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PlatformAPIPages; JOSArchitectureDiscussion

Introduction

For Java programmers, the architecture of JOS might be a little easier to understand by using Java-like pseudocode. These are not real Java classes. Instead, they illustrate the relationship between the kernel, Java virtual machine and JOSKernelModule.

These are not a run-time pictures; but a picture of class inheritence.

Module

A module is a native code plug-in for a kernel. The kernel must discover the native methods of a module, just like an application (like a classic Java virtual machine) discovers methods in a shared library.

public interface Module {
  public void init();
  public void destroy();
  public String[] listMethodName();
  public * getEntryPoint( String methodName );
:
}

Virtual machine

All of the JVMs must implement similar methods in order to be useful to a kernel.

public interface VirtualMachine {
:
}

Many implementations of a virtual machine are possible. Only one is required.

public class BasicVirtualMachine
	 implements VirtualMachine {
:
}

public class SmallVirtualMachine
	 implements VirtualMachine {
:
}

public class FastVirtualMachine
	 implements VirtualMachine {
:
}

A virtual machine module

When you combine a Module and VirtualMachine, you get a plug-in module for the kernel that happens to be a virtual machine.

public class BasicVirtualMachineModule
	 implements Module, VirtualMachine {
:
}

Kernel

A kernel uses modules and a virtual machine.

public interface Kernel {
  public void init(...);
  protected VirtualMachine getVirtualMachine();
  public Module createModule(...);
  public Enumeration getModules();
:
}

There might be many implementations of a kernel. Only one is required.

public class BasicKernel
	 implements Kernel {
  public native void init();
  public native VirtualMachine createVirtualMachine(...);
  public native Enumeration getVirtualMachines();
  public native Module createModule(...);
  public native Enumeration getModules();
:
}

Other relationships

The JOS architecture has other relationships. Could these be expressed in Java-like pseudocode, too? I would like to think so.

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