the registry is started on the host mycomputer
.
Microsoft Windows (use javaw
if start
is not available):
start rmiregistry
Solaris OS or Linux:
rmiregistry &
By default, the registry runs on port 1099. To start the registry on a different port, specify the port number on the command line. Do not forget to unset your CLASSPATH
environment variable.
Microsoft Windows:
start rmiregistry 2001
Solaris OS or Linux:
rmiregistry 2001 &
Once the registry is started, you can start the server. You need to make sure that both the compute.jar
file and the remote object implementation class are in your class path. When you start the compute engine, you need to specify, using the java.rmi.server.codebase
property, where the server's classes are network accessible. In this example, the server-side classes to be made available for downloading are the Compute
and Task
interfaces, which are available in the compute.jar
file in the public_html\classes
directory of user ann
. The compute engine server is started on the host mycomputer
, the same host on which the registry was started.
Microsoft Windows:
java -cp c:\home\ann\src;c:\home\ann\public_html\classes\compute.jar -Djava.rmi.server.codebase=file:/c:/home/ann/public_html/classes/compute.jar -Djava.rmi.server.hostname=mycomputer.example.com -Djava.security.policy=server.policy engine.ComputeEngine
Solaris OS or Linux:
java -cp /home/ann/src:/home/ann/public_html/classes/compute.jar -Djava.rmi.server.codebase=http://mycomputer/~ann/classes/compute.jar -Djava.rmi.server.hostname=mycomputer.example.com -Djava.security.policy=server.policy engine.ComputeEngine
The above java
command defines the following system properties:
java.rmi.server.codebase
property specifies the location, a codebase URL, from which the definitions for classes originating from this server can be downloaded. If the codebase specifies a directory hierarchy (as opposed to a JAR file), you must include a trailing slash at the end of the codebase URL.java.rmi.server.hostname
property specifies the host name or address to put in the stubs for remote objects exported in this Java virtual machine. This value is the host name or address used by clients when they attempt to communicate remote method invocations. By default, the RMI implementation uses the server's IP address as indicated by the java.net.InetAddress.getLocalHost
API. However, sometimes, this address is not appropriate for all clients and a fully qualified host name would be more effective. To ensure that RMI uses a host name (or IP address) for the server that is routable from all potential clients, set the java.rmi.server.hostname
property.java.security.policy
property is used to specify the policy file that contains the permissions you intend to grant.Once the registry and the compute engine are running, you can start the client, specifying the following:
Pi
class) by using the java.rmi.server.codebase
propertyjava.security.policy
property, which is used to specify the security policy file that contains the permissions you intend to grant to various pieces of codeCompute
remote object) and the number of decimal places to use in the
calculationStart the client on another host (a host named mysecondcomputer
, for example) as follows:
Microsoft Windows:
java -cp c:\home\jones\src;c:\home\jones\public_html\classes\compute.jar -Djava.rmi.server.codebase=file:/c:/home/jones/public_html/classes/ -Djava.security.policy=client.policy client.ComputePi mycomputer.example.com 45
Solaris OS or Linux:
java -cp /home/jones/src:/home/jones/public_html/classes/compute.jar -Djava.rmi.server.codebase=http://mysecondcomputer/~jones/classes/ -Djava.security.policy=client.policy client.ComputePi mycomputer.example.com 45
Note that the class path is set on the command line so that the interpreter can find the client classes and the JAR file containing the interfaces. Also note that the value of the java.rmi.server.codebase
property, which specifies a directory hierarchy, ends with a trailing slash.
After you start the client, the following output is displayed:
3.141592653589793238462643383279502884197169399
The following figure illustrates where the rmiregistry
, the ComputeEngine
server, and the ComputePi
client obtain classes during program execution.
When the ComputeEngine
server binds its remote object reference in the registry, the registry downloads the Compute
and Task
interfaces on which the stub class depends. These classes are downloaded from either the ComputeEngine
server's web server or file system, depending on the type of codebase URL used when starting the server.
Because the ComputePi
client has both the Compute
and the Task
interfaces available in its class path, it loads their definitions from its class path, not from the server's codebase.
Finally, the Pi
class is loaded into the ComputeEngine
server's Java virtual machine when the Pi
object is passed in the executeTask
remote call to the ComputeEngine
object. The Pi
class is loaded by the server from either the client's web server or file system, depending on the type of codebase URL used when starting the client.