gnu.cajo.utils.extra

Class Wrapper

java.lang.Object

gnu.cajo.utils.extra.Wrapper

All Implemented Interfaces:

Invoke, Serializable

public class Wrapper
extends Object
implements Invoke

The base class for wrapping objects, remote references, and proxies for syntactical transparency with the remaining codebase. It allows the object type to change between local object/remote/proxy, without affecting the calling code. It enforces compile time type checking, and the standard invocation systax, while still allowing invocation of methods via the reflection based invoke paradigm. It also promotes "lazy instantiation," as the wrapped reference will not be loaded into the runtime unless and until it is needed. If the wrapped object url is compatible, the wrapper may be freely passed between JVM instances.

Version:

1.0, 27-Apr-04 Initial release

Author:

John Catherino

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
protected Object	object The object being wrapped by the reflection based invocation paradigm.
protected String	url The URL where to get the wrapped object: file://, http://, ftp://, /path/name, path/name, or //[host][:port]/[name].

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	Wrapper() The no-arg constructor does nothing, it is protected for use only by subclasses.
	Wrapper(String url) The constructor loads an object, or a zipped marshalled object (zedmob) from a URL, a file, or from a remote rmiregistry.

Method Summary

Methods

Modifier and Type	Method and Description
String	getDescription() This method attempts to extract usage information about the inner object, if the method is supported.
Object	invoke(String method, Object args) This method must be called by all interface methods of the subclass, as it will load the wrapped object if it has not yet been loaded.
boolean	isRemote() This method is used to test if the inner object is a reference to a remote object.
protected void	load() This method is used to support lazy loading of remote object references, only when needed.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

object

protected transient Object object

The object being wrapped by the reflection based invocation paradigm. It may be a reference to a remote object, or a proxy sent by a remote object, or even an ordinary local object. It is not declared final, as to allow its representation to change, as required for performance or other application specific reasons, for example to be further wrapped by a MonitorItem. It is transient to force subclasses to manage application specific persistence.

url

protected String url

The URL where to get the wrapped object: file://, http://, ftp://, /path/name, path/name, or //[host][:port]/[name]. The host, port, and name, are all optional. If missing the host is presumed local, the port 1099, and the name "main". The referenced resource can be returned as a MarshalledObject, it will be extracted automatically. If the URL is null, it will be assumed to be ///.

Constructor Detail

Wrapper

protected Wrapper()

The no-arg constructor does nothing, it is protected for use only by subclasses.

Wrapper

public Wrapper(String url)

The constructor loads an object, or a zipped marshalled object (zedmob) from a URL, a file, or from a remote rmiregistry. If the object is in a local file, it can be either inside the application's jar file, or on its local file system.

Loading an item from a file can be specified in one of three ways:

• As a URL; in the format file://path/name
• As a class file; in the format path/name
• As a serialized item; in the format /path/name

File loading will first be attempted from within the application's jar file, if that fails, it will then look in the local filesystem. Note: any socket connections made by the incoming item cannot be known at compile time, therefore proper operation if this VM is behind a firewall could be blocked. Use behind a firewall would require knowing all the ports that would be needed in advance, and enabling them before loading the item. Note: for efficiency, the item will not be loaded at this time, rather on the first time it is need to perform in a particular method.

Parameters:

url - The URL where to get the object: file://, http://, ftp://, /path/name, path/name, or //[host][:port]/[name]. The host, port, and name, are all optional. If missing the host is presumed local, the port 1099, and the name "main". The referenced resource can be returned as a MarshalledObject, it will be extracted automatically. If the URL is null, it will be assumed to be ///.

Method Detail

load

protected void load()

This method is used to support lazy loading of remote object references, only when needed. It also absorbs the checked RemoteException, which will cause an unchecked NullPointerException, for the calling method's subsequent operation on the wrapped object.

isRemote

public boolean isRemote()

This method is used to test if the inner object is a reference to a remote object. This can be important to know as remote invocations are not time deterministic, and not assured of execution, as with local objects. If the wrapped object has not yet been loaded, this will cause it to happen, which could result in a NullPointerException on a load failure.

Returns:

True if the inner object is remote, false otherwise.

getDescription

public String getDescription()
                      throws Exception

This method attempts to extract usage information about the inner object, if the method is supported.

Returns:

A detailed guide to object usage, preferably with examples, with HTML markup permitted.

Throws:

NoSuchMethodException - If the inner object does not support the description method.

Exception - If the innter object rejected the invocation, for any application specific reasons.

invoke

public Object invoke(String method,
            Object args)
              throws Exception

This method must be called by all interface methods of the subclass, as it will load the wrapped object if it has not yet been loaded.

Specified by:

invoke in interface Invoke

Parameters:

method - The method name to be invoked.

args - The arguments to provide to the method for its invocation, possibly null.

Returns:

The resulting data, from the invocation, possibly null.

Throws:

RemoteException - if the remote registry could not be reached.

NotBoundException - if the requested name is not in the registry.

IOException - if the zedmob format is invalid.

InstantiationException - when the URL specifies a class name which cannot be instantiated at runtime.

IllegalAccessException - when the url specifies a class name and it does not support a no-arg constructor.

MalformedURLException - if the URL is not in the format explained

IllegalArgumentException - If the method argument is null.

NoSuchMethodException - If no matching method can be found.

ClassNotFoundException - If the wrapped object's codebase cannot be found on deserialization.

StreamCorruptedException - If control information in the stream is not consistent on deserialization.

Exception - If the inner object rejected the invocation, for any application specific reasons.