On teh way

Git Abstraction Git is widely used in current software development routine to do the version control of code. With the help of IDE like IntelliJ Idea , we can avoid manually inputing git command in command line, like git add , git commit , git push etc. But when some things goes wrong, it always need command line operations. Today, we are not going to introduce git command, but to understand how git works and what abstraction it have. As soon as we make it clear, we can understand many restrictions of git usage and many command in depth. So, when we met a new problem, we can infer a new solution from what we known, rather than just relying on Google. Abstraction As Torvalds explains, Git is first designed as a file system, so it has different concepts as old VCS system. In many ways you can just see git as a filesystem – it’s content-addressable , and it has a notion of versioning Git is a content-addressable filesystem, which means that we can retrieve th...

Iterator Design Pattern We have talked the usage of iterator when we need to interact with collections: Not expose internal state to outside world: Increasing cohesion of class and avoid mis-operation; A unified interface to operate on different kinds of operation; Enforce the order of method call; Iterator Pitfall public interface Iterator < E > { boolean hasNext ( ) ; E next ( ) ; default void remove ( ) { throw new UnsupportedOperationException ( "remove" ) ; } default void forEachRemaining ( Consumer < ? super E > action ) { Objects . requireNonNull ( action ) ; while ( hasNext ( ) ) action . accept ( next ( ) ) ; } } Checkout the interface of iterator (two more default method added in Java8), and we can see the common usage routine of Iterator . while ( hasNext ( ) ) T t = next ( ) ; // do with t But it is not so friendly for n...

SonarQube Gitlab Integration In order to improve the code quality, our team decided to install the SonarQube to do the static code analysis. Easy Install The installation can be done very easily using docker compose: version: "2" services: sonarqube: image: sonarqube ports: - "9002:9000" environment: - SONARQUBE_JDBC_URL = jdbc:postgresql://db:5432/sonar volumes: - ./sonarqube_conf:/opt/sonarqube/conf - ./sonarqube_data:/opt/sonarqube/data - ./sonarqube_extensions:/opt/sonarqube/extensions - ./sonarqube_bundled-plugins:/opt/sonarqube/lib/bundled-plugins db: image: postgres environment: - POSTGRES_USER = sonar - POSTGRES_PASSWORD = sonar volumes: - ./postgresql:/var/lib/postgresql # This needs explicit mapping due to https://github.com/docker-library/postgres/blob/4e48e3228a30763913ece952c611e5e9b95c8759/Dockerfile.template#L52 - ./postgresql...

Elasticsearch MySQL Sync Challenge (5): Redesign Multiple Syncer? “You may have already heard the new requirement: listening for the data change of main DB and send the data to auth system. What is your idea?” Leader said. "Em, the simple way is of course to write a similar config files as we did before like following: input : masters : - connection : address : 192.168.1.1 port : 3306 filter : ... output : mysql : - connection : address : auth - system --- - input : masters : - connection : address : 192.168.1.1 port : 3306 filter : ... output : elsticsearch : connection : search - system “Then we can start two separate syncer to finish the job of data synchronization for both search system and auth system.” Tony finished. “Yes, it works. But seems not so good.” “Em, yep. Two syncer connect to same data source in which they receive t...

GC Roots This blog is about the a question I received: Are the GC roots of Minor GC and Major GC the same? GC Algorithm If we are asked to come up with a GC collection algorithm, we may think about a solution : in which we maintain the count of reference to some objects, if count drop to 0, we claim the memory. This is called Reference Count , which can’t solve the cyclic reference, is a wrong algorithm. The GC collection algorithm adopted by JVM is marking algorithm from GC roots. The GC roots sounds very mysterious, but its meaning is very simple: GC roots is the object that live outside Java heap and referring to object in Java heap. From the Hotspot JVM source code, we can see the following GC roots types: enum RootType { universe = 1 , jni_handles = 2 , threads = 3 , object_synchronizer = 4 , system_dictionary = 5 , class_loader_data = 6 , management = 7 , ...

Java IO (4): AIO Before we continue to dive into the concepts and design of AIO (asynchronous IO), we need to make the differences between NIO (non-blocking IO) and AIO, from the kernel perspective. Difference Between Blocking and Synchronous We have already heard the phrase like Blocking IO and Synchronous IO , but what the differences here? In order to understand it, we need to see that when we do a IO operation, it generally has two phases: Our user process submit the IO operation to OS kernel, i.e. call a System Call of OS: If our user process is blocked when submitting IO job to OS kernel, we call it as Blocking IO . If it is not blocked, on the other hand, we call it Non-blocking IO . The OS kernel coordinate many kernel process, like Disk Driver & File System , to do the real IO operation: If our process still waiting when OS doing real IO operation, we call it Synchronous IO . Otherwise, it is called Asynchronous IO . So we can see how Java I...

Exception Handle in Executor Today, we start with a question: can following code compiles? // pre-condition: Thread#sleep throws InterruptedException ExecutorService service = Executors . newCachedThreadPool ( ) ; service . submit ( ( ) - > { Thread . sleep ( 1 ) ; return null ; } ) ; service . submit ( ( ) - > { Thread . sleep ( 1 ) ; } ) ; The short answer is no and the compiler will complain the second invocation of sleep but not the first one: service . submit ( ( ) - > { Thread . sleep ( 1 ) ; // <------ here } ) ; Submit A Task So what happens here? Let’s see the prototype of submit: < T > Future < T > submit ( Callable < T > task ) ; Very normal and meet our imagination. But in the second submit, do we really give a Callable ? No, we don’t return in the second submission. It’s actually a Runnable . Future < ? > submit ( Runnable task ) ; Comparing the prototype of ...