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Have you ever created or encountered a self modifying code in Java?
If yes, then please post the link or simply post the code.
Ignoring the world of grief you could be causing yourself via self-modifying code(!), it seems to me there are 3 options:
use the inbuilt compiler support of Java 6 and write/recompile/reload classes
use the Apache BCEL bytecode manipulation library to write your class directly
make use of Java 6's inbuilt scripting support (or use Apache BSF) to write methods in your JVM scripting language of choice, and execute these
Of the three above, my initial choice (in the absence of requirements) would be to take a look at option 3. I suspect it's the least painful way to start. I've used all of the above - unfortunately I can't post links to client code.
You can write (Java) code that generates new classes (byte code) at runtime using a library like bcel. That's not quite the same as self-modifying code. I suspect self-modifying code is not something the JVM supports.
For an example of generating new code at runtime, have a look at the source code of clojure.
That should be difficult to realize. But you can create at runtime new classes and load them with a custom classloader. If you want to modify the code again, you have to reload the class.
From BCEL:
The Byte Code Engineering Library is intended to give users
a convenient possibility to analyze, create, and manipulate
(binary) Java class files (those ending with .class).
Classes are represented by objects which contain all the
symbolic information of the given class: methods, fields and
byte code instructions, in particular.
I see these options for this purpose:
Generate the java source code and compile it with the external javac or the internal compiler tools (can't remember the name). And as you are responsible for the naming, just include a version count in the class name to avoid class loading anomalies.
Use the built in JavaScript engine support
Some scenarios can be solved using java Proxys
Edit: I once created a Java 1.4 program which took business rules from an existing legacy database, generated java files (basically implementations of a Predicate interface) with a bunch of println() from them and used the command line javac to compile them.
As an undergrad I got to work on the JikesRVM. It is a JVM implemented (mostly) in Java. At runtime it will JIT compile itself! It's a really cool piece of technology.
You could always just use a dynamic language...
Related
Can I make a Java program to generate another java application at runtime.
I want to make a "installer" program, which takes user input and generates an application as per user requirement, instead of just configuring the pre-built application according to the user needs.
I came across this solution - how to compile & run java program in another java program?, but I don't want to make clients install JDK on there computer.
Dynamically create table and Java classes at runtime -
which also need JDK, but I got a work around:
ToolProvider.getSystemJavaCompiler() returns null - usable with only JRE installed?
Can I make a complete application using above methods?
Is it a bad idea to generate such program?
Can I make Spring and Hibernate applications like that?
Or is there any existing framework for doing so?
(if possible it should create tables in db and generate html files as well. I came across http://velocity.apache.org/, so is it possible to generate java code using that.)
Your goal doesn't make a lot of sense from a practical perspective. I hope that my answer will help you to understand why.
Can I make a java program to generate another java application at runtime.
Yes you can. But it is a lot of work, especially if the application if complicated.
I want to make a "installer" program, which takes user input and generate an application as per user requirement, instead of just configuring the pre-build application according to the user needs.
That is possible ... in theory.
The problem is that you have to write a program that is capable of reading and understanding the user's requirements, and can then converting those requirements into code. Normally ... this is what a programmer does. Writing a program to do what a programmer does is not practical. (My guess is that it is 20 or more years beyond the "state of the art" of artificial intelligence to do such a thing.)
Now if the problem domain was sufficiently restricted, and the requirements were tightly specified in an unambiguous notation, then it might be feasible to do this. However, benefits of generating a program rather than configuring an existing one (based on the same requirement notation) are pretty small. And probably not worth the effort.
... but I don't want to make clients install JDK on their computer.
If you are generating Java programs you need a Java compiler. So if you insist on using a JRE (in Java 8), you need to include a 3rd party Java compiler in your application.
However, for Java 9 onward this is moot:
Oracle no longer provides JRE distributions for Java 9+ so you would need to get your client to use a 3rd-party source for their JRE.
You could (should) be using the Java 9+ jlink utility to produce a custom JRE for you application, and that can include the standard Java compiler.
If you are trying to generate code at the bytecode level, your problem is immediately ten times harder.
Sorry, I am using Java 8
Are you aware that Java 8 is "end of life" for commercial use? That is likely to affect your clients.
Can I make a complete application using above methods?
Maybe yes, maybe no. It depends on the problem domain. The more complicated it is, and the more diverse / general the requirements, the harder it will be.
Is it a bad idea to generate such program?
Yes. It is a bad idea. It is a lot more work than writing an application that is configured in the conventional way. (Noting that the configuration could include writing plugins in Java, rules in some scripting language, and so on.)
I would advise only generating source code or bytecodes if you already have a conventional application with most / all of the required functionality that you can use as a prototype for the generated generated code. (If you can't write such a prototype by hand, then writing a generator that will create one is not realistic.)
And even when it is feasible, I would question the wisdom of building a generator. There doesn't seem to be a significant pay-off for the extra effort. (For example, where is the benefit for the end user?)
Can I make spring and hibernate application like that?
I don't see why you couldn't generate such an application. But see 1) and 2).
Or is there any existing frameworks for doing so?
There are frameworks that could be used in some cases:
Templating frameworks like Velocity1 can be used to generate Java source code.
Bytecode engineering frameworks could be used to generate code directly.
1 - Indeed, I have used Velocity for Java source code generation. It worked, though I'm not convinced it was an ideal solution.
Sure you can. You can also leverage a project like GraalVM to generate native binaries for a given platform.
However, it is a lot of work, and the end result won't probably be as useful as you think. Any use case you have in mind will probably be a lot better served by an app that you just configure to do different tasks, so your efforts are probably best spent in that direction.
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Closed 11 years ago.
Possible Duplicate:
Convert String to code in Java
Dynamic code execution on Java
I have a String containing : "for(int i=0 ; i<5 ; i++){System.out.println(\"*\");}"
Can I execute the code in this String in Java?
Since Java 6, you can compile and run a Java compilation unit defined as a String or a File using standard APIs in the SDK (a compilation unit is basically everything that goes inside a .java file - package, imports, classes/interfaces/enumerations), take a look at this example. You can't run an arbitrary Java snippet like the one in your question, though.
If at all possible, it'd be a better idea to embed a different scripting language that allows you to run snippets of code from a Java program - for example, JavaScript, Groovy, MVEL, BeanShell, etc.
If you turn it into a full-blown source file, you can feed it to the java compiler programmatically, but last time I checked that was only available if you had the java SDK installed on your machine; it was not available on machines with the client distribution of Java. Of course, this may have changed since then. Look at package com.sun.tools.javac and you will find the java compiler API there.
Maybe you can run this as Groovy:
http://groovy.codehaus.org/Embedding+Groovy
There isn't a Java Core API function for doing this, but you can call javac either by using Runtime.exec or using some "unsafe" classes from com.sun.tools.javac Here's an example:
http://juixe.com/techknow/index.php/2006/12/12/invoke-javac-at-runtime/
I don't think you can execute a String containing a java code.
But it is worth a try if you can save that as a java source file and try to use ProcessBuilder class to execute.
Never tried it and not sure if it is best way to do it. So use it with caution :)
Good Luck!
Also found a similar post: Runtime class in java
No, you can not execute this code in your program.
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I want to detect if a JVM Class is a Kotlin class or not
(2 answers)
Closed 3 years ago.
I'm developing a Kotlin library for Android.
I want to provide a great developer experience for both Kotlin and Java developers, but because I don't have infinite resources I need to prioritize between those.
The library is interacting with a server I control, so I can add metadata to network calls that would help me know the split of my users.
But is there a way I can know when my library is called from Java VS Kotlin?
I see in this answer that there might be a way to inspect a given class, but as the library owner I don't think I can know which class is calling my own without using reflection which would add some runtime cost. Isn't there a way to get at runtime metadata from the JVM, that would indicate if the code was compiled from Kotlin (even at the cost of false-negatives, e.g. checking if KAPT was used at any point which would at least report some Kotlin users?)
Short answer: NO.
When your code is running, it's compiled in Java bytcode in both cases.
I have a question which I'm pretty confused from.
I am aware of the differences between Java Runtime Enviroment and Java Developement Kit.
I'm writing a program that uses the ToolProvider.getSystemJavaCompiler() method to compile java code from within the code.
Now, I've been answered that I can't compile code from client side if my client doesn't have JDK installed. My main question is, how can I do that? I don't want my clients having to install JDK on their computer just to run my program.
Thanks in advance!
You need to compile it on your system, and distribute the class file of corresponding java source file to anyone.
That class file doesn't require JDK but JRE must be installed on that system to run the class file.
If you want to compile code, you need a compiler, so if the user can't be expected to have the compiler you need, you'll simply have to bundle it.
I really can't say I know how to bundle the standard javac compiler, though it's probably possible, strictly speaking, to find the Jar file that contains it and bundle that along with your code. No idea how robust such a solution would be, though.
But depending on your needs, you may not need the standard javac. There are tons of byte-code generation libraries out there, with more or less high-level functionality. I wouldn't really want to recommend anything that I have no personal experience with, but examples include Byte Buddy or ASM. You could probably use ABCL too.
Eclipse's compiler is worth a look as well.
There is also an so question here.
So there really is no way to do what it is you are wanting to do unless you bundle the compiler itself with you application, or unless you find a library that has all of the Java compiler code in it already so it doesn't have to use the JDK compiler, you will not get what you want, and what you want is the ability to turn a String containing source code into a Java class.
I do not understand what you wish to accomplish, but the BEST option I can give you is asm. If you are up for the task, you can manually write new classes at runtime without the presence of the JDK compiler. HOWEVER, this does not involve you using a String full of source code and turning it into a Class object. This is you working at the low level with the Java bytecode for the most part.
This tutorial can get you started:
https://www.javaworld.com/article/2071777/design-patterns/add-dynamic-java-code-to-your-application.html
And here is the Java documentation for class files. You can use this to expand on what you learned from the first link:
https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html
That is the only instance creating classes on the fly that I can give you. That being said, you could try writing your own Java compiler that can turn source code into classes without ever getting the Java compiler, but at that point you are literally recreating the Java compiler yourself, and I assure you that is no easy feat for one person.
First, I have no experience doing this. But like the beginning of any good program, I have problem that I need to fix, so I'm willing to learn.
So many of you are probably already familiar with pdftk, the handy utility for handling various pdf-related tasks. So far as I can tell, most of these features are available in much newer, lighter libraries/extensions, except the one I need (and probably the only reason it still exists): merging form data files (fdf and xfdf) with a form PDF and getting a new file as the output.
The problem is that my server doesn't have gcj, which is fundamental to build/compile pdftk. I don't know if it's because I'm on Solaris or if it's for some other sysadmin-level reason, but I'm not getting gcj anytime soon. And there are no pre-compiled binaries for Solaris as far as I can find.
So I'm thinking that the MAKE file and C code can be rewritten to import the Java library (very ancient version of itext) directly, via javac.
But I'm not sure where to really start. All I know is:
I want a binary when I'm done, so that there won't be a need for a Java VM on every use.
The current app uses GCJ.
So my first thought was "Oh this is easy, I can probably just call the classes with some other C-based method", but instead of finding a simple method for doing this, I'm finding tons of lengthy posts on the various angles that this can be approached, etc.
Then I found a page on Sun's site on how to call other languages (like C) in a Java class. But the problems with that approach are:
I'd have to write a wrapper for the wrapper
I'd probably be better off skipping that part and writing the whole thing in Java
I ain't ready for that just yet if I can just import the classes with what is already there
I'm not clear on if I can compile and get a binary at the end or if I'm trapped in Java being needed every time.
Again, I apologize for my ignorance. I just need some advice and examples of how one would replace GCJ dependent C code with something that works directly with Java.
And of course if I'm asking one of those "if we could do that, we'd be rich already" type questions, let me know.
I'm not sure what you are looking for exactly, so I provided several answers.
If you have java code that needs to run, you must:
Run it in a jvm. You can start that vm within your own custom c-code, but it is still using a jvm
Rewrite it in another language.
Compile with an ahead-of-time compiler (eg gcj)
Incidentally, you could compile a copy of gcj in your home folder and use that. I believe the magic switch is --enable-languages=java,c (see: here for more)
If you have c-code you want to call from java, you have four options:
Java Native Interface (JNI). It seems you found this
Java Native Access (JNA). This is slower than JNI, but requires less coding and no wrapper c-code. It does require a jar and a library
Create a CLI utility and use Runtime.Exec(...) to call it.
Use some sort of Inter Process Communication to have the Java code ask the c-code to perform the operation and return the result.
Additional platform dependent options
Use JACOB (win32 only: com access)
I am not sure if I understand what you are looking for.
If you are looking to incorporate the C code into Java to make a native binary without the gcj, I think you are out of luck. You can include the C in Java, but it would be a primarily Java program meaning you would need the JVM on each run. Is there anything stopping you from compiling the gcj yourself?