与非JIT编译器相比,JIT编译器具体做什么?有人能给出简洁易懂的描述吗?


当前回答

即时编译器(JIT)是一种软件,它接收一个不可执行的输入,并返回要执行的适当机器代码。例如:

Intermediate representation    JIT    Native machine code for the current CPU architecture

     Java bytecode            --->        machine code
     Javascript (run with V8) --->        machine code

其结果是,对于特定的CPU体系结构,必须安装适当的JIT编译器。

区别编译器、解释器和JIT

虽然在一般情况下,当我们想要将源代码转换为机器码时可能会有例外,但我们可以使用:

Compiler: Takes source code and returns a executable Interpreter: Executes the program instruction by instruction. It takes an executable segment of the source code and turns that segment into machine instructions. This process is repeated until all source code is transformed into machine instructions and executed. JIT: Many different implementations of a JIT are possible, however a JIT is usually a combination of a compiler and an interpreter. The JIT first turn intermediary data (e.g. Java bytecode) which it receives into machine language via interpretation. A JIT can often measures when a certain part of the code is executed often and the will compile this part for faster execution.

其他回答

一开始,编译器负责将高级语言(定义为比汇编程序更高级别)转换成目标代码(机器指令),然后(通过链接器)将其链接成可执行文件。

At one point in the evolution of languages, compilers would compile a high-level language into pseudo-code, which would then be interpreted (by an interpreter) to run your program. This eliminated the object code and executables, and allowed these languages to be portable to multiple operating systems and hardware platforms. Pascal (which compiled to P-Code) was one of the first; Java and C# are more recent examples. Eventually the term P-Code was replaced with bytecode, since most of the pseudo-operations are a byte long.

即时(JIT)编译器是运行时解释器的一个特性,它不是在每次调用方法时都解释字节码,而是将字节码编译为运行机器的机器代码指令,然后调用此目标代码。理想情况下,运行目标代码的效率将克服每次运行时重新编译程序的低效率。

JIT代表Just-in-Time,这意味着代码在需要时才编译,而不是在运行时之前。

这是有益的,因为编译器可以生成针对特定机器优化的代码。静态编译器,就像普通的C编译器一样,将所有代码编译为开发人员机器上的可执行代码。因此,编译器将基于一些假设执行优化。它可以编译得更慢,做更多的优化,因为它不会降低用户执行程序的速度。

我知道这是一个老线程,但是运行时优化是JIT编译的另一个重要部分,这里似乎没有讨论它。基本上,JIT编译器可以在程序运行时监视它,以确定改进执行的方法。然后,它可以在运行时动态地进行这些更改。谷歌JIT优化(javaworld有一篇关于它的很好的文章。)

Jit代表即时编译器 Jit是一个将Java字节代码转换为可以直接发送到处理器的指令的程序。

在特定的系统平台上使用java即时编译器(实际上是第二个编译器)将字节码遵从为特定的系统代码,一旦代码被jit编译器重新编译,它通常会在计算机中运行得更快。

即时编译器随虚拟机一起提供,可选使用。它将字节码编译为立即执行的特定于平台的可执行代码。

代码被编译成某种IL(中间语言)。当你运行你的程序时,计算机不理解这段代码。它只理解本机代码。因此JIT编译器会动态地将IL编译为本地代码。它在方法级别上执行此操作。