This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Compiling a java program into an exe
How can I convert my java program to an .exe file ?
hi everyone . there are many programs such intellij idea or jedit which they have written in JAVA ,
How they have compiled the code into the exe file ?
How can ,I compile my java code to exe file and run it on windows?
And Why javac give us class file ? what is it ?
how about searching stack overflow:
How do I create an .exe for a Java program?
also, like the answer points out launch4j is the program you want to use. i have used it and it works well.
mkoryak took care of how to create an .exe for a Java program, but let me address "What is a Class file"?
In general, you don't have to compile Java down to byte code in the form of an exe (or anything else). You compile .java files into .class files. These files can't be run by an OS directly - rather, they are run by the Java Virtual Machine (JVM). The JVM is essentially a layer between your Java program and the underlying OS. The JVM will, on your behalf, interpret your .class files and pass the instructions contained in them to the underlying OS (Windows, OS X, Linux, etc).
So what does this do for you?
For one, it makes Java a little slower than other languages that are compiled right down to byte code. This was initially a big knock against Java but, as JVM interpreters have gotten better and people have migrated to slower platforms (i.e. Web Apps), this minor latency issue has become less and less of a problem.
On the flip side, however, you can write your Java code for a single OS - the JVM. You don't need to write one version of your app that targets Windows and another that targets OS X, and another, and another... Rather, you write one version of your application and you target that single OS (the JVM). The fact that there are versions of the JVM for every major OS out there means that, as soon as you write a Java program, you can run it on any platform (in theory, anyway).
That's one of the really cool things about Java, and also the meaning behind the often-heard tagline: "Write once, run anywhere."
So that's what the .class files are all about.
In Java, you write .java files and use a Java compiler (such as javac) to create .class files. These .class files are then run by a Java runtime (such as java), which executes your program on the JVM.
I hope that helps and, maybe, you don't need to compile your Java program down to an exe, at all.
Related
Sorry if this is off-topic. I am a java beginner and know java is supposed to be cross-platform consistent. But I wonder if the fact that my jar file or *class file executes on Ubuntu guarantees it will run fine on MacOS?
I basically do the following to create *class and *jar. Two java classes, MAIN.java depends on SIDE.java and both java files include package classes; header (javac creates a folder named classes and puts MAIN.class and SIDE.class in there):
javac -d . SIDE.java MAIN.java
jar cvfe MAIN.jar classes/MAIN classes/*.class
I tried running both:
java classes.MAIN -read number.logs
and
java -jar MAIN.jar -read number.logs
and they both run fine on Ubuntu. Is this good enough and it will run on MacOS? (I don't have MacOS, is there a simulator that I could use in this case to check things?)
Yes this is good enough until and unless you use some environment variables i.e. platform specific features like
System.getProperty("os.name").
This cross platform portability is ensured by JVM. You can read more about it here.
Java is cross-platform for a reason. Your java code is translated into a java bytecode (your .class files) and the JVM (Java Virtual Machine) is a machine which is running your bytecode.
This JVM comes with the Java installation on your operating system. Thus if you can install the JRE (which includes the JVM) you can run your code.
There is one exception, the Java language sometimes uses os-standards for representation of graphics (awt classes), so your program, if you are using those classes, can look different, but work the same.
How, for example, an operating system is saving files, is handled by the JVM and shouldn't concern you as a high-level developer unless you are running into restrictions from the OS-Side. Those restrictions are, depending on the case, handled by Exceptions. If you handle exceptions in your code, there is little to nothing that will not work on one os if it worked on another.
I know a good hacker can obtain the source code of my Java files but I want to at least make it difficult for any one else to do so.
I can use ProGuard to obfuscate my jar file and the next step is to convert the jar file into Windows exe, Ubuntu bin?, Mac Os ...
What are the free software to do so?
None at all. java-code is always compiled into .class-files, which are bundled into .jar-files. Just change the file-extension of a jar into .zip and try to open it. You can create a launcher for the app, but the app itself is written in java. Next point:
"A good hacker" isn't necessary. There are decompilers out there available to anyone, which can convert .class-files into readable java-files. Obfuscating code alters variable- and class-names in a way that makes them unusable for human readers; good obfuscators in addition alter the bytecode in a way that makes the code (nearly) decompileable. And .exe and the counterparts on Linux and Mac OS are decompile-able as well, so there isn't much use in it anyways.
In general: executable binaries more secure than a .jar-file. The vital point is to obfuscate the byte-code properly.
In Unix (or Linux), if I want to run a shell script, I can start the file with #!/bin/sh. With awk, I start the executable file with #!/usr/bin/awk -f and it treats the rest of the file as the program.
How do I do that with a Java program? I tried copying the simple.class to simple, putting #!/export/appl/Mail/java/bin/java at the top and making the file executable, but I get:
69> ./simple
Error: Could not find or load main class ..simple
I know this can be done with an executable shell script, or a C program that execs the java interpreter. Every other interpreter on Unix can be called with a #! load card, surely there's a way to do it with Java.
The most usual way is to have a wrapper for the Java. A shell script that executes the "java -jar yourJar.jar" or equivalent. And then you bundle the shell script and the windows equivalent bat file with your product.
Another option is to have a native launcher. For example you can see the Eclipse project which has gone that way. You download Eclipse and you have a native executable to run. The native executable will launch your Java program.
One more option is to compile Java into native code. For example you can use this commercial tool called Excelsior JET ( http://www.excelsior-usa.com/jet.html ).
The Java class file format doesn't allow text before the header, that's why the Java runtime no longer accepts the .class file after your modification.
On Linux you can use binfmt_misc to support additional executable formats, including .class files. It's basically a way to tell the Linux kernel how to detect executable formats and how to execute them.
This Archilinux Wiki article explains in more detail how to get this effect.
You cannot do it with a Java program. Firstly, the Java program needs to be compiled before execution. Secondly, even if compilation wasn't required, the hash sign is not a comment in Java, so that would be a syntax error.
I've never heard the term "load card". What you have is an "interpreter directive" designated by a shebang. This merely designates which interpreter the shell should invoke on a given script.
As for why C programs can be run directly in the shell, executables recognized by the operating system are passed to the loader. A Java class isn't an executable, at least to the OS anyway. So the shell must know which interpreter to pass control to.
But as you've noticed, the shebang doesn't work. The reason is that the class file is in a specific binary format that the JVM expects. Editing this file will break convention and lead to an error. Therefore, there is no way to do what you've asked.
However, you can create a "shortcut" to the program you want to run by creating an alias or perhaps writing a one-line Shell script to wrap the java command you need. This is the common practice as I understand it.
The other answers explain why you can't do what you are trying to do. However, if your shell is zsh, you can create a suffix alias. For example, to execute .jar files using java:
alias -s .jar="/usr/bin/java -jar"
Then, to execute blarg.jar, you just type ./blarg.jar. Of course, you must chmod +x your .jar file first.
Apart from the wrapper script and binfmt_misc solutions suggested by others, I'd like to suggest a potential solution which doesn't directly answer your question but maybe it solves your actual problem.
Since Scala does have an interpreter that can run code without you having to compile it first, and this code can reference any Java code, if your goal can be summed up as "using Java as a shell scripting language", you could use a .scala file as your starter script (which can include the shebang to be run with scala) from which you can call all your Java classes. This isn't any simpler tha having a bash-based starter script, but it's a good starting point to gradually move to scripting in Scala instead of Java in which case you can get rid of the need to compile to .class file in the first place.
The reason this doesn't work is that Java isn't really an interpreted language, it's partially compiled, partially interpreted.
The .java source code that you'd put the hashbang directive in needs to be compiled to a .class file before the java interpreter can run it. Comments are stripped out by the compiler, so there's no way to push a comment from the .java into the .class file. The .class file is "compiled" output in a specific format, so adding a hashbang directive to the top of it would break the format.
Java isn't really meant to be a scripting language - but some JVM languages are. For example Groovy supports hashbang and so does Clojure.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How can I convert my Java program to an .exe file?
Compiling a java program into an exe
I believe this is an easy question, though I cannot find a quick answer.
I've been learning in a Java class about GUI making (and really Java coding in general) which, when compiled, creates a .java file. That .java file can be opened with some sort of IDE but not just opened like a .exe file.
This brings me to my question. When/where do you actually use a Java GUI? I don't foresee me passing out .java files for my friends or co-workers to use the tools I've made. Are they supposed to work best with online applications?
Your .java file contains your Java code, which you then compile to a class file. That could be executed by your friends.
In most cases you would bundle your app with its resources in an executable jar file. If configured correctly, the operating system can run the Java app just by double clicking on it. One other solution would be to use Java webstart to distribute your GUI app but that is essentially the same thing. Java and the file association to jnlp files have to be configured correctly.
The easiest way is probably to use an exe wrapper like launch4j to create an exe file that loads the JVM and runs the Java application. Those wrappers can even create distributions with bundled jvms to make sure your application is able to run if the user doesn't have Java installed.
Hope it helps.
A .java file is source code which is meant for a programmer to write code in a format suitable to them, to define the behaviour of the program.
Source code is compiled into .class files which you can execute (run), although they are often packaged into JAR files (which are simply collections of .class files).
These can be executable and can be passed between people to share programs.
GUIs are irrelevant. You can either create a GUI or not, but the functionality will still exist in the code and can be executable. GUIs allow you to view and interact with a program, whereas without a GUI you have to use the command line if interaction is required.
When Java is compiled, it creates .class files, not .java files. The .java files are source code; the .class files are the result of compilation.
.java files can be opened by an IDE, but it doesn't make sense to open them as an .exe file because, again, .java files are source code.
To run Java applications, you have to run a Java Virtual Machine which is specific to your operating system. You provide the .class file as input, and the JVM runs it.
The .class files are the Java-equivalent to Windows .exe files, in the sense that they are "executable". However, while .exe files can be directly executed by Windows, .class files aren't directly executable by any operating system but by an operating system-specific JVM.
The "strength" of Java in that respect is that the same .class file (executable) can be run on any operating system that has a JVM installed on it.
It really doesn't matter if you develop a Java GUI (graphic user interface) or a Java CL (command line) program as in the end your software is supposed to run just like an *.exe as long as the user have the "Java Virtual Machine" installed.
Confused by java compilation process
OK i know this: We write java source code, the compiler which is platform independent translates it into bytecode, then the jvm which is platform dependent translates it into machine code.
So from start, we write java source code. The compiler javac.exe is a .exe file. What exactly is this .exe file? Isn't the java compiler written in java, then how come there is .exe file which executes it? If the compiler code is written is java, then how come compiler code is executed at the compilation stage, since its the job of the jvm to execute java code. How can a language itself compile its own language code? It all seems like chicken and egg problem to me.
Now what exactly does the .class file contain? Is it a abstract syntax tree in text form, is it tabular information, what is it?
can anybody tell me clear and detailed way about how my java source code gets converted in machine code.
OK i know this: We write java source code, the compiler which is platform independent translates it into bytecode,
Actually the compiler itself works as a native executable (hence javac.exe). And true, it transforms source file into bytecode. The bytecode is platform independent, because it's targeted at Java Virtual Machine.
then the jvm which is platform dependent translates it into machine code.
Not always. As for Sun's JVM there are two jvms: client and server. They both can, but not certainly have to compile to native code.
So from start, we write java source code. The compiler javac.exe is a .exe file. What exactly is this .exe file? Isn't the java compiler written in java, then how come there is .exe file which executes it?
This exe file is a wrapped java bytecode. It's for convenience - to avoid complicated batch scripts. It starts a JVM and executes the compiler.
If the compiler code is written is java, then how come compiler code is executed at the compilation stage, since its the job of the jvm to execute java code.
That's exactly what wrapping code does.
How can a language itself compile its own language code? It all seems like chicken and egg problem to me.
True, confusing at first glance. Though, it's not only Java's idiom. The Ada's compiler is also written in Ada itself. It may look like a "chicken and egg problem", but in truth, it's only a bootstrapping problem.
Now what exactly does the .class file contain? Is it an abstract syntax tree in text form, is it tabular information, what is it?
It's not Abstract Syntax Tree. AST is only used by tokenizer and compiler at compiling time to represent code in memory. .class file is like an assembly, but for JVM. JVM, in turn, is an abstract machine which can run specialized machine language - targeted only at virtual machine. In it's simplest, .class file has a very similar structure to normal assembly. At the beginning there are declared all static variables, then comes some tables of extern function signatures and lastly the machine code.
If You are really curious You can dig into classfile using "javap" utility. Here is sample (obfuscated) output of invoking javap -c Main:
0: new #2; //class SomeObject
3: dup
4: invokespecial #3; //Method SomeObject."<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #4; //Method SomeObject.doSomething:()V
12: return
So You should have an idea already what it really is.
can anybody tell me clear and detailed way about how my java source code gets converted in machine code.
I think it should be more clear right now, but here's short summary:
You invoke javac pointing to your source code file. The internal reader (or tokenizer) of javac reads your file and builds an actual AST out of it. All syntax errors come from this stage.
The javac hasn't finished its job yet. When it has the AST the true compilation can begin. It's using visitor pattern to traverse AST and resolves external dependencies to add meaning (semantics) to the code. The finished product is saved as a .class file containing bytecode.
Now it's time to run the thing. You invoke java with the name of .class file. Now the JVM starts again, but to interpret Your code. The JVM may, or may not compile Your abstract bytecode into the native assembly. The Sun's HotSpot compiler in conjunction with Just In Time compilation may do so if needed. The running code is constantly being profiled by the JVM and recompiled to native code if certain rules are met. Most commonly the hot code is the first to compile natively.
Edit: Without the javac one would have to invoke compiler using something similar to this:
%JDK_HOME%/bin/java.exe -cp:myclasspath com.sun.tools.javac.Main fileToCompile
As you can see it's calling Sun's private API so it's bound to Sun JDK implementation. It would make build systems dependent on it. If one switched to any other JDK (wiki lists 5 other than Sun's) then above code should be updated to reflect the change (since it's unlikely the compiler would reside in com.sun.tools.javac package). Other compilers could be written in native code.
So the standard way is to ship javac wrapper with JDK.
Isn't the java compiler written in java, then how come there is .exe file which executes it?
Where do you get this information from? The javac executable could be written in any programming language, it is irrelevant, all that is important is that it is an executable which turns .java files into .class files.
For details on the binary specification of a .class file you might find these chapters in the Java Language Specification useful (although possibly a bit technical):
Virtual Machine Startup
Loading of Classes and Interfaces
You can also take a look at the Virtual Machine Specification which covers:
The class file format
The Java Virtual Machine instruction set
Compiling for the Java Virtual Machine
The compiler javac.exe is a .exe file.
What exactly is this .exe file? Isn't
the java compiler written in java,
then how come there is .exe file which
executes it?
The Java compiler (at least the one that comes with the Sun/Oracle JDK) is indeed written in Java. javac.exe is just a launcher that processes the command line arguments, some of which are passed on to the JVM that runs the compiler, and others to the compiler itself.
If the compiler code is written is
java, then how come compiler code is
executed at the compilation stage,
since its the job of the jvm to
execute java code. How can a language
itself compile its own language code?
It all seems like chicken and egg
problem to me.
Many (if not most) compilers are written in the language they compile. Obviously, at some early stage the compiler itself had to be compiled by something else, but after that "bootstrapping", any new version of the compiler can be compiled by an older version.
Now what exactly does the .class file
contain? Is it a abstract syntax tree
in text form, is it tabular
information, what is it?
The details of the class file format are described in the Java Virtual Machine specification.
Well, javac and the jvm are typically native binaries. They're written in C or whatever. It's certainly possible to write them in Java, just you need a native version first. This is called "boot strapping".
Fun fact: Most compilers that compile to native code are written in their own language. However, they all had to have a native version written in another language first (usually C). The first C compiler, by comparison, was written in Assembler. I presume that the first assembler was written in machine code. (Or, using butterflies ;)
.class files are bytecode generated by javac. They're not textual, they're binary code similar to machine code (but, with a different instruction set and architechture).
The jvm, at run time, has two options: It can either intepret the byte code (pretending to be a CPU itself), or it can JIT (just-in-time) compile it into native machine code. The latter is faster, of course, but more complex.
The .class file contains bytecode which is sort of like very high-level Assembly. The compiler could very well be written in Java, but the JVM would have to be compiled to native code to avoid the chicken/egg problem. I believe it is written in C, as are the lower levels of the standard libraries. When the JVM runs, it performs just-in-time compilation to turn that bytecode into native instructions.
Short Explanation
Write code on a text editor, save it in a format that compiler understands - ".java" file extension, javac (java compiler) converts this to ".class" format file (byte code - class file). JVM executes the .class file on the operating system that it sits on.
Long Explanation
Always remember java is not the base language that operating system recognizes. Java source code is interpreted to the operating system by a translator called Java Virtual Machine (JVM). JVM cant understand the code that you write in a editor, it needs compiled code. This is where a compiler comes into picture.
Every computer process indulges in memory manipulation. We cant just write code in a text editor and compile it. We need to put it in the computer's memory, i.e save it before compiling.
How will the javac (java compiler) recognize the saved text as the one to be compiled? - We have a separate text format that the compiler recognizes, i.e .java. Save the file in .java extension and the compiler will recognize it and compile it when asked.
What happens while compiling? - Compiler is a second translator(not a technical term) involved in the process, it translates user understood language(java) into JVM understood language(Byte code - .class format).
What happens after compiling? - The compiler produces .class file that JVM understands. The program is then executed, i.e the .class file is executed by JVM on the operating system.
Facts you should know
1) Java is not multi-platform it is platform independent.
2) JVM is developed using C/C++. One of the reason why people call Java a slower language than C/C++
3) Java byte code (.class) is in "Assembly Language", the only language understood by JVM. Any code that produces .class file on compilation or generated Byte code can be run on the JVM.
Windows doesn't know how to invoke Java programs before installing a Java runtime, and Sun chose to have native commands which collect arguments and then invoke the JVM instead of binding the jar-suffix to the Java engine.
The compiler was originally written in C with bits of C++ and I assume that it still is (why do you think the compiler is written in Java as well?). javac.exe is just the C/C++ code that is the compiler.
As a side point you could write the compiler in java, but you're right, you have to avoid the chicken and egg problem. To do this you'd would typically write one or more bootstrapping tools in something like C to be able to compile the compiler.
The .class file contains the bytecodes, the output of the javac compilation process and these are the instructions that tell the JVM what to do. At runtime these bytecodes have are translated to native CPU instructions (machine code) so they can execute on the specific hardware under the JVM.
To complicate this a little, the JVM also optimises and caches machine code produced from the bytecodes to avoid repeatedly translating them. This is known as JIT compilation and occurs as the program is running and bytecodes are being interpreted.
.java file
compiler(JAVA BUILD)
.class(bytecode)
JVM(system software usually build with 'C')
OPERATING PLATFORM
PROCESSOR