Is the JVM a compiler or an interpreter?_问答_开发者

I have a v开发者_如何学编程ery 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.