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Peeking Inside the Box: Attribute-Oriented Programming with Java 1.5, Part 1
Pages: 1, 2

Instrumenting Bytecode

Rather than instrumenting source code and then compiling it, I could compile the original source code and then instrument the bytecode that is produced. Depending on the exact transformation required, this can be either much easier or much more difficult than source code instrumentation. The main advantage of bytecode instrumentation is that the code can be modified at run time, without having a compiler available.

Although Java's bytecode format is relatively simple, I will certainly want to use a Java library to do the parsing and generation of the bytecode (e.g., to insulate us from future changes in the Java class file format). I chose to use Jakarta's Byte Code Engineering Library (BCEL), but I could just as easily have picked CGLIB, ASM, or SERP.

Since I will be instrumenting bytecode in a number of different ways, I'll begin by declaring a generic interface for instrumentation. This will act as a simple framework for doing annotation-based instrumentation. This framework will support the transformation of classes and methods, based on annotations, so the interface will look something like this:

public interface Instrumentor
    public void instrumentClass (ClassGen classGen,
                                Annotation a);
    public void instrumentMethod (ClassGen classGen,
                                MethodGen methodGen,
                                Annotation a);

ClassGen and MethodGen are BCEL classes that implement the Builder pattern. That is, they provide methods for mutating an otherwise immutable object, and for transforming between the mutable and non-mutable representations.

Now I will need to write an implementation for this interface that replaces @Status annotations with the appropriate calls to StatusManager. As described previously, I want to wrap these calls in a try-finally block. Note that for this to work, the annotations that are used must be tagged with @Retention(RetentionPolicy.CLASS), which instructs the Java compiler not to discard the annotations while compiling. Since I declared @Status as @Retention(RetentionPolicy.SOURCE) earlier, I need to upgrade it.

As it turns out, in this case, instrumenting bytecode is significantly more difficult than instrumenting source code. The reason is that try-finally is a concept that exists in source code only! The Java compiler transforms try-finally blocks into a series of try-catch blocks and inserts calls to the finally block before every return. Thus, I will need to do something similar in order to add a try-finally block to existing bytecode.

This is the bytecode that represents an ordinary method call, flanked by StatusManager updates.

0:  ldc	#2; //String message
2:  invokestatic #3; //Method StatusManager.push:(LString;)V
5:  invokestatic #4; //Method doSomething:()V
8:  invokestatic #5; //Method StatusManager.pop:()V
11: return

This is the same method call, but in a try-finally block, so that StatusManager.pop() is called even if an exception is thrown.

0:  ldc	#2; //String message
2:  invokestatic #3; //Method StatusManager.push:(LString;)V
5:  invokestatic #4; //Method doSomething:()V
8:  invokestatic #5; //Method StatusManager.pop:()V
11: goto 20
14: astore_0
15: invokestatic #5; //Method StatusManager.pop:()V
18: aload_0
19: athrow
20: return

Exception table:
from   to  target type
 5     8    14   any
14    15    14   any

As you can see, I need to duplicate some instructions and add several jumps and exception table records just to implement a single try-finally. Luckily, BCEL's InstructionList class makes this fairly easy.

Instrumenting Bytecode At Compile Time

Now that I have an interface for modifying classes based on annotations and a concrete implementation of this interface, the last step is to write the actual framework that will call it. I'm actually going to write a few of these frameworks, starting with one that instruments all classes at a compile time. Since this is going to happen as part of my build process, I've decided to define an Ant task for it. The declaration of the instrumentation target in my build.xml file should look something like this:

<instrument class="com.pkg.OurInstrumentor">
  <fileset dir="$(classes.dir)">
    <include name="**/*.class"/>

To provide an implementation of this task, I need to define a class that realizes the org.apache.tools.ant.Task interface. Attributes and sub-elements of our task are passed in through set and add method calls. The execute method is called to implement the work of the task -- in this case, to instrument the class files given in the specified <fileset>.

public class InstrumentTask extends Task {

    public void setClass (String className) { ... }
    public void addFileSet (FileSet fileSet) { ... }

    public void execute () throws BuildException {
        Instrumentor inst = getInstrumentor();

        try {
            DirectoryScanner ds =

            // Java 1.5 "for" syntax
            for (String file : ds.getIncludedFiles()) {
                instrumentFile(inst, file);
        } catch (Exception ex) {
            throw new BuildException(ex);


The one problem with using BCEL for this purpose is that as of version 5.1, it does not support parsing annotations. I could load the classes that we're instrumenting and use reflection to view the annotations. However, then I would have had to use RetentionPolicy.RUNTIME instead of RetentionPolicy.CLASS. I'd also be executing any static initializers in those classes, which may load native libraries or introduce other dependencies. Luckily, BCEL provides a plugin mechanism that allows clients to parse bytecode attributes that it does not natively support. I've written my own implementation of the AttributeReader interface that knows how to parse the RuntimeVisibleAnnotations and RuntimeInvisibleAnnotations attributes that are inserted into bytecode when annotations are present. Future versions of BCEL should include this functionality without the need for a plugin.

This compile time bytecode instrumentation approach is shown in the directory code/02_compiletime of the sample code.

There are a number of disadvantages to this approach, however. For one thing, I had to add an additional step to my build process. I also cannot turn the instrumentation on or off based on command-line settings or other information that is not available at compile time. If both instrumented and non-instrumented code needs to be run in a production setting, two separate .jars will need to be built and the decision of which to use must be all or nothing.

Instrumenting Bytecode at Class Loading Time

A better solution would be to delay the instrumentation of our bytecode until after it is loaded from the disk. This way, the instrumented bytecode does not need to be stored. The start-up performance of our application may suffer, but the trade-off is that you can control what happens based on system properties, or other runtime configuration data.

Prior to Java 1.5, it was possible to do this type of class-file manipulation with a custom class loader. However, the new java.lang.instrument package added in Java 1.5 provides a few additional tools. In particular, it defines the concept of a ClassFileTransformer, which can be used to instrument a class during the standard loading process.

To register our ClassFileTransformer at the appropriate time (before any of our classes are loaded), I'll need to define a premain method. Java will call this before the main class is even loaded, and it is passed a reference to an Instrumentation object. I will also need to add a -javaagent option to the command line to tell Java about our premain method with Java. This argument takes the full name of your agent class (which contains the premain method) and an arbitrary string argument. In this case, we'll pass the full name of our Instrumentor class as the argument (this should all be on one line):


Now that I've arranged for a callback that occurs before any annotated classes are loaded, and I have a reference to the Instrumentation object, I can register our ClassFileTransformer.

public static void premain (String className,
                            Instrumentation i)
    throws ClassNotFoundException,
    Class instClass = Class.forName(className);
    Instrumentor inst = (Instrumentor)instClass.newInstance();
    i.addTransformer(new InstrumentorAdaptor(inst));

The adaptor that is registered will need to act as a bridge between the Instrumentor interface given above and Java's ClassFileTransformer interface.

public class InstrumentorAdaptor
    implements ClassFileTransformer
    public byte[] transform (ClassLoader cl,
                             String className,
                             Class classBeingRedefined,
                             ProtectionDomain protectionDomain,
                             byte[] classfileBuffer)
        try {
            ClassParser cp =
                new ClassParser(
                    new ByteArrayInputStream(classfileBuffer),
                    className + ".java");
            JavaClass jc = cp.parse();

            ClassGen cg = new ClassGen(jc);

            for (Annotation an :
                 getAnnotations(jc.getAttributes())) {
                instrumentor.instrumentClass(cg, an);

            for (org.apache.bcel.classfile.Method m :
                 cg.getMethods()) {
                for (Annotation an :
                     getAnnotations(m.getAttributes())) {
                    ConstantPoolGen cpg =
                    MethodGen mg =
                        new MethodGen(m, className, cpg);
                    instrumentor.instrumentMethod(cg, mg, an);


                    cg.replaceMethod(m, mg.getMethod());

            JavaClass jcNew = cg.getJavaClass();
            return jcNew.getBytes();
        } catch (Exception ex) {
            throw new RuntimeException("instrumenting " +
                className, ex);


This approach of instrumenting bytecode at startup is shown in the example code's /code/03_startup directory.


In this article, I have replaced a hard-coded solution with one that uses metaprogramming based on annotations and instrumentation. Although I've eliminated the need for any extra steps in our build process, my solution still has a number of limitations. In the next installment, I will explore a completely different implementation that uses thread sampling, and then combine these two techniques to create a solution that gives the best features of each. I will also discuss a number of additional requirements, including a progress bar and dynamic status messages.


Don Schwarz is a Java developer for a large investment bank who specializes in metaprogramming and language integration.

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