Extending the capability of a Java message processing or output node

Before you start

Ensure that you have read and understood the following topic:
After you have created a user-defined node, the following functions are available:
  1. Accessing ESQL
  2. Handling exceptions
  3. Writing to an output device

Accessing ESQL

Nodes can invoke ESQL expressions using Compute node ESQL syntax. You can create and modify the components of the message using ESQL expressions, and you can refer to elements of both the input message and data from an external database.

The following procedure demonstrates how to control transactions from the evaluate method in your user-defined node using ESQL:
  1. Set the name of the ODBC data source to use. For example:
    String dataSourceName = "myDataSource";
  2. Set the ESQL statement to execute:
    String statement = 
       "SET OutputRoot.XML.data = 
              (SELECT Field2 FROM Database.Table1 WHERE Field1 = 1);";
    Or, if you want to execute a statement that returns no result:
    String statement = "PASSTHRU(
                            'INSERT INTO Database.Table1 VALUES(
                                 InputRoot.XML.DataField1,
                                 InputRoot.XML.DataField2)');";
  3. Select the type of transaction you want from the following:
    MbSQLStatement.SQL_TRANSACTION_COMMIT
    Immediately commit the transaction upon execution of the ESQL statement.
    MbSQLStatement.SQL_TRANSACTION_AUTO
    Commit the transaction when the message flow has completed. (Rollbacks are performed if necessary.)
    For example:
    int transactionType = MbSQLStatement.SQL_TRANSACTION_AUTO;
  4. Get the ESQL statement. For example:
    MbSQLStatement sql = 
           createSQLStatement(dataSourceName, statement, transactionType);
    You can use the method createSQLStatement(dataSource, statement) to default the transaction type to MbSQLStatement.SQL_TRANSACTION_AUTO).
  5. Create the new message assembly to be propagated:
    MbMessageAssembly newAssembly = 
           new MbMessageAssembly(assembly, assembly.getMessage());
  6. Execute the ESQL statement:
    sql.select(assembly, newAssembly);
    Or, if you want to execute an ESQL statement that returns no result:
    sql.execute(assembly);

For more information about ESQL, see ESQL overview.

Handling exceptions

You use the MbException class to catch and access exceptions. The MbException class returns an array of exception objects representing the children of an exception in the broker exception list. Each element returned specifies its exception type. An empty array is returned if an exception has no children. The following code sample shows an example of how the MbException class could be used in the evaluate method of your user-defined node.

public void evaluate(MbMessageAssembly assembly, MbInputTerminal inTerm) throws MbException
  {
    try
      {

        // plug-in functionality

      }
    catch(MbException ex)
      {
        traverse(ex, 0);

        throw ex; // if re-throwing, it must be the original exception that was caught
      }
  }

  void traverse(MbException ex, int level)
  {
    if(ex != null)
      {
        // Do whatever action here
        System.out.println("Level: " + level);
        System.out.println(ex.toString());
        System.out.println("traceText:  " + ex.getTraceText());

        // traverse the hierarchy
        MbException e[] = ex.getNestedExceptions();
        int size = e.length;
        for(int i = 0; i < size; i++)
          {
            traverse(e[i], level + 1);
          }
      }
  }

Refer to the javadoc for more details of using the MbException class.

You can develop a user-defined message processing or output node in such a way that it can access all current exceptions. For example, to catch database exceptions you can use the MbSQLStatement class. This class sets the value of the 'throwExceptionOnDatabaseError' attribute, which determines broker behavior when it encounters a database error. When it is set to true, if an exception is thrown, it can be caught and handled by the evaluate method in your user-defined extension.

The following code sample shows an example of how to use the MbSQLStatement class.

public void evaluate(MbMessageAssembly assembly, MbInputTerminal inTerm) throws MbException
  {
    MbMessage newMsg = new MbMessage(assembly.getMessage());
    MbMessageAssembly newAssembly = new MbMessageAssembly(assembly, newMsg);

    String table = 
       assembly.getMessage().getRootElement().getLastChild().getFirstChild().getName();

    MbSQLStatement state = createSQLStatement( "dbName", 
       "SET OutputRoot.XML.integer[] = PASSTHRU('SELECT * FROM " + table + "');" );

    state.setThrowExceptionOnDatabaseError(false);
    state.setTreatWarningsAsErrors(true);

    state.select( assembly, newAssembly );

    int sqlCode = state.getSQLCode();
    if(sqlCode != 0)
      {
        // Do error handling here

        System.out.println("sqlCode = " + sqlCode);
        System.out.println("sqlNativeError = " + state.getSQLNativeError());
        System.out.println("sqlState = " + state.getSQLState());
        System.out.println("sqlErrorText = " + state.getSQLErrorText());
      }

    getOutputTerminal("out").propagate(newAssembly);
  }

Writing to an output device

To write to an output device, the logical (hierarchical) message needs to be converted back into a bitstream in your evaluate method. You do this using the getBuffer method in MbMessage, as follows:

public void evaluate( MbMessageAssembly assembly, MbInputTerminal in)
                                                     throws MbException
{
  MbMessage msg = assembly.getMessage();
  byte[] bitstream = msg.getBuffer();

  // write the bitstream out somewhere
  writeBitstream( bitstream );   // user method

 }

Typically, for an output node the message is not propagated to any output terminal, so you can just return at this point.

Note: You must use the supplied MQOutput node when writing to WebSphere MQ queues, because the broker internally maintains a WebSphere MQ connection and open queue handles on a thread-by-thread basis, and these are cached to optimize performance. In addition, the broker handles recovery scenarios when certain WebSphere MQ events occur, and this would be adversely affected if WebSphere MQ MQI calls were used in a user-defined output node.
Related reference
Exception list structure
Related information
Java user-defined node API