Katartal

Experiments Kotlin DSL for JVM class generation

Examples

Hello world!

_class("Test") {
    _method("main", listOf("args" to Array<String>::class.java), PUBLIC + STATIC) {
        _code {
            _getstatic(System::class.java, "out", PrintStream::class.java)
            _ldc("Hello world!")
            _invokeVirtual(PrintStream::class.java, "println", "(Ljava/lang/String;)V")
            _return()
        }
    }
}

Record

Following record:

public record Point(int x, int y) { 
}

Can be created using following Katartal code:

_record("Point", listOf("x" to Int::class.java, "y" to Int::class.java))

Enum

Enum with simple values:

public enum EnumWithPlainValues {
    A, B, C, D;
}

Using Katartal DSL:

_enum("EnumWithPlainValues") {
    _value("A")
    _value("B")
    _value("C")
    _value("D")
}

However, if values have fields, implementation becomes trickier. For example:

public enum EnumWithFields {
    A(1000), B(10000), C(100000);
    
    private final int num;
    
    EnumWithFields(int num) {
        this.num = num;
    }

    public int getNum() {
        return num;
    }
}

We have to implement constructor and partial field initializer by ourselves:

_enum("EnumWithFields") {
    _value("A") {
        _loadIntOnStack(1000)
    }

    _value("B") {
        _loadIntOnStack(10000)
    }

    _value("C") {
        _loadIntOnStack(100000)
    }

    _field("num", Int::class.java, FieldAccess.PRIVATE)

    _method("getNum") {
        _code {
            _instruction(ALOAD_0)
            _instruction(GETFIELD) {
                _indexU2(constantPool.writeFieldRef(className, "num", "I"))
            }
            _instruction(IRETURN)
        }
    } returns Int::class.java

    _constructor(listOf("name" to String::class.java, "ordinal" to Int::class.java, "num" to Int::class.java)) {
        _code {
            _instruction(ALOAD_0)
            _instruction(ALOAD_1)
            _instruction(ILOAD_2)

            _instruction(INVOKESPECIAL) {
                _indexU2(constantPool.writeMethodRef("java/lang/Enum", "<init>", "(Ljava/lang/String;I)V"))
            }

            _instruction(ALOAD_0)
            _instruction(ILOAD_3)
            _instruction(PUTFIELD) {
                _indexU2(constantPool.writeFieldRef(className, "num", "I"))
            }

            _return()
        }
    }
}

FizzBuzz

Solution in Java:

public class Test {
    public static String[] fizzBuzz(int count) {
        String[] result = new String[count];

        for (int i = 1; i <= count; i++) {
            if (i % 3 == 0 && i % 5 == 0) {
                result[i - 1] = "FizzBuzz";
            } else if (i % 3 == 0) {
                result[i - 1] = "Fizz";
            } else if (i % 5 == 0) {
                result[i - 1] = "Buzz";
            }
        }

        return result;
    }
}

Similar solution with Katartal:

_class("Test") {
    _method("fizzBuzz", listOf("count" to Int::class.java), PUBLIC + STATIC) {
        _locals {
            _var("result", Array<String>::class.java, 5u, 66u)
            _var("i", Int::class.java, 7u, 62u)
        }

        _code(maxLocals = 3, maxStack = 3) {
            // String[] result = new String[count]
            _instruction(ILOAD_0)
            _instruction(ANEWARRAY) {
                _referenceU2(constantPool.writeClass("java/lang/String"))
            }
            _instruction(ASTORE_1)

            // i = 1
            _instruction(ICONST_1)
            _instruction(ISTORE_2)

            _stackFrame {
                _append(ObjectVar(Array<String>::class.java), IntegerVar())
            }

            val forLabel = label()
            _instruction(ILOAD_2) // i
            _instruction(ILOAD_0) // count

            // i < count
            _if(IF_ICMPGT) {
                // i % 3 
                _mathOperation(IREM, ILOAD_2, ICONST_3)
                _if(IFNE) {
                    // i % 5 
                    _mathOperation(IREM, ILOAD_2, ICONST_5)
                    _if(IFNE) {
                        // then
                        _instruction(ALOAD_1)
                        _mathOperation(ISUB, ILOAD_2, ICONST_1)
                        _ldc("FizzBuzz")
                        _instruction(AASTORE)

                        _goto(32)
                    }
                }

                _stackFrame {
                    _same()
                }

                // else if(i % 3) 
                _mathOperation(IREM, ILOAD_2, ICONST_3)
                _if(IFNE) {
                    _instruction(ALOAD_1)
                    _mathOperation(ISUB, ILOAD_2, ICONST_1)
                    _ldc("Fizz")
                    _instruction(AASTORE)

                    _goto(16)
                }

                _stackFrame {
                    _same()
                }

                // else if(i % 3) 
                _mathOperation(IREM, ILOAD_2, ICONST_5)
                _if(IFNE) {
                    _instruction(ALOAD_1)
                    _mathOperation(ISUB, ILOAD_2, ICONST_1)
                    _ldc("Buzz")
                    _instruction(AASTORE)
                }

                _stackFrame {
                    _same()
                }

                // i++
                _instruction(IINC) {
                    _index(2u)
                    _const(1)
                }

                _goto(forLabel)
            }

            _stackFrame {
                _chop(1)
            }

            _instruction(ALOAD_1)
            _instruction(ARETURN)
        }
    } returns Array<String>::class.java
}

Misc

_class("Test") {

    _constructor(listOf("name" to String::class.java)) { }
    
    _method(
        name = "equals",
        parameters = listOf("other" to Object::class.java),
        returns = Boolean::class.java
    ) {
        _return(true)
    } throws FileNotFoundException::class.java
    
} extends ArrayList::class.java implements Serializable::class.java