VM Bridge protocol version 5

Requests are marked with "->", responses with "<-".  The server
continues to process requests until EOF. Only the first character is
significant.

The communication must begin with ReferenceClass, CreateInstance,
Invoke or PropertyAccess. If the client waits for a response
(predicate==1) the server sends back a single ARG, as defined
below. Returned object id's ("CID") are 32 bit unsigned integer values
starting with 1, increased by 1.


Example:

     $o = new java("java.lang.Long", 6);
     $o->toString(); // discard result

->  <CreateInstance value="java.lang.Long" predicate="I"> 
      <Long value="6"/> 
    </CreateInstance>

<-  <Object value="1" predicate="O"/>

->  <Invoke value="1" method="toString" predicate="I">
    </Invoke>

<-  <Object value="2" predicate="O"/>

->  <Unref value="2" />

->  <Unref value="1" />

A second example which shows how to avoid round-trips. 

     $o = new java("java.lang.Long", 6);
     $o->toString(); // discard result

->  <K p="2" v="java.lang.Long" i="0"> 
      <L v="6" /> 
    </K

->  <Y p="3" v="1" m="toString" i="0"></Y>

->  <U value="1" /> # result from CreateInstance


Optional requests:

All aquired [O]bject handles may be destroyed by invoking U (destroy).
However, the server tracks all handles and automatically destroys them
after EOF. It is an error to further reference destroyed handles.

If [F]inish is called, the server closes or recycles the connection
and responds with F p=A, if the connection has been recycled, p=E
otherwise. Instead of sending <F p=E/> and waiting for the response,
it is also possible to simply close the connection.



Main requests:


->
CreateInstance:
<C v=name p=C|I [i=ID]> ...ARGS...
</C>
v: string
p: char (Reference[C]lass, Create[I]nstance)
i: unsigned long

->
Invoke:
<I v=object m=method p=P|I> ...ARGS...
</I>
v: unsigned long
m: string
p: char (examine[P]roperty, [I]nvoke method)

When v=0, the call is directed to the current request-handling
instance of the java bridge.

->
ReferenceClass (alternate form)
<H p=1|2|3 v=name> ...ARGS...
</H>
p: char 
v: string
Predicate 2 does not write a result, but stores a result proxy under ++CID.
Predicate 3 does not write a result and does not create a result proxy.

->
CreateInstance (alternate form)
<K p=1|2|3 v=name> ...ARGS...
</K>
p: char
v: string
Predicate 2 does not write a result, but stores a result proxy under ++CID.
Predicate 3 does not write a result and does not create a result proxy.

->
PropertyAccess (alternate form)
<G p=1|2|3 v=object m=method> ...ARGS...
</G>
p: char 
v: unsigned long
m: string
Predicate 2 does not write a result, but stores a result proxy under ++CID.
Predicate 3 does not write a result and does not create a result proxy.

->
Invoke (alternate form)
<Y p=1|2|3 v=object m=method> ...ARGS...
</Y>
p: char (examine[P]roperty, [I]nvoke method)
v: unsigned long
m: string
Predicate 2 does not write a result, but stores a result proxy under ++CID.
Predicate 3 does not write a result and does not create a result proxy.


When v=0, the call is directed to the current request-handling
instance of the java bridge.



ARGS or response:
<->
String:
<S v=str/>
v: string

<->
Boolean:
<B v=T|F/>
v: char ([T]rue, [F]alse)

->
Boolean:(alternate form)
<T v=1]OTHER />
v: char [1]: true, everything else: false

<->
Long: 
<L v=l p=[O|A]/>
v: unsigned long
p: char (p[O]sitive, neg[A]tive)

->
Long: (alternate form)
<J v=l />
v: long (signed number)

<->
Double:
<D v=d/>
v: double

->
Object:
<O v=object />
v: unsigned long
NULL values can be sent as either v="" or v="0"

<-
Object:
<O v=object m=TYPE p=TYPE n=[T|F]/>
v: unsigned long
m: string
p: char ([A]rray/Map, [C]ollection, [O]bject, [E]xception)
n: char result can be cached (T) or not (F)

<-
Null:
<N/>

<-
Void:
<V m=[T|F]/>
n: char result can be cached (T) or not (F)

<-
Apply:
<A v=object p=cname m=fname n=param#> ...PAIRS...
</A>
v: unsigned long
p: string
m: string
n: unsigned long
Result can be used to send back the result of the current apply
call. If v is null, the function name p must be searched in the
"current" environment.

->
Result:
<R > ...ARG...
</R>

<->
Exception:
<E v=object m=stringRepresentation/>
v: unsigned long
m: string

->  (or <->, if a header has been sent, see below)
Composite:
<X t=A|H /> ...PAIRS...
</X>
t: char ([A]rray, [H]ashtable)

<->
Pair:
<P [t=N|S v=key]> ...ARG...
</P>
t: char (key is [N]umeric, [S]tring or doesn't exist (if X=A))
v: unsigned long (if X=H and t=N)
v: string (if X=H and t=S)



OPTIONAL requests:
->
Destroy:
<U v=object />
v: unsigned long

<->
EndConnection:
<F p=A|E />
p: char (Keep [A]live, [E]nd connection)




NOTES:

If the first byte is 0177, the following byte has a special meaning:

Bit 1 0
-------
         keep object  keep array,   binary             comment
          identity    Map, Coll.     data              
                      identity                         

    0 0    yes          yes        binary              default
    0 1    no           yes        binary              
    1 0    no           yes        base64 encoded      
    1 1    yes          yes        base64 encoded      default

Bit 2,3,4: log level (from java.log_level)

Bit 5: always 0

Bit 6: set to 1 if bit[0,1] are used
Bit 7: set to 1 if bit[2,4] are used


If the header is missing, the following default values are used

         keep object  keep array,   binary             comment
          identity    Map, Coll.     data              
                      identity                         
           no           no         base64 encoded      default


It is possible to send the header even though the connection has
already been established. The bridge will ignore the header if the
second byte is not 0xFF. This is convenient as some clients don't know
if they use a fresh or a recycled connection.

Otherwise the back end interprets the following two bytes (low/high)
as the length of the following context string. The client may initiate
such a context switch if it has received a context ID belonging to a
different ContextServer from a client (e.g.: PhpCGIServlet) while
keeping a persistent connection to a ContextRunner.

It is possible to "ping" the server by sending it the byte 0x0. If
alive it will send back a 0x0.