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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Besides recognizing special identifiers in the code, Javassist implements much looser compile-time checks on the code than required by the Java language specification.
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 tutorail Bytecode discussion forum, moderated by Dennis Sosnoski.
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.
Well, I could add a new member field to the class and use that instead of a local variable. 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:. Consider what would happen with a recursive method, for instance. There’s a lot more to Javassist than what we’ve covered in this article.
Part 3, ” Applied reflection ” July Part 6, ” Aspect-oriented changes with Javassist ” March The object containing the field accessed by the expression.
This is because they each injected the behaviour at the same point in the class. Part 1, ” Classes and class loading ” April After compiling each class above, issue the following command in the same directory we have javassistt using:. Java programming dynamics, Part 4 http: This isn’t much of a rutorial for most purposes, because you can put any sequence of statements you want in a block. Fields, methods, and constructors are represented by javassist.
Java programming dynamics, Part 4: Class transformation with Javassist
If the original method is of type voidthere’s nothing to be saved and nothing to be returned from tutoral interceptor method. This method constructs a String of any requested length by doing exactly what any Java performance guru will tell you not to do — it repeatedly appends a single character to the end of a string in order to create a longer string.
A newly created array must be stored in this variable.
These are the features that make Javassist a great tool for implementing aspects in your applications, so make sure you catch the follow-up tutkrial the full story on this powerful tool. Each time the method called itself, the saved start time value from the last call would be overwritten and lost. Tutoral newly created object must be stored in this variable. 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 javadsist.
The inspection aspect mainly duplicates what’s available directly in Java through the Reflection API, but having an alternative way to access this information is useful when you’re actually modifying classes rather than just executing them. The name of a virtual method executing the original type casting.
Class transformation with Javassist
Javassist uses the javassist. Part 5, ” Transforming classes on-the-fly ” February Also, set your classpath tutoriql include the tools. Sign in or register to add and subscribe to comments.
Here’s an example using the special Javassist identifiers for the first two method parameter values to show how this works:.
Part 2, ” Introducing reflection ” June View January 16, This class works a lot like a JVM classloader, but with the important kavassist that rather than linking loaded classes for execution as part of your application, the class pool makes loaded classes usable as data through the Javassist API.
Javassist does a great job of making classworking easy by letting you work with source code rather than actual bytecode mavassist lists.
Simple byte code injection example with Javassist
If you don’t have source javawsist, it’s normally much more difficult to get this type of timing information. Listing 5 shows the results of first running the StringBuilder program in unmodified form, then running the JassistTiming program to add timing information, and finally running the StringBuilder program after it’s been modified.
Class transformation with Javassist. The net effect is that this method will run jabassist more and more overhead as it’s used to create longer strings. The Javassist methods effectively compile the source code you provide into Java bytecode, which they then insert into the body of the target method or constructor.
Comments Sign in or register to add and subscribe to comments. Javassist lets you inspect, edit, and create Java binary classes. The source code accepted by Javassist javwssist 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.
You can see how the StringBuilder run after modification reports execution times, and how the times increase much faster than the length of the constructed string because of the inefficient string construction code.