Package condor.cedar

Classes to read and write binary data in a format compatible with the Condor Cedar C++ library.

See:
Description

Class Summary

CedarInputStream	A Java input stream that more-or-less corresponds to the C++ class ReliSock.
CedarOutputStream	A Java output stream that more-or-less corresponds to the C++ class ReliSock.

Package condor.cedar Description

Classes to read and write binary data in a format compatible with the Condor Cedar C++ library. The API (application programming interface) is meant to be compatible with Java rules and conventions and as such is quite different fromt he Cedar API, but the format of data transmitted and received "over the wire" is compatible with the C++ verison, with the exception of floats and doubles (which are hopelessly broken in the C++ version).

In particular,

• Floats and doubles are sent using the Java serialization, which means they are sent in their in-memory (IEEE 754) format, most significant byte (MSB) first, as four or eight bytes.
• All other primitive types are converted to 8-byte long integers and sent in network byte order (big-endian; MSB first). They are sign-extended on output and truncated on input. In particular,
  • booleans are sent as false = 0 and true = 1 and received as 0 = false, non-zero = true.
  • characters are zero-extended to 64 bits on output and truncated to 16 bits on input.
• Strings are sent by converting bytes using Java UTF-8 and then appending a zero byte. They are received by scanning up to a zero byte and reversing the UTF-8 encoding. In the particular case that the string has nothing but ASCII characaters (1..127), this is essentially the identity transformation (trailing zero added on output and stripped on input). Note that Java UTF-8 has the property that the encoded stream does not contain null bytes.

Applications are expected to use methods readUTF and writeUTF to transmit strings. The methods CedarOutputStream.writeChars(String) and readLine() are provided for compatibility with the DataOutput and DataInput interfaces, but they are deprecated.

The encoded data stream is divided into "messages", which are further sub-divided in to "packets". Message boundaries have end-to-end significance; packet boundaries are hidden in the implementation. No data element ever crosses a message boundary.

Each packet is preceded by a packet header: A one-byte EOM (end-of-message) indicator (non-zero indicates that this is the last packet of a message) followed by a length as a network-byte-order 32 bit unsigned integer. The length does not include the header itself, so a zero-length packet (one with no payload) has a length field containing 0. Packet boundaries are totally transparent on input.

On output, a packet is flushed when a fixed-size buffer fills, or on a call to flush(), close(), or endOfMessage(). If the current output buffer is empty, flush() has no effect. Message boundaries are indicated on output by the method

    void endOfMessage()

which forces out a packet with the EOM flag sent containing any buffered data. Note that an EOM packet may be empty, but a non-EOM packet is never empty. An implicit or explicit close implies an endOfMessage() if any data has been output since the last endOfMessage().

On input, the EOM "marker" acts like EOF: An attempt to read past it throws an EOFException. The method available() returns the number of unread bytes in the current input packet. If the current packet has been totally consumed (or was empty) and has the EOM bit sent, available() returns zero. If the current packet has been consumed but does not have EOM set, another packet is read, and available() returns the size of that packet (which will be non-zero unless it is an EOM packet). Thus available() only returns zero when the end of a message is encountered. The method

    boolean nextMessage()

skips past the end of the current message and positions the input stream to start reading the next message. If EOF is reached (last message in the stream), nextMessage() return false (and any attempt to read will throw an EOFException).

This package does not yet have the counterpart of "safesocks". Cedar supports both "relisocks", which are implemented on top of TCP, and "safesocks", which are implemented on UDP. With a safesock, each message is a single packet, which is a single UDP datagram. It is thus limited to the maxiumum datagram size, which is platform-dependent, but is generally a bit less that 64K. (As of 3/2000, this limitation has been lifted).