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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This is because the JVM design doesn’t provide you any access to the raw class data after it’s been loaded into the JVM. The construction of the interceptor method body uses a java. This varies depending on whether the original method jvassist a value or not.

The resulting value of the object creation. But this ease of use comes with some drawbacks. If you tried compiling this assignment directly in a Java program, you’d get a compile error because it violates one of the rules of the Java language: For an example of applying Javassist, I’ll use a task I’ve often handled directly in source code: The inspection aspect mainly duplicates what’s available directly in Java through the Reflection API, but having an alternative way to uttorial this information is useful when you’re actually modifying classes rather than just executing them.

I can keep the original method code unchanged and just change the method name, then add a new method using the original name. Consider what would happen with a recursive method, for instance.

In the same directory, create a file name SimpleTransform. The first one listed will get first cut at it. Lastly, we will create our jar file that contains our software above. Javassist does a great job of making classworking easy by letting you work with source code rather than actual bytecode instruction lists.


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. Part 2, ” Introducing reflection ” June If the original method is of type voidthere’s nothing to be saved and nothing to be returned from the interceptor method.

Simple byte code injection example with Javassist – AppCrawler

The main method body just mavassist the class information and then passes it to the addTiming method to handle the actual modifications. 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 and 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. The object containing the field accessed by the expression.

Class transformation with Javassist

Implementing the code tutorisl add method timing uses some of the Javassist APIs described in Javassist basics. A more substantial limitation on the source code is that there’s no way to refer to local variables declared outside the statement or block being added. This will also serve as the model for what I want to do using Javassist. This is a simple example that will time how long a method takes to execute, without changing the source code. Fortunately there’s a cleaner solution.

Java programming dynamics, Part 4: Class transformation with Javassist

This interceptor method can use the same signature as the original method, including tutprial the same value. Tuforial Sosnoski Published on September 16, This is easy enough to do in the source; you just record the current time at the start of the method, then check the current time again at the end of the method and find the difference between the two values.


It takes one parameter of the type java. A newly created array must be stored in this variable. The name of a virtual method executing the original type casting.

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. After compiling each class above, issue the following command in the same directory we have been using:. ClassPool class to track and control the classes you’re manipulating. Because I have the source code available for this method, I’ll show you how I would add the timing information directly.

Date ;” ; method. Depending on what you do in the source code, you can even get Javassist to generate invalid bytecode. Learn how your comment data is processed.

The first restriction is the actual format, which must be a single statement or block. Classes loaded in a class pool are represented by javassist. Generating the source code for the interception method is also easy — it only needs a few substitutions to work for any possible method.

Kavassist uses the javassist.

Javassist lets you completely replace the bytecode body of a method or constructor, or selectively add bytecode at the beginning or end of the existing body along with a couple of other variations for constructors. That’s where classworking comes javadsist handy — it lets you make changes like this for any method, without needing source code.

Listing 2 shows just the buildString method, with timing added.