Java Distributed Computing

Entry and EntryRep

Every JavaSpace consists solely of entries, which are represented by instances of the Entry class. An entry is a group of object references, which represent the fields in the Entry. When an entry is added to a JavaSpace, the entry is stored in serialized form by independently serializing each field in the Entry. Because of this, every field in an entry has to be public, has to be Serializable, and has to be an Object (not a primitive type).

EntryReps act as the conduit for Entrys into and out of JavaSpaces. They serialize Entrys before going into a JavaSpace during a write operation, and de-serialize Entrys returned as the result of read, take, or notify operations. A given EntryRep can be written multiple times to the same JavaSpace, which would result in multiple identical entries in the space.

EntryReps are used to specify JavaSpace entries in read or take operations. A client creates an Entry with the values and wildcards that it wants to match in a JavaSpace. Then it wraps it in an EntryRep, which generates the serialized form of the template Entry and passes it to the JavaSpace as an argument of the operation. The JavaSpace compares the serialized bytes of the template Entry to its own Entrys, and matches on the first one whose serialized bytes are the same as those of the non-null fields in the template Entry.

Another benefit of serializing each field of an Entry independently is that it allows for fault-tolerant retrieval of entries from the space. If ...