Setup

Install Maven by following the directions from Apache or using your favored package installer on Mac or Linux.

Build the code in this repo using

cd inst-plugin && mvn clean install
cd ../sample-programs && mvn clean compile
cd ..

The code in sample-programs and the goals package in inst-plugin is for running and testing purposes - you can and should look at this source code, but you won't need to modify it. The code to be modified is in the instrument package of inst-plugin.

Bytecode

Java compiles into JVM bytecode in .class files, and these (not the original .java files) are run on the JVM when a Java program is run.

To see what that bytecode actually looks like, we can use javap -c, which gives the bytecode stored in .class files. For example, in the sample-programs/target/classes directory, try running:

javap -c HelloWorld

It should print the following:

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

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String Hello, World!
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

The first method, HelloWorld(), is the constructor, which all Java objects have. It's recognizable by its invocation of <init>.

Of more interest is the second method, main(String[]), which is actually defined in the program in HelloWorld.java as a single invocation of the method System.out.println with the string Hello, World!. The bytecode for this method first gets the object System.out with getstatic, then loads the constant string Hello, World! with ldc, and finally invokes the println method from System.out before returning.

We can tell what constants are loaded and stored from the comments printed alongside the bytecode instructions. These are actually retrieved from the numbers next to the instructions, which make references to the constant pool (visible by running javap -v HelloWorld).

Not only does javap show us the general form of Java bytecode, it gives us the specific instructions we need in order to print something, which we'll use in the next section.

API: org.objectweb.asm

For manipulating bytecode, we'll be using the org.objectweb.asm package from ow2. It's built on a visitor pattern, providing the ability to change what the program does when processing each bytecode instruction. For an example of how this works, see ExploratoryClassVisitor in the inst-plugin folder, which overrides several methods of MethodVisitor. The ExploratoryClassVisitor will be helpful for this recitation's exercises by printing out what the arguments to each of these methods we want to use or override should look like.

To run it on a sample program (HelloWorld or SelectionSort), run

mvn recitation-10:explore -Dclass="HelloWorld"

or

mvn recitation-10:explore -Dclass="SelectionSort"

while in the sample-programs directory.

Challenge 1: Coverage Instrumentation

We're going to instrument bytecode in order to get code coverage information. The code we're going to change for this section is in CoverageClassVisitor.

To get line number information, we're going to override the ClassVisitor's method for visiting a line number by having it additionally execute the instructions that we saw print a string with javap:

• getstatic for System.out
• ldc the string we want to load (here, the line number)
• invokevirtual for println

When you think you've correctly overridden the method, build it by running

mvn clean install

in the inst-plugin folder and test it by running

mvn recitation-10:coverage -Dclass="HelloWorld"
mvn compile exec:java -Dexec.mainClass="HelloWorld"

in the sample-programs folder.

Challenge 2: Program Repair

Our next challenge is to change the program behavior itself. Try running the other sample program, an implementation of Selection Sort, using:

mvn compile exec:java -Dexec.mainClass="SelectionSort" -Dexec.args="3 2 1"

in the sample-programs folder. We want it to sort the output, printing [1, 2, 3], but, instead, it throws an ArrayIndexOutOfBoundsException! There's a bug in this sort method (available to view in SelectionSort.java). We could look for it to fix it manually, but, for this exercise, we're going to try instrumenting the code instead.

Through magic intuition, we know that the problem will be fixed by changing the subtractions in the program to additions. Try to do this by changing visitInsn in ArithmeticClassVisitor.

When you think you've correctly overridden the method, build it by running

mvn clean install

in the inst-plugin folder and test it by running

mvn recitation-10:repair -Dclass="SelectionSort"
mvn compile exec:java -Dexec.mainClass="SelectionSort" -Dexec.args="3 2 1"

in the sample-programs folder.

Resources

ASM Javadoc