I get path to .jar file as user input of my java application. I need to add one class that is on my class path to that .jar file and then send the .jar file to external service (over network). The service on the other site must be able t load classes from the .jar file including the one I added in my application.
The solution should have these properties:
universal: should work for any thinkable .jar containing classes user can give me.
automatic: class addition must by done by my java application.
The class to be added to .jar file has name that does not collide with any other class in that JAR.
Since jars are regular zip files, you can use the facilities provided by java.util.zip.
To add upon what others have said, if you are needing to load the JAR on the other end, you should examine http://docs.oracle.com/javase/7/docs/api/java/net/URLClassLoader.html
I understand that you have a class on your classpath, and what you want to do is to get its bytearray representation for the purpose of injecting it into an existing JAR file.
Neat.
Hava a look at the javax.tools package, available with JDK 1.6 and up:
http://docs.oracle.com/javase/6/docs/api/javax/tools/package-summary.html
You can programmatically compile code and serialize bytecode using mechanisms available there. If not, ASM could be of help as well: http://asm.ow2.org/
Related
I'm developing an Eclipse plugin in which I want to overwrite the functionality of a method contained in a class that is in the Eclipse library.
What I've tried so far is creating an identical (same package and file names) file in my plugin source, and making the changes I want there. Then, I set the build path order such that my source is above the Plug-in Dependencies. Based on my limited understanding, this should mean that when Java looks for that class, it should use mine over the one in the library.
However, this is not working. The behavior that I want to override is not changing, and I don't see the print statements I put in my code either.
How can I "replace" a class in the Eclipse library with one of my own?
I did it once (not proud of it :-)) in the following way:
Import the plugin you wish to hack by Import->Plugin Development->Plug-ins and Fragments (Import as Projects with source folders).
Set the project to build automatically, edit the file and find its resulting class file.
Open the jar of the plugin (the one containing its class files), inject your class file instead of the original one.
If jar file is signed remove all signature information from MANIFEST.MF (and maybe other files).
I admit it's ugly but it's the best way I've found.
I ended up using the JVM JavaAgent to achieve this, by overriding the class loader and loading in my own class to replace the one in the library.
This was a useful tutorial for me: https://stackoverflow.com/a/11898653/634324
I've looked here and the wider web to find a solution to this. There's related material, but I've been unable to find anything useful about my specific question.
I'm working on some Java software that needs to accept plugins. I don't want to use a fancy framework like OSGi, and ServiceLoader seems to offer the right level of support. I've basically got it working but am having a problem with classpaths. My directory structure is as follows:
progfolder
|___________ plugintest.jar
|___________/plugins
|________ plugin1.jar
|________ plugin2.jar
If I run plugintest.jar with java -jar plugintest.jar then it doesn't find the plugins even if I add ./plugins/ (or variations of this) to the Class-Path: in the manifest. Reading suggests that this only works for classes, not jars, so I've tried putting the classes from the two plugins inside plugins both directly and within their full path of directories, but with no success.
I'm not allowed to add -cp plugins/* to add the plugins folder to the classpath if I'm using the -jar option. To get round this, I can run using java -cp plugintest.jar;plugins/* com.plugin.test.Main and this works as expected - the two plugins are detected and accessible via code, but the command line is a bit clunky, although I could live with it, if it's the best option.
I found another solution where I create a classloader for jars found in plugins, which works in this simple case, but reading suggests I might run into security issues in a more complex application.
Is there a way to fix things so I can simply run with java -jar plugintest.jar without having to do my own class loading or is this just the way it is?
Ok, so at least a partial answer, following more experimentation. Putting the class files in the plugins directory does work, after all, but you have to remember to include the META-INF directory and META-INF/services. The file in the services directory has to include references to all the plugins.
It would be nice if there was a solution that allowed the plugin jar files to be used directly, but creating a class loader seems to be the only way to do this (that I've found, at least), and this may cause security issues, as previously noted.
last time I faced with similar problem [1]. I found answer in java documentation [2]:
Note: The Class-Path header points to classes or JAR files on the local network, not JAR files within the JAR file or classes accessible over Internet protocols. To load classes in JAR files within a JAR file into the class path, you must write custom code to load those classes. For example, if MyJar.jar contains another JAR file called MyUtils.jar, you cannot use the Class-Path header in MyJar.jar's manifest to load classes in MyUtils.jar into the class path.
[1] https://github.com/narvi-blog/01-exec-jar#dependency-jar-files-within-an-executable-jar-are-not-so-easy
[2] https://docs.oracle.com/javase/tutorial/deployment/jar/downman.html
I have a Mule project (created with Mule Studio, a flavor of Eclipse.) I would like to use the android.net MailTo class, which requires a few other files within the package, such as a ParseException and URI class. I could add the files into its own package in my project, however I've read I could import the package as a jar file. I can't find the jar file anywhere, and was wondering if there's a way to create a jar file out of only a portion of the package (without classes I don't need.)
Where can I find a copy of android.net as either source or JAR file? And if I get only the source, how can I compile into a JAR file?
Thanks,
Where can I find a copy of android.net as either source or JAR file?
There is no JAR file, strictly speaking, except as part of a firmware build.
The Android source code is available online, including the Java classes that are part of the android.net package.
However, getting this code to work on standard Java may be painful. I would recommend finding existing ordinary Java code.
I think this is a super easy question, but im new to this and not sure what to do. So I have added an external jar file into one of my projects, and was able to decompile the classes using JAD to see some of the class files, the thing is I want to add a new class to one of the directories in same JAR file, compile it and update the jar file. I was googling and I came up with things like an 'ANT build file' but not sure how that would work?
Thanks!
First of all i would check out if the developper of that JAR has provided the source to the classes. This reduces the possibility of JAD not beeing able to fully decompile all the class information.
However once you have all the .java files (including your changed or added class) you can let eclipse build the jar file as follows:
1.) Compile the .java files (Project -> clean..)
2.) File -> Export -> Java -> Jar file (click next)
3.) Select anything that belongs to the jar, input a name and click finish (Note the checkbox that says to export generated class files and resources).
Regarding Ant/ Maven and stuff: Those are just tools that help you. The same result you can archive by opening a console and use javac.exe to compile you can use ant or maven to compile your classes. Its just a matter of configuration.
I recomend you skip Ant right here and directly do the Maven in 5 Minutes tutorial :)
Your approach works. But you can have two classes with same name and package. This will remove the necessarily of updating the jar file.
You must set the loading order to make sure that your new modified version will be used instead of the one which is in the jar file.
Possible to use two java classes with same name and same package?
Before any tweak and class loader, test this approach, may be the default class loader behavior will do what you want.
in my Java project I am using an H2 in-memory database, for which I have to load the JDBC driver when I initialize my application. I want/need to load the H2 .jar file dynamically, so I do the following:
String classname = "org.h2.Driver";
URL u = new URL("jar:file:libs/h2.jar!/");
URLClassLoader ucl = new URLClassLoader(new URL[] { u });
Driver d = (Driver) Class.forName(classname, true, ucl).newInstance();
DriverManager.registerDriver(new DriverShim(d));
When I put the H2 .jar file into a "libs" folder outside my Java source code folder (that is, in Eclipse, this "libs" directory is on the same level as the "src" folder), then this approach works fine. However, unfortunately I have to put this H2 .jar file into a folder within the source code folder tree, but below the main class folder.
For example, my Java package structure looks like this in Eclipse:
<project>/src/my/app/MyApp.java // main class of my application
<project>/src/my/app/sub/package/h2.jar // how to access this?
<project>/libs/h2.jar // loading from here works
I know this is stupid, but unfortunately I have to work with this strange setup. But what I don't know: how can I edit my Java code (listed above) in order to work with this setup?
EDIT: This has to work outside Eclipse as well, so adding the JAR file to the Java Build Path in Eclipse is no option for me.
EDIT2: I already tried to load "jar:file:my/app/sub/package/h2.jar!/", but that did not work for me.
Thanks in advance for all helpful ideas!
Kind regards, Matthias
In some frameworks referring to files inside JARs can be done using the classpath: prefix. I doubt URLClassLoader supports it natively, but it's worth a try (e.g. classpath:/my/app/sub/package/h2.jar). But since that doesn't work with URLClassLoader, here are other ways:
One way to do it would be to write your own ClassLoader which reads the JAR file from classpath (using getResourceAsStream), uncompresses it (using ZipInputStream) to memory (e.g. a map of byte arrays) and loads the classes from there.
Another, slightly easier way, is to read the JAR file from classpath and write it into a temporary file. Then you can use the plain URLClassLoader to load classes from it. This has the disadvantage that the file must be written to a file and the file probably cannot be removed until the JVM exits (unless using Java 7 or higher).
I'm using the second approach (copying to a temp file) in one project, though I'm using it to launch an external process. I would be curious to hear why you have such a requirement. If it's just a matter of having the whole application in one JAR, there are numerous simpler methods for achieving that (Maven Assembly Plugin, Maven Shade Plugin, Jar Jar Links, One-JAR to name a few).
No it's not a homework, but an online build system that uses my classes under my/app/* and several other classes (not from me) to automatically build the whole solution. Anyway, I can't give you more details on the internals of this system, as I don't know them. As said, I simply have to live with it, and that is why I am asking here...
Sounds like you are working in a WTF environment (does it have a name?), so here are some ways to start hacking around it:
Find out more about your environment, especially absolute file paths of the following: directory where the source files are saved, directory where the generated .class files are saved, and the current working directory when the program is run.
If you can get any kind of output of what your program prints during runtime, you can put into your application some debug code where you use File.listFiles() to crawl the machine's directory trees. If you can get output only from what happens when compiling, it might be possible to execute your own code during compile by creating your own annotation processor (apt is part of javac since Java 6), though I'm not sure whether the annotation processor must be compiled first separately.
The working directory can be read from the user.dir system property and the location of class files can be probably gotten from the java.class.path system property (unless custom class loaders are used). There is no guarantee that a JAR file in the source directory would be copied to the classpath, so you might need to do some looking around.
Then when you know the file path of the JAR file, then you can get an URL to it using new File("path/to/h2.jar").toURI().toURL() which you can then pass to URLClassLoader.
If nothing else works, upload the source code of the libraries and compile them together with your project.
In the long run, try to replace the WTF build environment with one that uses a standard build tool (such as Maven) and a common CI server (such as Jenkins). It's normal for projects to have lots of library dependencies, so you shouldn't need to hack around a build environment to use them.