day1-jvm+leetcode

	 First, let's talk about why we wrote the article , I'm going to study second , No matter what the teacher said, I didn't study hard , I can't find internships now , What are you doing 
 no way , In order not to do five, Decided to start writing articles to record learning , By the way, review the content learned every day in the evening , I hope I can do better every day .
	 Simply talk about your current goal ,jvm+ Find out all kinds of other aspects , Brush problem top100, Write a web version of fighting landlord , Optimize 
 Previous projects , Deploy . First of all. , I hope I can stick to .

Today's summary

See the dark horse jvm 1-34
leetcode Did top 3 Tao simple
Look at a little websocket

JVM

jvm The benefits of

java Of （ Binary bytecode ） Running environment

1. Write once , Run anywhere
2. Automatic memory management , Garbage collection function
3. Array subscript out of bounds check
4. The bottom layer uses virtual methods to realize polymorphism

Memory structure

1 Program counter

Program Counter Register
Java Code flow ：
Code - Interpreter - Machine code -cpu

• effect ： Remember the address of the next instruction
• characteristic
  • Thread private
  • There will be no memory overflow

2 Virtual machine stack

Stack - The memory space required for thread operation
Stack frame - Memory required for each method to run ( Save parameters 、 local variable 、 The return address )
Each thread has only one active stack frame , Corresponding to the currently executing method

Relevant interview questions

1. Does garbage collection design stack memory ？
   • Unwanted , Will pop up the stack by itself
2. The larger the stack memory allocation, the better ？
   • No , Physical memory is certain , The larger the stack memory , The number of threads will be less .
3. Whether the local variables in the method are thread safe ？

• Local variables are thread private thread safe .
• There will be thread safety problems when passing as method parameters （ Will be shared ）.
• As a method return value, there are also problems .（ Will be shared ）
• Conclusion ： There is no escape method. The access scope is safe , Local variables reference objects and escape scope , There are thread safety issues .

Stack memory overflow

Wrong name ：java.lang.StackOverflowError
reason ：

• There are too many stacks （ Recursion has no end condition , Circular reference one with another ）
• The stack frame is too large

Thread running diagnostics

• Case study 1 ： cpu Take up too much
  • Positioning how to occupy

(1): linux Next
top： Check the system occupation , See which process is taking up too much
ps ： It depends on which thread takes up too much
ps H -eo pid,tid,%cpu | grep process id
(2)： use jdk Methods
according to jstack Find the problematic thread , Further locate the number of rows of the problem

• Case study 2 ： The program runs for a long time without results

jstack+ process id You can see the deadlock

3 Native Method Stack

native method stacks： use c/c++ Compiling api
such as ： Object Of clone()
wait()、hashcode()、wait()、notify()、notifyAll()

4 Pile up

heap Pile up ： use new The created object uses heap memory
characteristic ：

• Thread sharing , Thread safety needs to be considered
• There's a garbage collection mechanism

Heap memory overflow

java.lang.OutOfMemoryError:Java heap space

Heap memory diagnostics

1. jps Tools
   • See what's in the system java process
2. jmap Tools
   • Check heap memory usage
3. jconsole Tools
   • Continuous monitoring

---

Use jmap When something goes wrong ,
After searching, it may be jdk Version of the problem , So I changed it to jdk11



A mistake … Come again , Change the environment mac edition , Because it uses the command line
mac Next operation :
Enter the first /Library/Java/JavaVirtualMachines Find the version number name
vim ~/.bash_profile
modify JAVA_HOME Version number of
source /etc/profile Save it
Reference resources https://www.jianshu.com/p/4fd5f6bc6dfb
And then OK 了

But the problem remains unsolved …
So use the prompt

…?? There is another problem at the beginning , People are numb
Permission problems

Not yet. …
https://zhuanlan.zhihu.com/p/399352540
Brush to a post … Or last year , Also didn't succeed , Forget it, give up , Numb or use jconsole

---

• Case study ： Memory usage is still high after garbage collection

---

First loading jvisualVM
https://blog.csdn.net/Tanganling/article/details/119790892

---

Method area

Definition ： It's all java A zone shared by virtual machine threads （ Now in local memory ）, It stores some information related to the structure of the class ： Runtime constant pool 、 Member variables , Method data , Member method , Constructor method and some special methods . Created when the virtual machine starts , Logically, it is a part of the heap . The permanent generation and meta space is an implementation of the method area . The method area will also overflow

out of memory

• 1.8 Before the permanent generation
• 1.8 And then the meta space

scene ：spring and mybatis use cjlib Agents dynamically generate classes to generate many runtime classes , Permanent generation garbage collection ratio Poor .

Constant pool

It's actually a table , Find the class name to be executed according to this table 、 Method name 、 Parameter type 、 Literal quantity and other information .
Binary bytecode includes ： Class basic information , Constant pool , Class method definition , Virtual machine instructions
getstatic： Get static variables
ldc： Load a parameter
invokevirtual： Execute a virtual method call

Runtime constant pool

When a class runs, it should be put in memory , The location in memory is the runtime constant pool

StringTable String pool

The information in the constant pool will be loaded into the runtime constant pool , It hasn't become java String object .
ldc When , Will get to StringTable Go inside , If not, the symbol will be changed into a string object and put into the string pool . The zone is created only when it is used （ Lazy loading ）.
StringTable It's a hashtable Structure , It can't be expanded .

• Splicing

String s3 = s1+s2 ;
Create a new StringBuilder And call the constructor .


s3 It's in the string pool ,s4 yes new Of , therefore false

s5 It depends on the value after splicing （ Spliced in the compiler , Because they're all constants , Previous s1,s2 They're all constants ） Whether there is , So it is true.

Dynamically spliced elements will not exist in the string pool , Like this ,”a“ and "b" Will appear in the string pool ,"ab" It will not exist in the string pool （ Dynamic splicing ）, Will be put in the pile , Only constants can be put in

call s.intern() Method will try to put it into the string pool , And return the objects in the string pool

All two are true, Because they are all the same

On ture： Because they are all taken from the constant pool
Next false：s That's new Coming out

Leetcode

461 Hamming distance

Directly use the loop , Every time and 1 And it's ok

338 Bit count

1. Use the cycle , It's too complicated
2. Dynamic programming , Only the last one is different , So look Moves to the right one （ Already exist ）+ Whether the last one has changed

215 The... In the array K The biggest element

It's a sort question , Read the article by yourself and prepare to realize manual , It turned out to be quite difficult , I forgot everything I learned before , And theory and practice are two different things , Only now 5 A simple one. , Let's sum up tomorrow .

Finally, I want to say

I learned for a long time today , Maybe it's about the same time as the postgraduate entrance examination , Because I suddenly feel helpless , I hope I can stick to it ！！