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Chapter 4: Entry Objects

Contents

• Entry class
  • Simplistic
• Restrictions on entries
• Convenience Classes
• Further Uses of Entries
• Summary

Entries are used to pass additional information about services that clients can use to decide if a particular service is what it wants.

When a server registers a service, it places a copy of the service (or a suitable proxy) on the lookup service. This copy is an instance of an object, albeit in serialised form. The server can optionally register sets of attributes along with the service instance. Each set is given by an instance of a type or class. So what is stored on each service locator is an instance of a class along with a set of attribute entries. For example, a set of file editors may be available as services. Each editor is capable of editing different types of file as in

Figure 4.1: Editor class diagram

A client can search for a suitable editor in two ways

1. By asking for an instance of a specific class such as ImageEditor
2. By asking for an instance of the general class Editor with the additional information that it can handle a certain type of file

The Entry class also allows services to advertise their capabilities in ways that would otherwise require use of multiple inheritance, which is not available in Java. Suppose an editor belonged to class SuperEditor which can handle plain text, RTF files and image files. In a multiple inheritance system it would do so by subclassing from all of the classes. In Jini, it just advertises itself as being an Editor, but has entries attached for each of the types that it can handle.

To manage this, we would have a class FileType which gives information about the types of file handled:

public Class FileType implements Entry {
    public String type; // this is a MIME type

    public FileType(String type) {     
       this.type = type;
    }
}

For the SuperEditor, its capabilities would be given by using an array of entries:

Entry[] entries = new Entry[] {new FileType("plain/text"),
                               new FileType("application/rtf")
                               new FileType("image/gif")
                              };

On the other side, a client wishes to find services that can handle the attributes that it requires. The client uses the same Entry class for this. For any particular Entry, the client specifies

1. Which fields must match exactly (a non-null value)
2. Which fields it does not care about - a null value)

Entry[] entries = new Entry[] {new FileType("plain/text")};

Entry[] entries = new Entry[] {new FileType(null)};

The matching mechanism is pretty crude. A printer typically has the capacity to print a certain number of pages per minute. If it specifies this using an Entry, then it actually makes it rather hard to find! A client can request a printer service in which it does not care about speed, or request for a particular speed. It cannot ask for printers with a speed greater than some value. It cannot ask for a printer without a capability, such as anything except a color printer. An attribute must either match exactly or be ignored. The relational operators such as `<' and `!=' are not supported.

If you want to search for a printer at a particular speed, then printer speed capabilities may need to be given simpler descriptive values such as ``fast'', ``average'' or ``slow''. Then, once you have a ``fast'' printer service returned to the client, it can perform a query on the service itself for its actual speed. This would be done outside of the Jini mechanisms, using whatever interface has been agreed for the description of printers. A similar problem, that of finding a physically ``close'' service is taken up in the chapter on ``More Complex Examples''.

The mechanism chosen, of exact matches with wildcards, is comparatively easy to implement. It is a pity from the programmer's view that a more flexible mechanism was not used. One suggestion often made in the Jini mailing list is that there should be a boolean matches() method on the service object. However, it would involve un-marshalling the service on the locator in order to run the matches() method and this suffers from a variety of problems

1. What security permissions should the filter run with?
2. Slows the lookup service down
3. What happens if the filter modifies its arguments - deep copying to avoid this will cause further slow-downs

Entries are shipped around in marshalled form. Exported service objects are serialized, moved around and reconstituted as objects at some remote client. Entries are similarly serialized and moved around. However, when it comes to comparing them, this is usually done on the lookup service and they are not reconstituted on the lookup service. So when it comes to comparing an entry from a service and an entry from a client request, it is the serialized forms that are compared.

An entry cannot have as field one of the primitive types such as int or char. If one of these fields is required, then it must be wrapped up in a class such as Integer or Character. This to make it easier to perform ``wildcarding'' for matching (see next chapter for details). A wildcard for any object can be the ``pattern'' null which will work for any class, including wrapper classes such as Boolean (on the other hand, what is the wildcard for boolean: true or false?).

The only fields of interest are public, non-static, non-transient and non-final.

An entry class must have a no-args constructor.

The class AbstractEntry is a subclass of Entry, and is designed as a convenience class. It implements methods such as equals() and toString(). It is easy enough to use this for subclasses instead of Entry.

In addtion, Sun's implementation of Jini contains a further set of convenience classes, all subclassed out of AbstractEntry. These require the jini-ext.jar file and are

• Address - the address of the physical component of a service.
• Comment - a free-form comment about a service.
• Location - the location of the physical component of a service. This is distinct from the Address class in that it can be used alone in a small, local organization.
• Name - the name of a service as used by users. A service may have multiple names.
• ServiceInfo - generic information about a service. This includes the name of the manufacturer, the product, and the vendor.
• ServiceType - human-oriented information about the "type" of a service. This is not related to its data or class types, and is more oriented towards allowing someone to determine what a service (for example, a printer) does and that it is similar to another, without needing to know anything about data or class types for the Java platform.
• Status - the base class from which other status-related entry classes may be derived.

For example, the Address class contains

String country;
String locality;           // City or locality name.
String organization;       // Name of the company or organization that provides this service.
String organizationalUnit; // The unit within the organization that provides this service.
String postalCode;         // Postal code.
String stateOrProvince;    // Full name or standard postal abbreviation of a state or province.
String street;             // Street address.

The primary intention of entries is to provide extra information about services so that clients can decide whether or not they are the services they want to use. An expectation in this is that the information in an entry is primarily static. However, entries are objects, and could also implement behaviour as well as state. This could be used to extend the behaviour of a service, beyond that of the service itself.

The main use of this at present is to define the user interface for a service. This topic is looked at in detail in a much later chapter, but in brief: to allow for differing display devices, there may be a need for many user interfaces, or none. If all the possibilities have to be included in the service itself then it will become cumbersome and inflexible. It is better to put this elsewhere, such as in entry objects.

An entry is additional information about a service. A service may have any number of entries. Clients requests services by class and by entries, using a simple matching system. There are a number of convenience classes that subclass Entry.

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