[100 cases of JVM tuning practice] 01 - introduction of JVM and program counter

front said
Author's brief introduction ： Half old 518, Long distance runner , Determined to persist in writing 10 Blog of the year , Focus on java Back end
Column Introduction ： Case driven introduction JVM knowledge , Teach you how to use JVM Troubleshooting 、 Evaluation code 、 Optimize performance
The article brief introduction ： Introduce JVM The role of 、JVM、JRE、JDK The difference between , common JVM And JVM Learning route

1.JVM brief introduction

1.1 JVM The role of

JVM Its function is as follows ：

• Write once , Run everywhere
• Automatic memory management , Garbage collection
• Array subscript out of bounds check , If the memory allocation method of array subscript out of bounds is allowed , Then there may be content coverage , therefore JVM It provides a supporting mechanism for array subscript out of bounds checking .
• Provide polymorphic underlying support . The virtual method table holds the actual entry address of each method . If a method is not overridden in a subclass , The address entry in the virtual method table of that subclass is the same as the address entry of the same method of the parent class , All point to the actual entry address of the parent class . If a subclass overrides this method , The address in the subclass method table will be replaced with the entry address pointing to the subclass implementation version .

1.2 JVM、JRE、JDK

Differences among the three 、 The contact is shown in the figure below .

1.3 common JVM

JVM Comply with a series of specifications , If necessary , You can develop one yourself JVM, Many large companies have their own JVM, common JVM The following table . Its underlying implementation is different , We will focus on Hotspot Explain .

1.4 JVM Learning route of

Here's the picture ,JVM It mainly includes three parts ： Class loader ,JVM Memory structure and execution engine . A class is compiled , There must be a classloader to load . Class is placed in the method area , The instance object of the class is placed on the heap , Objects in the heap will use the virtual machine stack when calling methods 、 Program counter and local method stack . Method execution , Each line of code is interpreted and executed by the program interpreter in the execution engine , Hot code in method ( Call... Frequently ) Will be JIT Just in time compiler for optimized compilation ,GC Is responsible for garbage collection of objects that are no longer referenced in the heap . Sometimes JVM You also need to interact with the operating system , The local method interface is responsible for this responsibility . Learning will follow the order from easy to difficult , The following icon notes learn ： Study first JVM Memory structure , Introducing the process of garbage collection , To learn java The build process ( Such as optimization ), Then learn the process of class loading and common class loaders , Finally, it introduces the optimization of program runtime , That is, the interpreter and the immediate compiler .

2. Program counter

effect ：

The function of the program counter is to remember the next execution of the program jvm Address of instruction . In the computer java The process from code to execution is ： Source code to bytecode , Binary bytecode to jvm Instructions , Give it to the interpreter , The interpreter obtains the bytecode address to be interpreted through the program counter , Convert instructions into machine code , from CPU perform ; At the same time, the program counter moves to the next instruction to be executed .java It's multi-threaded , It is the program counter that ensures that if the thread is lost cpu, Recapture cpu The post interpreter can know where the next instruction to be executed is .

The program counter has the following characteristics ：

Thread private , Each thread has its own program counter , Record where the current thread code is executed .

java The only part of the virtual machine that will not have memory overflow .