Interpretation and compilation of JVM

1、 principle

Bytecode cannot be directly handed over to hardware for execution. Virtual machine is required translate become Machine code To perform ,“ translate ” There are two strategies ： Explain to perform and Compile implementation , also called Just in time compilation (JIT).

• Explain to perform ,

  • Run bytecode in interpreted mode ,
  • Explanation execution means Read a sentence , Execute a sentence .

  Advantage is Fast start-up efficiency ,
  The disadvantage is that The overall execution speed is slow .

• Compile implementation ,

  • Compile bytecode into machine code
  • Execute machine code directly
  • Compile at run time
  • After compiling , There's an order of magnitude improvement in performance .

  Its characteristics are opposite to interpretation and execution , Slow start , Fast execution .

stay jvm In the virtual machine, the two are mixed , There are both interpretation execution and compilation execution . First of all Explain to perform , Execute all bytecodes one by one , If JVM If a method is found to be called frequently, it will be compiled with the strategy of compilation and execution , Call the machine code directly the next time , This method is called Hot methods , thus it can be seen , Compile implementation Is in terms of method .

From a business perspective , The server and the client have different requirements for the execution speed and startup speed of the code .
such as , Mobile applications , Users want the program to start faster .
The server program , There may be higher requirements for the execution speed of the program .

So , from java7 Start , HotSpot Layered compilation is adopted , That is, two compilers are introduced ：C1、C2.
　　
C1 compiler , be called client compiler , For clients that require startup performance , Compilation period , The optimization strategy is simple ;

C2, be called Serve compiler , Server side with peak performance requirements , Long compilation time , The optimization strategy is complex .

Specifically, when compiling hotspot methods , First use C1 compiler , The hotspot method in the hotspot will be C2 The compiler compiles again .

2、JVM Related parameters

-Xint ： Explain the pattern , Do not activate JIT compiler ( namely CompileThreshold=0).

-Xcomp ： Compile mode , Appoint JVM Compile all bytecodes into local code when used for the first time .（ namely CompileThreshold=1）.
explain ：comp：Compile

-Xmixed ： Mixed mode execution ,-Xint、-Xcomp Two mixed use , That is, the interpretation mode and Compile mode A mixture of .

-Xbatch ： Compile methods in the foreground , The method cannot execute until the compilation is complete .

-XX:CICompilerCount=n ： Appoint JIT The number of threads the compiler uses to compile methods .

-XX:CompileThreshold=n ： Specifies the number of calls to a method , In order to make HotSpot and JIT The compiler can compile it .

3、JVM Three models of

int Pattern ： use -Xint Turn on , namely Explain the pattern , In this mode, all take the interpretation mode to run .

comp Pattern ： use -Xcomp Turn on , In this mode, notify JVM close Explain the pattern , use Compile mode function . But it often leads to poor automatic optimization .

mixed Pattern ： use -Xmixed Turn on , Namely, mixed operation mode , It's also Hotspot Default mode .