Swan Lake Preview 3

Overview of Ballerina Swan Lake Preview 3

This release is the third preview version of Ballerina Swan Lake. This release includes a new set of language features along with improvements and bug fixes to the compiler, runtime, standard libraries, and developer tooling.

You can use the update tool to update to Ballerina Swan Lake Preview 3 as follows.

For existing users:

If you are already using jBallerina, you can directly update your distribution to the Swan Lake channel using the Ballerina update tool. To do this, first, execute the command below to get the update tool updated to its latest version.

ballerina update

Next, execute the command below to update to Swan Lake Preview 3.

ballerina dist pull slp3

However, if you are using a jBallerina version below 1.1.0, install via the installers.

For new users:

If you have not installed jBallerina, then download the installers to install.


  • Support for defining external object method bodies
  • Order by clause for sorting
  • Inner/Nested query expressions
  • Support for executing stored procedures in the SQL connector
  • Azure Functions support

What’s new in Ballerina Swan Lake Preview 3


The language implementation is based on Ballerina Language Specifications Draft 2020-06-18.

External object method bodies

This release introduces support for defining object methods with external function bodies.

type Person object {
    string fname;
    string lname;

    function init(string fname, string lname) {
        self.lname = lname;
        self.fname = fname;

    function getFullName() returns string = @java:Method {
        class: "abc.Hello"
    } external;

The Java method to which the getFullName() method is bound:

public static BString getFullName(ObjectValue objectValue) {
        return objectValue.getStringValue(new BmpStringValue("fname")).concat(new BmpStringValue(" ")).concat(
                    objectValue.getStringValue(new BmpStringValue("lname")));
Order by clause for sorting

This release introduces the order by clause support for sorting in query expression/action. An order-by clause is executed by constructing a list of entries.

Student s1 = {id: 1, fname: "John", fee: 2000.56, age: 20};
Student s2 = {id: 2, fname: "John", fee: 2000.56, age: 22};
Student s3 = {id: 3, fname: "Roger", fee: 4000.56, age: 21};
Student s4 = {id: 4, fname: "Kate", fee: 2000.56, age: 24};

Student[] studentList = [s1, s2, s3, s4];

Student[] sortedList =  from var student in studentList
                        order by student.age ascending, student.fname
                        select student;
Inner/Nested query expressions

This release introduces the support to write inner/nested query expression/action.

Person[] personList = [
    {id: 1, fname: "Alex", lname: "George"},
    {id: 2, fname: "Ranjan", lname: "Fonseka"},
    {id: 3, fname: "Idris", lname: "Elba"},
    {id: 4, fname: "Dermot", lname: "Crowley"}

Department[] deptList = [
    {id: 1, name:"HR"},
    {id: 2, name:"Operations"},
    {id: 3, name:"Engineering"}

Employee[] empList = [
    {personId: 1, deptId: 2},
    {personId: 2, deptId: 1},
    {personId: 3, deptId: 3},
    {personId: 4, deptId: 3}

DeptPerson[] deptPersonList =
        from var emp in (from var e in empList select e)
        join Person psn in (from var p in personList select p)
            on emp.personId equals psn.id
        join Department dept in (from var d in deptList select d)
            on emp.deptId equals dept.id
        select {
            fname : psn.fname,
            lname : psn.lname,
            dept : dept.name

Standard library

SQL connectors

Stored procedures can now be executed through SQL connectors (JDBC & MySQL).

int uid = 10;
sql:OutParameter insertId = new;

var ret = dbClient->call(`call InsertPerson(${uid}, ${insertId})`);
if (ret is error) {
    io:println("Error occurred:", err.message());
} else {
    io:println("Out Parameter insert id: ", insertId.get(int));
    stream<record{}, sql:Error>? resultStr = ret.queryResult;
    if (!(resultStr is ())) {
        sql:Error? e = resultStr.forEach(function(record{} result) {
        io:println("Full Customer details: ", result);
    } else {
        io:println("Stored  procedure does not return anything.");

Module organization

The ballerina/nats library was moved to Ballerina Central. Previously, this module was packed in the Ballerina distribution. With this change, this library can now be released independently.

Developer tools

Language server
Introducing AI-based Data Mapping code action

Two record types can now be mapped automatically using the Data Mapping code action. Once a possible record mapping instance is identified, it suggests a mapping based on an AI algorithm. A mapping function will be generated automatically and added to the workspace to perform the record mapping.

The following is a sample in which the code action to generate a mapping function will appear when attempting to assign a mapping value to a variable of a type that is not directly assignable.

type Grade record {|
   int maths;
   int physics;
   int chemistry;
type NameAndGrade record {|
   string name;
   string surname;
   int maths;
   int physics;
   int chemistry;
public function main() {
   NameAndGrade student = {
	name: "Kamal",
	surname: “Perera”,
	maths: 90,
physics: 99,
chemistry: 95
   Grade grades = student;

By choosing the Generate mapping functioncode action, the following function will be added to the workspace.

function mapNameAndGradeToGrade(NameAndGrade nameAndGrade) returns Grade {
// Some record fields might be missing in the AI-based mapping.
   Grade grade = {
maths: nameAndGrade.maths, 
physics: nameAndGrade.physics, 
chemistry: nameAndGrade.chemistry};
   return grade;

Furthermore, the line with the error would be replaced with a function call as shown below.

Grade grades = mapNameAndGradeToGrade(student);

For more information, see Code Actions.

Test framework
Support single test execution

A single test function or a set of functions can now be executed using the --tests flag as follows.

$ ballerina test --tests <test_function> --all
API change in assertEquals and assertNotEquals functions

Deep value equality is supported only for anydata-typed values according to the language specification. The assertEquals function has been changed to accept only anydata-typed values to reflect this behavior.

Introduction of the assertExactEquals and assertNotExactEquals functions

The assertExactEquals function compares two values to assert whether they refer to the same entity (i.e., they are exactly equal).

import ballerina/test;

type Person object {
    public string name = "";
    public int age = 0;
    public Person? parent = ();
    private string email = "default@abc.com";
    string address = "No 20, Palm grove";

@test:Config {}
function testAssertObjectEquals() {
   Person p1 = new;
   Person p2 = p1;
   test:assertExactEquals(p1, p2);

@test:Config {}
function testAssertObjectNotEquals() {
    Person p1 = new;
    Person p2 = new ();
    test:assertNotExactEquals(p1, p2);

Introduction of the @test:BeforeGroups and @test:AfterGroups functions

These two new annotations can now be used when writing tests with the Ballerina test framework.

import ballerina/io;
import ballerina/test;

@test:BeforeGroups { value : ["group1"] }
function beforeGroupsFunc() {
	io:println(“I’m a before groups function!”)

@test:Config {}
function testFunction() {
	io:println(“I’m a test function!”)


@test:AfterGroups { value : ["group1"] }
function afterGroupsFunc() {
	io:println(“I’m a after groups function!”)

Introduction of the alwaysRun field to the @test:AfterSuite annotation

You can now specify alwaysRun : true|false in the @AfterSuite annotation, which enables running the @AfterSuite even if the @BeforeSuite function fails during the test execution. The default value is false.

import ballerina/io;
import ballerina/test;

function beforeSuiteFunc() {
	io:println("I’m the before suite function");
	int a = 2/0;

@test:AfterSuite {}
function afterSuiteFunc1() {
	io:println("I will be run only if before suite function executes successfully.");

@test:AfterSuite { alwaysRun:true }
function afterSuiteFunc2() {
	io:println("I will be run even if the before suite function fails.");

Code to Cloud

Azure Functions support

Ballerina now supports writing serverless functions using the Azure Functions framework.

import ballerina/http;
import ballerinax/azure.functions as af;

public function fromHttpToQueue(af:Context ctx,
        	@af:HTTPTrigger {} af:HTTPRequest req,
        	@af:QueueOutput { queueName: "queue1" } af:StringOutputBinding msg)
        	returns @af:HTTPOutput af:HTTPBinding {
	msg.value = req.body;
	return { statusCode: 200, payload: "Request: " + req.toString() };

For more information, see Azure Functions.