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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Simple byte code injection example with Javassist
This content is part of the series: As I mentioned back in The source of all bytecodethe source code used by Javassist is not exactly the Java language. Subscribe me to comment notifications. Fields, methods, and constructors jafassist represented by javassist. Class object representing the type on the right hand side of the instanceof operator.
The actual body text looks just like standard Java code except for the call to the renamed original method. It takes one parameter the jwvassist is java.
The construction of the interceptor method body uses a java. The type of the field if the expression is read access. Profile cancel Sign in with Twitter Sign in with Facebook.
Javassist Basic | java Tutorial
Java programming dynamics, Part 4 Stay tuned for additional content in this series. Listing 1 shows a bad example method that I’ll use as a guinea pig for my timing experiments: There’s a lot more to Javassist than what we’ve covered in this article. This will also serve as the model for what I want to ttutorial using Javassist.
Javassist does a great job of making classworking easy jqvassist letting you work with source code rather than actual bytecode instruction lists.
The target object of the method call. Because I have the source code available for this method, I’ll show you how I would add the timing information directly.
The first one listed will get first cut at it. The name of a virtual method executing the original instanceof expression. So what other approach can I use to get the same effect? Listing 2 shows just the buildString method, with timing added. This method constructs a String of any requested length by doing exactly what any Java performance guru will tell you not to javassit — it repeatedly appends a single character to the end of a string in order to create a longer string.
First, we need to download the javassist software from http: Also, set your classpath to include the tools. 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. 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.
These classes define methods for modifying all aspects of the item represented tuutorial the class, including the actual bytecode body of a method or constructor. 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: 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.
Listing 7 shows an example of this, where I’ve patched the JassistTiming code to always treat the timed method as returning an int value.
Sign in or register to add and subscribe to comments. 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.
Class transformation with Javassist. 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 javaxsist single block. This is tutoeial equivalent to thiswhich represents the caller-side this object.
The resulting value of the array creation. Part 6, ” Aspect-oriented changes with Javassist ” March 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.
You can even load binary classes directly from byte arrays or streams, and create new classes from scratch.
Java programming dynamics, Part 4: Class transformation with Javassist
A newly created array must be stored in this variable. It is used in a cast expression. 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.
This is easy enough to do in the source; you just record the current time at the start javzssist the method, then check the current time again at the end of the method and find the difference between the two values. The first restriction is the actual format, which must be a single statement or block.
Depending on what you do in the source code, you can even get Javassist to generate invalid bytecode. Java programming dynamics, Part 4 Class transformation with Javassist Using Javassist to transform methods in bytecode. Part 8, ” Replacing reflection with code generation ” June Javassist lets you inspect, edit, and create Java binary classes.
You can use a default class pool that loads from the JVM search path, or define one that searches your own list of paths. Class object representing the type of the exception caught by the catch clause. If the original method is of type voidthere’s nothing to be saved and nothing to be returned from the interceptor method. Fortunately there’s a cleaner solution.