Java Interview Summary - Basics 1

java multithreading - spin locks, lock bias

Benefits: You can give contrast Servlet and CGI 
difference between user threads and daemon threads: the user thread after the JVM will exit, then daemon threads will terminate (such as garbage collection thread), how to create a daemon thread in java 
creation methods: Recommended Runnable Interface; Inheritance thread; Callable (return value, used in conjunction with Future asynchronous) example
 lifecycle: New, Runnable, Running, Waiting , Blocked (join method), Dead (see below) 
priority: more dependent on OS threads scheduler, the best control of the code with their 
context switching (context-switching): return status of the CPU and memory 
to ensure the final termination of the main thread: join method of 
inter-thread communication: wait, notify, notifyAll (mainly between the thread-locking control) 
thread ways: sleep and yield is static, because you can only perform in executing thread

Thread Safety

How to ensure thread safety: atomic class, lock concurrency, volatile keyword reference links
 sync blocks and synchronization method: the tendency sync blocks, locks less 
ThreadLocal: do not want a lot of trouble when using synchronous global variable mechanism, you can use 
the deadlock: the investigation method (blocked thread locked resource id), java thread dump 
avoid deadlock: avoid nested locks, only lock used where needed, to avoid the wait indefinitely 
atomic operations: avoid data inconsistency (and other types of AtomicInteger) multiple threads, int ++ not atomic 
Lock Interface 
collection classes: are fail-fast ( a ConcurrentModificationException ), and the like may be used in a multithreaded ConcurrentMap

Thread Pool

Benefits: avoid running out of memory, can be recovered using the 
method of creating: Executors 
blocking queue: thread-safe, can be used for producer - consumer issues

spring boot - aop, ioc principle

IOC: Inversion of Control, Injection or call the Dependency 
the Spring of IOC: creating objects, controlled by the container are destroyed 
Principle: reflecting 
advantages: reducing the coupling, easier to maintain

AOP: Aspect Orientied Programming, based on IOC 
Features: Independent of the specific services, such as logging, permission checks, transaction management 
principle: dynamic proxy, static implantation (compiled after implantation) 
to achieve the Spring: JDK dynamic proxy (InvocationHandler and proxy class), CGLIB agent

spring boot - on jpa, if logic and envisioned entirely correct, but the result is not correct, it should be how the investigation process
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Configuration: spring.jpa.show-sql
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Mybatis familiarity
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Mybatis Common Knowledge
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Work commonly used design patterns

Creating type: Singleton, Factory, Abstract Factory, Builder 
structure type: adapter, combiner, agents, decorators 
behavior: Visitors, viewers, strategy

mysql - how to solve the million level data, queries are slow

Cache 
query optimization (index to avoid full table scan): Note that the index failed several cases 
database cluster (master / slave), database table hash, load balancing

Degree of familiarity redis database

Description: Redis is an open source (BSD license), the data structure stored in the memory system that can be used as a database, cache and messaging middleware. 
Advantages: speed, support for many types of data, support services, expiration settings 
supported data types: string, hash, list, set , zset 
persistent way: RDB and AOF (see below) 
Note: Master best not write snapshots (will block the main thread) and AOF 
other: redis is a single-threaded process, with the queue control serial access

Common Commands

redis-cli -h host -p port -a password to start a client (without the local parameter) 
of ping: Check redis service is started (return Pong) 
quit: Close connector 
select xx: Switch to the specified database 

keys pattern (such as keys * Search key) 
eXISTS key: whether key exists 
EXPIRE key seconds (timestamp): setting key expiration time 
PEXPIRE key milliseconds: invalidation of milliseconds 
the sET 
SETNX: If the key already exists, 0 is returned 
...

Cache Cache penetration and avalanche

Cache penetrate the 
general caching system, it is in accordance with the key to cache query, if the corresponding value does not exist, they should go to find the back-end systems (such as DB). Some deliberately malicious request query key is not present, a large amount of the request, it will cause a lot of pressure on the back-end systems. 
This is called caching penetration. 
How to avoid? 
1 : null query result is also the case where caching and set shorter, or the data key corresponding to the dirty cache after the insert.
2 : is not present on certain key filtered. We can put all the possible key into a large Bitmap by the bitmap filter query. Avalanche cache 
when the cache server reboot or a large number of cache concentrated in one time period fail, so that when failure, back-end systems will bring a lot of pressure. Cause the system to crash. 
How to avoid? 


1 : After a cache miss, by locking to control the number of threads or queue database read write cache. For example, a key for allowing only one thread to query the data and write cache, other threads wait.
2 : Make secondary cache, cache the original A1, A2 copy is cached, when A1 fails, access to A2, A1 cache expiration time is set short-term, A2 is set for long-term
 3 : Different different key, set the expiration time for the cache invalidation time points as uniform as possible.

Comparative RDB and AOF
 . 1 , aof file update frequency higher than rdb, aof preferably used to restore data.
2 , AOF of more secure than rdb is greater
 3 , rdb performance better than the AOF of
 4 , if both are loaded with priority AOF

RESP agreement

RESP is a communication protocol used before redis client and server; 
RESP features: simple, fast parsing and readable 
the For the Simple Strings at The First byte of Reply at The IS  " + " replies 
the For Errors at The First byte of Reply at The IS  " - " error 
the For integers at The First byte of Reply at The IS  " : " integer 
the For Bulk strings at The First byte of Reply at The IS  " $ " string 
the For arrays at The First byte of Reply at The IS  " * " array

Architecture type

1 Single.
 2 master copy from: reduce the pressure reading, but can not solve the availability 
3. Sentinel (Sentinel): automatic failover to ensure high availability 
4. Cluster (twemproxy) 
5. The clusters (straight type)