I am new to java so I dont know if my question makes sense. I have a jar which I got from another developer and this jar is compiled in java 1.7 and I am using 1.6. So when I run this application, it gives me a Unsupported major.minor version error. So,I would like to know is there a way that I can recompile the jar in 1.6 somehow? I cannot update my java version to 1.7 because, the development system cannot be connected to internet.
Thanks
There are some approaches like Retrotranslater, but it seems that it has not been updated for 1.7 => 1.6 (we used this some years ago for 1.5 => 1.4 or similar).
The general advice is to compile the source code with 1.6 (which should be easy if no 1.7 specific features are used).
Otherwise, you should really consider updating your java version to 1.7 somehow, since 1.6 is at its end of life already. Even if your development system is isolated, there must be some connectivity since somehow you also transferred the jar file from the other developer. With the same approach, you should be able to transfer the JDK installation package.
See also this question for more background information: Compiling Java 7 to Java 6
Related
I am new to Java and am a bit confused about how this is working/how I should be working. I am using intellij and a project that I am working on its pom.xml has:
<java.version>11</java.version>
<maven.compiler.source>$(java.version)</maven.compiler.source>
<maven.compiler.target>$(java.version)</maven.compiler.target>
When I go into the project structure on intellij the module is using language level 11.
on my computer I just downloaded the newest JDK (17)?
Does this cause issues working like this? Should I only be using a JDK that is associated with the version I am working?
I have not had any issues... but I am afraid my dependencies might be different than the ones I should be using...or the the build will be different if someone else is using another jdk.
The JDK version specified in the pom.xml specifies what source and target version is passed to javac. This specifies what system libraries and language features can be used.
The language level from IntelliJ matches this.
The installed JDK is the program (or set of programs) used for compiling and running the application.
Java allows to use a newer JDK to compile programs for older (source/target/release) versions.
The produced class files (bytecode) should be the same no matter what JDK version you use as long as the target/release version (specified in the pom.xml/language level in IntelliJ) is the same.
Furthermore, Java is (almost completely) backwards compatible. When writing code for an old Java version, it will likely also work in newer Java versions.
I regularly use MALLET for topic modeling in the classes that I teach. Running MALLET requires users to have the Java Development Kit installed. I currently have JDK 8 update 241 installed on my main computer, and I know that MALLET works properly in this environment. That said, JDK is now up to v14.
Which version(s) of JDK does MALLET support?
I'm not altogether sure that you do need the JDK. They never say that on the website. The tarfile that I downloaded already includes compiled classes - you aren't expected to build it from source - so the JRE should be enough.
Strangely enough, the compiled classes in the class directory are targeted at 1.7 (bytecode version 51) whereas the pom indicates that it's supposed to target Java 1.6. So it's quite probable that by rebuilding it you could support an older version of Java.
In any case, the JDK is backwards compatible by design. Any version from 7 onwards will be able to run it (6+ if you were to rebuild it).
Running it on a newer version will benefit from the new features of the JDK, such as improvements to the garbage collector, so you may see some performance improvement there. If you are not concerned about that then it doesn't matter.
I created a java desktop application using net beans in java version 8 environment. The application is running successfully in my machine. But if i try to run the application in a computer which is having java 7 or lower, it is throwing Unsupported major.minor version 52.0 error. What changes i have to make so that the application runs on other versions without errors? Do i need to develop the whole application again using lower versionof java?
With a bit of luck, you just need to recompile your code with a lower jdk version.
But, if you have used new technologies (Lambda's, ... ), yes, you will need to re-write certain parts of your code.
In NetBeans:
Right click on your project
Click on Properties
Go to Sources in the Properties window
Change Source/Binary Format to JDK 7
Make any changes in your code, necessary to make it compile
My question is if Java JDK and JREs have to be compatible to run?
I mean: will Java applications written using JDK version 8 in future work with current JRE's?
It is possible to use cross-compilation options when compiling. Do that and it will be possible to compile code with SDK 8 that is compatible with Java 1.1. It won't be very advanced code for 1.1, but it will run.
The short answer is No.
If you develop your application in JDK 8 and run it with JRE 7, you would get an UnsupportedClassVersionError.
This question is two part:
JDK vs JRE
forward / backward compatibility.
JRE is the acronym for Java Runtime Environment. JDK is the acronym for Java Development Kit: a set of tools which you use to develop Java programs. The JDK also contains a full JRE. In general there is no compatibility issue between the two. But you might want to take care not to use libraries which are only available in the JDK (for example code generation or the tools.jar)
Java itself is compiling to bytecode, which is forward compatible. That means you can use bytecode of any Java version and run it with any newer version. The other way around generally doesn't work and is checked by using the class file version ("java.lang.UnsupportedClassVersionError: Test : Unsupported major.minor version 51.0").
Then there are Java libraries, including the core libraries. So far there was never anything removed from them, so they are forward compatible. This is probably going to change with Java 9 where a very small usually unused library functions are removed.
Regarding to backwards compatibility, this is possible by setting the Java compiler to produce Bytecode of an older version. Up until Java 8, the compiler was always able to produce bytecode of the last two major versions as well. However, you might successfully compile a Java 8 source to Java 6, but not be able to run it. That is the case when you use libraries that are only available on a never Java. For such cases there is for example the maven animalsniffer plugin which will verify that when you compile against an older version, you actually only use libraries existing in said version.
I have a PowerMac and it is giving me bad version number on some .jars. I would love to make it seem like I am running Java 6. How would I spoof the version? Let me also say I am running PowerPC and Leopard
The most likely problem is that you have Java 6 JAR files and you are trying to run them on an old Java installation.
How would I spoof the version?
The answer to your question is that you can't. The way to run Java 6 specific JAR files it to use a Java 6 (or later) JRE or JDK.
The problem is that the format of Java class files has changed, and your installation can't cope with the new format. And this is not a gratuitous change that you can pretend doesn't exist. Java 6 (actually Java 5) has support for generic types, enums, annotations and other things. Assuming that the JARs contain code that uses these new language features, an older JRE simply won't know what to do with them.
There are two solutions:
Upgrade your Java installations to the required level on all machines. This is the best solution ... if it is an option ... because it means your users will get the benefit of security and bug fixes and performance enhancements. (And progress of your project won't be held back by the constraint of supporting legacy platforms.)
Compile all of your code for compatibility with the oldest version of Java that you still have to use. Either compile on the corresponding old JDK, or on a more recent JDK using appropriate -source / -target / -Xbootclasspath options ... as described by the javac manual page.
The catch with the second solution is that if the source code for the JAR files in question uses recently added Java language features or APIs, then recompiling for the older platform will fail. To fix this you will need to rewrite your code to replace the nice modern stuff with archaic stuff. Not a good solution, IMO.
The other possibility is that you are seeing corrupted JAR files. This is unlikely, but it can happen if you are using applets or webstart, and the server is delivering error pages instead of JAR files.
The third possibility is that you simply haven't configured your Mac's Java installation's correctly. Making Java 7 the default should allow you to run everything without class version problems. (Thanks #paulsm4) Note that I can't help you with that ... 'cos I don't use Java on a Mac.