Syntax of JavaFBP (Java Implementation of FBP)
and Component API

Checking has been added (Aug., 2014) to make sure that all mandatory input connections have been provided. Since this was not being checked before, it may be necessary to add the new optional parameter to the InPort metadata for some components. If this causes any problems for current users, please let us know.



In my book, "Flow-Based Programming", I describe the syntax of the network specifications of various FBP dialects that were in existence when the book was written. JavaFBP, the Java implementation of the FBP concepts, did not exist at that time, so this web page has been added describing the syntax of JavaFBP network definitions.

As described in my book, many non-trivial applications can be built using only the network definition language, so this web page starts with a description of the network definition language.  A running program can be built up in layers, using JavaFBP's subnet notation.  There is also a diagramming tool (DrawFBP), which can be used to define the network graphically, and which can actually generate the network definitions. 

If new components have to be coded, they will be coded in Java, using the Java component API.  Components also have attrbutes that have to be visible to the JavaFBP scheduler - these are specified using component metadata, described below.

The source code for the various constituents of JavaFBP is now being held on a public GitHub project, and is being managed using Gradle. There is also a README file on the GitHub web site.

One advantage of defining the network as executable code, as compared with other approaches that merely list connections in a language-independent way, is that the network can contain additional logic. This logic then controls the way the network is defined, rather than the way it runs. Some may regard this as a defect, rather than as an asset, and both views can certainly be defended, but one of the neat things it enables us to do is to adjust multiplexing levels of various structures in the diagram using a table of values (remember the multiplexing example in Sample DrawFlow Diagram). One merely retrieves a value from a table for the degree of multiplexing in a particular structure in the diagram, and this value is then used both as the index of a loop invoking the connect statement, and as the index for the elements of an array-type port (see below for both of these terms).

Version 2.5 of JavaFBP has the following additional features (including enhancements to metadata, also described below in Component Metadata) :

  • isConnected() now works on element basis
  • port name without element number can be used in place of port_name[0]
  • optional array size on openArrayPort (In/Out); annotation must specify fixedSize = true
  • if non-optional output array, at least one element must be connected
  • for input array ports, no elements need to be connected
  • send to unconnected port element should generate message and crash  (to avoid doing the send, do an isConnected() test - the port must have been declared optional in the metadata)
  • added metadata annotation @SelfStarting, indicating that the process starts at the beginning of job even if it has input ports (some copies of my book incorrectly state that this is the MustRun attribute)
  • setDimension parameter in metadata, described below
  • check count of entities received on a connection, specified via an optional boolean parameter specified on connect() method; capacity and optional boolean can be in either order (*)
  • new public method on Network called getIPCounts(), which returns a Map<String, BigInteger>. (*)
  • process names can now be specified using Unicode, e.g. Chinese:

As of version 2.6, JavaFBP now checks that components only receive and send packets containing data of the type specified in the class annotations. This uses the Java isAssignableFrom() method - e.g. if the port annotations specify Object (the default), this port may send or receive packets containing anything.

(*) Connection counts are identified by the name of the connection's downstream port, as there can only be one of these for a given connection.

(*) A sample component has been added to the jar file, called DispIPCounts, using the getIPCounts() method, which outputs the requested counts at intervals specified in an IIP.  This is just to give an idea of what could be done.  Here is some sample output from this component (connections are identified by the name of their downstream - i.e. input.port):

2011.03.21T14:07:25:647 TestIPCounting.Discard.IN                       9763
2011.03.21T14:07:25:656 TestIPCounting.ReplStr.IN                       9787
2011.03.21T14:07:26:160 TestIPCounting.Discard.IN                      37865
2011.03.21T14:07:26:160 TestIPCounting.ReplStr.IN                      37887
2011.03.21T14:07:26:660 TestIPCounting.Discard.IN                      67349
2011.03.21T14:07:26:660 TestIPCounting.ReplStr.IN                      67360

Network Definitions

Since the way the syntax relates to the underlying diagram may not be all that clear, a brief description is in order.  At the end of this page, I have given an extremely simple JavaFBP component.

Any JavaFBP network definition starts as follows:

public class xxxxxx extends Network {

protected void define() {

where xxxxxx is the Network name, including the usual imports, copyright statements, etc. Of course you will have to import classes for JavaFBP objects, such as Network and Component, as well as any JavaFBP "verb" classes you may be using.

The network definition is terminated with:

public static void main(String[] argv) throws Exception {
new xxxxxx().go();

In between the beginning and the ending statements defining the network, you specify a list of connections, using the following methods, which I will refer to as "clauses":

  • component - define an instance of a component (an FBP "process")
  • connect - define a connection
  • initialize - define a connection including an Initial Information Packet (IIP)
  • port - define a port on a process

Every component instance must have a unique character string identifying it, which allows other component instances or initial information packets (IIPs) to be attached to it via a connection.

The following method call: 


returns a reference to a component instance. The first reference to this particular component instance must specify the component class to be executed by that occurrence. This is done by coding

component("xxxx", cccc.class)

where cccc  is the name of the Java module to be executed.

Similarly, a port is identified by a port clause, e.g. port("xxxx").

A port may be an array-type port, in which case the port clauses referring to its elements have index values, as follows: 


where "n" runs up monotonically from 0. Each element of the port array will be connected to a different component occurrence or IIP.

A connect or initialize clause may contain the relevant component clauses, together with their corresponding port clauses, embedded within it, as e.g.

     connect(component("Read", ReadText.class),
         component("Splitter1", Splitter1.class),

or the connect and component portions may be in separate statements, provided component precedes any connects that reference it.

A connect contains:

  • "from" component clause
  • "from" port clause
  • "to" component clause
  • "to" port clause
  • (optional) connection capacity, specified as an int
  • (optional) IP count facility desired, specified as a boolean

These last two may appear in either order if present, and one or both may be omitted. If the optional fifth parameter (connection capacity) is omitted, the default value is used: 1 for testing, or 10 for production (the static int value defaultCapacity in the Network class has to be set either to DEBUGSIZE (1) or PRODUCTIONSIZE (10)). 

If the optional sixth parameter (IP count facility desired) is specified as true, the number of IPs passing across this connection is continuously updated in a synchronized internal HashMap, keyed on the full process name (process name preceded by the name of containing subnet process (separated by a period) and so on recursively) and port name.  An example of a sample component that displays any specified counts at some interval specified in an IIP is included in jar files - versions 2.5 and later - under the name DispIPCounts.  The network demonstrating the use of this function is in test.networks, called TestIPCounting.

If an asterisk (*) is specified for the "from" port, this is called an "automatic output port", and indicates a signal generated when the "from" component instance terminates (actually the port is just closed, so no packet has to be disposed of). 

If an asterisk (*) is specified for the "to" port, this is called an "automatic input port", and indicates a delay - the "to" component instance does not start until a signal or a close is received at this port.

If *SUBEND is specified as a port name on a subnet, a packet containing null is emitted at this port every time the subnet deactivates, i.e. all the contained components terminate.  It doesn't have to be named in the port metadata.  This null packet is emitted for all activations, including the last one.

An initialize clause contains:

  • a reference to any object
  • a component clause
  • a port clause

as e.g.

     initialize(new FileReader(

However, it has been recommended that IIPs should be strings, rather than arbitrary objects, to facilitate future graphical management of networks.

One last point: any number of "from" ports can be connected to a single "to" port; only one "to" port can ever be connected to a given "from" port.

A "named global" facility has been provided - putGlobal(String, Object) - which can be used to make an object available to all components, using the getGlobal(String): Object method.

putGlobal(String, Object) adds an object to a global HashMap using a String as identifier; getGlobal(String): Object can be used by a component to retrieve said object using the same String value.

CAUTION: this is extremely dangerous, as it runs counter to FBP philosophy. It should be used very carefully!

Sample Network

Let us code up a network implementing the following picture:


First list the component clauses, together with the component classes they are to execute (assuming that component classes have been written to execute the various nodes of the diagram), e.g.:

component("Read Masters",Read.class)
component("Read Details",Read.class)
component("Process Merged Stream",Proc.class)
component("Write New Masters",Write.class)
component("Summary & Errors",Report.class)

Now these component clauses may either be made into separate statements or they can be imbedded into the connect statements that follow.  Here are the connections in the diagram, without imbedded component clauses:

  connect(component("Read Masters"),port("OUT"),component("Collate"),
port("IN",0));    // array port
connect(component("Read Details"),port("OUT"),component("Collate"),
port("IN",1));      // array port
component("Process Merged Stream"), port("IN"));
connect(component("Process Merged Stream"),port("OUTM"),
component("Write New Masters"),port("IN"));
connect(component("Process Merged Stream"),port("OUTSE"),
component("Summary & Errors"),port("IN"));

So you can either separate your component and connect statements, or add the class designation to the first component clause referencing a particular component occurrence, giving the following:

connect(component("Read Masters",Read.class),port("OUT"),
    component("Collate",Collate.class), port("IN",0)); // array port
  connect(component("Read Details",Read.class),port("OUT"),
     component("Collate"),port("IN",1)); // array port
component("Process Merged Stream",Proc.class),port("IN"));
connect(component("Process Merged Stream"),port("OUTM"),
component("Write New Masters",Write.class),port("IN"));
connect(component("Process Merged Stream"),port("OUTSE"),
component("Summary & Errors",Report.class),port("IN"));

Now "Read Masters" and "Read Details" use the same Java class, so we need some way to indicate the name of the file that each is going to read. This is done using Initial Information Packets (IIPs). In this case they might usefully specify FileReader objects, so we need to add two initialize clauses, as follows:

initialize(new FileReader("c:\\mastfile"),
component("Read Masters"),
initialize(new FileReader("c:\\detlfile"),
component("Read Details"),

Note that, since both "Read" component occurrences use the same class code, they naturally have the same port names - of course, the ports are attached to different IIPs.

Remember that back-slashes have to be doubled in Java character strings! Process names can contain any character - but double quotes in the name must be "escaped" using a back-slash.

"Write New Masters" will have to have an IIP to specify the output destination - perhaps:

initialize(new FileWriter("c:\\newmast"),
component("Write New Masters"),

Note also that this IIP is not a destination for the Writer - it is an object used by this component occurrence so that the latter can figure out where to send data to.

Add the beginning and ending statements, and you're done!   The actual sequence of connect and initialize statements is irrelevant.

Here is the final result:

public class xxxxxx extends Network {

protected void define() {
connect(component("Read Masters",Read.class),port("OUT"),
component("Collate",Collate.class),port("IN",0)); // array port
connect(component("Read Details",Read.class),port("OUT"),
component("Collate"),port("IN",1));// array port
component("Process Merged Stream",Proc.class),port("IN"));
connect(component("Process Merged Stream"),port("OUTM"),
component("Write New Masters",Write.class),port("IN"));
connect(component("Process Merged Stream"),port("OUTSE"),
component("Summary & Errors",Report.class),port("IN"));
initialize(new FileReader("c:\\mastfile"),
component("Read Masters"),
initialize(new FileReader("c:\\detlfile"),
component("Read Details"),
initialize(new FileWriter("c:\\newmast"),
component("Write New Masters"),


public static void main(String[] argv) throws Exception{

new xxxxxx().go();

Simplified Notation (JavaFBP-2.0+)

In the latest release of JavaFBP, we have introduced a new, simplified notation, in addition to that shown above.  In this notation connect specifies two character strings, and initialize specifies an object and a character string.   In both cases, the second character string specifies a combination of component and port, with the two parts separated by a period. Array port indices, if required, are specified using square brackets, e.g.


The old port notation will still be supported, but is only really needed when the port index is a variable.  When debugging, it will be noted that the square bracket notation is used in trace lines, even when it was not used in the network definition.

Component names must of course not include periods or most special characters, but they may include blanks, numerals, hyphens and underscores, and they must be associated with their implementing class using a (preceding) component statement.

Here is the above network using the new notation:

public class xxxxxx extends Network {

protected void define() {
component("Read Masters",Read.class);
component("Read Details",Read.class);
component("Process Merged Stream",Proc.class);
component("Write New Masters",Write.class);
component("Summary & Errors",Report.class);
connect("Read Masters.OUT", "Collate.IN[0]");
connect("Read Details.OUT", "Collate.IN[1]");
connect("Collate.OUT"), "Process Merged Stream.IN");
connect("Process Merged Stream.OUTM", "Write New Masters.IN");
connect("Process Merged Stream.OUTSE", "Summary & Errors.IN");
initialize(new FileReader("c:\\mastfile"), "Read Masters.SOURCE");
initialize(new FileReader("c:\\detlfile"), "Read Details.SOURCE");
initialize(new FileWriter("c:\\newmast"),
"Write New Masters.DESTINATION");

public static void main(String[] argv) throws Exception {
new xxxxxx().go();

Here is a network example showing how variable port numbers can be used with the LoadBalance function to define an (admittedly fairly trivial) self-balancing network. 

This also shows a slightly different way of specifying the define function, which avoids having to remember to change the network name in the main() method, when you do cut and paste.  Its disadvantage is that trace output will show a "generated" network name, rather than the actual one.

public class TestLoadBalancer {

public static void main(final String[] args) {
try {
new Network() {
protected void define() {
int multiplex_factor = 10;
component("generate", Generate.class);
component("display", WriteToConsole.class);
component("lbal", LoadBalance.class);
connect("generate.OUT", "lbal.IN");
initialize("100 ", component("generate"), port("COUNT"));
for (int i = 0; i < multiplex_factor; i++) {
connect(component("lbal"), port("OUT", i),
component("passthru" + i, Passthru.class), port("IN"));
connect(component("passthru" + i), port("OUT"), "display.IN");
} catch (Exception e) {

Simple Subnet

As described in the book, networks can be built up level by level, using what we call "subnets" - they may be thought of as networks with "sticky" connections.  Here is a very simple subnet.

Note the metadata - a subnet can act as a component, so metadata is required.

public class SubnetX extends SubNet {
protected void define() {
component("SUBIN", SubInSS.class); // substream-sensitive
component("SUBOUT", SubOutSS.class); // do.
component("Pass", Passthru.class);
initialize("IN", component("SUBIN"), port("NAME"));
connect(component("SUBIN"), port("OUT"), component("Pass"), port("IN"));
connect(component("Pass"), port("OUT"), component("SUBOUT"), port("IN"));
initialize("OUT", component("SUBOUT"), port("NAME"));

Component Coding

Components are basically simple Java classes, which extend Component. The class Component really describes a JavaFBP "process", (or instance of a component), but has been retained for historical reasons.

Sample Component

This component generates a stream of 'n' IPs, where 'n' is specified in an InitializationConnection (specified by an initialize clause in the foregoing). Each IP just contains an arbitrary string of characters, in order to illustrate the concept.  Of course any copyright information included is up to the developer.

The statements starting with @OutPort are called the component metadata, and are described below in the section called "Component Metadata".

A JavaFBP component basically consists of 5 sections:

  • package
  • import statements
  • metadata
  • declares for ports
  • openPorts() method
  • execute() method

Here is a sample component:

package com.jpmorrsn.fbp.components;

import com.jpmorrsn.fbp.engine.*;

/** Component to generate a stream of 'n' packets, where 'n' is
* specified in an InitializationConnection.

@OutPort(value = "OUT", description = "Generated stream",
type = String.class)
"Generates stream of packets under control of a counter")
@InPort(value = "COUNT",
description = "Count of packets to be generated",
type = String.class)

public class Generate extends Component {

static final String copyright = "Copyright .....";

OutputPort outport;

InputPort count;

protected void openPorts() {
outport = openOutput("OUT");
count = openInput("COUNT");

protected void execute() {
Packet ctp = count.receive();
if (ctp == null) {

String cti = (String) ctp.getContent();
cti = cti.trim();
int ct = 0;
try {
ct = Integer.parseInt(cti);
} catch (NumberFormatException e) {

for (int i = 0; i < ct; i++) {
int j = ct - i;
Integer j2 = new Integer(j);
String s = j2.toString();
if (j < 10) {
s = "0" + s;
if (j < 100) {
s = "0" + s;
s = s + "abc";

Packet p = create(s);




The scheduling rules for most FBP implementations are described in the chapter of my book called Scheduling Rules.

In earlier JavaFBP versions, the scheduler presented an IIP to a component once per invocation. This has been changed as of JavaFBP-2.6 to once per activation.  In practice this will only affect "non-loopers" (components that get reactivated multiple times).  

There are a few other minor changes to component code as of JavaFBP-2.0:

  • As good programming practice, we now feel that IIP ports should be closed after a receive has been executed, in case it is attached to an upstream component (rather than an IIP), and that component mistakenly sends more than one IP - this statement has accordingly been added to the above example.
  • The drop statement now takes the packet as a parameter, rather than being a method of Packet.
  • The send is now unconditional - it either works or crashes (it can be made conditional by means of the isConnected() method).
  • We are adding a "long wait" state to components, specifying a timeout value in seconds. This is coded as follows:
  •         double _timeout = 2;   // 2 secs
      //   activity taking time goes here                                                       
  • Typically, the timeout value is given a default value in the code, or it can be overridden (if desired) by an IIP.
  • While the component in question is executing the activity taking time, its state will be set to "long wait". If one or more components are in "long wait" state while all other components are suspended or not started, this situation is not treated as a deadlock. However, if one of the components exceeds its timeout value, an error will be reported (complain).
  • A major change - metadata, as shown in the above component. 
  • As of JavaFBP-2.8 we have added a new attribute of JavaFBP connections: the dropOldest attribute. This is specified by having connect return a Connection object, and then invoking setDropOldest() on it. Here is an example:

            Connection c = connect(component("Generate", GenerateTestData.class), 
                            component("SlowPass", SlowPass.class), port("IN"));

    A connection tagged with dropOldest will still activate an inactive downstream process, but, if the feeding process would normally suspend because there is no room for new data packets in the connection, the oldest IP will be dropped. This attribute is useful when a measuring device is generating a stream of measurements, and it is not necessary to process every single one. A similar case might be that of mouse movement events during a "drag" function.

Component Metadata:

  • Input and output port names will be coded on components using Java 5.0 "attribute" notation. This metadata can be used to do analysis of networks without having to actually execute the components involved. Here is an example of the attributes for the "Collate" component:
  •     @OutPort("OUT") 
    @InPort(value = "IN", arrayPort = true)

    public class Collate extends Component {

  • Note that, as Java metadata does not support multiple entries with the same name, we have provided the additional metadata terms @InPorts and @OutPorts. When only one is needed (as for @OutPort in this example) the "plural" term can be omitted.
  • As shown above for "CTLFIELDS", when no attributes are needed within an @InPort or @OutPort statement, the short notation (no "value" clause) can be used.
  • Input ports do not necessarily have to be connected, even though attributes are specified for them; output ports, however, must be (unless optional is specified on the @OutPort term - see below).
  • MustRun is also specified as metadata, rather than as an interface, as it was in version 1.5.3, i.e.
  • There is a complete list of the metadata annotations in metadata below.
  • The following addition was made in version 2.2:
    • <output port name>.isConnected returns boolean
    • To support isConnected, a new metadata attribute called optional has been added to @OutPort, e.g.
    • @OutPort(value = "OUT", optional = true)

  • The following changes and additions have been made in version 2.5:
    • Network definitions are Java executable code, so port names can be computed rather than explicitly spelled out (as in the example above).  However,  @InPort and @OutPort do not support this kind of symbol manipulation, so a parameter has been added to these classes called setDimension, which allows a series of port names consisting of a string followed by numbers from 0 on up to 'n-1' to be specified, where the setDimension argument is 'n'.  
    • In this case, the value string must end with an asterisk, and the asterisk will be replaced with the numbers 0, 1, 2, ... 9, 10, 11, etc.  Thus, if the name is specified as "IN*", and setDimension(4), this will be equivalent to "IN0", "IN1", "IN2", "IN3".
    • A new parameter, valueList(), has been added to the @InPort and @OutPort annotations, allowing a list of names of ports with the same attributes to be specified.  If valueList() is used in conjunction with setDimension(), at least one list element must end with an asterisk.  List elements without the asterisk will just ignore the setDimension parameter.
    • A new annotation, @SelfStarting, has been added, indicating that the process starts at the beginning of job even if it has input ports.

    • A new Exception has been added as of JavaFBP 2.5: ComponentException.  This can be used to terminate a component occurrence from inside a nested subroutine.  It is thrown with an integer argument, and will be caught by the nearest "catch" going up the call stack.  If there are no "catch"es lower down, the throw will be caught by the scheduler, and the component occurrence will deactivate.  At this point, its behaviour, after deactivating, will be as follows for various values of the integer argument:
      • 0:                        none
      • 1-999:                generate a trace line if tracing enabled
      • 1000 or greater:  bring down the network with a "complain" call.
    • A ComponentException can also be thrown with a string argument, in which case the integer value is treated as zero.

Input ports and input array port elements are always treated as optional.

Component Metadata

Component Metadata:

Note: when "value" is the only parameter, the "short" form
(see above) can be used

- value (String)

- value (String)
- valueList({String, String, ... })
- arrayPort (boolean)
- description (String)
- type (class)
- setDimension (int) --- requires * at end of value, or some entries in valueList - fixedSize (boolean) - only relevant if arrayPort
- value (String)
- valueList({String, String, ... })
- arrayPort (boolean)
- description (String)
- type (class)
- setDimension (int) --- requires * at end of value, or some entries in valueList - optional (boolean) - fixedSize (boolean) - only relevant if arrayPort - cannot coexist with optional

parameter: list of @InPort references, e.g. @InPorts( { @InPort("IN"),
@InPort("TEST") })

parameter: list of @OutPort references, e.g. @OutPorts( { @OutPort("ACC"),
@OutPort("REJ") })

@MustRun // Must run at least once

@SelfStarting // Start process at beginning of run, even if it has input ports

@Priority(Thread.MAX_PRIORITY) // default is NORM_PRIORITY


This was alluded to in "Flow-Based Programming" (2nd ed.), Chap. 21, but development of these ideas is currently suspended (as of May 2014). If anyone wants to revisit this idea, please let us know!

Component API

Packet class:

* A Packet may either contain an Object, when type is NORMAL,
* or a String, when type is not NORMAL. The latter case
* is used for things like open and close brackets (where the
* String will be the name of a group. e.g. accounts)

Object getAttribute(String key); /* key accesses a specific attribute */
Object getContent(); /* returns null if type <> NORMAL */

Component class:

* All verbs must extend this class, defining its two abstract methods:
* openPorts, and execute.

Packet p = create(Object o);

Packet p = create(Packet.type (int) t, String s);

drop(Packet p); // Note this change!

longWaitStart(double interval); // in seconds

/** 3 stack methods - as of JavaFBP-2.3

push (Packet p);

Packet p = pop(); // return null if empty

int stackSize();

InputPort interface:

Packet = receive();

void close();

Connection class:

void setDropOldest(); // as of 2.8

OutputPort class:

void send(Packet packet);

boolean isConnected(); // as of 2.2

void close();