Transcript Case Study A Global Compute Engine

Case Study
A Global Compute Engine
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Here we will discuss some implementation
details:
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Class Loaders
Compute Engine
Security Managers
Refer to Chapter 6 of the book “Distributed
Programming with Java” for more details
Copyright © 2001 Qusay H. Mahmoud
Class Loaders
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Java is a dynamic language capable of loading
classes as they are needed (locally or from a
remote machine)
Classes are introduced in the Java
environment when they are referenced by
name in a running class
The first class that gets to run is the one with
main()
Future attempt at loading classes is carried out
by the class loader
Copyright © 2001 Qusay H. Mahmoud
Class Loaders
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java.lang.ClassLoader is an abstract class
It provides a number of methods, e.g.:
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defineClass: converts an array of bytes to an
instance of Class (final)
findSystemClass: find the system class with the
specified name (final)
loadClass: resolve the specified name to a class or
load a class (abstract)
Refer to docs for exact method signatures….
Copyright © 2001 Qusay H. Mahmoud
Class Loaders
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When working with class loaders:
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loadClass(String, boolean) must be defined by a
subclass of ClassLoader. If the flag is set to true,
then the method should call the resolveClass
defineClass converts an byte b[] to an instance of
Class. It must be resolved before used to determine
if any referenced classes are to be defined
The system class loaded by the findSystemClass
method is from the local file system
To avoid loading the same class more than once,
use a hashtable
Copyright © 2001 Qusay H. Mahmoud
Building a network class loader
public class NetClassLoader extends ClassLoader {
}
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Provide impl’n for loadClass(String, boolean)
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Verify the class name
Check if the class request has already been loaded
Check if class is a system class
Define the class for the JVM
Resolve the class
Return the class to the caller
Copyright © 2001 Qusay H. Mahmoud
Network Class Loader
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Hidden issue: inability to cast an object created
from a loaded class into its original class. The
object to be returned needs to be casted.
Example:
NetClassLoader ncl = new NetClassLoader();
Object o; Class c;
c = ncl.loadClass(“Hello”);
o = c.newInstance();
((Hello)o).someMethod(); // this is a problem
Copyright © 2001 Qusay H. Mahmoud
Network Class Loader
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Solutions:
Have the loaded class extends an abstract class
– Have the loaded class implements an interface
public interface Compute {
void run();
}
If a class is to be loaded by the NetClassLoader, it
must implement the Compute interface
– Use Reflections APIs (explore this)
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Copyright © 2001 Qusay H. Mahmoud
Network Class Loader
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Example:
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NetClassLoader.java
Compute.java
Fetch.java
HelloTest.java
For the latest info on network class loaders, see:
http://java.sun.com/developer/technicalArticles/Networ
king/classloaders/
Copyright © 2001 Qusay H. Mahmoud
Compute Engine
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Responsible for loading a remote client class
It receives the URL of the class to be loaded
then it uses the network class loader to load
the class and all its associated classes
Collects the output and sends it back to the
client
Security: how to protect the compute engine’s
host file system from malicious client’s code??
Copyright © 2001 Qusay H. Mahmoud