Fullexamples

Full Examples

Hello World

The good old:

class HW {
  public static void main(String [] args) {
    System.out.println("Hello world!");
  }
}

...can be generated as follows:

import cafebabe.ByteCodes._
import cafebabe.AbstractByteCodes._

val classFile = new cafebabe.ClassFile("HW", None)

classFile.setSourceFile("HW.java")

classFile.addDefaultConstructor

val codeHandler = classFile.addMainMethod.codeHandler
    
codeHandler << 
  GetStatic("java/lang/System", "out", "Ljava/io/PrintStream;") << 
  Ldc("Hello world!") <<
  InvokeVirtual("java/io/PrintStream", "println", "(Ljava/lang/String;)V") <<
  RETURN
    
codeHandler.freeze
classFile.writeToFile("./classfiles/HW.class")

Factorial

The following class:

class Factorial {
  public int fact(int value) {
    if(value <= 1) {
      return 1;
    } else {
      return value * this.fact(value - 1);
    }
  }
}

...can be generated this way:

import cafebabe._
import cafebabe.ByteCodes._
import cafebabe.AbstractByteCodes._

val classFile = new ClassFile("Factorial", None)
classFile.setSourceFile("Factorial.java")
classFile.addDefaultConstructor

val codeHandler = classFile.addMethod("I", "fact", "I").codeHandler

val labelName = codeHandler.getFreshLabel("else")

codeHandler << ILoad(1) << Ldc(1) << If_ICmpGt(labelName) << Ldc(1) <<
               IRETURN << Label(labelName) << ILoad(1) << ALoad(0) <<
               ILoad(1) << Ldc(1) << ISUB << InvokeVirtual("Factorial", "fact", "(I)I") <<
               IMUL << IRETURN

codeHandler.freeze

classFile.writeToFile("Factorial.class")

The bytecode corresponding to the method is:

 0:  iload_1
 1:  iconst_1
 2:  if_icmpgt   7
 5:  iconst_1
 6:  ireturn
 7:  iload_1
 8:  aload_0
 9:  iload_1
 10: iconst_1
 11: isub
 12: invokevirtual  #XX; //Method fact:(I)I
 15: imul
 16: ireturn

Showing that Booleans are Meaningless in Byte Code

object TestGen {
  import cafebabe._
  import AbstractByteCodes._
  import ByteCodes._

  def main(args : Array[String]) = {
    val classFile = new cafebabe.ClassFile("HW", None)
    
    classFile.addDefaultConstructor

    val testMethod = classFile.addMethod("Z", "test", "I", "Z").codeHandler
    testMethod <<
    ILoad(1) <<
    ILoad(2) <<
    Ldc(37) <<
    IADD <<
    IADD <<
    IRETURN
    testMethod.freeze

    val codeHandler = classFile.addMainMethod.codeHandler   
    codeHandler << 
    GetStatic("java/lang/System","out","Ljava/io/PrintStream;") <<
    DefaultNew("HW") <<
    Ldc(3) <<
    Ldc(2) <<
    InvokeVirtual("HW", "test", "(IZ)Z") <<
    InvokeVirtual("java/io/PrintStream","println","(I)V") <<
    RETURN
    codeHandler.freeze

    classFile.writeToFile("./classfiles/HW.class")
  }
}

generates

class HW extends java.lang.Object {
  public HW();
    Code:
    0:   aload_0
    1:   invokespecial   #9; //Method java/lang/Object."<init>":()V
    4:   return

  public boolean test(int, boolean);
    Code:
    0:   iload_1
    1:   iload_2
    2:   bipush  37
    4:   iadd
    5:   iadd
    6:   ireturn

  public static void main(java.lang.String[]);
    Code:
    0:   getstatic       #19; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:   new     #3; //class HW
    6:   dup
    7:   invokespecial   #20; //Method "<init>":()V
    10:  iconst_3
    11:  iconst_2
    12:  invokevirtual   #22; //Method test:(IZ)Z
    15:  invokevirtual   #28; //Method java/io/PrintStream.println:(I)V
    18:  return
}