Creating Custom Java Simple Functions

Creating Custom Java Simple Functions

Simple custom Java functions can be called in expressions in the context of most StreamBase components, other than the Aggregate operator. Follow these steps to create a custom Java simple function:

·         Use the New StreamBase Java Function wizard to create the base code, as described in Using the StreamBase Java Function Wizard.

·         Implement a public static in a public Java class.

·         Observe the guidelines in Method Parameter and Return Types.

For example, Hypotenuse.java in the sample application declares the following class and method:

public class Hypotenuse {

    public static double calculate(double a, double b) {

        return Math.sqrt(a*a + b*b);

    }

}

At compile time, the calljava() implementation looks for only a single method matching the exact types of the function call in the StreamBase expression. But there can be multiple matching methods, such as these two functionally equivalent ones:

public static boolean isZero(int i) { return i == 0; }

public static Boolean isZero(Integer i) {

    return i == null ? null : Boolean.valueOf(i.intValue() == 0);

}

If this case occurs, StreamBase throws an error.

Creating Custom Java Aggregate Functions

Follow these steps to create a custom Java aggregate function:

·         Use the New StreamBase Java Function wizard to create the base code, as described in Using the StreamBase Java Function Wizard.

·         Define a Java class that extends the com.streambase.sb.operator.AggregateWindow class.

·         Observe the guidelines in Method Parameter and Return Types.

·         Observe the guidelines in the annotations to the example below.

Consider the following annotated example:

package com.mycompany;

import com.streambase.sb.operator.AggregateWindow;

public class MyStdev extends AggregateWindow {                               

    private double sum;

    private double sumOfSquares;

    private int count;

    public void init() {                                                      

        sum = sumOfSquares = 0.0;

        count = 0;

    }         

    public double calculate() {                                              

        return Math.sqrt((count * sumOfSquares - sum*sum) / count*(count-1));

    }

    public void accumulate(double value) {                                   

        sum += value;

        sumOfSquares += value*value;

        ++count;

    }

    public void release() { /* nothing to release in this example */ }       

}

The following annotations describe points of interest in the preceding example:

	Declare a public class that extends the AggregateWindow class (as documented in the StreamBase Java Client library).
	The init() method is called at the start of each new use of the class. Since custom aggregate objects are likely to be reused, perform all initialization ininit() rather than in the constructor. (The constructor is called only once, while init() is called before each use.)
	Your implementation must contain a calculate() method that takes no arguments and returns a value that is convertible to a StreamBase data type. The calculate() method may be called several times, or not at all.
	Your implementation must provide at least one accumulate() method, and can optionally provide several overloaded accumulate() methods, one per data type. calljava() determines which one to call based on type. The argument types for accumulate() and the return type for calculate() can be any of the types described in the table in the next section.
	The release() method is called at the end of each use of the class.

Method Parameter and Return Types

The method can have any number of parameters, including none. Each parameter must be a Java primitive or object type corresponding to a StreamBase data type as shown in the following table:

StreamBase Data Type	Java Primitive	Java Object
blob	—	com.streambase.sb.ByteArrayView
bool	boolean	java.lang.Boolean
double	double	java.lang.Double
int	int	java.lang.Integer
long	long	java.lang.Long
list	—	java.util.List
string	byte[]	java.lang.String
timestamp	—	com.streambase.sb.Timestamp
tuple	—	com.streambase.sb.Tuple

Notes

·     For simple functions, the return type cannot be void, and must be one of the Java primitive or Java Object types shown above.

·     You can use java.lang.String anywhere a byte[] is acceptable as an argument or return value. In this case, the StreamBase string is transparently converted to or from a java.lang.String using the system default encoding.

·     If a parameter's type is byte[] and its value is null, it is represented as a Java null. Likewise, if a Java method with a byte[] return type returns a null, the calling StreamBase expression will see the return value as string(null).

·     If the parameter or return type is list or tuple, you must either provide a custom function resolver, or must define the argument types in acustom-function element in the server configuration file. See Custom Functions with Complex Data Types for details.

·     If any value of a parameter with a primitive type is null at runtime, the method that implements the custom function is not invoked. However, Java Object parameter types can be used to pass in null parameter values.

For example, if a StreamBase custom function call would involve converting a StreamBase int(null) or bool(null) value to a primitive Java int or boolean, the method is not called, and null is assumed as the return value.

public static boolean isZero(int i) { return i == 0; }

calljava("TheClass", "isZero", 1)          /* false *

calljava("TheClass", "isZero", 0)          /* true */

calljava("TheClass", "isZero", int(null))  /* null */