JVM 是编译器还是解释器？

关于 JVM，我有一个非常基本的问题: 它是一个编译器还是一个解释器？

如果它是一个解释器，那么存在于 JVM 中的 JIT 编译器怎么办？
如果都不是，那么 JVM 到底是什么？(我不想要 jVM 的基本定义，比如把字节码转换成机器特定的代码等等)

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小开

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.

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It's both. It can interpret bytecode, and compile it to native code.

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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.

小开

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.

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最佳答案

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. It's main purpose is to do heavy optimizations in performance.

So now, Let's find answers to your questions..

1)JVM: is it a compiler or an interpreter? -- Ans: Interpreter

2)what about JIT compiler that exist inside the JVM? -- Ans: If you read this reply completly, you probably know it now

3)what exactly is the JVM? -- Ans:

  • 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 principle reasons why java is secure)
  • Next, the Execution Engine component converts the Bytecode into executable machine code

Hope this helped you..

小开

Like @delnan already stated in the comment section, it's neither.

JVM is an abstract machine running Java bytecode.

JVM has several implementations:

...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.

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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.


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