JVM easy start-02

One .JVM Architecture diagram

Throughout JVM Learning process , This picture should be remembered deeply , So that you won't lose your way

Two .Native Method

In a multithreaded Thread Of start Method source code , There's a way 【private native void start0()】, There is a keyword native.

1. I've brought native Keywords , explain Java It doesn't reach the scope of , Go back and call the underlying C The library of languages .
2. It goes to the local method stack , Call the local method interface JNI(Java native interface)

Historical reasons ：Java It was born C、C++ When you're walking around , Want to have a foothold , Must call C、C++ The program , So a special area is opened in the memory and marked as native Code for , Its specific approach is to Navitive Method Stack Registered in navtive Method , Load when executing the engine Native Libraries.

3、 ... and . Method area

Method area （Method Area） Is shared by all threads , Information about all defined methods is stored in this area .

Static variables 、 Constant 、 Class information （ Construction method 、 Interface definition ）、 The runtime constant pool exists in the method , But instance variables exist in heap memory and are independent of the method area （static、final、class、 Constant pool ）

Four . Stack

1. To explain it in a popular language is ： If you drink too much, you will vomit , If you eat too much, you will be in line .
2. Stack storage is faster than heap , Next to registers , Stack data can be shared .
3. There is no garbage collection problem in the stack , As soon as the thread ends , The stack is Over, The life cycle is consistent with the thread , Threads are private .

5、 ... and . Pile up （sun-hotspot）

One JVM There's only one heap memory , The size of heap memory can be adjusted ; After the class loader reads the class file , Need to put class 、 Method 、 Constant variables in heap memory , Save real information for all reference types .

5.1 Pile up

• New Area Young/New
• Retirement area Tenure/Old
• The permanent zone Perm

GC Garbage recycling is mainly carried out in new areas and old-age areas , It is also divided into light GC And heavy GC, If there's not enough memory , Or there is an endless cycle , It will lead to java.lang.OutOfMemoryError:Java heap space.

5.2 New Area

The new area is divided into two parts ： Eden （Eden） And the survival zone （Survivor Space）, All classes are in Eden Quilt new Coming out .

When Eden When you run out of space , The program has to create objects ,JVM Your garbage collector will be on Eden Zone for garbage collection （Minor GC）, Similar operation 0 The district is full , Move to 1 District , Retirement area . If the elderly care area has implemented Full GC The object cannot be saved after , It will produce OOM abnormal “Out of MemoryError”.

If it appears OOM, explain Java There is not enough heap memory for the virtual machine , Here's why ：

1. Java The heap memory setting of the virtual machine is not enough , You can use the parameter -Xms( Initial value size ),-Xmx( Maximum size ) To adjust
2. A lot of objects are created in the code , And it can not be collected by garbage collector or dead loop for a long time

5.3 The permanent zone

1. To hold JDK What you carry Class,Interface Metadata , It stores the class information necessary for the running environment
2. The data in this area will not be collected by the garbage collector , close JVM The memory occupied by this area will be released
3. Java.lang.OutOfMemoryError:PerGen space explain Java Virtual machine to permanent generation Perm Not enough memory settings
4. JDK1.8 And there is no permanent generation after , Constant pool 1.8 In meta space

5.4 Heap memory tuning

-Xms： Set the initial allocation size , Default physical memory 1/64

-Xmx： Maximum allocated memory , Default physical memory 1/4

-XX:PrintGCDetails： Output detailed GC Processing logs

6、 ... and . Method area + Pile up + Stack interaction

The illustration ：https://blog.csdn.net/thera_qing/article/details/110544168

Learning comes from ：B standing - Madness theory