JVM actually performs compilation steps during runtime for performance reasons. This means that Java doesn't have a clean compile-execution separation. It first does a so called static compilation from Java source code to bytecode. Then this bytecode is passed to the JVM for execution. But executing bytecode is slow so the JVM measures how often the bytecode is run and when it detects a "hotspot" of code that's run very frequently it performs dynamic compilation from bytecode to machinecode of the "hotspot" code (hotspot profiler). So effectively today Java programs are run by machinecode execution.

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。

非jit编译器获取源代码，并在编译时将其转换为特定于机器的字节代码。JIT编译器获取编译时生成的与机器无关的字节代码，并在运行时将其转换为特定于机器的字节代码。Java使用的JIT编译器允许一个二进制文件无需修改就能在多种平台上运行。

20%的字节码在80%的时间内被使用。JIT编译器获得这些统计数据，并优化这20%的字节代码，通过添加内联方法、删除未使用的锁等，并创建特定于这台机器的字节码来更快地运行。我引用这篇文章，我发现它很方便。http://java.dzone.com/articles/just-time-compiler-jit-hotspot

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.

即时(JIT)编译(也称为动态翻译或运行时编译)是一种执行计算机代码的方式，它涉及在程序执行期间(在运行时)而不是在执行之前进行编译。

IT编译结合了两种传统的机器代码转换方法——预先编译(AOT)和解释，并结合了两者的一些优点和缺点。JIT编译结合了编译代码的速度和解释的灵活性。

让我们考虑在JVM中使用的JIT，

例如，HotSpot JVM JIT编译器生成动态优化。换句话说，它们在Java应用程序运行时做出优化决策，并生成针对底层系统架构的高性能本机机器指令。

When a method is chosen for compilation, the JVM feeds its bytecode to the Just-In-Time compiler (JIT). The JIT needs to understand the semantics and syntax of the bytecode before it can compile the method correctly. To help the JIT compiler analyze the method, its bytecode are first reformulated in an internal representation called trace trees, which resembles machine code more closely than bytecode. Analysis and optimizations are then performed on the trees of the method. At the end, the trees are translated into native code.

跟踪树是在编程代码的运行时编译中使用的数据结构。跟踪树用于一种“即时编译器”，它跟踪在热点期间执行的代码并编译它。提到这一点。

参考:

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html https://en.wikipedia.org/wiki/Just-in-time_compilation