I have a very basic question about JVM: is it a compiler or an interpreter?
If it is an interpreter, then what about JIT compiler that exist inside the JVM?
If neither, then what exactly is the JVM? (I dont want the basic definition of jVM of converting byte code to machine specific code etc.)
First, let's have a clear idea of the following terms:
Javac is Java Compiler -- Compiles your Java code into Bytecode
JVM is Java Virtual Machine -- Runs/ Interprets/ translates Bytecode into Native Machine Code
JIT is Just In Time Compiler -- Compiles the given bytecode instruction sequence to machine code at runtime before executing it natively. Its main purpose is to do heavy optimizations in performance.
So now, Let's find answers to your questions:
JVM: is it a compiler or an interpreter?
An interpreter
What about JIT compiler that exist inside the JVM?
If you read this reply completely, you probably know it now.
What exactly is the JVM?
JVM is a virtual platform that resides on your RAM
Its component, Class loader loads the .class file into the RAM
The Byte code Verifier component in JVM checks if there are any access restriction violations in your code. (This is one of the principal reasons why java is secure)
Next, the Execution Engine component converts the Bytecode into executable machine code
It is a little of both, but neither in the traditional sense.
Modern JVMs take bytecode and compile it into native code when first needed. "JIT" in this context stands for "just in time." It acts as an interpreter from the outside, but really behind the scenes it is compiling into machine code.
The JVM should not be confused with the Java compiler, which compiles source code into bytecode. So it is not useful to consider it "a compiler" but rather to know that in the background it does do some compilation.
Like #delnan already stated in the comment section, it's neither.
JVM is an abstract machine running Java bytecode.
JVM has several implementations:
HotSpot (interpreter + JIT compiler)
Dalvik (interpreter + JIT compiler)
ART (AOT compiler + JIT compiler)
GCJ (AOT compiler)
JamVM (interpreter)
...and many others.
Most of the others answers when talking about JVM refer either to HotSpot or
some mixture of the above approaches to implementing the JVM.
It is both. It starts by interpreting bytecode and can (should it decide it is worth it) then compile that bytecode to native machine code.
It's both. It can interpret bytecode, and compile it to native code.
Javac is a compiler but not a traditional compiler.
A compiler typically converts source code to Machine level language for execution and that is done in a single shot i.e. entire code is taken and converted to machine level language at ONCE. (more on this below).
While, JavaC converts it to Bytecode instead of machine level language.
JIT is a Java compiler but also acts as an interpreter. A typical compiler will convert all the code at once from source code to machine level language. Instead, JIT goes line by line (line by line execution is a feature of Interpreters) and converts bytecode generated by JavaC into machine level language and executes it. JVM which has JIT in it has multiple implementations. Hotspot being one of the major ones for Java programming. Hotspot implementation makes JIT optimize the execution by converting chunks of code which are repetitive into Machine level language at once (like a compiler as mentioned above) so that they can be executed faster instead of converting each line of code 1 by 1.
So, the answer is not Black and White with respect to the typical definitions of Compiler and Interpreter.
This is my understanding after reading several online answers, blogs, etc. If somebody has suggestions to improve this understanding, please feel free to suggest.
JVM have both compiler and interpreter. Because the compiler compiles the code and generates bytecode. After that the interpreter converts bytecode to machine understandable code.
Example: Write and compile a program and it runs on Windows. Take the .class file to another OS (Unix) and it will run because of interpreter that converts the bytecode to machine understandable code.
Related
After hours of research I haven't found a concrete answer for my question and I'm going maddd!:
The steps from editing to execution:
1 . (Compilation step) After writing the source code, i compile the program. In this step it is converted into bytecode. A java.class file (the bytecode) is generated.
2 .(Execution step) Now i execute the program.
(Interpretation step) When I do this, the JVM interprets the bytecode into machine code. So I understand that the machine code is only generated after execution!??
Now the steps are: code-->bytecode-->execution-->machinecode
All these steps are hardware- and software-independent.
Am i right?
This is called JIT (just in time compilation), so that when I execute the program the bytecode is compiled into machinecode, and only then.
So why is this step called interpretation?
I'm thanking you in advance for your answers!
In short because JVM doesn't have to have JIT. It can interpret the bytecode instead of compiling it. Of course an interpret-only JVM would be slow, but the JIT part is just an extra feature to improve performance, not a required property of a Java Virtual Machine. The -Xint command line parameter can be used to run a java program in interpret-only mode.
The reason it's compiled to bytecode and not machine code is to get the platform independence. Bytecode is platform independent, so the same code can run on any platform (as long as there's the JVM to interpret it). If it were compiled into machine code, it would be operating system and processor architecture dependent.
(Interpretating step) When i do this, the JVM interprets the bytecode into machine code. So i understand that the machine code is only generated after execution!??
Not exactly, and no. A JVM operating strictly as a bytecode interpreter does not transform bytecode into machine code and then execute that. The machine code executed by such a JVM is (comprised by) the pre-existing machine code of the JVM itself. The byte code is used to provide some of the data on which to operate and to direct which of the JVM's machine code is executed.
Now the steps are: code-->bytecode-->execution-->machinecode
All these steps are hardware- and software-independent. Am i right?
No, not at all. The particulars of the Java code --> bytecode transformation are somewhat dependent on which Java compiler (software) you use. The Java virtual machine you use must be specific to the hardware on which it runs, and it is itself a piece of software. Moreover, the operating environment is influenced by a lot of other software.
Java hardware independence, such as it is, means that a Java program (bytecode) will behave consistently on any hardware, but the details of how that consistent behavior is provided on any given machine are all kinds of hardware- and software-dependent.
This is called JIT(just in time compilation), so that when I execute the program the bytecode is compiled into machinecode, and only then. But why is this step called interpretating?
JIT is something else, and a JVM that performs JIT (as in fact most do) is not strictly an interpreter. Most such JVMs run some bytecode in an interpretative manner as described above, but compile some bytecode to native (machine) code, and run that machine code directly when subsequently needed. The latter manner of execution generally isn't called "interpreting".
To simplify the question, lets assume that in our JRE we have a Hotspot JVM implementation that doesn't use JIT.
Meaning we won't have machine code precompiled.
I am trying to understand, the JVM when it encounters byte code, does the interpreter uses the Hotspot compiler to compile the code to machine code and only then can the JVM run the code?
Or does the interpreter simply runs the code as the JVM has some kind of native methods that correspond to each byte code command?
Meaning we won't have machine code precompiled.
The above sentence of yours indicates your misunderstanding of what the JIT compiler is. It doesn't compile code ahead of time, but just in time, hence its name.
This makes the rest of your question harder to understand because the "HotSpot compiler" is the JIT compiler.
But, to sum this up for you: in a normal case, HotSpot begins by interpreting the bytecode, then JIT-compiles those pieces whose runtime profiling data indicates that they are the "hot spots", being executed a lot (default threshold is 10,000 passes over a piece of code).
If you disable JIT compiling, HotSpot will indeed just interpret all of the Java bytecode.
On the EN Wiki I read that both C# and Java are interpreted languages, however at least for C# I think it is not true.
Many interpreted languages are first compiled to some form of virtual
machine code, which is then either interpreted or compiled at runtime
to native code.
From my understanding, it is compiled into CIL and when run, using JIT its compiled to target platform. I have also read that JIT is an interpreter, is that really so?
Or are they called interpreted as they are using intermediate code? I do not understand it.
Thanks
JIT is a form of compilation to native (machine) code. Typically (but not as a necessity), implementations of either the CLI and JVM are compiled in two steps:
the language compiler compiles code to something intermediate (IL/bytecode)
the JIT compiles that to native/machine code at runtime
However, interpreters for both do exist. Micro Framework operates as an IL interpreter, for example. Equally, tools like (looking .NET here) NGEN and "AOT" (mono) allow compilation to native/machine code at the start.
They are considered JIT languages which is different from interpreting. JIT simply compiles to native code when needed during execution. The common strategy is to compile into an intermediate representation (bytecode) beforehand which makes the JIT faster.
However, there is nothing that prevents them from being interpreted, or even statically compiled. Languages are simply languages - how they are executed is irrelevant from a language perspective.
On the EN Wiki I read that both C# and Java are interpreted languages
Can you pls provide the link?
May be the interpreted word means different here. It perhaps means that these languages are first interpreted to convert source code into platform-independent code.(VM Specific)
are they called interpreted as they are using intermediate code
I too think so.
I have also read that JIT is an interpreter
JIT is a compiler. See this
Is something "interpreter" or not depends on context of discussion.
From purely abstract view interpreter can be defined as any intermediate program present in runtime which dynamically translates program code written in one language to a target code of hardware/software of other language. Think about runing java bytecode on x86 hardware, or running Python on CLR VM what exactly IronPython is. In this view every virtual machine is an interpreter of some kind. As it is program present in runtime it clearly differs from static compilers or hardware implemented VM-s.
Now there are many different ways to achieve this functionality where accent is on "dynamically" and "present in runtime".
In discussions where implementation of VM matters, people make clear distinction between "classical" interpreter and JIT-ed one. Classical interpreter is something which for every instruction of hosted program emits routine of target code. This design is simple to build, but hard to optimize. JIT-ed design reads bunch of instruction of original code, and then translates all those instructions to a one native compiled routine. So it "interprets" faster. It is like micro static compiler within VM. There are many different ways to accomplish behavior labeled as JIT, and then there are other approaches like tracing compilers.
Modern VM's like CLR, HotSpot and J9 JVM's are even more complex than to be tagged with simple labels as JIT or Interpreter. They can be at a same time static compilers (AOT execution), classical interpreters and JIT-ed VMs.
For example CLR can compile code Ahead-Of-Time (static compiler), and store native code as bunch of more or less excutable files on disk to be used for faster future startups of hosted program. I believe "ngen" is AOT process used in windows for this functionality. If AOT is not used CLR behaves as JIT VM.
J9 and HotSpot are able to switch in runtime between purely interpreted execution or JIT-ed on depending of code analysis and current load. So it's is quite gray area. J9 even has AOT functionality similar to CLR.
Some other VMs like Maxine JVM or PyPy are socalled "metacircular" VM. This means they are (mostly) implemented in a same language they host (Maxine is JVM written in Java). In order to provided good code they usually have some JIT like behavior implemented in host language which is than bootstrapped and optimized by a very low, close to machine, interpreter.
So actual definition of interpreter varies on context of discussion. When labels like JIT are used then there is clear accent of discussion to an implementation details of VM being discussed.
From wiki: In computing, just-in-time compilation (JIT), also known as dynamic translation, is a technique for improving the runtime performance of a computer program.
So I guess JVM has another compiler, not javac, that only compiles bytecode to machine code at runtime, while javac compiles sources to bytecode,is that right?
That is precisely correct.
javac compiles .java source code to .class bytecode for JVM (Java Virtual Machine)
HotSpot, at run-time, identifies which portion of the bytecode is worth further compiling to the running platform instructions for performance
See also
HotSpot FAQ
Even more compilers can get involved!
Note that other languages other than Java can also join in on the fun, by having their own compilers that compile to JVM bytecode, and then use whatever JVM runtime to run on. On HotSpot, this too mean that they'll get JIT-compiled.
See also
Wikipedia/List of JVM Languages
Yes, JIT works in runtime.
Javac translates java source to java bytecode. While JVM interprets that bytecode or compiles it to native code. But this isn't a step like translating the source, as the JIT compiler hasn't any user frontend. Also, the JIT runs only for hot methods - the most called ones.
That's precisely right.
Suns JVM (and most other ones too I suppose), doesn't compile entire class-files into machine-code right away, but runs the application for a period of time in order to detect hotspots in the code, which would benefit from being compiled (instead of interpreted), and compiles those "just in time".
This two step compilation process to get to native machine code actually also happens with most statically compiled languages such as C and C++. First they will compile the code into a temporary format such as 2-3 code, then a second compiler will translate this into native machine code. The purpose of this separation into frontend and backend compiler is to make it much easier to port the compiler to a different machine architecture or to accept a different input language. GCC is a good example of a static compiler that is very versatile due to this architecture.
The benefit from doing the final translation at runtime, other than not having to select the target machine architecture until you run the application, is that you have additional information available about how the program is actually being run. This can be used very effectively to improve the final compilation.
Compile time - javac compiles java code into bytecode (.class files).
Run-time - the JVM interprets bytecode into machine code. JIT is an optimization that allows faster execution of bytecode, by detecting hotspots (for example, in the Sun JVM) and compiling the code beforehand.
In the past I have used C++ as a programming language. I know that the code written in C++ goes through a compilation process until it becomes object code "machine code".
I would like to know how Java works in that respect. How is the user written Java code run by the computer?
Java implementations typically use a two-step compilation process. Java source code is compiled down to bytecode by the Java compiler. The bytecode is executed by a Java Virtual Machine (JVM). Modern JVMs use a technique called Just-in-Time (JIT) compilation to compile the bytecode to native instructions understood by hardware CPU on the fly at runtime.
Some implementations of JVM may choose to interpret the bytecode instead of JIT compiling it to machine code, and running it directly. While this is still considered an "interpreter," It's quite different from interpreters that read and execute the high level source code (i.e. in this case, Java source code is not interpreted directly, the bytecode, output of Java compiler, is.)
It is technically possible to compile Java down to native code ahead-of-time and run the resulting binary. It is also possible to interpret the Java code directly.
To summarize, depending on the execution environment, bytecode can be:
compiled ahead of time and executed as native code (similar to most C++ compilers)
compiled just-in-time and executed
interpreted
directly executed by a supported processor (bytecode is the native instruction set of some CPUs)
Code written in Java is:
First compiled to bytecode by a program called javac as shown in the left section of the image above;
Then, as shown in the right section of the above image, another program called java starts the Java runtime environment and it may compile and/or interpret the bytecode by using the Java Interpreter/JIT Compiler.
When does java interpret the bytecode and when does it compile it? The application code is initially interpreted, but the JVM monitors which sequences of bytecode are frequently executed and translates them to machine code for direct execution on the hardware. For bytecode which is executed only a few times, this saves the compilation time and reduces the initial latency; for frequently executed bytecode, JIT compilation is used to run at high speed, after an initial phase of slow interpretation. Additionally, since a program spends most time executing a minority of its code, the reduced compilation time is significant. Finally, during the initial code interpretation, execution statistics can be collected before compilation, which helps to perform better optimization.
The terms "interpreted language" or "compiled language" don't make sense, because any programming language can be interpreted and/or compiled.
As for the existing implementations of Java, most involve a compilation step to bytecode, so they involve compilation. The runtime also can load bytecode dynamically, so some form of a bytecode interpreter is always needed.
That interpreter may or may not in turn use compilation to native code internally.
These days partial just-in-time compilation is used for many languages which were once considered "interpreted", for example JavaScript.
Java is compiled to bytecode, which then goes into the Java VM, which interprets it.
Java is a compiled programming language, but rather than compile straight to executable machine code, it compiles to an intermediate binary form called JVM byte code. The byte code is then compiled and/or interpreted to run the program.
Kind of both. Firstly java compiled(some would prefer to say "translated") to bytecode, which then either compiled, or interpreted depending on mood of JIT.
Java does both compilation and interpretation,
In Java, programs are not compiled into executable files; they are compiled into bytecode (as discussed earlier), which the JVM (Java Virtual Machine) then interprets / executes at runtime. Java source code is compiled into bytecode when we use the javac compiler. The bytecode gets saved on the disk with the file extension .class.
When the program is to be run, the bytecode is converted the bytecode may be converted, using the just-in-time (JIT) compiler. The result is machine code which is then fed to the memory and is executed.
Javac is the Java Compiler which Compiles Java code into Bytecode. JVM is Java Virtual Machine which Runs/ Interprets/ translates Bytecode into Native Machine Code. In Java though it is considered as an interpreted language, It may use JIT (Just-in-Time) compilation when the bytecode is in the JVM. The JIT compiler reads the bytecodes in many sections (or in full, rarely) and compiles them dynamically into machine code so the program can run faster, and then cached and reused later without needing to be recompiled. So JIT compilation combines the speed of compiled code with the flexibility of interpretation.
An interpreted language is a type of programming language for which most of its implementations execute instructions directly and freely, without previously compiling a program into machine-language instructions. The interpreter executes the program directly, translating each statement into a sequence of one or more subroutines already compiled into machine code.
A compiled language is a programming language whose implementations are typically compilers (translators that generate machine code from source code), and not interpreters (step-by-step executors of source code, where no pre-runtime translation takes place)
In modern programming language implementations like in Java, it is increasingly popular for a platform to provide both options.
Java is a byte-compiled language targeting a platform called the Java Virtual Machine which is stack-based and has some very fast implementations on many platforms.
Quotation from: https://blogs.oracle.com/ask-arun/entry/run_your_java_applications_faster
Application developers can develop the application code on any of the various OS that are available in the market today. Java language is agnostic at this stage to the OS. The brilliant source code written by the Java Application developer now gets compiled to Java Byte code which in the Java terminology is referred to as Client Side compilation. This compilation to Java Byte code is what enables Java developers to ‘write once’. Java Byte code can run on any compatible OS and server, hence making the source code agnostic of OS/Server. Post Java Byte code creation, the interaction between the Java application and the underlying OS/Server is more intimate. The journey continues - The enterprise applications framework executes these Java Byte codes in a run time environment which is known as Java Virtual Machine (JVM) or Java Runtime Environment (JRE). The JVM has close ties to the underlying OS and Hardware because it leverages resources offered by the OS and the Server. Java Byte code is now compiled to a machine language executable code which is platform specific. This is referred to as Server side compilation.
So I would say Java is definitely a compiled language.