JVM quick start

JVM brief introduction

What is? JVM？

JVM yes Java virtual machine （Java Virtual Machine） For short . It's called a virtual machine , Because it is an abstract computer , There is a complete hardware architecture inside （ register , Stack , Instruction system, etc ）.

Simply speaking ,Java A virtual machine is a perform Java Bytecode virtual computer .

What is a virtual machine ？

Definition ： A virtual machine is a virtual computer . It's a software , Can simulate a series of virtual computer instructions .

Virtual machines can be divided into System virtual machine and Program virtual machine Two kinds of .

JVM The role of

Java Virtual machine is The running environment of binary bytecode , Responsible for loading bytecode into its interior , explain / Compile to the corresponding platform （Linux,Mac,Windows etc. ） Execution of machine instructions on .

JVM Characteristics

• Platform independence
• A compilation , Run anywhere
• Automatic memory management
• Automatic garbage collection

JVM The location of

It was said that , Virtual machine is a software that simulates the instructions of virtual computer . So it It's essentially a piece of software , Running on the operating system .

JVM Structure

JVM The structure of can be simply represented by the figure below ：

Java Memory optimization mainly focuses on heap , The method area is a special heap area .

Class loader

Class loader （ClassLoader） be used for Load the bytecode file into JVM Middle to run .

JVM Three layers are provided in ClassLoader:

1. Bootstrap classLoader： Boot class loader , Mainly used for loading Java Core class library of .
2. ExtClassLoader： Extend the classloader , Used for loading jre/lib/ext Some of the extended jar package .
3. AppclassLoader： system class loader , Used to load classes in the application .

Parent delegate mechanism

The parental appointment mechanism is JVM load .class The mechanism of . When loading a class , The system class loader delegates upward to the extension class loader , The extension class loader delegates upward to the boot class loader . If the boot class loader contains this class , Load successfully . Otherwise, it is handed down to the extension class loader to load , The extension class loader cannot load , It will be loaded by the system class loader .

Advantages of parental delegation ：（1） Avoid class reloading , Once a class is loaded by the parent loader , It will not be loaded by the subclass loader （2） Protect program security , Prevent the core file from being tampered

Sandbox security mechanism

Sandbox is a security mechanism in the field of computer security , Provide an isolated environment for running programs . If JVM It's a big box , If I run an unknown program directly inside , Is likely to JVM Other internal procedures have adverse effects . therefore , We can do it in JVM Zhongjian has a small box （ The sandbox ）, Let it run in this small box .

If the code in the sandbox needs to access the operating system and local resources , How to ensure safety ？ Sandbox security mechanism provides different for different sandboxes jurisdiction , Call the operating system and local resources according to permissions . for example , Core classes have the highest permissions , Remote code has lower permissions .

native keyword

native It means local , When the method is native Keyword modification , Express Java The scope of action of has not reached , Will call libraries in other languages . Execute to be native The modified method will enter the local method stack , call JNI （Java Native Interface） To operate other programming languages .

Native Method Stack ： For registration native Method , Load the local method library when the execution engine executes

JNI： Local method interface , Call the class libraries of other languages through this interface

Java Use native Keywords are for extension Java Use , Can be in Java Using other programming languages , So that it can stand .

Program counter

Each thread has a program counter , Threads are private . It's a pointer , Point to the bytecode in the method area , Tell the execution engine which instruction to execute now .

Method area

Method area Shared by all threads , All field and method bytecodes , And some special methods , Such as constructor , The interface code is also defined here . Simply speaking , Information about all defined methods is stored in this area , This area is a shared area .

Static variables 、 Constant 、 Class object （ Construction method 、 Interface definition ） The runtime constant pool exists in the method area , But variable instances exist in heap memory

Stack

A stack is a last in, first out data structure . The operation of stack supervisor , Life cycle is synchronized with thread .

Thread end , Stack memory is also released , So for the stack , There is no recycling .

The contents stored in the stack are ：

• Eight basic data types
• References to objects in the heap

Pile up

Instances in the heap used to store objects . The stack is thread level , There is only one heap in the virtual machine .

The memory of the heap can be further subdivided into : New Area 、 Elderly area and meta space . The new area can be subdivided into Eden Park 、 survive 0 Area and survival 1 District .

New Area

Eden Park is the place where objects are generated , When Eden Park is full , Will trigger a lightweight GC, Objects that have not been cleared will be sent to the survival area . When the new area is full , Will trigger a reset GC, heavy GC The surviving objects will be sent to the elderly area . Besides , Without setting , The new area has experienced 15 Second light GC The target will be sent to the elderly .

Old age area

The old area is used to store the heavy experience in the new area GC Or reach light GC Objects that still survive after the threshold number .

Meta space

Meta space is resident in memory , What's stored is Java Some environment or class information at runtime .

IDEA Set in JVM Size

At the project construction site , double-click Edit Configurations

In the pop-up page , Set the size of the virtual machine

​ -Xms8m -Xmx8m -XX:+PrintGCDetails in -Xms、-Xmx Used to set the initial memory allocation size and the maximum allocated memory ,-XX:+PrintGCDetails For printing GC Details of .

Garbage collection mechanism

Garbage recycling is mainly concentrated in Eden Park and the elderly area .

Types of garbage collection include light GC and heavy GC, light GC Only clean the new areas , and heavy GC It's the whole picture GC.

GC Common algorithms of include Replication Algorithm 、 Mark removal method and mark sorting method .

Copy algorithm

Here we will be survivors 0 District And survivors 1 The district is called from Area and to District . When Eden Park is full , Will trigger a light GC, The surviving object enters to District , And then from Copy the existing objects in the area to to In the area ; The final will be from Empty the area and Exchange from Area and to The location of the area .

The advantage of this method is There will be no memory fragmentation , And the time complexity is low , The disadvantage is to maintain an empty to District , It's a waste of resources .

Mark removal method

Mark removal method includes two steps , First scan , Mark the elements to be cleared , Then scan again to clear the object .

The advantages of this method will not cause a waste of resources , But it has high time complexity , And will produce memory fragments .

Mark up

Mark sorting method is based on mark clearing method , By moving objects, memory fragmentation is avoided .

Compared with mark removal , The time complexity of this algorithm is higher , But there will be no memory fragmentation .