JVM family - overview, program counter day1-1

JVM（Java virtual machine ）

Java The running environment of binary bytecode

advantage （ characteristic ）

1. Write once , Run everywhere , Realize cross end
2. Memory management mechanism （ Automatic garbage collection ）
3. Array subscript out of bounds check
4. polymorphic

JVM,JRE,JDK

JVM： Running environment
JRE：JVM+Java Base class library
JDK：JVM+Java Base class library + Compiler tools

JVM Usefulness

1. It is helpful to understand the underlying implementation principle
2. Trend frame , The optimizer , solve the problem

common JVM

JVM Just a set of specifications

JVM Three core parts

1. ClassLoader： Class loader
2. JVM Memory structure （ Method area , Pile up , Stack , Program counter , Native Method Stack ）
3. Execution engine （ Interpreter , Just in time compiler , Garbage collection ）

Program counter

Program Counter Register Program counter （( register )

yes Java Shielding and abstraction of program hardware

Basic process of program execution

take Java The source code is compiled into binary bytecode （JVM Instructions ）, After the binary bytecode passes through the interpreter, the machine code is formed , Finally let CPU perform

effect

Remember the next one JVM The execution address of the instruction , Execute in order according to the address information
If there is no program counter , Then I don't know which command to execute next

Program counters are physically dependent on registers
The register is CPU The fastest reading unit in

characteristic

1. Thread private （ Each thread has its own program counter ）
2. There is no memory overflow