April 13, 2021 interview with beaver family

The interview process

1. Introduce yourself first

2. Let's talk about the project first

   Talk about the modules you are responsible for in your project

   Start to follow the project module and ask in detail

   When it comes to multithreading updating cached data , I was asked if there would be concurrent problems , How to solve （ Mainly redis The update operation is not atomic , There may be concurrent operations ）

   Add synchronized On the way , Or if it's just coverage setnx The order guarantees atomicity

3. Start asking about your resume

   Design patterns

   ​ Observer mode

   ​ I talked about the listener mode , I didn't make it clear

   ​ （ You can talk about proxy mode ）

   Linux

   ​ How to check the log tail -f -n Number Query the log of the latest rows

   ​ Look up the key words Add | grep

   ​ Ignore case when querying keywords （ Can't ） grep -i file name

   ​ Number of query log lines （ Can't ）nl file name

   ​ Have you checked the system parameters （ Can't ）

   ​ df -h View disk space

   ​ linux The order added

   cloud

   ​ Tell me about your understanding of the registry （ Service registration process , Discovery process , Self-protection mechanism ）

   sql The experience of optimization

   ​ Said explain Use type keys Explanation of the field ,

   ​ Said The function found in the work slows down the query speed , Index addition , The leftmost matching principle and the associated field reference

   ​ sql Optimize and supplement

4. Talk about how you learn

   My brain went out of my head and said that I had been watching jvm The book of , Isn't this digging ,

   Then the five steps of the class loading mechanism are put aside

   When it comes to verification , Asked only class Is the file format verified ？

   ​ The class loading mechanism complements

   I was asked how I learned when I was working , What has been learned ？

   Just learn from yourself gitlab Version of ci ,docker To write dockerfile Said , But the interviewer didn't go on

5. The interviewer said, is there anything you want to ask ？

   1. What kind of business
   2. Those used in the technology stack

   summary ：

   Generally speaking, the interview was a failure , Be asked a little more by the interviewer , It won't be , It seems that the depth of learning is not enough , We should strengthen our study .

   Knowledge supplement

   linux The order added

   grep Use

   Regexp selection and interpretation:
     -E, --extended-regexp     PATTERN is an extended regular expression (ERE)
     -F, --fixed-strings       PATTERN is a set of newline-separated fixed strings
     -G, --basic-regexp        PATTERN is a basic regular expression (BRE)
     -P, --perl-regexp         PATTERN is a Perl regular expression
     -e, --regexp=PATTERN      use PATTERN for matching
     -f, --file=FILE           obtain PATTERN from FILE
     -i, --ignore-case         ignore case distinctions
     -w, --word-regexp         force PATTERN to match only whole words
     -x, --line-regexp         force PATTERN to match only whole lines
     -z, --null-data           a data line ends in 0 byte, not newline
   
   Miscellaneous:
     -s, --no-messages         suppress error messages
     -v, --invert-match        select non-matching lines
     -V, --version             display version information and exit
         --help                display this help text and exit
   
   Output control:
     -m, --max-count=NUM       stop after NUM matches
     -b, --byte-offset         print the byte offset with output lines
     -n, --line-number         print line number with output lines
         --line-buffered       flush output on every line
     -H, --with-filename       print the file name for each match
     -h, --no-filename         suppress the file name prefix on output
         --label=LABEL         use LABEL as the standard input file name prefix
     -o, --only-matching       show only the part of a line matching PATTERN
     -q, --quiet, --silent     suppress all normal output
         --binary-files=TYPE   assume that binary files are TYPE;
                               TYPE is 'binary', 'text', or 'without-match'
     -a, --text                equivalent to --binary-files=text
     -I                        equivalent to --binary-files=without-match
     -d, --directories=ACTION  how to handle directories;
                               ACTION is 'read', 'recurse', or 'skip'
     -D, --devices=ACTION      how to handle devices, FIFOs and sockets;
                               ACTION is 'read' or 'skip'
     -r, --recursive           like --directories=recurse
     -R, --dereference-recursive
                               likewise, but follow all symlinks
         --include=FILE_PATTERN
                               search only files that match FILE_PATTERN
         --exclude=FILE_PATTERN
                               skip files and directories matching FILE_PATTERN
         --exclude-from=FILE   skip files matching any file pattern from FILE
         --exclude-dir=PATTERN directories that match PATTERN will be skipped.
     -L, --files-without-match print only names of FILEs containing no match
     -l, --files-with-matches  print only names of FILEs containing matches
     -c, --count               print only a count of matching lines per FILE
     -T, --initial-tab         make tabs line up (if needed)
     -Z, --null                print 0 byte after FILE name
   
   Context control:
     -B, --before-context=NUM  print NUM lines of leading context
     -A, --after-context=NUM   print NUM lines of trailing context
     -C, --context=NUM         print NUM lines of output context
     -NUM                      same as --context=NUM
         --group-separator=SEP use SEP as a group separator
         --no-group-separator  use empty string as a group separator
         --color[=WHEN],
         --colour[=WHEN]       use markers to highlight the matching strings;
                               WHEN is 'always', 'never', or 'auto'
     -U, --binary              do not strip CR characters at EOL (MSDOS/Windows)
     -u, --unix-byte-offsets   report offsets as if CRs were not there
                               (MSDOS/Windows)
   
   

   Query system commands

   uname -a #  View kernel / operating system /CPU The information of linux system information  
   head -n l /etc/issue #  View operating system version  
   cat /proc/cpuinfo #  see CPU Information  
   hostname #  Check the linux System information command  
   lspci -tv #  List all PCI equipment  
   lsusb -tv #  List all USB The equipment linux System information command  
   lsmod #  List loaded kernel modules  
   env #  View environment variable resources  
   free -m #  View memory usage and swap usage  
   df -h #  View the usage of each partition  
   du -sh #  View the size of the specified directory  
   grep MemTotal /proc/meminfo #  View total memory  
   grep MemFree /proc/meminfo #  View the amount of free memory  
   uptime #  View system running time 、 The number of users 、 load  
   cat /proc/loadavg #  View system load disks and partitions  
   mount | column -t #  View attached partition status  
   fdisk -l #  View all sections  
   swapon -s #  View all swap partitions  
   hdparm -i /dev/hda #  View disk parameters ( Only applicable to IDE equipment ) 
   dmesg | grep IDE #  View on startup IDE Device detection status network  
   ifconfig #  View properties of all network interfaces  
   iptables -L #  View firewall settings  
   route -n #  View routing table  
   netstat -lntp #  View all listening ports  
   netstat -antp #  View all established connections  
   netstat -s #  View network statistics process  
   ps -ef #  View all processes  
   top #  Real time display of process status users  
   w #  view active user  
   id #  View specified user information  
   last #  View user login log  
   cut -d: -f1 /etc/passwd #  View all users of the system  
   cut -d: -f1 /etc/group #  View all groups in the system  
   crontab -l #  View the scheduled task service for the current user  
   chkconfig –list #  List all system services  
   chkconfig –list | grep on #  List all started system services  
   rpm -qa #  View all installed packages  
   cat /proc/cpuinfo ： see CPU Of related parameters linux System commands   
   cat /proc/partitions ： see linux System information command for hard disk and partition information    
   cat /proc/meminfo ： see linux System memory information linux System commands   
   cat /proc/version ： View version , similar uname -r   
   cat /proc/ioports ： View device io port   
   cat /proc/interrupts ： View interrupt    
   cat /proc/pci ： see pci Device information   
   cat /proc/swaps ： View all swap Partition information   
   

sql Optimize and supplement

explain

If it is mysql Words , We can learn about Use How to view the execution plan

--  see SQL Use index or not , prefix explain that will do 
explain select * from emp where name = 'Jefabc';

-- expain The information coming out is 10 Column , Namely id、select_type、table、type、possible_keys、key、key_len、ref、rows、Extra

One 、 id

SELECT Identifier . This is a SELECT The query serial number of

My understanding is that SQL Identification of the order of execution ,SQL From big to small execution

1. id Phase at the same time , The order of execution is from top to bottom

2. If it's a subquery ,id The serial number of will increase ,id The higher the value, the higher the priority , The first to be executed

3. id If the same , It can be thought of as a group , From top to bottom ; In all groups ,id The bigger the value is. , The higher the priority , Execute first

Two 、select_type

Each select The type of clause

(1) SIMPLE( Simple SELECT, Don't use UNION Or subquery, etc )

(2) PRIMARY( The outermost query in the subquery , If the query contains any complex sub parts , The outermost select Marked as PRIMARY)

(3) UNION(UNION The second or later of SELECT sentence )

(4) DEPENDENT UNION(UNION The second or later of SELECT sentence , Depends on external queries )

(5) UNION RESULT(UNION Result ,union The second... In the sentence select Start all the back select)

(6) SUBQUERY( First in subquery SELECT, The results don't depend on external queries )

(7) DEPENDENT SUBQUERY( First in subquery SELECT, Depends on external queries )

(8) DERIVED( Derived from table SELECT, FROM A subquery of a clause )

(9) UNCACHEABLE SUBQUERY( The result of a subquery cannot be cached , The first line of the outer link must be reevaluated )

3、 ... and 、table

Show the table names in the database accessed in this step （ Show which table this row's data is about ）, Sometimes it's not a real name , It may be abbreviated as , For example, above e,d, It may also be the abbreviation of the result of the step

Four 、type

How to access the table , Express MySQL How to find the desired row in the table , also called “ Access type ”.

Common types are ： ALL、index、range、 ref、eq_ref、const、system、NULL（ From left to right , Performance from poor to good ）

ALL：Full Table Scan, MySQL Will traverse the entire table to find the matching rows

index: Full Index Scan,index And ALL The difference for index Type only traverses the index tree

range: Retrieve only rows in the given range , Use an index to select rows

ref: Indicates the join matching condition of the above table , That is, which columns or constants are used to find values on index columns

eq_ref: similar ref, The difference is that the index used is the only index , For each index key value , Only one record in the table matches , Simply speaking , It is used in multi table connection primary key perhaps unique key As a condition of Association

const、system: When MySQL Optimize some part of the query , And convert to a constant , Use these types to access . For example, place the primary key in where In the list ,MySQL You can convert the query to a constant ,system yes const Special case of type , When the query table has only one row , Use system

NULL: MySQL Decompose statements during optimization , Execute without even accessing tables or indexes , For example, selecting the minimum value from an index column can be done through a separate index search .

5、 ... and 、possible_keys

Pointed out that MySQL Which index can be used to find records in the table , If there is an index on the field involved in the query , Then the index will be listed , But not necessarily used by queries （ The index that this query can utilize , If there is no index display null）

This column is completely independent of EXPLAIN Output the order of the table shown . This means that possible_keys Some of the keys in cannot actually be used in the order of the generated tables .
If the column is NULL, There is no relevant index . under these circumstances , It can pass the inspection WHERE Clause to see if it references certain columns or columns suitable for indexing to improve your query performance . If so , Create an appropriate index and use it again EXPLAIN Check query

6、 ... and 、Key

key Columns show MySQL Actually decide which key to use （ Indexes ）, Must be included in possible_keys in

If index is not selected , The key is NULL. To coercive MySQL To use or neglect possible_keys Index in column , Use in query FORCE INDEX、USE INDEX perhaps IGNORE INDEX.

7、 ... and 、key_len

Represents the number of bytes used in the index , You can use this column to calculate the length of the index used in the query （key_len The displayed value is the maximum possible length of the index field , It's not the actual length , namely key_len It is calculated according to the table definition , It is not retrieved from the table ）

Without loss of accuracy , The shorter the length, the better

8、 ... and 、ref

Comparison of columns and indexes , Indicates the join matching condition of the above table , That is, which columns or constants are used to find values on index columns

Nine 、rows

Estimate the number of rows in the result set , Express MySQL According to table statistics and index selection , Estimated number of rows to read to find the required record

Ten 、Extra

This column contains MySQL Solve query details , There are several situations ：

Using where: Don't read all the information in the table , Just index to get the data you need , This happens when all the request columns of the table are part of the same index , Express mysql The server will filter the rows after the storage engine retrieves them

Using temporary： Express MySQL You need to use temporary tables to store result sets , Common in sorting and grouping queries , common group by ; order by

Using filesort： When Query Contained in the order by operation , And sorting operations that can't be done with indexes are called “ File sorting ”

sing join buffer： The change emphasizes that no index is used to get the connection conditions , And you need to connect buffers to store intermediate results . If this value appears , That should pay attention to , Depending on the specific situation of the query, you may need to add indexes to improve the performance .

Impossible where： This value emphasizes where Statement will result in no eligible rows （ There can be no results by collecting statistics ）.

Select tables optimized away： This value means only by using the index , The optimizer may only return one line from the aggregate function result

No tables used：Query Use in statement from dual Or without any from Clause

sql Optimize

① SQL Optimize

 avoid  SELECT *, Just query the required fields .
 Small tables drive large tables , That is, small datasets drive large datasets ：
 When B The data set ratio of the table A Watch hour , use in Optimize  exist The execution order of the two tables is to check first B Check the table again A Table query statement ：SELECT * FROM tb_dept WHERE id in (SELECT id FROM tb_dept) ;
 When A The data set ratio of the table B Watch hour , use exist Optimize in , The execution order of the two tables is to check first A surface , check B surface , Query statement ：SELECT * FROM A WHERE EXISTS (SELECT id FROM B WHERE A.id = B.ID) ;
 Try to use joins instead of subqueries , Because use  join  when ,MySQL  No temporary tables are created in memory .

② Optimize the use of indexes

 Try to use primary key query , Not other indexes , Because the primary key query does not trigger a back table query .
 Don't do column operations , Put the calculation into each business system to realize 
 The query statement is as simple as possible , Big sentences split small ones , Reduce lock time 
or  The query is rewritten into  union  Inquire about 
 No functions and triggers 
 avoid  %xx  Inquire about , have access to ：select * from t where reverse(f) like reverse('%abc');
 To use less  join  Inquire about 
 Use same type comparison , such as  '123'  and  '123'、123  and  123
 Try to avoid  where  Used in clauses  !=  perhaps  <>  The operator , The query reference discards the index and performs a full table scan 
 The list data uses paging query , The amount of data per page should not be too large 
 Avoid using... On index columns  is null  and  is not null

③ Table structure design optimization

 Use the smallest data type that can hold data .
 Use as much as possible  tinyint、smallint、mediumint  As an integer type instead of  int.
 Use as much as possible  not null  Define fields , because  null  Occupy  4  Byte space . Numbers can be defaulted  0 , String default  “”
 Use less as far as possible  text  type , When you have to use it, you'd better make a separate table .
 Use as much as possible  timestamp, Instead of  datetime.
 Don't have too many fields in a single table , It is suggested that  20  Within fields .

The class loading mechanism complements

Class loading time

1. encounter new、getstatic、putstatic or invokestatic These four bytecode instructions , If the type has not been initialized

   turn , You need to trigger the initialization phase first .

2. Use java.lang.reflect When a package's method makes a reflection call on a type , If the type is not initialized , Then

   First trigger its initialization

3. When initializing a class , If it is found that its parent class has not been initialized , You need to trigger the initialization of its parent class first .

4. When the virtual machine starts , The user needs to specify a main class to execute （ contain main() Method type ）, Virtual opportunity first

   Initialize this main class .

5. When using JDK 7 New dynamic language support , If one java.lang.invoke.MethodHandle The final solution of the example

   The analysis result is REF_getStatic、REF_putStatic、REF_invokeStatic、REF_newInvokeSpecial There are four types of method sentences

   Handle , And the class corresponding to this method handle has not been initialized , You need to trigger its initialization first .

6. When an interface defines JDK 8 New default method added （ By default Keyword decorated interface method ） when , If there is

   The implementation class of this interface is initialized , The interface must be initialized before it .

   For the six scenarios that trigger type initialization ,《Java Virtual machine specification 》 A very strong qualifier is used in

   ——“ Yes and only ”, The behavior in these six scenarios is called active reference to a type . besides , For all reference types

   Expressions do not trigger initialization , It's called passive reference

The class loading mechanism has five main stages

load

“ load ”（Loading） The stage is the whole “ Class loading ”（Class Loading） A stage in the process , There is no hope that readers will be confused

These two nouns look very similar . In the loading phase ,Java There are three things virtual machines need to do ：

1） Get the binary byte stream that defines this class by using the fully qualified name of a class .

2） Convert the static storage structure represented by the byte stream into the runtime data structure of the method area .

3） Generate a representation of this class in memory java.lang.Class object , As the access to various data of this class in the method area

mouth .

in general , Virtual machine get class file （ Or some other form ） Convert to class objects and store them in the method area .

verification

1. File format validation
2. Metadata validation
3. Bytecode verification
4. Symbol reference validation

Get ready

The preparation phase is formally defined for variables in the class （ That is, static variables , By static Decorated variable ） Allocate memory and set class variables

Initial value stage .

analysis

The parsing phase is Java The process of virtual machine replacing symbolic reference in constant pool with direct reference .

· Symbol reference （Symbolic References）： A symbolic reference describes the referenced target with a set of symbols , The symbol can be any

The literal quantity of form , As long as it can be used to locate the target unambiguously . Symbol references are independent of the memory layout of the virtual machine implementation , lead

The target used is not necessarily what has been loaded into the virtual machine memory . The memory layout of various virtual machines can be different ,

But the symbol references they can accept must be consistent , Because the literal form of symbolic reference is clearly defined in 《Java Virtual machine specification

Fan 》 Of Class In file format .

· Direct reference （Direct References）： A direct reference is a pointer that points directly to the target 、 A relative offset or an energy

Handle to indirectly locate the target . Direct reference is directly related to the memory layout of the virtual machine , The same symbol is referenced in different virtual

Direct references translated from pseudo machine instances are generally not the same . If there is a direct quote , The target of the reference must already be in the virtual machine

There is... In your memory .

initialization

The initialization phase is to execute the class constructor () Method process

·() The compiler automatically collects the assignment actions and static statement blocks of all class variables in the class （static{} block ） Medium

Statement merge , The order in which the compiler collects is determined by the order in which statements appear in the source file , Only... Can be accessed in a static statement block

To variables defined before a static statement block , Define the variable after it , In the previous static statement block, you can assign values , But you can't visit

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