Java virtual machine (java virtual machine, JVM), a virtual machine capable of running java bytecode. Virtual machine as a programming language, in fact
Not only is it dedicated to the Java language, any language can be compiled and run by the JVM as long as the generated compiled file matches the JVM's requirements for the format of the loaded compiled file.
Such as kotlin, scala, etc.
** There are many JVMs, not just Hotspot, but also JRockit, J9, etc. The basic structure of JVM ** JVM is composed of three main subsystems
Class loading subsystem
Runtime data area (memory structure)
Execution engine 
runtime data area (memory structure) 1. Method Area
The bytecode of all fields and methods of the class, as well as some special methods such as constructors, interface codes are also defined here. Simply put, all the defined methods
Information is stored in this area, static variable constant class information (construction method/interface definition) runtime constant pool is stored in the method area, although Java virtual
The machine specification describes the method area as a logical part of the heap, but it has an alias called Non-Heap (non-heap), the purpose should be to match Java
Heap division
2. Heap
The virtual machine is automatically allocated and created when it starts to store the instance of the object. Almost all objects are allocated memory on the heap. When the object cannot be applied for in the space
The memory will throw OutOfMemoryError exception. It is also the main area managed by the garbage collector. 
2.1 Young Generation
The area where classes are born, grow, and die, where a class is generated, applied, and finally collected by the garbage collector.
End of Life.
The Cenozoic is divided into two parts: Eden space and Survivor space. All classes are newly created in Eden space.
The surviving area is divided into From and To areas. When the space in the Eden area is used up, the program needs to create objects again, and the garbage collector of the JVM will garbage back the Eden area
Receive (Minor GC), destroy the objects in the Eden area that are no longer used by other objects. Then move the remaining objects in the Eden area to From
Survivor area. If the From Survivor area is also full, garbage collect the area and move to the To Survivor area.
2.2 Old Generation
The objects in the new generation that are still in stock after many GCs are moved to the old area. If the old age is also full, Major GC (also called Full GC) will occur at this time,
Clean up the memory of the elderly zone. If the elderly area executes Full GC and finds that the object still cannot be saved, it will throw
OOM (OutOfMemoryError) exception
2.3 Meta Space
After JDK1.8, the meta space replaced the permanent generation. It is an implementation of the method area in the JVM specification. The difference is that the metadata area is not in the virtual machine, but is used
The local memory of the permanent generation is in the virtual machine, and the permanent generation also belongs to the heap in the logical structure, but not physically.
Why was the permanent generation removed?
Refer to the official explanation http://openjdk.java.net/jeps/122
It probably means that the removal of the permanent code is an effort to integrate HotSpot and JRockit, because JRockit does not have a permanent code, and there is no need to configure a permanent code.
3. Stack
The memory model of Java thread execution method, a thread corresponds to a stack, each method will create a stack frame (used to store local variables
Table, operand stack, dynamic link, method export and other information) there is no garbage collection problem, as long as the thread ends, the stack will be released. The life cycle and thread
To
4. Native Method Stack (Native Method Stack)
The function is very similar to the stack, except that the Java stack serves the JVM to execute Java methods, while the local method stack serves the JVM to execute native methods. Register native
Method, load the local method library when Execution Engine executes 5. Program Counter Register
It is a pointer to the method bytecode in the method area (used to store the address of the starter instruction, which is the instruction code to be executed), which is executed by
The engine reads the next instruction, which is a very small memory space, almost negligible
JDK performance tuning monitoring tool Jinfo
View the extended parameters of the running Java program
View the parameters of the JVM
View java system properties
Equivalent to System.getProperties()
The Jstatjstat command can view the usage of each part of the heap memory and the number of loaded classes. Command format:
jstat [-command options] [vmid] [interval time/ms] [number of queries]
Jmap
Can be used to view memory information
Heap object statistics are shown in the 
figure: 
Num: serial number Instances: number of instances Bytes: occupied space size Class Name: class name heap information
heap memory dump
jmap -dump:format=b,file=temp.hprof
You can also automatically export the dump file when setting the memory overflow (when the project memory is large, it may not be exported)
1.-XX: HeapDumpOnOutOfMemoryError
2.-XX:HeapDumpPath=output path
You can use the jvisualvm command tool to import file analysis
Jstack
jstack is used to generate a thread snapshot of the java virtual machine at the current moment. 
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