JVM learning notes: three local method interfaces and execution engines

Local method interface

Local method

• Java Medium native The method of decoration
• native method Namely java Call not java Code interface
• Local methods are provided by the operating system （ from C or C++ To write ）
• Definition native Method , No implementation provided , What is called is true or false Java Code implementation
• native Can be divided with abstract Use with identifiers other than

The benefits of using local methods

1. Java Can interact with the underlying operating system
   Some tasks use Java It's not easy to achieve
   Don't worry about implementation details , Just call
2. Java Data interaction with hardware ( start(), read(), hashCode() )
   JVM The interpreter for C Realization , Can better interact with the outside
   jre For the most part Java Realization , Interact with the outside world through some local methods
   for example java.lang.Thread Of setPriority() The method is to use Java Realized , But it does the calling thing Local methods in this class setPriority0()
   
   

Execution engine

summary

The execution engine is JVM One of the core components of

JVM Load bytecode into memory , Bytecode is not an executable machine code , need The bytecode is interpreted by the execution engine / Compile to machine instructions on the corresponding platform .

The execution engine is in JVM Play a role in translating high-level languages into machine languages

• Front end compilation ：.java --> .class
• Back end compilation ：.class explain / Compile as machine code

The mechanism of the execution engine

1. Interpreter
   JVM Startup time , The bytecode is interpreted and executed line by line according to the predefined specification , Translate the bytecode content into the machine code of the corresponding platform for execution .
2. JIT compiler
   Compile bytecode as a whole into machine code , But not immediately

Why? Java It's a semi interpreted, semi compiled language ？

JIT The real-time compiler is Java Later , Originally, it was to explain and implement

1. The original simple line by line explanation of bytecode execution is inefficient
2. JIT Translate bytecode into machine code , Cached in the method area JIT In the code cache , Improve the efficiency of implementation
3. Decide whether to start according to the frequency of code execution JIT The compiler directly compiles the bytecode into the machine code of the corresponding platform

JVM It will be compiled for the frequently used hot code , Cache to method area , High execution efficiency

JIT Why do you need an interpreter when the compiler is efficient ？

JIT Compiler compilation takes time , But efficient execution .

As soon as the program starts , The interpreter responds immediately , To explain the execution .

Combine the two ,JVM After starting , First, explain and execute the code through the interpreter , Then compile and execute with the compiler . Adopt the architecture of simultaneous interpreter and real-time compiler to balance .