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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Sign in or register to add and subscribe to comments. These are the features that make Javassist a great tool for implementing aspects in your applications, so make sure you catch the follow-up for the full story on jzvassist powerful tool. CtMethodand javassist.
Simple byte code injection example with Javassist
This month I’ll start in on javaswist second part of the series, jagassist the Java class information becomes just another form of data structure to be manipulated by applications.
This site uses Akismet to reduce spam. If you don’t have source code, it’s normally much more difficult to get this type of timing information. This is not equivalent to thiswhich represents the caller-side this object.
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.
To test this, simple create two directories with the SimpleMain. There are ways around this limitation, but the workarounds are messy — you generally need to find a way to merge the separate code inserts into a tuotrial block. The resulting value of the field access if the expression is read access. Class transformation with Javassist.
This type of issue isn’t a problem as long as you’re careful with the source code you supply to Javassist.
Listing 1 shows a bad example method that I’ll use as a guinea pig for my timing experiments: Fortunately there’s a cleaner solution. Listing 4 javassst this code, in the form of an application that takes a pair of command-line arguments giving the class name and method name to be timed.
Either way, the new titorial is passed as a Java-like source code statement or block in a String. Class class, CtClass provides methods for inspecting class data such as fields and methods. 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. Part 8, ” Replacing reflection with code generation ” June After covering the basics of the Java class format and runtime access through reflection, it’s time to move this series on to more advanced topics.
Class transformation with Javassist
The main method body just finds the class information and then passes it to the addTiming method to handle the actual modifications.
Lastly, we will create our jar file that contains our software above. This is easy enough to do in the source; javassst 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.
The object containing the field accessed by the expression. Object and returns true if the parameter value is an instance of the type on the right hand side of the original instanceof operator.
The name of a virtual method executing the original type casting. The net effect is that this method will javasssist into more and more overhead as it’s used to create longer strings. Part 2, ” Introducing reflection ” June This is because they each injected the behaviour at the same point in the class.
When I described how Javassist lets you add code, I mentioned that the added code could not reference local variables defined elsewhere in the method.
The first one listed will get first cut at it. The value that would be stored in the field if the expression is write access.
Javsasist varies depending on whether the original method returns a value or not. We also need a custom manifest with the following contents, which you should create in the same directory as the one in which we have been working:. Otherwise, it returns false. It takes one parameter of the type java. 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 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.