Extracting basic blocks information from Java Bytecodes
   
I was looking for some way to extract the basic block information from Java bytecodes, when I found the Javassist library. According to the Javassist website

"Javassist is a class library for dealing with Java bytecode."

Following is simple example, showing how to use it. Our objective here is to find basic block information in an method, in a given class file. A basic block can be described as a portion of code with a single entry and exit point. This implies that if the execution enters a basic block, all the instruction in that basic block would be executed.

I downloaded Javassist from the following website :
http://sourceforge.net/projects/jboss/files/Javassist/3.18.0-GA/

So first we need a class, which we wish to study. I used the following class file.

---

public class SampleCode {

    private int varA;
    private int varB;
    private int toggle;
    private int result;
   
    public SampleCode() {
        super();
        this.varA = 0;
        this.varB = 0;
        this.toggle = 0;
        this.result = -1;
    }

    public void setVarA(int varA) {
        this.varA = varA;
    }

    public void setVarB(int varB) {
        this.varB = varB;
    }

    public void setToggle(int toggle) {
        this.toggle = toggle;
    }
   
    public int calculate(){       
        if(toggle==1){
            result = varA * varA + varB;
        }else{
            result = varA + varB;
        }
        return result;
    }
}

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Now to extract the basic block, we can use the following class file

---

import javassist.CtClass;
import javassist.CtMethod;
import javassist.NotFoundException;
import javassist.bytecode.BadBytecode;
import javassist.bytecode.analysis.ControlFlow;
import javassist.bytecode.analysis.ControlFlow.Block;
import static javassist.ClassPool.getDefault;

public class BasicBlockExtractor {

    public static final String CLASSNAME = "SampleCode";
    public static final String METHODNAME_1 = "calculate";
    public static final String METHODNAME_2 = "setVarA";

    public static void main(String[] args) throws NotFoundException, BadBytecode {

        extractBasicBlocks();

    }
   
    public static void extractBasicBlocks() throws NotFoundException, BadBytecode{
        //Get the Class implementation byte code
           CtClass ctClass = getDefault().get(CLASSNAME);

           //Get the method from the Class byte code
           CtMethod ctmethod = ctClass.getDeclaredMethod(METHODNAME_1);
          
           ControlFlow cf = new ControlFlow(ctmethod);
           Block[] blocks = cf.basicBlocks();
          
           System.out.println("Number of basic blocks in class ' "+ CLASSNAME + " ' in method ' " + METHODNAME_1 + " ' is " + blocks.length);

          
    }
}

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You must add the Javassist jar file to you project, for this to work. Execution the above code gives the following output 

"Number of basic blocks in class ' SampleCode ' in method ' calculate ' is 4"

Subsequently, if you wish to obtain the line numbers for different basic blocks in a method you could do it as follows :-

---

import javassist.CannotCompileException;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.NotFoundException;
import javassist.bytecode.BadBytecode;
import javassist.bytecode.analysis.ControlFlow;
import javassist.bytecode.analysis.ControlFlow.Block;
import static javassist.ClassPool.getDefault;

public class BasicBlockExtractor{

    public static final String CLASSNAME = "SampleCode";
    public static final String METHODNAME_1 = "calculate";
    public static final String METHODNAME_2 = "setVarA";

    public static void main(String[] args) throws NotFoundException, BadBytecode, CannotCompileException {

        extractBasicBlocks();

    }
   
    public static void extractBasicBlocks() throws NotFoundException, BadBytecode, CannotCompileException{
        //Get the Class implementation byte code
           CtClass ctClass = getDefault().get(CLASSNAME);

           //Get the method from the Class byte code
           CtMethod ctmethod = ctClass.getDeclaredMethod(METHODNAME_1);
          
           ControlFlow cf = new ControlFlow(ctmethod);
           Block[] blocks = cf.basicBlocks();
          
           System.out.println("Number of basic blocks in class ' "+ CLASSNAME + " ' in method ' " + METHODNAME_1 + " ' is " + blocks.length);

           for(Block thisbb : blocks){
               int pos=ctmethod.getMethodInfo().getLineNumber(thisbb.position()); //Source code line position from binary line position
               System.out.println("Basic block "+thisbb.index() +" is at "+pos);
           }
    }
}

---

For me, the output of the above code was as follows :-

Number of basic blocks in class ' SampleCode ' in method ' calculate ' is 4
Basic block 0 is at 35
Basic block 1 is at 36
Basic block 2 is at 38
Basic block 3 is at 41