Bci for Beginners

© 2013 IBM Corporation
Iain Lewis
30 Sept 2013
BCI for Beginners

Important Disclaimers
THE INFORMATION CONTAINED IN THIS PRESENTATION IS PROVIDED FOR INFORMATIONAL PURPOSES ONLY.
WHILST EFFORTS WERE MADE TO VERIFY THE COMPLETENESS AND ACCURACY OF THE INFORMATION
CONTAINED IN THIS PRESENTATION, IT IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED.
ALL PERFORMANCE DATA INCLUDED IN THIS PRESENTATION HAVE BEEN GATHERED IN A CONTROLLED
ENVIRONMENT. YOUR OWN TEST RESULTS MAY VARY BASED ON HARDWARE, SOFTWARE OR
INFRASTRUCTURE DIFFERENCES.
ALL DATA INCLUDED IN THIS PRESENTATION ARE MEANT TO BE USED ONLY AS A GUIDE.
IN ADDITION, THE INFORMATION CONTAINED IN THIS PRESENTATION IS BASED ON IBM’S CURRENT PRODUCT
PLANS AND STRATEGY, WHICH ARE SUBJECT TO CHANGE BY IBM, WITHOUT NOTICE.
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- CREATING ANY WARRANT OR REPRESENTATION FROM IBM, ITS AFFILIATED COMPANIES OR ITS OR THEIR
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2

About me
 Iain Lewis
 QA Engineer, IBM Java Technology Center,
IBM Hursley, U.K.
 13 years experience developing and
deploying Java SDKs

 Contact info
– iain_lewis@uk.ibm.com
 Visit the IBM booth #5112 and meet other
IBM developers at JavaOne 2013
3

Agenda
 Who am I, and why am I here
 What is BCI?
 BCI and the JVM
 Practical BCI

Who am I?
 Test IBM's Java SDK
– I break Java
 Focus on system testing in QA
– Multi-threaded load testing
– Third-party applications
 System testing the Java SDK means writing Java
 Can the VM handle any application or Java code?
 Recently wrote some BCI based testing

Why are you here?
 Have heard of BCI
 Would like to know more about
– What BCI is
– What BCI can do
 Would like practical help on
– How to get started
– Where to get help
– What tools are available
–


What is BCI?
 Bytecode Instrumentation is modifying a Java class without the source
 Can be done offline, at load time, or at any point afterwards, and multiple times
 Application and most SDK classes can be changed

Class
File
BCI
Modified
Class
JVM
Disk/Network Runtime

Why BCI?
 Support for BCI at runtime is built into the Java SDK
 Java bytecodes are well documented and easy to work with
 BCI is supported by many tools
 BCI is widely used

What can you do with BCI?
 Offline/at load time
– Replace the entire class with a different one!
– Add/Remove fields and methods
 At any time
– Modify the logic of existing methods
– Insert new logic into existing methods
 Allows you to do
– Profiling/monitoring
– Code coverage
– Code generation
– Runtime logging


What can you do with BCI? (cont)
public void service(ServletRequest req, ServletResponse res)
throws ServletException, java.io.IOException
{
// insert new logic here
// existing method logic
}

Why BCI?
 Doesn't have to be compiled in to a class
 Doesn't need access to the source
 Can be turned on and off
 Can be applied to an arbitrary application
 When turned off, has no performance impact

How do you do it? JVM interface
 BCI is supported at the JVM level
 Supported via 'agents' and the Instrumentation interface
 java -javaagent:/path/to/agent.jar[=options] MyClass
 Native agents also possible via JVMTI


How do you do it? JVM interface (cont.)
import java.lang.instrument.Instrumentation;
public class MyJavaAgent {
public static void premain(String agentArgument,
Instrumentation instrumentation)
{}
}
Manifest-Version: 1.0
Premain-Class: MyJavaAgent
Can-Retransform-Classes: true

How do you do it? JVM interface (cont.)
 Allows your agent to:
– Register as a ClassFileTransformer
– Transformers get a call back when a class is loaded, in order to transform it
 Can also trigger retransformations via retransformClasses()
•
 Documentation of Instrumentation :
– http://docs.oracle.com/javase/7/docs/api/index.html?java/lang/instrument/Instrumentation.htm
void addTransformer(ClassFileTransformer transformer)

How do you do it? JVM Interface (cont)
 A class is an array of bytes
 A transformer returns a modified copy of the byte array
Class byte array
from disk Transformer 1
Copied/Modified
byte array Transformer 2
Final
byte array
public static byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer)
{}

How do you do it? Modifying Bytes (the hard way)
 Read the Java Virtual Machine Specification
– http://docs.oracle.com/javase/specs/jvms/se7/html/
 Parse the class byte array
– http://docs.oracle.com/javase/7/docs/api/java/io/DataInputStream.html
 Return a modified version

How do you do it? Modifying Bytes (the hard way)
DataInputStream is = new DataInputStream(new
ByteArrayInputStream(classFileBuffer))

int magic = is.readInt();
System.out.println("Magic number is " + magic); // 0xCAFEBABE

int minor = is.readShort();
System.out.println("minor number is " + minor); // 0 for Java 7

int major = is.readShort();
System.out.println("major number is " + major); // 51 for Java 7

How do you do it? Modifying Bytes (the easier way)
 Use a framework
– e.g. ASM http://asm.ow2.org
 Use javac/javap
 Use a bytecode plugin for your IDE
– Bytecode outline plugin for Eclipse http://andrei.gmxhome.de/bytecode/index.html
 Write as much of your code in Java as possible

Using a Framework (ASM)
 Provides an abstraction layer to hide the details of a class
 Easier to learn (provides symbolic constants, helper APIs)
 Widely used
 Under active development

 Is an additional dependency
 Doesn't tell you what bytecodes to create!

A transformer in 3 classes and 3 methods – class 1
/** Implementation of transform() in ClassFileTransformer */
public byte[]transform(...) throws IllegalClassFormatException
{
ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
ClassAdapter1 adapter = new ClassAdapter1(writer);
ClassReader cr = new ClassReader(classfileBuffer);
cr.accept(adapter, ClassReader.SKIP_FRAMES);
return writer.toByteArray();
}

/** Override of visitMethod in ClassVisitor */
public MethodVisitor visitMethod(...) {
MethodVisitor mv = cv.visitMethod(...);
return new MethodAdapter1(mv);
}

/** Override of visitCode in MethodVisitor */
public void visitCode() {
mv.visitCode();
// Use ASM api calls to add profiler logic here
}

Writing bytecodes with javac/javap
 Writing logic in bytecode is possible, but not simple
 Get your computer to do it for you!
 Write your logic in Java
 javac turns Java code into bytecode, packaged up in a class file
 javap -v turns a class file into a human-readable format
 Why javap? (and why not)
– Shipped with the Java SDK
– Easy to use
– Shows you the whole of the class, not just methods (helpful for learning)
– Not easy to translate javap output into the Java code to produce it

Javap on HelloWorld
public static void main(String[] args) {
System.out.println(“Hello World”);
}
javap -v HelloWorld.class

Javap on HelloWorld
(...)
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #22 // String Hello World
5: invokevirtual #24 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 7: 0
line 9: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
(...)

Writing bytecodes with the Bytecode Outline Eclipse Plugin
 Plugin for Eclipse
 Turns Java source code into bytecodes
 Also shows you the Java code using ASM API calls to create the equivalent
bytecode
 (demo)

Demo
 Very simple counting profiler
 Prints out the number of times each method in your application was called
 (demo)

Recap
 Use a framework
– e.g. ASM http://asm.ow2.org
 Get your computer to write your bytecode for you
– Use javac and javap
– Use the Bytecode Outline Plugin for Eclipse (or similar)
 Write as much of your code in Java as possible

 BCI is very powerful and used in many products
 JVM knowledge is required but...
 There are many tools available to make it easier

References
 Eclipse Bytecode Outline plugin (allows you to use asmifier in Eclipse)
– http://andrei.gmxhome.de/bytecode/index.html
 ASM
– http://asm.ow2.org/index.html
 java.lang.instrument package documentation
– http://docs.oracle.com/javase/7/docs/api/index.html?java/lang/instrument/package-
summary.html

References
 Inside the Java Virtual Machine – Bill Venners
– http://www.artima.com/insidejvm/ed2/jvm.html
 Java VM Spec
– http://docs.oracle.com/javase/specs/jvms/se7/html/index.html

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