This topic contains sections marked as revised for this release
A message processing node is used to process a message, and an output node is used to output a message as a bit stream.
Before you start
WebSphere Message Broker provides the source for two sample user-defined nodes called SwitchNode and TransformNode. You can use these nodes in their current state, or you can modify them.
When you code a message processing node or an output node, the two types provide essentially the same functions. You can perform message processing within an output node, and likewise you can propagate an output message to a bit stream from a message processing node. For simplicity, this topic refers mainly to the node as a message processing node, but it does discuss the functionality of both types of node.
A Java user-defined node is distributed as a .jar file.
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Any class that implements MbNodeInterface, and is contained in the broker's LIL path, is registered with the broker as a message processing node. When you implement MbNodeInterface, you must also implement an evaluate method for this class. The evaluate method is called by the broker for each message that passes through the flow.
package com.ibm.jplugins; import com.ibm.broker.plugin.*; public class BasicNode extends MbNode implements MbNodeInterface
When the node is instantiated, the constructor of the user's node class is called. Create the terminals of the node, and initialize any default values for attributes in this constructor.
A message processing node has a number of input terminals and output terminals that are associated with it. Use the methods createInputTerminal and createOutputTerminal to add terminals to a node when the node is instantiated.
For example, to create a node with one input terminal and two output terminals:
public MyNode() throws MbException { // create terminals here createInputTerminal ("in"); createOutputTerminal ("out"); createOutputTerminal ("failure"); }
In many cases, the user-defined node needs to access the contents of the message received on its input terminal. The message is represented as a tree of syntax elements. Use the supplied utility function to evaluate methods for message management, message buffer access, syntax element navigation, and syntax element access.
The MbElement class provides the interface to the syntax elements. For further details of the Java API, see the Javadoc information.
For example:
The received input message is read-only, so before you can transform a message, you must write it to a new output message. You can copy elements from the input message, or you can create new elements in the output message.
The MbMessage class provides the copy constructors, and the methods to get the root element of the message. The MbElement class provides the interface to the syntax elements.
MbMessage newMsg = new MbMessage(assembly.getMessage()); MbMessageAssembly newAssembly = new MbMessageAssembly(assembly, newMsg);
MbElement rootElement = newAssembly.getMessage().getRootElement(); MbElement switchElement = rootElement.getFirstElementByPath("/XML/data/action");
String elementValue = (String)switchElement.getValue(); if(elementValue.equals("add")) switchElement.setValue("change"); else if(elementValue.equals("change")) switchElement.setValue("delete"); else if(elementValue.equals("delete")) switchElement.setValue("hold"); else switchElement.setValue("failure");
MbElement tag = switchElement.createElementAsLastChild(MbElement.TYPE_NAME, "PreviousValue", elementValue);
tag.createElementAsFirstChild(MbElement.TYPE_NAME_VALUE, "NewValue", switchElement.getValue()); MbOutputTerminal out = getOutputTerminal("out");
createElementAfter(String) createElementAsFirstChild(String) createElementAsLastChild(String) createElementBefore(String) createElementAsLastChildFromBitstream(byte[], String, String, String, String, int, int, int)
createElementAfter(int) createElementAfter(int, String, Object) createElementAsFirstChild(int) createElementAsFirstChild(int, String, Object) createElementAsLastChild(int) createElementAsLastChild(int, String, Object) createElementBefore(int) createElementBefore(int, String, Object)
Before you propagate a message, decide what message flow data you want to propagate, and whether to propagate to a node terminal, or to a Label node.
MbOutputTerminal out = getOutputTerminal("out"); out.propagate(newAssembly);
MbRoute label1 = getRoute ("label1"); Label1.propagate(newAssembly);
Call the clearMessage() function within the final try/catch block to clear the memory that is allocated for the message tree.
Declare the name using the following method. The name of the node must be the same as the one that is used in the workbench. All node names must end with "Node".
public static String getNodeName() { return "BasicNode"; }
package com.ibm.pluginsamples; public class BasicNode extends MbNode implements MbNodeInterface { ...
Declare node attributes in the same way as Java Bean properties. You must write getter and setter methods for the attributes. The API framework infers the attribute names using the Java Bean introspection rules. For example, if you declare the following two methods:
private String attributeVariable; public String getFirstAttribute() { return attributeVariable; } publc void setFirstAttribute(String value) { attributeVariable = value; }
the broker infers that this node has an attribute called firstAttribute. This name is derived from the names of the get or set methods, not from any internal class member variable names. Attributes can only be exposed as strings, therefore, you must convert any numeric types to and from strings in the get or set methods. For example, the following method defines an attribute called timeInSeconds:
int seconds; public String getTimeInSeconds() { return Integer.toString(seconds); } public void setTimeInSeconds(String value) { seconds = Integer.parseInt(value); }
The evaluate method, defined in MbNodeInterface, is called by the broker to process the message. All the processing function for the node is included in this method.
public void evaluate(MbMessageAssembly assembly, MbInputTerminal inTerm) throws MbException { // add message processing code here getOutputTerminal("out").propagate(assembly); }
The message flow data, which consists of the message, environment, local environment, and exception list, is received at the input terminal of the node.
public void onDelete() { // perform node cleanup if necessary }
This method is called by the broker immediately before it deletes the node.