Java Bytecode Instrumentation Using Agent Breaking into Java Application at Runtime. Intro. In this blog, I would like to describe one of techniques that can be used to flexibly change application logic executed by a Java application server or, to be more precise, within Java Virtual Machine JVM of its server nodes. JVM executes preliminary compiled and deployed platform agnostic bytecode which is an outcome of Java source code compilation, and the technique described below is based on the concept of bytecode manipulation. Using this technique, it is possible to introduce nearly any changes to already deployed Java application by operating on its bytecode level which is interpreted by JVM at runtime, without modifying applications source code since latter would mean necessity of re compilation, re assembly and re deployment of an application. This blog will also illustrate one of reasons why static analysis of decompiled code obtained from original resources like applications class files located on an application server and loaded by Java class loader may sometimes be misleading and why static reverse engineering of a Java application may differ from its observed runtime behaviour. Sample Program In Java Using Switch Case' title='Sample Program In Java Using Switch Case' />Core Java Interview Questions and Answers for Beginner, Advanced Experienced programmers from my 10 years of java programming and interviewing experience. Java Data Structures 2nd Edition End of the World Production, LLC. What is the SAP JCo and how can I get it SAP JCo SAP Java Connector is a middleware component and a development library that enables a Java application to. Updating TestLink Test Case Execution Status Remotely Through Selenium WebDriver Using TestLink API. In first two TestLink tutorials part 1 and part 2 we learned. 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YoSghxkxBVQ/0.jpg' alt='Sample Program In Java Using Switch Case' title='Sample Program In Java Using Switch Case' />When this technique may be useful and why not to simply make necessary changes in source code of a Java application and promote them to an application server Here are some examples We dont have consistent original development project for example, if original project is not available, and decompilation doesnt allow us to reproduce complete project structure and artefacts necessary for successful build and assembly of an application We need to apply an ad hoc patch logic amendment to an already running application in order to make rapid test before developing and assembling complete patch We need to collect runtime specific information about executed classes all or just selected ones Or we just want to break into already deployed application and hack into its logic. The blog mostly contains examples illustrating usage of bytecode instrumentation and manipulation technique. Several remarks regarding presented demo applications shall be made upfront In order to avoid irrelevant complexity, examples are based on a standalone Java application. Since the described capability is a part of JVM features and is not specific to a particular application server implementation, it can be adopted and used in real life scenarios in conjunction with various application servers SAP Application Server Java being one of them All development is simplified, so that amount of code lines is reduced to reasonable minimum and we may stay focused on core subject, even though resulting in heavy usage of hard coded values and simplistic class model design. In real life development, majority of hard coded values shall be exposed as configurable parameters In a standalone program and complementary developed classes, output to console is used intensively to make it transparent and informative, when corresponding objects are called and what their state is. In real life development, such verbose output shall be disabled or implemented using logging framework of an application server with respective appropriate logging level severity. In sake of better readability and clarity, following values are inserted before corresponding log entries in console output Output of calls coming from program main class is preceded with Application Main Output of calls coming from class that is responsible for displaying text in console and that is called from program main class is preceded with Application Text display Output of calls coming from subsequent instrumentation examples is preceded with Instrumentation Output of calls coming from subsequent agent specific examples are preceded with Agent. In order to make segregation of functionality used in demonstrations more obvious, developed classes are located in following packages Java application that we are going to break into and instrument, is located in a package vadim. Java agent that is used in demonstration of instrumentation via agent loading, is located in a package vadim. Java agent loader application that is used in demonstration of attachment to a running JVM from external application, is located in a package  vadim. Correspondingly, instrumented Java application, Java agent and Java agent loader are located in three different Java projects, leading to following projects structure I will start from a basic application and will gradually enhance implemented features in order to illustrate various practical aspects of discussed topics and techniques, so used projects and their content will change over time across this blog. Demo application. Lets use following small standalone Java program as a starting point for future enrichment and manipulation. The program consists of two classes main class Demo. Application and class Text, called from a main class. Class Demo. Application implements method main and is an entry point for the called Java program. Demo. Application. String args. System. Application Main Start application. String value Demonstration of Java bytecode manipulation capabilities. Text text new Text. System. out. printlnApplication Main Value passed to text display value. Role Management Tool To Install .Net 2.0 more. System. out. printlnApplication Main Complete application. Class Text is called from a main class and issues given text to console output after waiting for a second. Text. public void displayString text. Time 1. 00. 0. long sleep. Start. Time. long sleep. Blog Magazine Template Blogger here. End. Time. System. Application Text display Text display is going to sleep for sleep. Time ms. sleep. Start. Time System. Time. Thread. sleepsleep. Time. catch Interrupted. Exception e. e. Stack. Trace. sleep. End. Time System. Time. System. out. Application Text display Text display wakes up. System. out. printlnApplication Text display Text display sleep time. End. Time sleep. Start. Time 1. System. out. Application Text display Output text. Sample execution of this program will result into following console output. Application Main Start application. Application Main Value passed to text display Demonstration of Java bytecode manipulation capabilities. Application Text display Text display is going to sleep for 1. Application Text display Text display wakes up. Application Text display Text display sleep time 1. Application Text display Output Demonstration of Java bytecode manipulation capabilities. Application Main Complete application. Now lets see what is bytecode instrumentation and how it can be applied. Bytecode instrumentation and manipulation. Starting from Java 5, JDK provides developers with functionality of so called instrumentation of bytecode. This technique aims possibility of modification of bytecode that is loaded into JVM and executed by it for example, its extension with additional instructions or other changes of original bytecode. It shall be noted that bytecode instrumentation doesnt cause any change to original resource of bytecode class file. It manipulates bytecode on the fly at a time when class loader attempts to access and load bytecode of corresponding queried class into JVM, extending or substituting bytecode obtained from an original resource, with its instrumented version. Main interface that is used for instrumentation, is java.