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.
|Published (Last):||18 April 2005|
|PDF File Size:||10.29 Mb|
|ePub File Size:||13.49 Mb|
|Price:||Free* [*Free Regsitration Required]|
Javassist isn’t the only library for working with bytecode, but it does have one feature in particular that makes it a great starting point for experimenting with classworking: Class object representing the type of the exception caught by the catch clause.
Object and returns true if the parameter value is an instance of the type on the right hand side of the original instanceof operator. View January 16, By using this identifier in the call to tutkrial original method, all the arguments supplied in the call to the interceptor method are passed on to the original method.
It should be javazsist that you can pass more than one javaagent on the command line to your JVM.
There’s a lot more to Javassist than what we’ve covered in this article. 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.
But this ease of use comes with some drawbacks. Class transformation with Javassist. The resulting value of the object creation. To test this, simple create two directories with the SimpleMain. After compiling each class above, issue the following command jjavassist the same directory we have been using:.
Java programming dynamics, Part 4 Stay tuned for additional content in tutorrial series. The main method body just finds the class information and then passes it to the addTiming method to handle the actual modifications.
Simple byte code injection example with Javassist
This is a simple example that will time how long a method takes to execute, without changing the source code. A more substantial limitation on the source code is that there’s no way to refer to local variables declared outside javaxsist statement or block being added.
You can see how the StringBuilder run after modification reports execution javassost, and how the times increase much faster than the length of the constructed string because of the inefficient string construction code. That’s where classworking comes in handy — it lets you make changes like this for any method, without needing source code.
It takes one parameter the type is java.
Part 3, ” Applied reflection ” July This varies depending on whether the original method returns a value or not. Here’s an example using the special Javassist identifiers for the first two method parameter values to show how this works:. After tuutorial the basics of the Java class format and runtime access through reflection, it’s time to move this series on to more advanced topics. Date ;” ; method.
Lastly, we will create our jar file that contains our software above.
Simple byte code injection example with Javassist – AppCrawler
Subscribe me to comment notifications. If the original method is of type voidthere’s nothing to be saved and nothing to javsssist returned from the interceptor method.
That’s just the start for CtClassthough, which also defines methods for adding new fields, methods, and constructors to the class, and for altering the class name, superclass, and interfaces. Jacassist is easy enough to do in the source; you just record the current time at the start of the javasssit, then check the current time again tutorrial the end of the method and find the difference between the two values.
Java programming dynamics, Part 4 http: The source code accepted by Javassist doesn’t exactly match the Java language, but the main difference is just the addition of some special identifiers used to represent the method or constructor parameters, method return value, and other items you may want to use in your inserted code.
The value that would be stored in the field if the expression is write access. This type of issue isn’t a problem as long as you’re careful with the source code you supply to Javassist. Also, set your classpath to include the tools.
A newly created array must be stored in this variable. This approach of using an interceptor method works well with Javassist. The first one listed will get first cut at it. Besides recognizing special identifiers in the code, Javassist implements much looser compile-time checks on the code than required by the Java language specification. Javassist lets you completely replace the bytecode body of a method or constructor, or selectively add bytecode at the beginning tutorixl end of the existing body along with a couple of other variations for constructors.
You can even load binary classes directly from byte arrays or streams, and create new classes from scratch.
Tutoriaoand javassist. Depending on what you do in the source code, you can even get Javassist to generate invalid bytecode. Because I have the source code available for this method, I’ll show you how I would add the timing information directly.