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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Each time the method called itself, the saved start time value from the last call javaseist be overwritten and lost. This will also serve as the model for what I want to do using Javassist. Listing 3 shows what a source code version of this approach would look like:.
Notice gutorial lines for each, but they were pulled from the agent1 jar file. There’s a lot more to Javassist than what we’ve covered in this article.
Part 5, ” Transforming classes on-the-fly ” February The net effect is that this method will run into more and more overhead as it’s used to create longer strings. Well, I could add a new member field to the class and use that instead of jaavassist local variable. Part 1, ” Classes and class loading ” April It takes one parameter the type is java.
Simple byte code injection example with Javassist
This varies depending on whether the original method returns a value or not. The value that would be stored in the field if the expression is write access. It’s important to realize that Javassist won’t necessarily catch any errors in the code, though, and that the results of an error may be difficult to predict. This site uses Akismet to reduce spam.
Java programming dynamics, Part 4: Class transformation with Javassist
Either way, the new bytecode is passed as a Java-like source code statement or block in a String. 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. If you’re going to work with classes as data, you need to do so outside of the JVM. 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.
A newly created array must be stored in this variable.
I’ll call this whole topic area classworking. Getting the same effect by using Javassist to manipulate the class bytecode seems like it should be easy. 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.
This month I’ll start in on the second part of the series, where the Java class information becomes just another form of data structure to be manipulated by applications. 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. Listing 4 shows 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.
Classes loaded in a class pool are represented by javassist. Javassits what would happen with a recursive method, for instance. Part 6, ” Aspect-oriented changes with Javassist ” March To test this, simple create two directories with the SimpleMain. 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.
This isn’t much of a restriction for most purposes, because you can put any sequence of statements you want in a block.
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. This is a simple example that will time how long a method takes to execute, without changing the source code.
That’s a smelly kind of solution, though, and suffers from some limitations for general use. This interceptor method can use the same signature as the original method, including returning the same value.
Comments 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 this powerful tool.
The resulting value of the object creation.