Example. Javassist is a bytecode instrumentation library that allows you to modify bytecode injecting Java code that will be converted to bytecode by Javassist. Simple byte code injection example with Javassist. by Steve Please the file in your classpath. Also, set your . i used tutorial. Java consultant Dennis Sosnoski takes a look at Javassist, the bytecode manipulation library that’s the basis for the aspect-oriented.

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First, we need to download the javassist software from http: The added code just saves the start time to a local variable, then computes the elapsed time at the end of the method and prints it to the console.

Simple byte code injection example with Javassist – AppCrawler

This site uses Akismet to reduce spam. This will also serve as the model for what I want to do using Javassist. A newly created object must be stored in this variable. The value that would be stored in the field if the expression is write access.

View January 16, By using this identifier in the call to the original method, all the arguments supplied in the call to the interceptor method are passed on to the original method. For an example of applying Javassist, I’ll use a task I’ve often handled directly in source code: Part 2, ” Introducing reflection ” June I’ll start classworking coverage with the Javassist bytecode manipulation library.

This type uttorial issue isn’t a problem as long as you’re careful with the source code tytorial supply to Javassist. Subscribe me to comment notifications.

Javassist uses the javassist. This approach of using an interceptor method works well with Javassist. This is a mixed blessing in some respects — I don’t generally advocate messing with technology you don’t understand — but it certainly makes bytecode manipulation much more accessible than with frameworks where you work at the level of individual instructions.


This limitation blocks me from implementing the timing code in Javassist the same way I did in the source code; in that case, I defined a new local variable in the code added at the start and referenced that variable in the code added at the end.

Java programming dynamics, Part 4 http: It should be noted that you can pass more than one javaagent on the command line to your JVM. So what other tutoriaal can I use to get the same effect?

Because strings are immutable, this approach means a new string will be constructed each time through the loop, with the data copied from the old string tutlrial a single character added at the end. Javassist again accepts the source code without complaint, but the resulting bytecode fails verification when I try to execute it.

Part 8, ” Replacing reflection with code generation ” June Listing 1 shows a bad example method that I’ll use as a guinea pig for my timing experiments: Javassist provides ways to add code at the beginning and end of methods, after all, which is javzssist what I did in the source code to add timing information for the method. StringBuffer to accumulate the body text showing the proper way to handle String construction, as opposed to the approach used in StringBuilder.

For comments or questions about the material covered in this article series, as well as anything else that pertains to Java bytecode, the Java binary class format, or general JVM issues, visit the JVM and Bytecode discussion forum, moderated by Dennis Sosnoski. The object containing the field accessed by the expression. Javassist accepts the source code and converts it into valid bytecode, but the resulting times are garbage.


Also, set your classpath to include the tools. The net effect is that this method will run into more and more overhead as it’s used to create longer strings.

Simple byte code injection example with Javassist

You can even load binary classes directly from byte arrays or streams, and create new classes from scratch. Generating the source code for the interception method is also easy — it only needs a few substitutions to work for any possible method.

Part 3, ” Applied reflection ” July Next month, we’ll delve a little deeper with javasaist look at some of the special hooks Javassist provides for bulk modification of classes and for on-the-fly modification as classes are loaded at runtime.

This is a simple example that will time how long a method takes to execute, without changing the source code. Implementing the code to add method timing uses some of the Javassist APIs described in Javassist basics. Object and returns true if the parameter value is an instance of the type on the javasaist hand side tuutorial the original instanceof operator.

Getting the same effect by using Javassist to manipulate the class bytecode seems like it should be easy. Lastly, we will create our jar file that contains our software above.